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Accurately predicting the estimated ultimate recovery (EUR) of shale gas wells is key to formulating a shale gas reservoir development plan. However, in practice, determining the EUR remains challenging due to the complex dynamic characteristics of shale gas production, which first decreases rapidly and then slowly. In this study, based on material balance theory and equivalent seepage resistance theory and considering crucial factors including primary water, adsorption, and pore effects, a new production model for fractured horizontal shale gas wells is developed. The calculation process is designed by using Newton's iterative method. The shale gas well EUR prediction method is verified, and the factors influencing the EUR are analyzed. The results show that adsorption has a significant effect on production, especially on the Langmuir volume. Moreover, ignoring the influence of primary water, which exists in shale gas reservoirs in the form of bound water, results in an overestimation of the EUR. Furthermore, production positively correlates with the fracture half-length and the number of fractures, but the action mechanisms of these two factors differ. Unlike the number of fractures, which predominantly affects the initial stage of production, the fracture half-length has a more nuanced role. It is capable of altering the stimulated reservoir volume zone, thereby exerting influence over the entire production life cycle.

The lithofacies and thermal maturity of the over-mature Lower Cambrian marine shale in the Northern Guizhou Region, and their impacts on reservoir properties in this shale were analyzed by combining geochemistry, mineralogy, and gas adsorption methods. Ten lithofacies were identified, and the dominant lithofacies in the studied shale are lean-total organic carbon (TOC) argillaceous-rich siliceous shale (LTAS), medium-TOC siliceous shale (MTSS), and rich-TOC siliceous shale (RTSS). Since the gas generation potential of organic matter was weak, meso- and macro-pores were compressed or filled during the thermal evolution stage with a vitrinite reflectance (RO) range of 3.0%–4.0%. The controlling factors for methane adsorption capacity in the shale samples are significantly influenced by TOC content rather than thermal maturity. Among the RTSS, MTSS, and LTAS samples, RTSS exhibits the highest favorability for preserving hydrocarbon gas, followed by MTSS. The shale types in this study play a significant role in determining the properties of shale reservoirs, serving as an effective parameter for evaluating shale gas development potential. The RTSS and MTSS with a RO range of 2.0%–3.0% stand out as the most favorable target shale types for shale gas exploration and development.

The Lower Silurian Longmaxi Formation is a major horizon for shale gas development in the Changning block in Sichuan Basin. In this study, the fracture features in the Longmaxi Formation in the Changning block were investigated through outcrop observation, core description, inclusion testing, and Fullbore Formation MicroImage logging. The results showed that tectogenetic shear fractures dominated the Longmaxi Formation, with maximum principal stress in the SSW, NW, and NE directions. According to fracture features and stress analysis, three phases of faults were generated by tectonic movements after the buried depth reached a maximum in the study area: phase 1 nearly EW-trending faults formed in the middle-late Yanshanian, phase 2 NE-trending faults formed from the end of the Yanshanian to the early Himalayan, and phase 3 NW-trending faults formed from the middle Himalayan to the present. According to the regression analysis of the tested shale gas production and faults, the faults with a throw of > 200 m could have a great effect on gas production, and high-yield wells were generally located over 1 km away from faults. Based on the dissection of tectonic styles and typical wells drilled in the Changning block, three shale gas accumulation models were established: wide gentle syncline + internal smalxl fault, subdued anticline + large fault, and slope + internal fault. The study reveals the mechanism of fault influence on shale gas accumulation in the Longmaxi Formation in the Changning block and provides a reference for efficient shale gas production in the Sichuan Basin.

Preservation conditions are key to enriching shallow shale gas. Therefore, the veins of two typical wells in the Shixi syncline in northern Guizhou as research objects. Based on core observation, vein thin-section observation, cathodoluminescence, calcite in situ U-Pb dating, fluid inclusion microthermometry, Raman spectrum shift, single well basin simulation, and C-O isotope geochemical analysis, the researchers clarified the characteristics of the veins and the differences in paleofluid activity, as well as their significance for shale gas preservation. The results showed that: 1) a small number of high-angle fractures had developed mainly in the Shixi syncline in northern Guizhou. The width of viens is small and filled primarily with early calcite-quartz and late calcite. The inclusions were mainly methane and brine. The proportion of brine inclusions in the SD1 well was greater than that in the SX1 well. The SD1 well has experienced more intense second-stage uplift and denudation, and the consequent gas loss is serious. 2) The results of the C-O isotope analysis showed that most of the vein-forming fluids were derived from marine carbonate rocks. The O3b had obvious negative deviations of δ13C and δ18O, which were modified by exogenous or deep hydrothermal fluid. The C-O isotope difference between the vein bodies and the surrounding rock of the SX1 well was small, and the preservation conditions were better. 3) The difference in gas content in the Shixi syncline depends on the degree of damage to gas reservoir preservation conditions caused by burial depth and other factors. Shixi syncline is a wide and gentle syncline with central retention enrichment mode. Therefore, clarifying the development of shale veins, the characteristics of paleofluid activity, and their significance for shale gas preservation can lay a foundation for studying the enrichment mechanism of shallow shale gas and thus guide further exploration and development.

In the calculation of shale gas resources/reserves, isothermal adsorption parameters are mainly determined by dry sample data at the laboratory temperature, and the regression effect of total organic carbon content (TOC) isothermal adsorption parameters is poor. Based on the physical significance of key parameters in Langmuir and Henry adsorption models, the problems in determining the isothermal adsorption parameters in the calculation of shale gas reserves/reserves are analyzed by taking the marine shale in the Upper Ordovician Wufeng Forming–Lower Silurian Longmaxi Formation in the Sichuan Basin as an example. Based on a large number of measured isothermal adsorption data, the effects of TOC, temperature and water saturation on the shale gas adsorption parameters are analyzed. The influence model of TOC, temperature and water saturation on isothermal adsorption parameters of shale gas is established, and a novel method for calculating the adsorption parameters of shale gas from under experimental conditions to original reservoir conditions is proposed. The results are obtained as follows. First, there are problems in the calculation of shale gas resources/reserves, such as poor and contradictory fitting relation between isothermal adsorption parameters and TOC, difficulty in the conversion of Van't Hoff equation, and difficulty in direct application of water saturation (Sw) in resource/reserves calculation. Second, the problem of poor direct fitting relation between Langmuir pressure (pL) and TOC is solved by integrating the Langmuir adsorption model with the Henry adsorption model and by establishing a new parameter – Henry constant a for methane adsorption capacity of shale. Third, when Sw30 MPa), the effect of Sw on the methane adsorption capacity of shale shows a similar trend, and the effect of water molecules on the adsorption capacity on shale pore surface is constant. It is concluded that the proposed novel method for determining the isothermal adsorption parameters of shale with different TOC, temperature and water saturation conditions provides a technical support for the accurate calculation of shale gas resources/reserves and contributes to the scientific development of shale gas industry.

In 2012, China's first national shale gas demonstrations areas were set up in the Sichuan Basin. After 10 years' construction and practice, the giant marine shale gas area of 10 × 1012 m3 level is built up in the Sichuan Basin, and shale gas steps into a new stage of large-scale benefit exploration and development. In order to systematically summarize the achievements in shale gas exploration and development and provide guidance and reference for the exploration and development of deep shale gas and shale oil & gas in other areas, this paper systematically summarizes the main characteristics, development models and key identification and evaluation technologies for quality reservoirs of Wufeng Formation–Longmaxi Formation shale gas by analyzing the electric property, lithofacies, reservoir parameters and microscopic porosity of key wells in the basin. And the following research results are obtained. First, biogenetic siliceous shale, calcareous shale and mixed shale are main lithofacies types in quality shale gas reservoirs, and they are formed in the environment of semi-deep and deep water continental shelf. Their lateral distribution is controlled by the paleogeomorphology and their vertical development is influenced by provenance, redox condition and paleo productivity. They are 25–90 m thick. Second, in the quality shale gas reservoirs develop organic pores, inorganic pore and microfractures (including lamina/bedding fractures), among which, organic pore is one of the main reservoir spaces and microfracture is not only indispensable reservoir space, but also production pathway. The reservoir space of shale gas is overall micro-nano pore, and macropores play an important role in shale gas enrichment. Third, three development models of quality reservoir are established, including sedimentary type, diagenesis type and reworking type. The sedimentary type is the foundation. Multiple quality reservoirs are developed in the high U/Th interval of graptolite belt at the bottom of Longmaxi Formation, and their thickness is mainly controlled by paleogeomorphology and especially greater in the depression area. The diagenesis type is divided into three forms, i.e., syngenetic-early diagenetic rigid support, middle-late diagenetic mineral-organic matter transformation, and overpressure relief compaction. The reworking type is dominated by quality reservoirs with microfractures. Fourth, the core technologies for identifying and evaluating quality shale gas reservoir include large-size core and rock slice observation, high-accuracy rock mineral identification, experimental gas content test and simulation, SEM microscopic characterization, 3D microscopic pore reconstruction, comprehensive geophysical interpretation and prediction and big data analysis. In conclusion, nearly 10 years' research and practice achievements in demonstration area construction can deepen the understanding on domestic quality shale gas reservoirs, promote the effective development of the theories and technologies related to shale gas reservoirs, improve the prediction accuracy of shale gas sweet spot zones/intervals, and expand the shale gas exploration and development achievements of demonstration areas.

During volume fracturing of shale gas reservoirs, hydraulic fractures may readily communicate with natural fractures to propagate forward and induce the formations to slip along the fracture surfaces. The resulted inter-well frac-hit and casing deformation affect the safe and efficient operation of shale gas fracturing. In addition, unpropped fractures caused by small natural fracture width lead to deteriorating fracture conductivity, which in turn impacts the stimulation effect of shale gas reservoirs. This paper discusses the three key issues, i.e. inter-well frac-hit, casing deformation and unpropped microfractures, that impact the economic exploration and exploitation of shale gas, and proposes engineering prevention and control measures through literature review and research on mechanism by integrating theoretical and experimental analysis, which have been applied on site. Firstly, after clarifying the mechanism and main controlling factors of inter-well frac-hit, an evaluation model and prediction method of frac-hit based on machine learning were established. The measures for preventing and controlling inter-well frac-hit, including temporary plugging at fracture tip and shut-in of old wells, were determined after evaluation with the well-cluster fracture model. Secondly, an analysis model of casing deformation caused by fracture shear and slippage was established after stress analysis. According to the analysis of stress on casing intersected with fractures during fracturing, it is ascertained that increase of fluid pressure within natural fractures is the main factor that causes casing deformation. The methods for preventing casing deformation were proposed in terms of fracturing operation and well construction. Thirdly, the mechanism of micro-proppant migration was analyzed by integrating the model of particle migration and the transport experiment in large-scale plate, and the experiment confirms that micro-proppant can effectively improve the fracture conductivity. It is concluded after field application that the prevention and control measures proposed for inter-well frac-hit and casing deformation can mitigate frac-hit and casing deformation significantly, and micro-proppants are conducive to improving post-frac shale gas production. The measures provide a support for large-scale and economic development of deep shale gas.

Although significant progress has been made in the geological theory and exploration and development of shale gas in China, the research on the geological particularity and enrichment regularity associated with shale gas accumulation remains weak. With this in mind, we analyzed the shale gas accumulation patterns in China. The findings show that: (1) A variety of organic-rich shales were deposited in the continental margin-type marine and transitional environments during the Pre-Mesozoic period and the basin-type terrestrial environment during the Mesozoic and Cenozoic eras. (2) The basins dominated by Mesozoic and Cenozoic terrestrial sediments superimpose on the Pre-Mesozoic continental margin-type marine–or transitional-facies sediments. The former developed the terrestrial shales with large thickness, high organic matter abundance, new age, and low thermal maturity; meanwhile, the deep parts of the basins are favorable for shale gas enrichment. However, the latter preserves the shale with old age, high thermal maturity, and undergoing multiphase tectonic movements; and the tectonically stable regions are favorable for shale gas enrichment. (3) The aulacogens mainly formed in the Meso-Neoproterozoic, Sinian-Ordovician, and Devon-Triassic periods are favorable for marine organic-rich shale deposition; moreover, this type of shale and other types including transitional facies shales with favorable reservoir-forming assemblages, coal-measure shales in small and medium-sized basins, and low-medium mature shales affected by magma intrusion, have jointly constituted the various shale gas accumulation patterns in China. (4) Based on the drilling discoveries in China, shale gas accumulation patterns can be categorized into two types: tectonic-controlled and stratigraphic-controlled. The former is mainly related to structural features, whereas the latter is mainly related to the variation of lithology and strata. (5) Shale development and gas content are mainly affected and controlled by geological factors including sedimentation, diagenesis, and later tectonic movements. Favorable sedimentary environments, appropriate thermal maturity, good sealing capacity of caprock, and excellent structural preservation are the necessary conditions for shale gas enrichment.

Affected by reservoir heterogeneity, developed natural fractures, and bedding fractures, the fracturing pressure curves in fracturing of shale gas horizontal wells present complex shapes. A large amount of information contained in the fracturing curves is still not fully excavated. Based on the theory of shale gas fracture network fracturing, the calculation model of bottom hole net pressure is established by integrating the real-time data such as casing pressure, pump rate, and proppant concentration. Net pressure slope and net pressure index are constructed as key parameters, and the net pressure curve is divided dynamically to describe the mechanical conditions corresponding to the fracture propagation behavior during the fracturing process. Six fracture propagation modes were identified, including fracture network propagation, fracture propagation blockage, normal fracture propagation, fracture propagation long bedding, fracture height growth, and rapid fluid filtration, and then the operation pressure curve diagnosis and identification method were formed for shale gas fracture network fracturing in horizontal wells. The shortcomings of conventional operation curve diagnosis and identification methods are abandoned and the fracture network complexity index is presented. The higher index indicates more time of fracture network propagation and fracture propagation along bedding and the better reservoir stimulation effect. The model is applied to shale gas wells in the southeastern margin of Sichuan Basin, and the average fracture network complexity index of a single well is 0.3, which is in good agreement with the microseismic monitoring results. This proves the good reliability of the method developed. The method is helpful to improve the potential and level of fracturing stimulation of shale reservoirs and is of great significance for improving the post-fracturing evaluation technology of fracture network and guiding the real-time dynamic adjustment of field fracturing operations.

When deep and ultra-deep shale gas well fracturing is carried out, multi-cluster fracturing can hardly realize synchronous initiation and propagation of hydraulic fractures due to the combined effects of heterogeneity of deep in-situ stress and ''dense cluster'' fracture arrangement, and the strong interference between fractures aggravates the unbalanced fracture propagation degree. Field practice proves that the fracture-opening temporary plugging fracturing technology can effectively control the unbalanced propagation of multiple fractures. In addition, the application effect of temporary plugging process can be improved by developing a method for simulating fracture control during fracture-opening temporary plugging fracturing of deep/ultra-deep shale-gas horizontal wells. Based on rock mechanics, elasticity mechanics, fluid mechanics and fracture propagation theory, combined with the flow distribution equation of horizontal-well multi-cluster fracturing and the plugging equation of temporary plugging balls, this paper establishes a fracture propagation model and a fracture control simulation method for the fracture-opening temporary plugging fracturing of deep/ultra-deep shale gas horizontal wells. Then, the influences of the number of temporary plugging balls and the times and timing of temporary plugging on temporary plugging control are simulated, and the influences of temporary plugging balls on fracture propagation morphology and SRV (stimulated reservoir volume) distribution are analyzed by taking Sinopec's one deep shale gas well in Dingshan-Dongxi structure of southeast Sichuan Basin as an example. And the following research results are obtained. First, fracture-opening temporary plugging can significantly promote the balanced propagation of multiple fractures, and the simulation confirms that the number of temporary plugging balls and the times and timing of temporary plugging play an important role in fracture control. Second, as the number of temporary plugging balls increase, the SRV increases firstly and then decreases, so there is an optimal number of temporary plugging balls. Third, increasing the times of temporary plugging can improve the fault tolerance rate of temporary plugging and diverting process, but it is necessary to increase the number of temporary plugging balls appropriately. Fourth, when the timing of temporary plugging is appropriate, the balanced propagation of multiple fractures is achieved and the maximum SRV is reached. In conclusion, this method is of great significance to optimizing the design of temporary plugging fracturing, improve the implementation level of field process and develop deep and ultra-deep shale gas efficiently.

Shale, which is a fine-grained sedimentary rock, differs in reservoir quality (RQ) and completion quality (CQ) as the result of variations in its mineral and organic-matter compositions. The RQ and CQ directly affect shale gas exploitation. Therefore, classification of the lithofacies types of shale, analysis of the reservoir differences of different lithofacies, and identification of the superior shale lithofacies with the best RQ and CQ are important for decisions on well placement, targeting the “sweet spot” zone and optimizing the expected ultimate recovery of a shale gas well. In this study, several methods including microscopic observation of thin sections, X-ray diffraction analysis, core analysis and elemental geochemistry were used to investigate the lithofacies types, reservoir characteristics and silica origin of the marine shales of the Wufeng Formation–Longmaxi Formation (WF Fm.–LMX Fm.) in the Luzhou area, southern Sichuan Basin, China. The lithofacies characteristics exhibit marked differences through the WF–LMX shale succession. Specifically, the lower gas-bearing zone (LMX-1) is mainly composed of siliceous shales and siliceous–argillaceous mixed shales, whereas the upper gas-bearing zone (LMX-4) mostly contains argillaceous shales and argillaceous–siliceous mixed shales. The lower shale gas reservoirs have a higher silica content and lower clay and carbonate contents than the upper zones, and thus superior petrophysical qualities. The upper part of the WF Fm. (WF-2) and the lower part of the LMX Fm (LMX-1, LMX-2) are rich in bio-chemical authigenic silica, whereas the silica minerals of other intervals are mainly derived from allochthonous terrigenous material. The siliceous shales and siliceous–argillaceous mixed shales with a biogenic quartz content greater than 40% have the best RQ and CQ and are the most promising lithofacies for shale gas in the Luzhou area. Although the upper gas-bearing zones represented by LMX-4 are also good shale gas reservoirs, the RQ, CQ and gas content are generally worse than those of the lower gas-bearing zones because of the silica origin and lithofacies types; thus, extracting shale gas from the upper zones would be a greater challenge.

Unconventional reservoirs have become the main alternative for increasing oil and gas reserves around the world. Owing to their ultralow permeability properties and special pore structure, hydraulic fracturing technology is necessary to realize the efficient development and economic management of unconventional resources. To maximize the production capacity of wells, several fracture parameters, including fracture number, length, width, conductivity, and spacing, need to be optimized effectively. The optimization of hydraulic fracture parameters in shale gas reservoirs generally demands intensive computations owing to the necessity of numerous physicalmodel simulations. This study proposes a machine learning (ML)–assisted global optimization framework to rapidly obtain optimal fracture parameters. We employed three supervised ML models, including the radialbasis function, K-nearest neighbor, and multilayer perceptron, to emulate the relationship between fracture parameters and shale gas productivity for multistage fractured horizontal wells. Firstly, several forward shale gas simulations with embedded discrete fracture models generate training samples. Then, the samples are divided into training and testing samples to train these ML models and optimize network hyper parameters, respectively. Finally, the trained ML models are combined with an intelligent differential evolution algorithm to optimize the fracture parameters. This novel method has been applied to a naturally fractured reservoir model based on the real-field Barnett shale formation. The obtained results are compared with those of conventional optimizations with high-fidelity models. The results confirm the superiority of the proposed method owing to its very low computational cost. The use of ML modeling technology and an intelligent optimization algorithm could greatly contribute to simulation optimization and design, prompting progress in the intelligent development of unconventional oil and gas reservoirs in China.

The first fractured shale gas well of China was constructed in 2010. After 10 years of development, China has become the second country that possesses the core technology of shale gas development around the world, realized the shale gas fracturing techniques from zero to one and from lagging to partially leading, and constructed the fracturing theory and technology system suitable for middle–shallow marine shale gas exploitation. In order to provide beneficial guidance for the efficient exploitation of shale gas in China in the future, this paper comprehensively reviews development history and status of domestic fundamental theories, optimized design methods, fluid systems, tools and technologies of shale gas fracturing and summarizes the research results in fundamental theories and optimized design methods, such as fracturing sweet-spot cognition, fracture network propagation simulation and control, rock hydration and flowback control, and SRV (stimulated reservoir volume) evaluation and characterization. The development and application of slick-water fracturing fluid system and new fracturing fluid with little or no water is discussed. The development and independent research & development level of multi-stage fracturing tools are evaluated, including drillable composite plug, soluble plug, large-diameter plug and casing cementing sleeve. The implementation situations of field technologies and processes are illustrated, including the early conventional multi-stage multi-cluster fracturing and the current “dense cluster” fracturing and temporary plugging fracturing. Based on this, the current challenges to domestic shale gas fracturing technologies are analyzed systematically, and the development direction of related technologies is forecast. In conclusion, it is necessary for China to continuously research the fracturing theories, technologies and methods suitable for domestic deep and ultra-deep marine shale gas, terrestrial shale gas and transitional shale gas to facilitate the future efficient development of shale gas in China.

China's shale gas production in 2020 exceeds 200 × 108 m³, which creates a miracle in the history of natural gas development in China. The Sichuan Basin has already been and will be the main battlefield of shale gas exploration and development in China. In order to further promote the large-scale efficient development of shale gas in China, under the new situation of global COVID-19 spread and domestic “carbon peak and carbon neutrality” goal, this paper analyzes the progress and challenges of shale gas exploration and development in the Sichuan Basin from four aspects, including resource exploration, gas reservoir engineering, drilling and production engineering and industry regulation, and puts forward countermeasures and suggestions for achieving large-scale efficient development of shale gas. The following research results are obtained: First, the large-scale efficient development of shale gas in the Sichuan Basin has to take the sustainable and stable production of middle–shallow shale gas and the large-scale productivity construction of deep shale gas as the base. Second, compared with the shale gas exploration and development in the North America, the Sichuan Basin has its own characteristics in terms of geographical setting, geological condition, drilling and production technology and industry regulation, which makes it difficult to copy the development pattern of large scale, high density and continuous well deployment from the North America, so it is necessary to adopt the strategy of “high production with few wells”. On the one hand, continue to apply the geology and engineering integration technology to carry out “integrated research, integrated design, integrated implementation and integrated iteration” in the whole life cycle of shale gas well; and on the other hand, carry out problem-oriented continuous researches from the aspects of geological evaluation, development policy, engineering technology and industry regulation, so as to improve geological evaluation theory and technology, innovate gas reservoir engineering theory and method, research and develop engineering technology for cost reduction and efficiency improvement, improve shale gas industry regulation, and form a new pattern of collaborative promotion of technical and non-technical elements. In conclusion, the research results provide important reference and guidance for the large-scale efficient development of shale gas in the Sichuan Basin and even the whole country.

To understand the shale gas reservoirs in the first member of the Permian Maokou Formation (Mao 1 Member) in Nanchuan area of the Sichuan Basin, we used outcrops, cores, mineralogical and geochemical datas, thin-section analysis, and argon-ion-polishing scanning electron microscopy results with logging and seismic attribute prediction techniques to study the sedimentary facies, source rocks, reservoir properties and types. The geological characteristics and enrichment model of shale gas reservoirs of the Mao 1 Member were revealed and are summarized as follows: (1) the Maokou Formation can be divided into four members from bottom to top. The Mao 1 Member is located in the relatively deep-water gentle slope of a carbonate platform, comprising well-developed dark-gray/grayish-black carbonaceous mudstone, calcareous mudstone, argillaceous limestone, and bioclastic limestone. Carbonaceous and calcareous mudstones, with a relatively high total organic carbon content (TOC0.5–5.1%) and moderate thermal maturity (Ro: 2.0–2.3%), are potentially good hydrocarbon source rocks with possibly new type of atypical shale gas reservoir; (2) organic pores, dissolution pores, grain margin fractures, and shrinkage fractures are developed in the Mao 1 Member and may form a reservoir of a complex “pore–fracture–network,” which is beneficial to gas storage and migration; (3) the shale gas in the Mao 1 Member has the accumulation and enrichment model of “integrated source and reservoir, continuous distribution, and structure-fracture dual reservoir control” with a continuous distribution area exceeding 500 km2 and a geological resource volume exceeding 70 billion cubic meters; therefore, it has great exploration potential for shale gas. The research results are crucial in improving the understanding of new type of shale gas geology and guide shale gas exploration in southeastern Sichuan.

China is highly dependent on foreign oil and gas, and the exploration and exploitation of shale gas is an important way to alleviate the energy and environmental pressure of China. Although a series of significant progress has been made in a short time, China is still at the crossroads of shale gas exploration direction. Therefore, this paper systematically reviews the particularity of shale gas geological conditions in China, and points out China's exploration direction of shale gas in different fields combined with the research achievements of shale gas accumulation mechanism. The following results were obtained. (1) China's shale gas exploration fields can be divided into three major parts, i.e., marine, continental and marine–continental transitional facies. The geological characteristics of shale gas in North China, Yangtze and Tarim plates are different. The interaction of multiple factors, including tectonic and sedimentary evolution, shale gas accumulation, late-stage transformation and etc., has led the complexity of the geological distribution and the diversity of exploration fields of shale gas in China. (2) The organic pores and fractures of marine shale are developed. The Ordovician Wufeng-Silurian Longmaxi Formation will still be the primary target of shale gas exploration in future. Meanwhile, the Cambrian Niutitang and Sinian Doushantuo Formation in the Yangtze region, the Devonian Luofu and Carboniferous Jiusi Formation in the Yunnan-Guizhou-Guangxi region, the Jixianian Hongshuizhuang and Mesoproterozoic Xiamaling Formation will be the targets of strategic breakthrough in shale gas exploration. (3) The structural deep and slope of middle-large basins and the sedimentation centers of small-middle basins are the main directions of continental shale gas exploration. Inter-laminar cracks and intergranular pores are well developed in continental shale. The coupling of various organic matter types and thermal evolution results in the symbiosis of shale oil and shale gas, which will become an important field for further exploration of continental shale gas. (4) The marine–continental transitional shale, which mainly formed in Late Carboniferous-Permian, is characterized by cyclic association with sandstone, mudstone, coal and carbonate. Diagenetic fractures are well developed in transitional shale. The favorable structural and diagenetic preservation are the two major factors for the enrichment of this type of shale gas. The flat-lagoon and delta sedimentary systems, middle-large superimposed and small-middle residual basins, and deep layers with quality caprock and good structural preservation are the favorable directions for shale gas exploration in transitional facies.

Shale of the Middle–Upper Permian Dalong Formation and Gufeng Formation in the Lower Yangtze region is characterized by large thickness, high total organic carbon (TOC), wide distribution and moderate organic thermal evolution degree, so it may be the next important field of shale gas exploration. In order to point out the target and direction of shale gas exploration and development in this region, this paper selects the Dalong Formation and Gufeng Formation shale in the Xuanjing area of Lower Yangtze as the research object to quantitatively describe the development characteristics of shale pores of different scales by means of scanning electron microscopy (SEM), high pressure mercury injection, CO2 and N2adsorption. Then, the fractal dimension of sample pores is calculated by using the FHH (Frenkel–Halsey–Hill) model. Finally, combined with TOC and mineral compositions, the relationship between pore structure and fractal dimension is discussed. And the following research results are obtained. First, dominant lithofacies of Dalong Formation are argillaceous-rich siliceous shale and argillaceous-rich/siliceous mixed shale, which has smaller specific surface area and pore volume, larger average pore diameter. The pore development is mainly controlled by clay mineral content. Dominant lithofacies of Gufeng Formation is siliceous shale, which has a larger specific surface area and pore volume, and smaller average pore diameter. The pore development is controlled by organic matter and brittle mineral content. Second, the influencing factors of fractal dimension can be attributed to the controlling factors of micropore development. The fractal dimension D1of Dalong Formation ranges from 2.4515 to 2.5513 (average 2.5227), and D2ranges from 2.5817 to 2.6578 (averagely 2.6246). The fractal dimension D1of Gufeng Formation ranges from 2.5817 to 2.6578 (averagely 2.6246), and D2ranges from 2.7227 to 2.871 (averagely 2.813). Gufeng Formation shale presents the characteristics of more complex pore structure. Third, the fractal dimension D1is more sensitive to specific surface area, pore development and mineral composition, while D2is more effective in characterizing the average pore diameter. In conclusion, Dalong Formation shale with high D1and D2and Gufeng Formation shale with low D1and high D2can be taken as the favorable exploration targets of Permian marine shale gas in the Lower Yangtze region, and the regional overpressure zones (belts) with weaker structural deformation in hydrocarbon rich sags will be the favorable shale gas exploration targets in this area.

Porosity is a key parameter in shale gas reservoir evaluation and reserve calculation and its accurate test is the basis for calculating geological reserves of shale gas and determining development plans. In order to clarify the differences between different porosity test methods and their influences on the calculation results of shale gas reserves, we collected 65 shale samples of Lower Silurian Longmaxi Formation from six shale gas wells in the Zhaotong National Shale Gas Demonstration Area of the southern Sichuan Basin for comparative experiments using three porosity test method, including gas injection porosimetry (GIP) method, water immersion porosimetry (WIP) method and nuclear magnetic resonance (NMR) method. Then, these three methods were comparatively analyzed based on the test results. Finally, it was proposed to optimize the key parameters of these three shale porosity test methods. And the following research results were obtained. First, in terms of the GIP method, the particle size of shale sample shall be in the range of 20–60 mesh and the helium saturation equilibrium time shall be over 1800 s. Second, in terms of the WIP method, the sample shall be dried for at least 48 h under 110 °C and saturated for 24 h under the confining pressure of 15 MPa. Third, in terms of the NMR method, NMR porosity calculation shall not be conducted until the NMR signal of the dried sample is deducted, on the basis of echo time and waiting time optimization. Fourth, porosity average and median value obtained by these three shale porosity test methods follow the relationship of WIP porosity > NMR porosity > particle GIP porosity > plunger GIP porosity. Fifth, different shale porosity test methods have greater influences on the calculation results of shale gas geological reserves, whose difference can reach 20%. In conclusion, during the application of NMR method and WIP method, fluid is introduced for saturation, which may damage the shale pores. However, the particle GIP porosity can reflect the entire space of shale more comprehensively and is not influenced by the properties of the applied fluid. Therefore, it is suggested to adopt the particle GIP method to calculate shale gas geological reserves.

In order to accurately evaluate the fracturing stimulation effect of horizontal wells in shale gas reservoirs, it is suggested to establish a model to predict the temperature distribution in the wellbore of horizontal well and an inversion model of DTS data by using MCMC algorithm, and optimize the production profile interpretation process. In this paper, the characteristics of temperature profile of fractured horizontal wells in shale gas reservoirs were analyzed and the main factors affecting the temperature profile were figured out. Finally, the newly established inversion model was applied to the production profile interpretation of one case well in a certain shale gas reservoir. And the following research results were obtained. First, the temperature profile of fractured horizontal wells is in the shape of irregular “saw tooth”, and each “saw tooth” corresponds to an effective hydraulic fracture with fluid inflow. Second, the longer the fracture is, the greater the wellbore temperature drop at the corresponding fracture location is, and the gas flow rate in the fracture is positively correlated with the temperature drop. Third, from the perspective of influence degree, the factors influencing the temperature profile of fractured horizontal wells in shale gas reservoirs are ranked from the strong to the weak as follows: fracture half-length, gas flow rate, permeability of stimulated area, wellbore diameter, fracture conductivity, horizontal inclination angle, and comprehensive thermal conductivity, among which, the first three are main controlling factors. Fourth, the MCMC inversion method is applied to invert the DST temperature data of the case well. The temperature profile predicted in the model is better accordant with the measured DTS profile, and the absolute error of the predicted temperature at different levels of effective hydraulic fractures is less than 0.02 °C. The interpreted gas flow rate of each fracturing stage is closer to the field measurement, and the deviation of the maximum gas flow rate of a single fracturing stage is only 180.35 m3/d. The absolute error between single-well gas production rate and gas production rate measured at the wellhead is less than 3 m3/d, which proves the reliability of this newly developed inversion model.

With the rapid development of shale gas exploration and development in China, casing deformation in shale gas horizontal wells happens frequently, which directly impacts the development efficiency and benefits of shale gas. In order to explore casing deformation prediction, prevention and treatment methods, this paper analyzes the geological and engineering causes of casing deformation in shale-gas horizontal wells through laboratory work, such as the casing resistance to internal pressure alternating test, the ground simulation test and systematical casing deformation characteristic analysis of MIT24 caliper logging, and the large-scale physical simulation test and numerical simulation of casing deformation. Then, combined with the generalized shear activity criterion, a new method for evaluating casing deformation risk points and some technical measures for preventing casing deformation were formulated. And the following research results were obtained. First, the deformation characteristics of 119 casing deformation points in 23 wells interpreted by MIT24 caliper logging are consistent with the mechanical behaviors of shear compression deformation test. Second, the large-scale physical simulation test shows that natural fault-fractures slip obviously under the state of strike-slip stress. Third, numerical simulation shows that the compression stress on casing increases with the increase of fault-fracture slip. When the fault-fracture slip is between 7.5 mm and 9.0 mm, the casing reaches the critical yield strength and begins to undergo plastic deformation. The “temporary fracture plugging + long segment and multi-cluster” and other technologies are field tested in 28 wells in Weiyuan area of southern Sichuan Basin. The casing deformation rate decreases from 54% (before this research) to 14.3%, and the segment loss rate decreases from 7.8% to 0, which reveals remarkable achievements in casing deformation treatment. In conclusion, the shear slip of fault and macro fractures (referred to as fault-fracture) is the main cause of casing deformation in shale gas horizontal wells, and some measures (e.g. “temporary fracture plugging + long segment and multi-cluster”, reducing fracturing scale and releasing wellbore pressure properly) shall be taken in advance to reduce the fault-fracture activity before the risk point of casing deformation is fractured, so as to reach the goal of casing deformation prevention.

Cuttings accumulation in horizontal drilling of a shale gas well is one of the main reasons for high drilling torque and drag and serious backing pressure and consequently influencing the rate of penetration (ROP), so inhibiting the generation of a cuttings bed and keeping the hole clean is an important prerequisite to ensure the smooth and safe drilling of horizontal section. In order to improve the hole cleaning efficiency of horizontal sections, this paper studied the decay laws of helical flow induced by helix angles and rotation speed of a newly developed cuttings bed remover with V-shaped blades for horizontal wells and their influences on the resuspension distribution of cuttings particles by virtue of the CFD numerical simulation method and Euler–Euler binary-fluid model, combined with the theory of particle dynamics. Then, the parameters such as blade rotation speed and blade helix angle were optimized. And the following research results were obtained. First, with the increase of the rotation speed, cuttings deposit at the bottom of the annulus deflects to a certain degree. And the higher the rotation speed, the more obvious the deflection. Second, annulus pressure drop loss increases with the increase of the helix angle. And this phenomenon is more obvious when the helix angle is larger. Third, the field test results in Well Changning H25-7 of the southern Sichuan Basin show that after the application of the newly developed cuttings bed remover for horizontal wells, the drilling friction is decreased by 33 %, the tripping is smooth without sticking. And in the process of its application, no complex downhole accident happened. Obviously, it provides good hole cleaning to ensure the safe and smooth implementation of the later casing running operation. In conclusion, this newly developed cuttings bed remover can satisfy the demand of cutting bed cleaning and drilling cost reduction and efficiency improvement of horizontal wells, so it is worth popularizing and applying. What's more, it provides guidance and reference for the design of similar cuttings bed removers.

The Wufeng Formation–Longmaxi Formation in the Sichuan Basin in South China is the key stratum for shale gas exploration and production. To date, three national shale gas demonstration zones have been developed. Nevertheless, there are still some test wells that have not yet been commercialized. In this study, the geological characteristics of commercial and non-commercial zones are analyzed, as are the main controlling factors of high-producing wells (high estimated ultimate recovery; EUR), and the reasons for low-production wells (low EUR) by dissecting the three national shale gas demonstration zones and the main shale gas exploration wells. The results of this study indicate the following: (1) The black shale in the WF2–LM4 graptolite zone is deposited in the Craton depression on the Upper Yangtze plate, which provides a relatively stable tectonic environment for tectonic deformation and uplift destruction. The large shale thickness and weak tectonic activity jointly result in shale gas being enriched mainly in the deep-water shelf. (2)The regional fault has a destructive effect on shale gas preservation, and the shale gas reservoir is likely to be destroyed. In the areas close to the regional fault, multiple fracture-fluid migration activities caused by multistage tectonic movements are also detrimental to shale gas preservation. Conversely, shale gas is generally well preserved in areas far from regional faults. (3) The black shale thickness in the WF2–LM4 graptolite zone in the deep-water shelf area controls the shale gas field distribution. Furthermore, the horizontal well trajectory in the WF2–LM4 graptolite zone determines the shale gas well test production and EUR. The results of this work will provide a reference for shale gas exploration and development of the Wufeng Formation–Longmaxi Formation in the Sichuan Basin, as well as the Silurian strata in other parts of the world.

In the design of a shale-gas cluster horizontal well, it is necessary to consider the bypass of the fracturing influence domains of existing wells and the interference between fracturing influence domains when the wellbore trajectories of infill adjustment wells in the fracturing areas are designed. In order to quickly evaluate the rationality of the design scheme of fracturing wellbore trajectory in an infill adjustment well, this paper adopted the vector algebra method to build a geometric model of the obstacles in the shale gas fracturing area. In this geometric model, the influence domains of hydraulic fractures are taken into account. Then, based on this geometric model, the optimization design model of bypass trajectory in the shale gas fracturing area was established by taking the minimization of total trajectory length and trajectory potential energy as the optimization objective and the anti-collision between trajectories as the constraint. Besides, the geometric check method to judge if there is any interference between fracturing influence domains was provided. Finally, the established optimization design model was verified based on the actual drilling data of Fuling Shale Gas Field in the Sichuan Basin. And the following research results were obtained. First, the obstacle sizes in fracturing areas will be seriously underestimated if the fracturing influence domains are neglected. Second, if the fracturing influence domains are neglected, the designed bypass trajectory can bypass the wellbore trajectories of old wells, but may intersect the fracturing influence domains of existing wells, thus inducing drilling accidents. In conclusion, the proposed optimization design model of bypass trajectory in the shale gas fracturing area can satisfy the constraint of anti-collision and bypass and achieve the optimization objective of minimizing total trajectory length and trajectory potential energy, and the corresponding design calculation avoids complex calculation and check.

The Upper Ordovician WufengFm and the Lower Silurian LongmaxiFm are important strata for shale gas exploration and development in the Sichuan Basin, but the genetic mechanism, evolutionary history and the controlling effect of mineral diagenetic evolution on the formation of shale gas reservoirs are not clear. In this paper, the evolution history of organic matter pores and the diagenetic evolution of minerals were analyzed based on the analysis of petrology, mineralogy and organic geochemistry, combined with basin simulation and practical shale gas exploration and development. Then, the types and genetic mechanisms of high-quality shale gas reservoirs were discussed, and the development intervals of high-quality shale gas reservoirs were determined. And the following research results are obtained. First, the shale gas development intervals of Wufeng–LongmaxiFms in the Sichuan Basin are mainly dominated by siliceous shale, limy siliceous shale and clayey shale. Rock type has an important controlling effect on the types and characteristics of shale reservoir space. Siliceous shale and limy siliceous shale have the highest reservoir capacity with the most developed organic pores. Second, the diagenetic evolution of minerals controls the formation of shale gas reservoirs. Biogenic silica, formed in the early diagenetic stage, together with terrestrial detrital silica and pyrite, constitutes particle support lattices in the form of microcrystalline aggregates, so as to resist the compaction effectively and preserve a great number of residual intergranular pores, which is beneficial to the formation of high-quality shale gas reservoirs. Third, siliceous shale in the WF2–LM4 graptolite zone (from WufengFm to the bottom of LongmaxiFm) presents a high-quality reservoir genetic mechanism of “multicellular algae controlling hydrocarbon source, biogenic silica controlling framework, and co-evolution controlling a high-quality reservoir”. In conclusion, the siliceous shale and limy siliceous shale in the WF2–LM4 graptolite zones are the main development intervals of high-quality shale gas reservoirs in the Sichuan Basin. It is also a valuable reference for the Upper Ordovician-Lower Silurianshale gas exploration and development in other countries and regions worldwide.

The Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation in the central Yangtze area of southern China has a good prospect of shale gas exploration. So far, however, its complicated geological conditions and shale gas exploration and development potentials have not been understood completely, which affects its exploration achievements. In order to determine the main factors controlling shale gas enrichment in the Wufeng–Longmaxi Fms in this area, this paper studied the key shale gas enrichment conditions in the black shale there (e.g. sedimentary environments, reservoir development characteristics and preservation conditions) based on single-well comprehensive reservoir evaluation, combined with drilling, logging, core, outcrop and test data. In addition, it was compared with the main shale gas blocks in the Sichuan Basin, such as Jiaoshiba and Changning. And the following research results are obtained. First, the organic-rich shale section in this area was formed during the sedimentation from Wufeng Formation to the third submember of the first member of Longmaxi Formation. It is the deposit of deepwater continental facies, and its thickness is in a range of 15–39 m, presenting a trend of increasing from south to north. Second, the reservoir rocks in the high-quality shale sections are dominated by siliceous shale, and the reservoir porosity is in a range of 1.60–7.44%. The reservoir spaces are dominated by organic pores with good connectivity and high total organic carbon (TOC) content. The organic matter is of a sapropel–sapropel prone hybrid type, with a high thermal evolution degree, better gas bearing property and good fracturability. Third, the Dangyang synclinorium is characterized by better strata preservation, rock occurrence flat, less developed faults and thick and stable roofs and floors, so its shale gas preservation conditions are better. Fourth, different from the Changning and Jiaoshiba Blocks, the central Yangtze area is characterized by great burial depth, large bidirectional stress difference, and low formation pressure coefficient. In conclusion, the Wufeng–Longmaxi Fms is better in shale gas enrichment conditions and has a potential of further exploration. However, its commercial shale gas development and scale production increase in the future face challenges due to its large stress difference, great burial depth, and lower pressure coefficient.

In order to maximize the resource utilization rate, it is common to adopt one-time overall deployment of well pattern to develop shale gas, and the design of horizontal well spacing is the key to the deployment of shale gas well pattern. To determine the optimal well spacing, it is not only necessary to understand both geological characteristics and drilling fracturing technology, but also take into consideration the influences of economic factors, such as gas price and cost. At present, there is no reliable method for designing the well spacing of shale-gas horizontal wells at home and abroad. In this paper, a method for analyzing the well spacing of shale-gas horizontal wells based on the integration of geology, engineering and economy was established for the first time. Then, by means of geological modeling, numerical simulation and cash flow analysis, the well spacing of shale-gas development wells in Well Block Ning 209 in the Changning–Weiyuan National Shale Gas Demonstration Area in the Sichuan Basin was comprehensively evaluated by using estimated ultimate reserve (EUR), recovery factor and internal rate of return (IRR). And the following research results were obtained. First, under the current geological, engineering and economic conditions of Well Block Ning 209, the IRR of shale gas platform development can be kept greater than 8% if the well spacing is larger than 240 m. Second, when the well spacing is controlled between 330 m and 380 m, single well EUR, recovery rate of the platform and economic benefit can be considered simultaneously. In conclusion, the research results support the formulation of the shale gas development technology policy of Well Block Ning 209 and lay a foundation for the realization of its scale efficient development of shale gas.

Fracturing equipment is the core device in shale gas reservoir stimulation. As domestic shale gas exploration and development steps into deep layers, the development of fracturing technology puts forward higher requirements for relevant equipment. Electric drive is an important development direction of fracturing equipment technology. In this paper, the technical progress of electric drive fracturing equipment in China and abroad was investigated. It is shown that a high-power frequency conversion system is the key technology to determine the performance of electric drive fracturing equipment. After the adaptability of the high-power frequency conversion technology to electric drive fracturing equipment was analyzed, based on the fracturing operation of Model 2500 electric drive fracturing trucks in the Weiyuan Block of the Sichuan Basin for shale gas reservoir stimulation, electric drive and diesel-driven fracturing equipment were comparatively analyzed from the aspects of economic and technical indicators. And the following research results were obtained. First, compared with a diesel-driven fracturing truck of the same power, an electric drive fracturing truck can realize a full power coverage and a continuous adjustment of output displacement, and can better meet the operation requirements of fracturing process for a precise control of the pumping displacement, while reducing the power cost by 68% and the equipment purchase cost by 10–20%. Second, compared with the skid mounted equipment, an electric drive fracturing truck has a better transport performance, being suitable for the fracturing well sites with poor road conditions, such as loess gullies, hills and mountains. It is suggested that the following development direction of the electric drive fracturing equipment should focus on the improvement of single machine power density. In addition, it is recommended to enhance the basic studies on high-pressure and high-power semiconductor devices and make a good plan for the power demand of shale gas platform construction in advance, so as to give a better play to the operating cost advantage of electric drive fracturing equipment.

So far, how imbibition effect changes the occurrence state of shale gas has not been quantitatively understood, and the law of gas–water dynamic displacement caused by the retention of abundant fracturing fluid in a shale reservoir has not been determined clearly. In this paper, a gas–water displacement experiment was performed to simulate the change in the water content of shale near the wellbore after hydraulic fracturing. Then, based on the low field nuclear magnetic resonance spectrum analysis technique of hydrogen-bearing fluid (1H-NMR), the occurrence state of methane in shale reservoirs was dynamically monitored and methane volume in different occurrence states was calculated. Finally, the law of imbibition effect on the occurrence state of shale gas was studied. And the following research results were obtained. First, the process of shale saturation with methane is divided into adsorption-dominant phase and pore-filling phase. And adsorption and pressure gradient play a role simultaneously in the process of shale saturation with methane. Second, in the early stage of the process of shale saturation with methane, priority is given to saturated adsorbed gas, and free methane exists in shale pores as an intermediate state in which the external methane is converted to the adsorbed methane. After the adsorbed gas reaches the saturated state, methane fills shale pores under the pressure gradient until the pressure inside and outside the pores is balanced. Third, imbibition effect leads to the occurrence of gas–water displacement in shale. The adsorbed methane is partially desorbed into free methane, and the proportion of adsorbed gas is reduced. After 80 h' imbibition, the proportion of adsorbed gas is reduced from 63.58% to 45.87%. The increase of free gas volume results in the rise of shale pore pressure. And at the same time, water occupies partial pore volume to compress the storage space of free gas and expel some free gas out of shale pores. In this way, the gas bearing property of the reservoir is deteriorated, and the gas content of the shale sample is reduced to 7.34 mL/g from 7.91 mL/g which is the value before the beginning of imbibition. Fourth, the retention of abundant fracturing fluid in the process of hydraulic fracturing makes the shale reservoir near the wellbore in the state of rich water, the gas–water displacement induced by imbibition effect increases the volume of free methane in the external space (such as shale pore and wellbore), and the increase of pore pressure leads to the rise of the formation pressure, which is beneficial to the exploitation of shale gas to a certain extent.

Shale gas is as an emerging industry and its development is highly concerned in China. Under the guidance and support of national industrial policies, shale gas industry develops rapidly and has already stepped into the stage of large-scale exploitation of shale gas buried less than 3500 m deep underground. In order to standardize the healthy and rapid development of shale gas industry, ensure available standards for the national supervision on shale gas exploration and development and quickly copy the technologies in the efficient exploration and development of shale gas, it is necessary to formulate the technical standards and technical standard system of the whole shale gas industry chain. In this paper, the current situations of shale gas exploration and development technologies in China were analyzed. Then the role and significance of the formulation of shale gas technical standards in promoting the progress of shale gas technologies were described in detail. Finally, the development direction of shale gas exploration technology and standard formulation was predicted. And the following research results were obtained. First, China has established the exploration and development theories and technical systems that are in line with national conditions, and 27 distinguished technologies of 6 technical series have been developed, including comprehensive geological evaluation of shale gas, development optimization, optimized and fast drilling of horizontal wells, stimulated reservoir volume of horizontal wells, factory-like operation, and distinguished, efficient and clean exploitation. Second, 173 distinguished shale gas standards have been formulated, and they provide a powerful support for the quality and fast development of China's shale gas industry. The annual output of shale gas in 2019 is 143.6 × 108 m3. In conclusion, the construction of shale gas technical standard system promotes the continuous improvement of shale gas exploration and development technologies, impels the significant upgrading of technical level and ensures the all-around HSE control.

As shale gas development is advancing continuously and rapidly, how to deeply analyze the production performance of shale gas wells and evaluate their production characteristics has become an urgent problem in the evaluation of shale gas productivity construction zone, the formulation of new area development scheme and the preparation of planning program. Some scholars have applied the Logical Growth Model (LGM) in the production decline analysis of unconventional gas wells, but the influences of shale gas reservoir and development characteristics are not taken into consideration. Therefore, this method still has some space of further development and improvement. In this paper, a Logistic Growth Model considering shale gas reservoirs and development characteristics (RB-LGM) was established based on the previous research results. Then, it was applied to the shale gas development wells in the Changning Block of the Sichuan Basin to analyze their production performance, and the analysis results were compared with the fitting and prediction results provided by the Arps hyperbolic decline model. Finally, the optimal well spacing of horizontal wells was determined using RB-LGM. And the following research results were obtained. First, shale gas is produced by deploying horizontal wells in the clustered pattern in a large number, so on the basis of LGM, RB-LGM takes shale gas reservoir parameters (thickness, shale density, gas content) and development parameters (horizontal section length, well spacing and recovery factor) as the logic control factors of horizontal-well gas production fitting, so that the production prediction result of gas well is more reasonable. Second, RB-LGM can not only well fit the early production data of gas well, but ensure the convergence of the later prediction results under the control of logical conditions. Third, RB-LGM takes into account the influence of shale gas reservoir and development characteristics so as to optimize the horizontal well pattern and analyze the change trend of reservoir parameters in the development area through data inversion.

In recent years, major breakthroughs have been made in natural gas exploration in deep and ultra-deep strata in the Sichuan Basin, but the overall successful rate is low. To further clarify the prospects there, it is necessary to make an in-depth analysis of the previously discovered large-scale reef-shoal gas fields such as Puguang, Yuanba, Anyue and Longgang and deep shale gas discovery in Dingshan and Dongxi, southern Sichuan Basin. On one hand, large high-energy facies are the basis for controlling the development of large-scale reef-shoal reservoirs in conventional reef-shoal areas. The reservoir original porosity is high. The atmospheric freshwater dissolution in the early diagenetic stage, dolomitization, unconformity karst, and “pore–fracture coupling” mainly control the development of secondary pores. The contribution of hydrothermal fluids to reservoir is double-sided, and such early pores can be preserved till present due to those retention processes such as early hydrocarbon charging. Apart from continuous preservation as the key factor, most gas reservoirs are featured by “near-source enrichment, phase transformation, and dynamic adjustment”. On the other hand, deep shale gas generally has the characteristics of “high pressure, high porosity, and high gas content”, that is, “overpressure and rich gas”. The key to the development of high-quality deep shales with high pores are “quartz compression retaining pore'' and “reservoir fluid overpressure”. The weak tectonic effect in the late stage is the main reason for deep shale gas to maintain the “high pressure and high gas content”. In conclusion, technological advances like geological target identification and “sweet spot” prediction, as well as deep, high-temperature and high-pressure engineering processes, are the guarantee for efficient exploration of conventional and unconventional deep and ultra-deep natural gas, which has great potential in the Sichuan Basin.

At present, standard discharge has been adopted in the treatment of shale gas drilling wastewater and the electro-flocculation pretreatment has a good application prospect due to its high efficiency, cleanness, low dosage, etc. To improve its adaptability to drilling wastewater treatment, we conducted experiments to investigate the effects of current densities and reaction time on hardness, turbidity and organic matter removal, and the mechanism of electro-flocculation was further explored and a comparative analysis was made with chemical coagulation and ultrafiltration. The following findings were achieved. (1) The drilling wastewater is rich in dissolved salts, among which the hardness ions are mainly Ca2+, and the Ca2+ concentration varies little at the reaction time of 10 min, but decreases significantly with the increase of reaction time. (2) During the electro-flocculation process, the harness ions are usually removed with suspended matters, and their removal trend is different. Lower current densities and longer reaction time will be good for higher hardness removal rates, while higher current densities can help decrease the turbidity quickly before the reaction time of 10 min (3) The pH value of the wastewater is negatively correlated with the concentration of Ca2+ and iron ions. The TOC decreases with the increase of reaction time, and the larger the current density, the greater the decrease of TOC. A linear correlation is found between ΔTOC and ΔCl– and ΔTurbidity. Compared with chemical coagulation and ultrafiltration, electro-flocculation presents significant removal effects of hardness and turbidity. When the current density is 8 mA/cm2 and the reaction time is 20 min, the removal rates of Ca2+, turbidity and TOC are 53.4%, 98.3%, and 62.7%, respectively. Especially for those macromolecular substances with conjugated double bonds, electro-flocculation has an obvious effect and has other advantages like no chemical dosing, no membrane pollution and short reaction time, and so on.

In order to improve the calculation method of the isosteric adsorption heat, clarify thermodynamic characteristics of CH4 adsorption by continental shale and reveal the adsorption mechanism, this paper selected shale samples from the seventh Member of Yanchang Formation of Upper Triassic in the Yanchang Gasfield of the Ordos Basin as the research object. The isothermal adsorption experiment was carried out on the CH4 adsorption by continental shale and the excess adsorption curves were plotted. Then, the characteristics of the isosteric adsorption heat of the shale with different types of adsorption capacity were illustrated by analyzing and comparing the difference between excess and absolute adsorption capacity. And the following research results were obtained. First, under the same temperature and pressure, absolute adsorption capacity is higher than the excess values. The difference between them is higher under low temperature and high pressure and it is in the relationship of exponential function with the equilibrium pressure. If excess adsorption capacity is used to evaluate the adsorptive property of shale reservoirs, the evaluation result will be underestimated. Second, for CH4 adsorption by Yanchang Formation shale, the absolute and excess isosteric adsorption heat values have a linear positive correlation with absolute and excess adsorption capacity respectively, and the intermolecular force of adsorbate has a dominant effect on isosteric adsorption heat values. Third, absolute isosteric adsorption heat value is less than the excess heat. The relative error is in the range of 18.18–49.79%, and it is higher in the stage with low adsorption capacity. If excess adsorption capacity is taken as the basic data to calculate initial isosteric adsorption heat values, the calculation result will be overvalued, and consequently, the evaluation of the intermolecular force of adsorbent and adsorbate is overestimated.

At present, shale gas exploration and development in China is faced with some problems, such as the imperfect evaluation system of reservoir effectiveness and the limitations of reservoir evaluation system on efficient development of shale gas. In order to improve the content and the standard of reservoir evaluation, this paper analyzed the shortcomings and challenges in the static evaluation of shale gas reservoirs on the basis of existing reservoir evaluation, and established a method for evaluating shale gas reservoir effectiveness and a scheme for classifying pore systems. Then, the dynamic evaluation parameters after shale fracturing and their effects on drainage and production measures were discussed. In addition, the potential evaluation parameters of “automatic mitigating water blocking” were studied, and a comprehensive reservoir evaluation system of “static–dynamic” combination was established. And the following research results were obtained. First, new challenges to the shale gas reservoir evaluation are emerged as the lack of in-depth study on “reservoir effectiveness, dynamic evaluation parameter system after fracturing and drainage and production measures after fracturing”, which leads to the serious lag of existing shale gas reservoir evaluation system behind production. Second, the evaluation of reservoir effectiveness is mainly presented as the evaluation on the lower limit of effective porosity, and is embodied in the influence of clay bound water and unconnected pores on the development of shale gas. Third, the development of shale gas reservoir evaluation follows the trend of refining the static reservoir evaluation parameters, defining the potential evaluation indexes of “automatic mitigating water blocking” and establishing the reservoir comprehensive evaluation system of “static–dynamic” combination. Fourth, a post-frac dynamic evaluation system is determined for the potential evaluation indexes of “automatic mitigating water blocking” (e.g., wettability, water imbibition retention capacity, water imbibition expansion mode, expansion rate, and water imbibition cracking capacity). Fifth, a reservoir evaluation idea is put forward that “static evaluation of shale gas reservoir is the foundation and postfrac dynamic evaluation is the complement”, and a comprehensive reservoir evaluation system is established of “static–dynamic” combination suitable for the evaluation of marine shale gas reservoirs in China.

So far, large-scale development of shale gas wells above 3500 m in the Changning Block of the Sichuan Basin has been realized by means of the large-scale hydraulic fracturing technology. As the main process parameters are finally set, however, the improvement rate of its stimulation effect tends to slow down, while in North America, single-well production has increased significantly by shortening cluster spacing and increasing proppant volumes. In order to provide a reliable practice basis for optimizing the parameters of shale gas fracturing process, this paper analyzes the productivity increasing mechanism of the new fracturing technology of intensive stage + high-intensity proppant injection by virtue of the model for calculating induced stress and multi-stage and multi-cluster productivity of horizontal wells. And accordingly, the main engineering factors of fracturing stimulation technology were ascertained. Then, after the pilot test scheme on this new fracturing technology was formulated according to the geological parameters of this block, the pilot test was carried out. Finally, the fracturing process parameters were optimized based on actual production effects and experimental results. And the following research results were obtained. First, shortening the spacing between main fractures, increasing the interference degree of induced stress and improving the stimulation degree of hydraulic fractures to shale reservoirs are the technical keys to the intense stage, and increasing the proppant volume, reducing the influence degree of proppant embedding and breaking on the attenuation of fracture conductivity and ensuring the long-term conductivity of propped fractures are the internal causes of significant production increase through high-intensity proppant injection. Second, the optimized implementation parameters of this new process in the Changning Block are as follows. The cluster spacing is in the range of 15–20 m, the proppant injection intensity is 2.0–2.5 t/m, and the liquid consuming intensity is 30–35 m3/m. It is concluded that this new process increases the single-well production of shale gas wells and the development benefit of the Changning Block and provides technical support for improving the comprehensive development benefit of shale gas wells in this block.

The Zhaotong National Shale Gas Demonstration Area along the southern margin of the Sichuan Basin is located in the complex marine tectonic area of South China, where shale deformation and reformation are intense and the factors controlling sweet spots are complex, so the preservation conditions have an important impact on the enrichment of shale gas. In order to support the selection and evaluation of shale gas sweet spots in this area and improve the success rate of drilling, this paper carried out a geological survey on field outcrops. Then, based on drilling, mud logging and physical property test data, the structural deformation pattern and the regional deformation characteristics of this demonstration area were analyzed, and the development characteristics of formation joints and fractures, the sealing capacity of shale surrounding rock and the distribution characteristics of gas reservoirs were studied. Finally, the preservation conditions of shale gas in the Wufeng Formation of Upper Ordovician and the Longmaxi Formation of Lower Silurian were discussed. And the following research results were obtained. First, in the Zhaotong National Shale Gas Demonstration Area, three structural deformation patterns are developed from south to north, including trough type, equal amplitude type and baffle type, which are distributed in three major deformation zones, respectively, i.e., the shear deformation zone of Central Guizhou Uplift, the compression–torsion deformation zone of northern Yunnan–Guizhou Depression and the compression deformation zone of Southern Sichuan Depression. Second, three types of joints and fractures whose relationships with the direction of strata are high angle, middle–low angle and bedding intersection are developed in the Wufeng–Longmaxi formations and its overlying strata, and their occurrence characteristics are basically consistent with those of the three major deformation zones. Third, the shale of Wufeng–Longmaxi formations is thick in the north and thin in the south and possesses the preservation conditions of source–reservoir integration and self-sealing hydrocarbon accumulation. And combined with the sealing ability of the overburden strata and the roof and floor, its preservation conditions are overall better. Fourth, from the perspective of shale gas component, this area can be divided into three belts, i.e., methane, methane + nitrogen mixture and nitrogen from north to south. And the preservation conditions of shale gas are generally better in the north and worse in the south. In conclusion, the shale in the central–northern part of Zhaotong National Shale Gas Demonstration Area (compression deformation area and its southern margin) is the most favorable area because of its large shale thickness, weak reformation and deformation, bedding development of joints and fractures, good sealing performance and excellent preservation conditions. The compression–torsion deformation zone of northern Yunnan–Guizhou Depression in the central part is moderate in preservation conditions, and it is the relatively favorable area. The shear deformation zone of Central Guizhou Uplift in the southern part has poor preservation conditions, and it is a prospective area.

Commercial production of unconventional oil and gas resources will not be easily achieved without large-scale engineering measures, let alone the additional operation cost, increasingly stricter requirement for safety and environment, fluctuating low oil and gas prices, etc., defeating the confidence of those investors. Therefore, unconventional measures are urgently needed to guide the exploration and exploitation of unconventional oil and gas resources. Thus, we put forward the concept of joint exploration and development by integrating research methodologies and operating techniques for a variety of oil and gas resources to simultaneously achieve analysis, construction, gathering and exploitation of multiple hydrocarbon sources. In this way, the annoying interference between the produced mixture of hydrocarbon flow resulting in the reduction of single-well flowrate will be possibly turned into a dynamic mutual force to enhance the well's flowrate. We also point out that the inevitability of joint exploration and development is determined by the occurrence conditions of oil and gas resources, its feasibility relies on the advancement of technologies, and its arduous and long-term nature is attributed to the current energy market and environment. In spite of various problems and difficulties, we believe that joint exploration and development will be a feasible option to achieve both cost reduction and production & benefit enhancement, boost investors' confidence, raise energy comprehensive utilization, and enhance energy supply efficiency. In conclusion, the advantages of joint exploration and development outweigh its disadvantages for both countries and enterprises.

By horizontal well multi-staged fracturing technology, shale rocks can be broken to form fracture networks via hydraulic force and increase the production rate of shale gas wells. Nonetheless, the fracturing stimulation effect may be offset by the water phase trapping damage caused by water retention. In this paper, a technique in transferring the negative factor of fracturing fluid retention into a positive factor of changing the gas existence state and facilitating shale cracking was discussed using the easy oxidation characteristics of organic matter, pyrite and other minerals in shale rocks. Furthermore, the prospect of this technique in tackling the challenges of large retention volume of hydraulic fracturing fluid in shale gas reservoirs, high reservoir damage risks, sharp production decline rate of gas wells and low gas recovery, was analyzed. The organic matter and pyrite in shale rocks can produce a large number of dissolved pores and seams to improve the gas deliverability of the matrix pore throats to the fracture systems. Meanwhile, in the oxidation process, released heat and increased pore pressure will make shale rock burst, inducing expansion and extension of shale micro-fractures, increasing the drainage area and shortening the gas flowing path in matrix, and ultimately, removing reservoir damage and improving gas recovery. To sum up, the technique discussed in the paper can be used to “break” shale rocks via hydraulic force and to “burst” shale rocks via chemical oxidation by adding oxidizing fluid to the hydraulic fracturing fluid. It can thus be concluded that this method can be a favorable supplementation for the conventional hydraulic fracturing of shale gas reservoirs. It has a broad application future in terms of reducing costs and increasing profits, maintaining plateau shale gas production and improving shale gas recovery.

For a full and accurate understanding of the position and role of natural gas in China's energy revolution and providing a decision-making reference for the scientific establishment of natural gas development strategies and the deepening of the energy revolution, this paper analyzed the role of natural gas in guiding the energy revolution in China and points out the main direction of natural gas utilization in the Sichuan–Chongqing gas province which takes the lead in China's natural gas industry. The study provides the following findings. First, with a huge space for development, natural gas will play an important role in China's third energy revolution. Second, natural gas resources are abundant and have a huge potential for development in China. Third, natural gas can be used for a large range of purposes in the Sichuan–Chongqing gas province. In addition to be used as city gas, natural gas can also be used for industrial fuel mainly as a substitute for coal, transportation, distributed energy, peak-shaving power generation, and high-end natural gas chemical raw materials. Some conclusions are made. First, as the cleanest fossil energy and the best fuel, natural gas will play an irreplaceable role in China's third energy revolution in the 21st century. Second, the Sichuan–Chongqing gas province is rich in natural gas, hydropower and other renewable resources. By 2020, it will become the largest gas production province in China, with natural gas production over 550 × 108 m3. By 2030, driven by the rapid development of shale gas, gas production in the province is expected to exceed 800 × 108 m3. Third, the Sichuan–Chongqing gas province is likely to become the leader in China's third energy revolution.

Deep shale gas reservoirs buried underground with depth being more than 3500 m are characterized by high in-situ stress, large horizontal stress difference, complex distribution of bedding and natural cracks, and strong rock plasticity. Thus, during hydraulic fracturing, these reservoirs often reveal difficult fracture extension, low fracture complexity, low stimulated reservoir volume (SRV), low conductivity and fast decline, which hinder greatly the economic and effective development of deep shale gas. In this paper, a specific and feasible technique of volume fracturing of deep shale gas horizontal wells is presented. In addition to planar perforation, multi-scale fracturing, full-scale fracture filling, and control over extension of high-angle natural fractures, some supporting techniques are proposed, including multi-stage alternate injection (of acid fluid, slick water and gel) and the mixed- and small-grained proppant to be injected with variable viscosity and displacement. These techniques help to increase the effective stimulated reservoir volume (ESRV) for deep gas production. Some of the techniques have been successfully used in the fracturing of deep shale gas horizontal wells in Yongchuan, Weiyuan and southern Jiaoshiba blocks in the Sichuan Basin. As a result, Wells YY1HF and WY1HF yielded initially 14.1 × 104 m3/d and 17.5 × 104 m3/d after fracturing. The volume fracturing of deep shale gas horizontal well is meaningful in achieving the productivity of 50 × 108 m3 gas from the interval of 3500–4000 m in Phase II development of Fuling and also in commercial production of huge shale gas resources at a vertical depth of less than 6000 m.

Due to downhole complexities, shale-gas horizontal well fracturing in the Sichuan Basin suffered from casing deformation and failure to apply the technique of cable-conveyed perforation bridge plug. In view of these problems, a new technique of staged volume fracturing with temporary plugging by sand filling is employed. Based on theoretical analyses and field tests, a design of optimized parameters of coiled tubing-conveyed multi-cluster sand-blasting perforation and temporary plugging by sand filling was proposed. It was applied in the horizontal Well ZJ-1 in which casing deformation occurred. The following results are achieved in field operations. First, this technique enables selective staged fracturing in horizontal sections. Second, this technique can realize massive staged fracturing credibly without mechanical plugging, with the operating efficiency equivalent to the conventional bridge plug staged fracturing. Third, full-hole is preserved after fracturing, thus it is possible to directly conduct an open flow test without time consumption of a wiper trip. The staged volume fracturing with temporary plugging by sand filling facilitated the 14-stage fracturing in Well ZJ-1, with similar SRV to that achieved by conventional bridge plug staged fracturing and higher gas yield than neighboring wells on the same well pad. Thus, a new and effective technique is presented in multi-cluster staged volume fracturing of shale gas horizontal wells.

Casing deformation occurs during the development of Changning–Weiyuan national shale gas demonstration Area in the Sichuan Basin. In view of this, the correlation between the casing deformation and the geological characteristics as well as hydraulic fracturing was analyzed. It is shown that fracture and bedding (hereinafter collectively referred to as fractures) in faults are the internal cause for casing deformation and hydraulic fracturing is the external cause. Then, the mechanism of casing deformation was clarified. As the fracturing fluid flows along a certain passage into the natural fracture, the pore pressure in the fracture rises. When the critical pressure is reached, the natural fracture is activated to move, and consequently casing deformation occurs. Fluid may flow along three pathways, i.e., the hydraulic fracture, the axial fracture along the borehole axis created during hydraulic fracturing and the micro-annulus of cement sheath induced by repeated fracturing. Finally, some specific preventive measures were put forward. First, install packers in the whole sections with fractures and beddings. Second, improve the properties of slurry to avoid the formation of micro-annulus of cement sheath. And third, adopt the fracturing technologies to avoid the casing from repeated high pressure. The study results provide the guidance for the solution to shale gas casing deformation.

Zipper hydraulic fracturing in multiple wells with long horizontal sections is a primary solution means to increase the shale gas production rate and efficiency and to reduce the cost in Southern Sichuan Basin. Microseismic based fracturing monitoring can be used for real-time imaging of hydraulic fractures, so it has been widely used to evaluate the fracturing effect of shale gas reservoirs and to direct the optimization and adjustment of fracturing parameters. In China, however, the microseismic fracturing monitoring on fracturing of shale gas reservoirs cannot be used to evaluate the fracturing results until the fracturing operation in the pad wells is completed according to the parameters which are designed prior to the fracturing monitoring. Its evaluation results can merely provide a guidance for the fracturing parameters of the next pad wells instead of the wells in operation. As a result, the real-time effect of microseismic fracturing monitoring is out of work. In view of this, the fractures induced by zipper hydraulic fracturing in multiple shale gas wells with long horizontal sections in the southern Sichuan Basin, was real-time imaged by using the combined technology of radially arranged microseismic surface monitoring and microseismic well monitoring on the basis of real-time positioning method. The fracturing results were assessed and used in real time for the optimization of prepad fluid parameter, perforation and temporary plugging additive releasing time, so as to effectively avoid repeated fracturing and uneven fracturing effects and improve fracturing stimulation effects. This method is applied in two well groups. It is shown that the average shale gas production rate is increased by 2–5 times. Furthermore, microseismic fracturing real-time monitoring plays a vital role in real-time evaluation of fracturing effect and real-time optimization of fracturing parameters, so it can be used as the reference and should be popularized further.

In recent years, great progress has been made in geologic evaluation, engineering test and development optimization of the Lower Cambrian Wufeng Fm–Lower Silurian Longmaxi Fm shale gas in the Sichuan Basin, and the main shale gas exploitation technologies have been understood preliminarily. In addition, scale productivity construction has been completed in Jiaoshiba, Changning and Weiyuan blocks. In this paper, the Wufeng Fm–Longmaxi Fm shale gas wells in Changning Block were taken as the study object to provide technical reference for the development design of similar shale-gas horizontal wells. The technology combining geology with engineering, dynamic with static, and statistical analysis with simulation prediction was applied to quantify the main factors controlling shale-gas well productivity, develop the shale-gas well production prediction model, and optimize the key technical parameters of geologic target of shale-gas horizontal wells in the block (e.g. roadway orientation, location and spacing, horizontal section length and gas well production index). In order to realize high productivity of shale gas wells, it is necessary to maximize the included angle between the horizontal section orientation and the maximum major stress and fracture development direction, deploy horizontal-well roadway in top-quality shale layers, and drill the horizontal section in type I reservoirs over 1000 m long. It is concluded that high productivity of shale gas wells is guaranteed by the horizontal-well wellbore integrity and the optimized low-viscosity slickwater and ceramsite fracturing technology for complex fracture creation. Based on the research results, the technical policies for shale gas development of Changning Block are prepared and a guidance and reference are provided for the shale gas development and productivity construction in the block and the development design of similar shale-gas horizontal wells.

The marine shale gas resource in South China is abundant, but the existing conventional methods cannot meet the needs of predicting the future production development of shale gas. With the marine shale of the Lower Silurian Longmaxi Fm in this region as an example and based on its development potential, the development features of the existing shale gas resources were analyzed. It is considered that the Longmaxi shale gas accumulation zones in the Sichuan Basin and its neighboring areas contain resources of about 17.4 × 1012 m3 and recoverable resources of about 2.9 × 1012 m3. In particular, the shale gas above 3500 m underground is the main body for recent development targets, with a potential production scale about 300 × 108 m3. On this basis, the development features of foreign and domestic shale gas wells were studied and the drilling workload analytical method for the prediction of shale gas production was established: (1) the initial production of a single well can be used to characterize the productivity of the shale gas well, and its test production is approximate to its initial production. Due to the similarity of decline rate among shale gas wells, the single-well estimated ultimate recovery (EUR) value of a single well can be roughly speculated from its initial production, thus the production scale of a shale gas field can be speculated according to the analysis of drilling workload; (2) currently, the expected test production of South China marine shale gas is 17.6 × 104 m3/d, and the predicted single-well EUR value is roughly 1.5 × 108 m3. It is concluded that the marine shale gas in South China represents high initial production of a single well, high declining rate, longer production cycle and the drilling workload is closely related with the production of a shale gas field. Therefore, the analysis of drilling workload shows better applicability to the evaluation of shale gas production. Based on this method, the shale gas production of the Sichuan Basin and its neighboring area in 2020 was estimated to be about 200 × 108 m3.

An oriented cluster perforating technology, which integrates both advantages of cluster and oriented perforating, will help solve a series of technical complexities in horizontal well drilling. For realizing its better application in oil and gas development, a series of technologies were developed including perforator self-weight eccentricity, matching of the electronic selective module codes with the surface program control, axial centralized contact signal transmission, and post-perforation intercluster sealing insulation. In this way, the following functions could be realized, such as cable-transmission horizontal well perforator self-weight orientation, dynamic signal transmission, reliable addressing & selective perforation and post-perforation intercluster sealing. The combined perforation and bridge plug or the multi-cluster perforation can be fulfilled in one trip of perforation string. As a result, the horizontal-well oriented cluster perforating technology based on cable conveying was developed. This technology was successfully applied in unconventional gas reservoir exploitation, such as shale gas and coalbed methane, with accurate orientation, reliable selective perforation and satisfactory inter-cluster sealing. The horizontal-well oriented cluster perforating technology benefits the orientation of horizontal well drilling with a definite target and direction, which provides a powerful support for the subsequent reservoir stimulation. It also promotes the fracturing fluid to sweep the principal pay zones to the maximum extent. Moreover, it is conductive to the formation of complex fracture networks in the reservoirs, making quality and efficient development of unconventional gas reservoirs possible.

Identification of sweet spot is of great significance in confirming shale gas prospects to realize large-scale economic shale gas development. In this paper, geological characteristics of shale gas reservoirs were compared and analyzed based on abundant data of domestic and foreign shale gas reservoirs. Key elements of sweet spots were illustrated, including net thickness of gas shale, total organic carbon (TOC) content, types and maturity (Ro) of organic matters, rock matrix and its physical properties (porosity and permeability), and development characteristics of natural fractures. After the data in Changning and Weiyuan blocks, the Sichuan Basin, were analyzed, the geologic laws of shale gas enrichment were summarized based on the economic exploitation characteristics of shale gas and the correlation between the elements. The elements of favorable “sweet spots” of marine shale gas reservoirs in the Changning block and their distribution characteristics were confirmed. Firstly, the quality of gas source rocks is ensured with the continuous thickness of effective gas shale larger than 30 m, TOC > 2.0% and Ro = 2.4–3.5%. Secondly, the quality of reservoir is ensured with the brittle minerals content being 30–69%, the clay mineral content lower than 30% and a single lamination thickness being 0.1–1.0 m. And thirdly, the porosity is higher than 2.0%, the permeability is larger than 50 nD, gas content is higher than 1.45 m3/t, and formation is under normal pressure–overpressure system, which ensures the production modes and capacities. Finally, the primary and secondary elements that control the “sweet spots” of shale gas reservoirs were further analyzed and their restrictive relationships with each other were also discussed.

Shale gas reservoirs are different from conventional ones in terms of their bedding architectures, so their hydraulic fracturing rules are somewhat different. In this paper, shale hydraulic fracturing tests were carried out by using the triaxial hydraulic fracturing test system to identify the effects of natural bedding directions on the crack propagation in the process of hydraulic fracturing. Then, the fracture initiation criterion of hydraulic fracturing was prepared using the extended finite element method. On this basis, a 3D hydraulic fracturing computation model was established for shale gas reservoirs. And finally, a series of studies were performed about the effects of bedding directions on the crack propagation created by hydraulic fracturing in shale reservoirs. It is shown that the propagation rules of hydraulically induced fractures in shale gas reservoirs are jointly controlled by the in-situ stress and the bedding plane architecture and strength, with the bedding direction as the main factor controlling the crack propagation directions. If the normal tensile stress of bedding surface reaches its tensile strength after the fracturing, cracks will propagate along the bedding direction, and otherwise vertical to the minimum in-situ stress direction. With the propagating of cracks along bedding surfaces, the included angle between the bedding normal direction and the minimum in-situ stress direction increases, the fracture initiation and propagation pressures increase and the crack areas decrease. Generally, cracks propagate in the form of non-plane ellipsoids. With the injection of fracturing fluids, crack areas and total formation filtration increase and crack propagation velocity decreases. The test results agree well with the calculated crack propagation rules, which demonstrate the validity of the above-mentioned model.

Due to the special physical properties of shale gas reservoirs, it is necessary to adopt unconventional and standardized technologies for its surface engineering construction. In addition, the surface engineering design of shale gas reservoirs in China faces many difficulties, such as high uncertainty of the gathering and transportation scale, poor adaptability of pipe network and station layout, difficult matching of the process equipments, and boosting production at the late stage. In view of these problems, the surface engineering construction of shale gas reservoirs should follow the principles of “standardized design, modularized construction and skid mounted equipment”. In this paper, standardized surface engineering design technologies for shale gas reservoirs were developed with the “standardized well station layout, universal process, modular function zoning, skid mounted equipment selection, intensive site design, digitized production management” as the core, after literature analysis and technology exploration were carried out. Then its application background and surface technology route were discussed with a typical shale gas field in Sichuan–Chongqing area as an example. Its surface gathering system was designed in a standardized way, including standardized process, the modularized gathering and transportation station, serialized dehydration unit and intensive layout, and remarkable effects were achieved. A flexible, practical and reliable ground production system was built, and a series of standardized technology and modularized design were completed, including cluster well platform, set station, supporting projects. In this way, a system applicable to domestic shale gas surface engineering construction is developed.