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Research on the type, size, structure, and other characteristics of shale micro pore-fracture and their genesis is one of the core index for Shale gas study. Based on systematically collected shale samples from outcrop profiles and well cores, the experiments of thin-section observation, scanning electron microscopy, energy-dispersive X-ray spectroscopy, whole-rock analysis, rock–eval pyrolysis and basin simulation analysis were performed to study the micro pore-fracture characteristics and its main controlling factors for the development of shale pores in Tiemulike Formation in Yining Basin. The results show that four types of micro pore-fractures were identified: organic hydrocarbon-generating micro pores, granular dissolved micro pores, intergranular micro pores, and micro-fracture. The development of micro pores in shales is influenced by the internal material composition of the shale reservoir and external temperature conditions. The high organic carbon content with a high degree of thermal evolution led to the development of numerous micro pores, and the main micro pores were produced by shrinking the hydrocarbon generation volume due to the thermal evolution of organic matter. The development of micro-fractures was found to be favoured by the high content of brittle minerals and overpressure in the formation. [ABSTRACT FROM AUTHOR]

This study focuses on the implications of a global hyperthermal event, the Paleocene–Eocene Thermal Maximum (PETM) for the origin, preservation and maturation of organic matter (OM) in the Late Paleocene Patala Formation, Himalayan fold-and-thrust belt, Pakistan. The OM present in the full array of lithofacies of the Patala Formation is characterized by petrographic (vitrinite reflectance) and geochemical techniques (total organic carbon (TOC), total organic nitrogen (TON), Rock-Eval pyrolysis and stable C and N isotopes). These analyses record deposition of the formation in a Late Paleocene shallow marine shelf environment. The organic geochemical proxies indicate the presence of Type III mixed with subordinate Type II kerogen as well as thermally immature to early mature source rocks. OM is derived both from terrestrial and marine sources. Rather high TOC (> 2 wt.%) and a negative carbon isotope excursion (CIE) reveal that the prevalence of anoxic conditions, at least for short periods, enhanced OM preservation. The quantity and quality of OM, as well as the thickness and thermal maturity of the Patala Formation, make it a potential target for shale-gas exploration. During PETM, the extensive biological activity and addition of organic carbon into shallow marine eastern Tethyan settings provided precursor materials for hydrocarbon source rocks and shale-gas generation within the Patala Formation. The high organic influx and suboxic to anoxic conditions during PETM also facilitated OM preservation within the formation. Thus, the transient global warming PETM event significantly contributed to the OM accumulation and its preservation, which implies the suitability of such source rocks for shale-gas exploration within the Potwar Basin (Pakistan) and similar basins in neighboring areas. [ABSTRACT FROM AUTHOR]

This paper explores the reasons for the perforation failure of the shale gas gathering pipeline in the E Gas Mine and proposes preventive measures. EDS experiment found that the corrosion products were mainly Fe2O3, FeS, and FeCO3. Shale gas contains 24.908 g/m3 CO2 and 0.384 g/m3 H2S, and formation water contains 20.445 g/m3 Cl−. Therefore, CO2/H2S corrosion has occurred in the pipeline, and Cl− exacerbated localized damage to the material matrix, accelerated corrosion of the pipeline. The base material was more corrosion resistant than the weld, but the weld was more erosion resistant than the base material. The CFD simulation results found that the main reason of pipeline erosion is that, the shale gas contains grit and the gas volume exceeds the designed gas volume. So, the perforation failure of the pipeline was a result of both corrosion and erosion, and the effect of erosion is stronger than that of CO2/H2S corrosion on the pipeline. [ABSTRACT FROM AUTHOR]

After shale gas well sites are exploited, remediation is essential to restore the ecological environment. Effective bioremediation often has long cycles, so reducing this period is a research focus. To elucidate the intrinsic mechanisms between microorganisms and oil removal and to support the acceleration of bioremediation, gas chromatography–mass spectrometry and high-throughput sequencing technologies were utilized. The oil transformation and microbial response mechanisms during the natural remediation process from August to December at an abandoned shale gas well site in Weiyuan County, Neijiang City, Sichuan Province, were analyzed, revealing the directions of microbial succession. The Results showed that from August to September, the greatest degradation of oil components (C10-C20、C21-C30 alkanes) occurred, with Actinobacteriota, Gemmatimonadota, Proteobacteria, Acidobacteriota, Bacteroidota, and Ascomycota playing major roles. Key contributors to oil degradation included Sphingomonas, Flavisolibacter, Ramlibacter, Mortierella, Fusarium, and Rectifusarium. These microorganisms, along with those such as Chloroflexi, Gemmatimonas, Ellin6067, Cercospora, Sarocladium, Preussia, Calyptrozyma, Staphylotrichum, and Exophiala, which facilitate the cycling of nutrients like carbon, nitrogen, and phosphorus, collectively promote the degradation of oil. Moisture content, electrical conductivity, total nitrogen, total phosphorus, and pH affect the activity of oil-degrading microbes and thus oil degradation. Conversely, microbes alter soil chemistry during degradation, impacting those physicochemical properties. This feedback mechanism influences the activity of other oil-degrading microbes, creating a dynamic interaction network. Ultimately, the microbial community shifts towards populations that aid soil ecosystem restoration. This study reveals microbial succession and its role in oil degradation, offering insights for improving and accelerating bioremediation. [ABSTRACT FROM AUTHOR]

Conventional logging interpretation methods qualitatively identify shale reservoirs using shale attribute parameters and interpretation templates. However, improving the identification accuracy of complex shale reservoirs is challenging due to the numerous evaluation parameters and the complexity of model calculations. To quantitatively characterize high-quality shale reservoirs effectively, this study utilizes two wells in the Fuling shale gas field as examples and establishes a comprehensive evaluation method for identifying high-quality shale gas reservoirs utilizing multi-fractal spectral analysis of well logs. First, the conventional well logs are qualitatively analyzed and evaluated via multiple fractals and R/S analysis. Subsequently, a gray relational analysis is employed to combine the production well logging, which reflects dimensionless productivity contributions, with the fractal characteristics of conventional well logs to obtain the corrected weight multifractal spectrum width ∆α' and fractal dimension D'. Comprehensive fractal evaluation indices λ and γ are introduced, forming three categories of productivity evaluation standards for shale gas reservoirs characterized by fractals. Finally, a validation well is employed to demonstrate the effectiveness of the evaluation method. The results indicate that the identification of high-quality shale gas reservoirs based on the above comprehensive fractal evaluation method can reflect the productivity classification level of fractured well sections, simplify the calculation of formation evaluation parameters, and avoid the problem of poor correlation between predicted sweet spot zones and gas production. This approach has wide applicability and value for identifying high-quality reservoir areas in shale gas reservoirs and provides technical support for the effective large-scale development of shale reservoirs. [ABSTRACT FROM AUTHOR]

Predicting ground motions due to induced seismicity is a challenging task owing to the scarcity of data and heterogeneity of the uppermost crust. Dealing with this requires a thorough understanding of the underlying physics and consideration of inter-site variability. The most common ground motion model used in practice is the parametric ground motion prediction equation (GMPE), of which hundreds exist in the literature. However, relatively few are developed with a focus on induced seismicity. Developing GMPEs that are specific to an appropriate magnitude-distance range ( R < 30 km; 2 ≤ M ≤ 6 ) is important for induced seismicity applications. This paper proposes a framework for the development of physically-based GMPEs to provide more accurate and reliable estimates of the potential induced-seismicity ground motion hazard, allowing for better risk assessment and management strategies. To demonstrate this approach, a new set of GMPEs for the 2018-2019 induced seismicity sequence at the Preston New Road (PNR) shale gas site near Blackpool, United Kingdom, is presented. The physically-based GMPE was developed based on a pseudo-finite-fault stochastic ground motion simulation, calibrated with parameters derived from the spectral analysis of weak-motion records from induced seismic events. An optimization-based calibration technique using the area metric (AM) was subsequently performed to calibrate optimal parameters for simulating ground motion at the PNR site. Finally, using a suite of forward simulations for events with 1 ≤ M ≤ 6 recorded at distances up to 30 km, combined with empirical data, a location-specific GMPE was derived through adjustment of an existing model. [ABSTRACT FROM AUTHOR]

To solve the problem of thinning of the wall thickness of the sewage pipeline on the side of the shale gas separator of Sichuan E Oil and Gas Field Company, this study explores the reasons for the failure of sewage pipelines by macroscopic inspection, mechanical property experiment, metallographic structure analysis, (electron microscope scanning) SEM, (energy-dispersive spectrometry) EDS, (X-ray polycrystalline diffractometer) XRD, and CFD simulation analysis. Inspection found corrosion on the weld seam and inner wall surface of the pipeline. The elements in the corrosion products are mainly C, O, Fe, and Cl, and the corrosion products are mainly FeCO3. Therefore, one of the causes of pipeline failure is CO2 corrosion. Cl- exists in the transportation medium of the pipeline, which promotes CO2 corrosion under the action of acidification and autocatalysis; CFD simulation analysis found that the erosion of pipeline by grits is concentrated at the diameter change location of the pipeline, which is the same as the location of corrosion. Therefore, the cause of pipeline failure is the combined effect of CO2 corrosion and grits erosion. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing is an effective technology for hydrocarbon extraction from unconventional shale and tight gas reservoirs. A potential risk of hydraulic fracturing is the upward migration of stray gas from the deep subsurface to shallow aquifers. The stray gas can dissolve in groundwater leading to chemical and biological reactions, which could negatively affect groundwater quality and contribute to atmospheric emissions. The knowledge of light hydrocarbon solubility in the aqueous environment is essential for the numerical modelling of flow and transport in the subsurface. Herein, we compiled a database containing 2129 experimental data of methane, ethane, and propane solubility in pure water and various electrolyte solutions over wide ranges of operating temperature and pressure. Two machine learning algorithms, namely regression tree (RT) and boosted regression tree (BRT) tuned with a Bayesian optimization algorithm (BO) were employed to determine the solubility of gases. The predictions were compared with the experimental data as well as four well-established thermodynamic models. Our analysis shows that the BRT-BO is sufficiently accurate, and the predicted values agree well with those obtained from the thermodynamic models. The coefficient of determination (R2) between experimental and predicted values is 0.99 and the mean squared error (MSE) is 9.97 × 10−8. The leverage statistical approach further confirmed the validity of the model developed. [ABSTRACT FROM AUTHOR]

In order to acknowledge the enrichment mechanism of organic matter in the black shale of the Middle Ordovician Yanxi Formation in central and southern Hunan area, the shale samples of the Yanxi Formation in the Maduqiao, Jinweixiang, Hejiadong and Mianhuaping are selected to test abundance of organic matter, major, trace and rare earth elements. The results show that the average total organic carbon (TOC) of black shale in study area is 2.03%, indicating the high abundance of organic matter in the Yanxi Formation. The CIA values and w(Sr)/w(Cu) ratios show that the Yanxi Formation had a warm and humid climate. The concentrations of Zn, V, Ni and Cu imply a medium primary productivity in the Yanxi Formation, whereas they have weak correlations with TOC. The element concentrations of V, Cr, Ni, and Mo reveal that the black shales were deposited in anoxic-euxinic conditions. The accumulation of organic matter is jointly affected by paleoclimate, primary productivity, redox conditions, sea level change and terrigenous debris. The organic matter enrichment model of the Yanxi Formation shales is a 'deepwater retention basin' controlleded by anoxic conditions. [ABSTRACT FROM AUTHOR]

To fully exploit the technical advantages of the large-depth and high-precision artificial source electromagnetic method in the complex structure area of southern Sichuan and compensate for the shortcomings of the conventional electromagnetic method in exploration depth, precision, and accuracy, the large-depth and high-precision wide field electromagnetic method is applied to the complex structure test area of the Luochang syncline and Yuhe nose anticline in the southern Sichuan. The advantages of the wide field electromagnetic method in detecting deep, low-resistivity thin layers are demonstrated. First, on the basis of the analysis of physical property data, a geological–geoelectric model is established in the test area, and the wide field electromagnetic method is numerically simulated to analyze and evaluate the response characteristics of deep thin shale gas layers on wide field electromagnetic curves. Second, a wide field electromagnetic test is conducted in the complex structure area of southern Sichuan. After data processing and inversion imaging, apparent resistivity logging data are used for calibration to develop an apparent resistivity interpretation model suitable for the test area. On the basis of the results, the characteristics of the electrical structure change in the shallow longitudinal formation of 6 km are implemented, and the transverse electrical distribution characteristics of the deep shale gas layer are delineated. In the prediction area near the well, the subsequent data verification shows that the apparent resistivity obtained using the inversion of the wide field electromagnetic method is consistent with the trend of apparent resistivity revealed by logging, which proves that this method can effectively identify the weak response characteristics of deep shale gas formations in complex structural areas. This experiment, it is shown shows that the wide field electromagnetic method with a large depth and high precision can effectively characterize the electrical characteristics of deep, low-resistivity thin layers in complex structural areas, and a new set of low-cost evaluation technologies for shale gas target layers based on the wide field electromagnetic method is explored. [ABSTRACT FROM AUTHOR]

The key scientific problem to be solved in this paper is the optimal development and utilization model and the economic evaluation model of China's land-phase shale oil and gas resources, and the purpose of the research is to promote the large-scale commercial development and utilization of China's shale oil and gas resources, and to safeguard China's oil and gas energy security and the sustainable development of the economy. The article proposes to adopt the small surface element volume method (oil content rate method) to evaluate the pure shale oil resources, adopt the Cobb–Douglas production function model as the optimization model to measure the boundary production capacity of shale oil and gas, construct the optimal development and utilization model for shale oil and gas resources considering the five first-level safeguard indexes, namely, science and technology (A), capital (K), talents (L), reserves (S) and ecological environment (E), and establish the basic constraint model for the optimal development and utilization of shale oil and gas resources. The basic constraint model, as well as the evaluation model of economic coefficients for the development and utilization of shale oil and gas resources were established. The pure shale oil resources are mainly calculated based on the movable oil content of shale. In the paper, the S1 of normal pyrolysis (300 °C) is regarded as movable oil, and the sum of S1 and evaporated hydrocarbon (light hydrocarbon) loss is the movable oil content of shale. The integrated geological-physical exploration-engineering comprehensive evaluation of China's land-phase shale oil-rich and high-yielding "sweet spot" is an important prerequisite for the realization of shale oil and gas resources to build production scale and effective development, and the least-squares method is used to estimate the average production function, the distance to the maximum value of the residuals, and the boundary capacity production function. The average production function and residual maximum distance are estimated by the least squares method, and the production function of the boundary capacity is derived, and the quotient of the boundary capacity and the actual capacity is calculated to get the capacity utilization rate, which can be used to analyze the potential of future shale oil and gas growth. The development of shale oil and gas resources in a target block requires comprehensive consideration of the first-level guarantee indicators such as science and technology (A), capital (K), talents (L), reserves (S) and ecological environment (E), as well as more than 10 s-level indicators and a number of third-level indicators, in order to ensure that the oil companies maximize their profits by organizing the development and production. The economic coefficient can be expressed as the ratio of economically recoverable resources to geological reserves. The larger the economic coefficient for the development and utilization of shale oil and gas resources is, the better the economy of the area is, and the larger the proportion of shale oil and gas resources that can be exploited. There is little special literature on the optimal development and utilization model of shale oil and gas resources and energy security among many research results at home and abroad. The evaluation of pure shale oil using the small surface element volume method (oil content rate method) and the construction of the boundary capacity calculation model, the optimal development of the basic constraints model and the economic evaluation model that we have determined, although they can not yet fully cover all the links and factors related to the development and utilization of shale oil and gas resources, are not yet fully covered by our research work. However, our research work has given the model more geological and economic theoretical connotations, and provided an economic basis and technical reference for the large-scale and commercial development and utilization of shale oil and gas resources as an effective alternative to oil imports. [ABSTRACT FROM AUTHOR]

Research shows that conservatives support fossil fuel extraction and distrust regulation. Yet scholarship overlooks where environmentalist and conservative interests may align—a question I explore through studying a rural, white, and conservative Pennsylvania community where many leased their land for shale gas drilling (fracking). Landowners endorsed fracking and discredited state regulators and environmentalists. Eventually, however, many became troubled by how their land sovereignty and community "home rule" were eroded by petroleum companies and state zoning preemption laws. Though few rejected fracking outright, most believed their town should be allowed to manage the industry's footprint. Although climate advocates increasingly view local sovereignty as enabling NIMBYism that stymies climate action, communities often experience state-led energy siting policies as a procedural injustice. Regarding fracking, community empowerment would abet climate action by enabling municipal checks on industrial expansion. Environmentalists could forge alliances with rural, conservative towns by supporting greater local democratic decision-making over fracking. [ABSTRACT FROM AUTHOR]

Soluble bridge plug is a key tool to improve the speed and quality of staged fracturing, which requires system design of pressure-bearing capacity, low manufacturing cost and dissolution rate to achieve different downhole environments. In order to study the application of soluble bridge plugs made in China under real working conditions and put forward improvement measures. In this paper, with the help of field engineering examples, relying on the working steps of "quality experimental detection → application collection → stability study," the systematic study of soluble bridge plugs from material determination to damage is realized. The results of this study are as follows: The release pressure is 15.6 MPa (converted release value is 19.66t). Matching with the driving force of the hydraulic release tool 23–25t under production conditions. The effective sealing time in various working environments is 24 h. Under the condition of different KCL solution concentrations at 90 °C, the dissolution time is 9–11 days, and 0.19–0.23 kg of cast iron and ceramic residues remain. The next step is to construct different stability research schemes under the premise of long time span, optimize the performance of bridge plug and strengthen the strength of anti-collision ring and spacer ring. At the same time, ensure the perfection of ground equipment in the construction process. [ABSTRACT FROM AUTHOR]

Fluid injection associated with unconventional resource extraction activities can alter the friction-permeability relationship of fractures, thereby significantly affecting the stability of fractures/faults. In this study, we utilized the three-dimensional (3D) carving technique to reconstruct natural shale fractures recovered from the Longmaxi shale reservoir at a depth of 3700 m in the southeastern Sichuan Basin. Subsequently, we conducted friction-permeability coupling experiments to elucidate the relationship between friction, permeability, and stability for natural shale fractures. In the experiments, confining pressure was maintained at 3 MPa, temperature was maintained at 25 °C, the constant shearing velocity was set to 1 μm/s, and the velocity step was set to 10–1–10–1–10 μm/s. Our results show that the frictional coefficient of natural Longmaxi shale fractures ranges from 0.67 to 0.71, which is higher than that of saw-cut fractures of 0.61–0.62. Furthermore, frictional stability parameter (a–b) of the natural Longmaxi shale fracture exhibits velocity strengthening behavior under all conditions, indicating aseismic slip possibly caused by the abundant phyllosilicate content in shale fractures. However, the decrease of roughness and the effect of fluid injection reduce the frictional coefficient of natural fractures during the shearing process. At the same time, fluid injection reduces frictional stability, leading to a transition from velocity strengthening to velocity neutral. In addition, the increase in permeability of the natural fracture is more pronounced due to the velocity step shear compared to constant velocity shear. We confirm that fracture stability is influenced by a combination of factors including fracture surface roughness and fluid injection. Highlights: Three-dimensional (3D) scanning and carving techniques were employed to reproduce natural Longmaxi shale fractures. Fluid injection leads to a transition of frictional stability from velocity strengthening to velocity neutral. Frictional coefficient and stability of shale fractures are affected by both roughness and fluid flow. Compared with constant velocity shear, velocity step shear has more obvious effect on permeability enhancement. [ABSTRACT FROM AUTHOR]

Pore volume, pore size and adsorption capacity are fundamental factors that control gas storage and production in shale reservoirs. We investigated the evolution of pore structure and adsorption capacity as functions of lithology and present-day burial depth. Nineteen shale samples from the Wufeng-Longmaxi Formation from shallow depths (present depths < 700 m) in the Sichuan Basin, China, were analyzed to elucidate the evolution of the pore system, applying N2/CO2 isotherm adsorption, high-resolution FE-SEM, FIB-SEM, and CH4 isotherm adsorption. Two comparison basin models with 700 m and 4000 m present-day burial depth were constructed to illustrate the impact of uplift on pore structure and methane adsorption. The results indicate that mineral structure, rock fabric, and TOC content are essential factors affecting the pore structure. Pores of various sizes are preserved through different mechanisms during the compaction. The development of micropores is mainly controlled by thermal maturity and TOC content. In contrast, mesopores and macropores are stress-sensitive and prone to be compacted during deposition. Methane adsorption capacity is highly related to the burial depth. During the process of reservoir uplift, the methane adsorption capacity of the reservoir gradually increases with decreasing burial depth, with a transition point occurring between 700 and 1300 m. Furthermore, as the reservoir continues to uplift to burial depths below 700 m, the methane adsorption capacity of the reservoir in normally pressured shale gas formations rapidly decreases. However, for overpressured shale gas reservoirs, there is no significant change in the methane adsorption capacity of the reservoir. Highlights: Mechanism of pore structure development during subsidence and uplift was analyzed, and the pores with different sizes were also taken into consideration. The effect of burial depth and uplift in terms of pressure and temperature on pore gas storage capacity was analyzed, and three stages of shallow shale gas adsorption were reconstructed. An empirical formula was proposed for considering methane adsorption in ultra-shallow shale reservoirs. Both over pressured and hydrostatic pressure condition was considered in our models. [ABSTRACT FROM AUTHOR]

Deep shale has the characteristics of large burial depth, rapid changes in reservoir properties, complex pore types and structures, and unstable production. The whole-rock X-ray diffraction (XRD) analysis, reservoir physical property parameter testing, scanning electron microscopy (SEM) analysis, high-pressure mercury intrusion testing, CO2 adsorption experimentation, and low-temperature nitrogen adsorption–desorption testing were performed to study the pore structure characteristics of marine shale reservoirs in the southern Sichuan Basin. The results show that the deep shale of the Wufeng Formation Longyi1 sub-member in the Luzhou area is superior to that of the Weiyuan area in terms of factors controlling shale gas enrichment, such as organic matter abundance, physical properties, gas-bearing properties, and shale reservoir thickness. SEM is utilized to identify six types of pores (mainly organic matter pores). The porosities of the pyrobitumen pores reach 21.04–31.65%, while the porosities of the solid kerogen pores, siliceous mineral dissolution pores, and carbonate dissolution pores are low at 0.48–1.80%. The pores of shale reservoirs are mainly micropores and mesopores, with a small amount of macropores. The total pore volume ranges from 22.0 to 36.40 μL/g, with an average of 27.46 μL/g, the total pore specific surface area ranges from 34.27 to 50.39 m2/g, with an average of 41.12 m2/g. The pore volume and specific surface area of deep shale gas are positively correlated with TOC content, siliceous minerals, and clay minerals. The key period for shale gas enrichment, which matches the evolution process of shale hydrocarbon generation, reservoir capacity, and direct and indirect cap rocks, is from the Middle to Late Triassic to the present. Areas with late structural uplift, small uplift amplitude, and high formation pressure coefficient characteristics favor preserving shale gas with high gas content and production levels. [ABSTRACT FROM AUTHOR]

Extracting gas from unconventional shale reservoirs with low permeability is challenging. To overcome this, hydraulic fracturing (HF) is employed. Despite enhancing shale gas production, HF has drawbacks like groundwater pollution and induced earthquakes. Such issues highlight the need for ongoing exploration of novel shale gas extraction methods such as in situ heating through combustion or pyrolysis to mitigate operational and environmental concerns. In this study, thermally immature shales of contrasting organic richness from Rajmahal Basin of India were heated to different temperatures (pyrolysis at 350, 500 and 650 °C) to assess the temperature protocols necessary for hydrocarbon liberation and investigate the evolution of pore structural facets with implications for CO2 sequestration in underground thermally treated shale horizons. Our results from low-pressure N2 adsorption reveal reduced adsorption capacity in the shale splits treated at 350 and 500 ºC, which can be attributed to structural reworking of the organic matter within the samples leading to formation of complex pore structures that limits the access of nitrogen at low experimental temperatures. Consequently, for both the studied samples BET SSA decreased by ∼58% and 72% at 350 °C, and ∼67% and 68% at 500 °C, whereas average pore diameter increased by ∼45% and 91% at 350 °C, and ∼100% and 94% at 500 °C compared to their untreated counterparts. CO2 adsorption results, unlike N2, revealed a pronounced rise in micropore properties (surface area and volume) at 500 and 650 ºC (∼30%–35% and ∼41%–63%, respectively for both samples), contradicting the N2 adsorption outcomes. Scanning electron microscope (SEM) images complemented the findings, showing pore structures evolving from microcracks to collapsed pores with increasing thermal treatment. Analysis of the SEM images of both samples revealed a notable increase in average pore width (short axis): by ∼4 and 10 times at 350 °C, ∼5 and 12 times at 500 °C, and ∼10 and 28 times at 650 °C compared to the untreated samples. Rock-Eval analysis demonstrated the liberation of almost all pyrolyzable kerogen components in the shales heated to 650 °C. Additionally, the maximum micropore capacity, identified from CO2 gas adsorption analysis, indicated 650 °C as the ideal temperature for in situ conversion and CO2 sequestration. Nevertheless, project viability hinges on assessing other relevant aspects of shale gas development such as geomechanical stability and supercritical CO2 interactions in addition to thermal treatment. Highlights: Insitu thermal treatment of shales for liberation of hydrocarbons and CO2 sequestration. Significant changes in shale geochemistry and pore properties with increasing temperatures of treatment. Visualizing thermally induced pore evolution in shale: microcracks to collapsed pores. Limitations of N2 in accessing complex pore structures. [ABSTRACT FROM AUTHOR]

With the progression of many shale gas wells in the Sichuan-Chongqing region of China into the middle and late stages of exploitation, the problem of sand production in these wells is a primary factor influencing production. Failure to implement measures to remove sand from the gas wells will lead to a sharp decline in production after a certain period of exploitation. Moreover, As the amount of sand produced in the well increases, the production layer will be potentially buried by sand. To boost the production of shale gas wells in the Sichuan-Chongqing region and improve production efficiency, a novel downhole jet sand-washing device has been developed. Upon analyzing the device's overall structure, it is revealed that the device adopts a structural design integrating a jet pump with an efficient sand- washing nozzle, providing dual capabilities for jet sand- washing and sand conveying via negative pressure. To enhance the sand- washing and unblocking performance of the device, various sand- washing fluids and the structures of different sand- washing nozzles are compared for selection, aiming to elevate the device's sand- washing and unblocking performance from a macro perspective. Subsequently, drawing on simulation and internal flow field analysis of the device's sand- washing and unblocking process through CFD and the control variable method, it is ultimately found that the length diameter ratio of the cylindrical segment of the nozzle outlet, the outlet diameter, and the contraction angle of the nozzle greatly influence the device's sand- washing and unblocking performance. And the optimum ranges for the length-diameter ratio of the cylindrical segment of the nozzle outlet, the outlet diameter, the contraction angle of the nozzle, and the inlet diameter are 2 to 4, 6 mm to 10 mm, 12° to 16°, and 18 mm and 22 mm, respectively. The findings of the research not only provide new insights into existing sand removal processes but also offer a novel structure for current downhole sand removal devices and a specific range for the optimal size of the nozzle. [ABSTRACT FROM AUTHOR]

This study focuses on the characterization of shale gas reservoirs in the Longmaxi Formation and their controlling factors in the east Sichuan Basin. Detailed mineralogical and reservoir characterization of shale samples in the region was carried out by various methods, including X-ray diffraction analysis and nitrogen adsorption experiments. The results show that the shale is mainly composed of clay minerals (illite content ranges from 34.9% to 55.7%), quartz and calcite. In terms of reservoir characteristics, the shale mesopore morphology is mainly "slit-type", with BET specific surface area ranging from 7.12-25.63 m2/g and BJH pore volume from 0.0095-0.0262 mL/g. These reservoir characteristics show a significant positive correlation with the organic carbon content (1.82-3.87%). correlation. Petrographic analysis further reveals that the brittle mineral content has a significant effect on the brittleness, pore development and fracturing effectiveness of the rocks. In addition, diagenesis (including compaction, cementation, dissolution, and thermal evolution of organic matter) had a significant impact on the formation and characterization of shale pores. These findings provide a key scientific basis for understanding the geological characteristics and development potential of shale gas reservoirs in the Longmaxi Formation in the Sichuan Basin. [ABSTRACT FROM AUTHOR]

This study focuses on the shale fracture characteristics and their developmental master factors in the southeastern Sichuan Basin, aiming to gain a deeper understanding of the fracture network in shale gas reservoirs and its impact on hydrocarbon exploration. Through geological investigation and core sample analysis of Longmaxi Formation shales within the Dingshan-Dongxi and Songkan formations, the characteristics and main controlling factors of shale fractures were deeply studied by using fracture measurements, chi-square projection method and electron microscopy. The results show that: the inclination of cracks is mainly dominated by high-angle cracks and upright cracks, with the upright cracks of the Silurian system accounting for 57.35%, and the high-angle cracks of the Triassic system accounting for 43.85%. The fracture filler is mainly calcite and pyrite, in which the calcite and the analysis of fracture density shows that the density of Silurian fracture lines is mainly 3-8 lines/m, accounting for 70.5% of the total number; the development of microfractures also plays an important role in shale gas storage and transportation, and there are various types of microfractures, including open and filled seams, which provide an important storage space for shale gas exploration; the degree of development of shale fractures is related to the mineral components, organic carbon content and thickness of the rock formation. The degree of shale fracture development is closely related to the mineral fraction, organic carbon content and formation thickness. Shales with high brittle mineral content and organic carbon content have a higher degree of fracture development, while the greater the thickness of the formation, the lower the fracture density. These findings indicate that the development degree and types of shale fractures vary significantly among different geological periods and tectonic blocks, and that their complexity and diversity have important implications for shale gas storage and transportation. [ABSTRACT FROM AUTHOR]

Horizontal drilling and multistage hydraulic fracturing have seen widespread application in shale formations during the past decade, leading to significant economic productivity gains through the creation of extensive fracture surfaces. The determination of the ideal cluster spacing in shale gas wells is contingent upon the unique geological and formation characteristics. Generally, reducing the spacing between clusters has the potential to augment gas recovery, albeit at the expense of higher drilling and completion costs, as well as the influence of stress shadows on fracture propagation. This study introduces an integrated methodology designed to explore the impact of cluster interference on well performance. Commencing with a fracture propagation model accommodating stress shadow effects for an equivalent slurry volume injection, analytical rate transient analysis (RTA) was amalgamated with reservoir numerical simulation to compute the effective fracture surface area (Aca.) for hydrocarbon production. The correlation between the effective fracture surface area determined by RTA and the actual stimulated fracture area (Aca.) derived from numerical simulations was established in relation to cluster spacing. The findings of this research reveal that wells featuring a greater number of stages and tighter cluster spacing tend to exhibit elevated cluster interference, resulting in a lower effective-to-actual fracture surface area ratio and heightened stress shadow effects impeding fracture propagation. A cluster spacing of 33 feet with six clusters per stage emerges as the optimal choice at formation permeability of 0.00005 md that decreased to 18 ft at formation permeability of 0.00001 md. ACe either stabilizes or decreases above the optimal value, suggesting that more clusters would not have a major impact on increasing the effective stimulated area. Allowing 20% interference, regardless of the permeability of the formation, maximized cumulative production while preventing thief zones and excessive cluster interference. The insights gained from this study will serve as a valuable resource for completion and reservoir engineers, enabling them to fine-tune cluster spacing to maximize well revenue in the dynamic landscape of shale gas extraction. [ABSTRACT FROM AUTHOR]

The critical pressure (Pc) and critical temperature (Tc) of shale gas depend on the characteristic pore size because of the importance of fluid–rock interactions in the matrix. This size dependency is neglected in highly permeable formations, where gas composition is only implemented because the fluid–fluid interactions are dominant. This study determines the critical properties by accounting for the characteristic pore size in the shale matrix and gas composition. The analyzed components are carbon dioxide, ethane, methane, n-butane, nitrogen, pentane, and propane. It shows that the bulk properties overestimate the actual critical properties. The overestimation varies between 15 and 26% in a uniform 5 nm conduit with a circular cross section, and it increases nonlinearly when decreasing the conduit size. Overestimation versus size is presented to provide a convenient tool for correcting the existing data. This study also determines the critical properties of Midra shale by accounting for the pore-throat size and pore-body size distributions. The former distribution is based on the mercury injection capillary pressure measurements of eight samples, whereas the latter is based on the nitrogen adsorption measurements of six samples. This study indicates that common bulk properties overestimate the critical properties of the studied shale between 5 and 22%. The results have applications in characterizing multiphase transport in shale gas reservoirs. [ABSTRACT FROM AUTHOR]

The integrity of shale gas wells is crucial in ensuring safety and efficiency throughout the development process. Such integrity spans the entire process of drilling and fracturing horizontal wells and is an essential indicator for ensuring safe and stable production throughout the lifespan of the well. This study investigates methods for assessing the integrity of shale gas wells by employing the analytic hierarchy process combined with experimental data to establish evaluation criteria and weights. The assessment is carried out specifically on shale gas wells in Changning Block. Results indicate that the integrity of these shale gas wells is influenced by various factors, such as drilling and fracturing processes. Moreover, the integrity assessment of indicators such as oil layer casing/technical casing, liquid carrying capacity, and tube column deformation is relatively low, indicating a need for enhanced monitoring and management. The comprehensive evaluation results indicate that, overall, the integrity rating of shale gas wells is generally considered "common," but some potential safety hazards still remain that require timely attention and resolution. Case analysis reveals varying levels of integrity risks in shale gas wells. Case 1's score of 93.51 warrants attention but is still deemed generally safe. However, Case 2's score of 73.89 indicates a disaster level, emphasizing urgent intervention needs. Critical factors such as pressure, cementation quality, and corrosion demand proactive management for safe, sustainable operations. [ABSTRACT FROM AUTHOR]

In this study, the corrosion behavior of N80 and TP125V steels was delved firstly into produced water from shale gas fields containing CO2-O2. Moreover, the localized corrosion of these steels was investigated to elucidate the effects of aerobic and anoxic on steel corrosion. The results indicated that the corrosion rates of N80 and TP125V steels under aerobic conditions were lower compared to those in the presence of CO2-O2. Specifically, at temperature of 100 °C and with dissolved oxygen (DO) concentration of 4 mg/L in the CO2-O2 environment, the N80 and TP125V steels exhibited the highest corrosion rate, with values of 0.13 mm/y and 0.16 mm/y, respectively, as determined by specific weight loss measurements. Conversely, these rates decreased to 0.022 mm/y and 0.049 mm/y under aerobic conditions. Furthermore, severe localized corrosion of N80 and TP125V steels with a DO concentration of 4 mg/L was also observed in the CO2-O2 environment. Finally, it was evident that pitting corrosion is the predominant type of corrosion affecting N80 and TP125V steels in the produced water from shale gas fields. [ABSTRACT FROM AUTHOR]

Fracturing horizontal well casing deformation has become very prominent, particularly in tectonic stress-concentrated shale gas fields, limiting the efficient development progress of shale gas. The main failure mode of casing shearing deformation had been attributed to fault slip caused by multi-fracturing. The current research did not provide a clear picture of the dynamic evolution relationship between hydraulic fracturing, fault slip, and casing deformation. In this paper, the dynamic model of fault slip induced by formation pressure change is established, incorporating the effects of stress drop, physical change of friction, and casing and cement-sheath resistance loads. The discontinuous displacement approach and explicit/implicit coupling iteration methods are used to reveal the relationships between the effective normal stress, shear stress, friction coefficient, and sliding velocity during the fault slip process. Furthermore, the microscopic process of casing deformation sheared by fault slip is investigated using static equilibrium theory, and a characterization method for determining the amount casing deformation caused by real-scale fault slip is proposed. The results show that three stages exist in the process of casing deformation sheared by fault slip, including trigger activation stage, accelerated slip stage, and deceleration slip stage. Fault slip is clearly influenced by fault strike. To reduce the amount of fault slip, the fault direction with the maximum in-situ stress should be avoided as much as possible. Serious casing deformation still occurs for large-scale activated faults even though the optimization measure of wellbore structure has been well taken. To fundamentally reduce the possibility of casing shear deformation, it is necessary to prevent fault slip through optimizing the design of hydraulic fracturing. This study lays the theoretical groundwork for the casing deformation control method in shale gas wells. [ABSTRACT FROM AUTHOR]

Storage capacity and differential molar enthalpy of adsorption (or isosteric heat of adsorption) are important parameters to understand the characteristic of heterogeneous materials and catalysts for chemical and energy industry applications. Langmuir isotherm and other single site, dual site, and multilayer isotherms are developed for the prediction of adsorption and enthalpy, with a main assumption of constant isosteric heat of adsorption. However, some experimental and simulation data showed some inconsistencies in terms of heat of adsorption. Exploiting molecular simulation, we provide a first principle estimate of gas hosting capacity and associated thermodynamic properties of nanopores as present in type IID kerogen. The adsorption capacity and enthalpy of adsorption of the organic matter was computed using the grand canonical ensemble combined with the fluctuation method. The data obtained were utilized to assess the predictive power of industry standard models such as the Langmuir isotherm and other single site, dual site, and multilayer isotherms with respect to adsorption and enthalpy. The obtained results suggest that the sorption and thermodynamic properties of kerogen nanostructures are best described by monolayer-multisite isotherms rather than multilayer models. In short, for an adsorption theory to be physically consistent, it should capture both adsorption and isosteric heat. [ABSTRACT FROM AUTHOR]

Shale matrix permeability is vital to the evaluation of shale petrophysical property. In this study, an analytical model that considers pore size distribution, tortuosity, adsorption, and slippage effects is developed, to calculate the permeability of shale samples. Specifically, field emission scanning electron microscope (FE-SEM), the ImageJ software, and the MATLAB programming and numeric computing environment are employed to experimentally characterize the pore structure and to fit and optimize the pore size distribution. The discrete pore size distributions of organic matter and inorganic matter pores for Longmaxi and Qiongzhusi shale samples are determined based on FE-SEM images. According to the sum of squares error (SSE), coefficient of determination (R2), root-mean-square error (RMSE), and outcome of the fit, the Gaussian 2 correlation function is optimized to fit the continuous pore size distribution. Finally, the permeabilities of five shale rock samples are calculated and compared with the measured results from the Longmaxi and Qiongzhusi Formations. The results illustrate that the multi-term Gaussian fit is more suitable for calculating the permeability of samples with multimodal pore size distributions than the multi-segment Gaussian fit. The calculated results from the pore size distribution are more accuracy than those from the representative mean pore radius. The influence of pore size distribution on permeability and how seepage mechanisms in different pore sizes work together to represent permeability changes at the core scale will be investigated in the future. The study provides a more reliable calculation of the shale matrix permeability compared to previous methods. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing of shale reservoirs is one of the important technologies in the oil and gas industry. To ensure the safe operation of oil and gas recovery, it is important to study the shale-fluid interactions on the geomechanical behavior of shale. This study investigated the effect of fracturing fluid treatment on the mechanical and elastic properties of the Mowry Shale formation, Wyoming, USA. Cylindrical Mowry Shale specimens with a diameter of 12.5 mm collected from the United States Geological Survey (USGS) and School of Energy Resources (SER) of the University of Wyoming were treated with brine and brine + stimulation fluid for one month each at pressures of 9 and 11.7 MPa and temperatures of 96 and 66 °C, respectively. Triaxial compression experiments were conducted on the specimens. Results showed that all Mowry Shale specimens experienced an increase in maximum volumetric strain with the increase in effective confining pressure (Pd). Regardless of aging fluids, the maximum deviatoric stress (Δσd) of most Mowry Shale specimens increases with the increase in Pd. At a lower Pd, the USGS specimen aged with brine and stimulation fluid exhibits higher maximum Δσd than those aged with brine only. However, at a higher Pd, the USGS specimen aged with brine exhibits a higher maximum Δσd. SER specimens aged with brine and stimulation fluid exhibit higher maximum Δσd values than those aged with brine for all three Pd values. Regardless of the aging fluids, most USGS specimens experience a brittle failure mode, while SER specimens aged with brine and stimulation fluid experienced a more ductile behavior. [ABSTRACT FROM AUTHOR]

This study investigated the impact of pore accessibility and complexity on gas storage, transport, and recovery potential in the little-studied thermally mature Raghampuram shale samples collected from 2930 to 2987 m depth of Krishna–Godavari basin, India. Our findings reveal that sample nature (powdered, chipped, or cores) and assessment methods significantly influence pore accessibility evaluation, highlighting a research gap in the interpretation of irregularity, complexity, and heterogeneity of shale pore structure using unreliable monofractal theories. Employing a multiscale methodology involving low-pressure N2 and CO2 sorption, synchrotron small-angle scattering, and He-pycnometry techniques, we estimated accessibility in powder and core samples. Powder samples displayed a pore accessibility range of 36.07–106.94%, which was a substantial increase (154.54–423.07%) compared to that of solid core samples (1.61–4.16%). Total organic carbon was found to influence closed pore formation, while clay, carbonate, and quartz + K-feldspar contributed to open pores. Multifractal analyses comparing pore heterogeneity and complexity between accessible and inaccessible pores demonstrated higher heterogeneity and complexity in the latter, with accessible pores exhibiting simpler characteristics. Pore size distributions of both accessible and total pores (includes both accessible and inaccessible pores) exhibited multifractal behavior. Our findings emphasize the significance of evaluating pore accessibility and heterogeneity in shale-gas analysis, providing fresh insights into the interlinked elements of pore structure, composition, and gas recovery potential, thus advancing reservoir characterization understanding. [ABSTRACT FROM AUTHOR]

Although the recent debate in energy economics on the importance of oil price indexation versus shale gases suggest that big data can be used in predictive analysis in energy economics, little is known particularly in the context of shale gas and oil price interlinkages. Grounding our investigations in such directions we investigate in this paper the relationship between gas and crude oil markets and the impact US shale gas by employing time-varying causality method by Shi et al. (J Time Ser Anal 39(6):966–987, 2018; J Financ Econom 18(1):158–180, 2020) and cross-quantilogram correlation approach by Han et al. (J Econom 193(1):251–270, 2016). In particular, as a representative of the crude oil market, we use OPEC oil; WTI; Crude oil Oman; Crude oil Dubai while for the gas market, we use natural gas prices of UK NBP (National Balancing Point), NYMEX HH (Henry Hub) and US shale gas prices. Data period is from 11th January 2013 to 8th September 2020. We find significant negative spillovers from crude oil markets to natural gas markets particularly during moderate market conditions. The results suggest crucial implications in energy economics literature, to diversify assets to hedge against risks. We further find strong causality association between oil markets, natural gas markets and further oil markets and shale gas markets. Our findings describe that aftermath of the shale-gas boom the predictability nexus between oil and natural gas increased. Once we condition for shale gas the significant negative spill overs from oil markets to natural gas markets increases in the long-run. We suggest important policy prescriptions which have interconnected market repercussions. [ABSTRACT FROM AUTHOR]

It is greatly significant to evaluate the stability of the shales with characteristic defects that the propagation and crack initiation behavior under the dynamic impact. For studying the influence of bedding and preset cracks on the dynamic fracture characteristics of shales, the dynamic tensile mechanical test of cracked layered shales was operated using the Split Hopkinson Pressure Bar. Using high-speed photography technology to record this whole process of crack propagation and dynamic failure. According to the results, the bedding angles and pre-existing crack angles of the specimen affected the characteristics of crack initiation points, crack initiation angles, and propagation paths. As the pre-existing crack angle increases, the crack initiation points of shale specimens gradually shift from a pre-existing crack tip to a pre-set hole edge region. When these pre-existing crack angles increased, it is important to the influence of bedding angles on crack initiation angles. The cracks of the specimen mainly propagated along or across the bedding plane to both sides of the loading end. According to the crack growth and propagation characteristics, we divided the failure modes of the shale specimen into five typical modes. The study can supply some theoretical support for the actual exploitation of deep shale gas. [ABSTRACT FROM AUTHOR]

Shale gas is an extremely important unconventional oil and gas resource, and its efficient development can effectively alleviate the current tense energy situation. However, shale gas reservoirs often have extremely poor permeability, and reservoir transformation has become a key technology for achieving their efficient development. However, the commonly used hydraulic fracturing technology is difficult to achieve its target production capacity, and other engineering technologies related to reservoir transformation urgently need to be proposed and attempted. The synergistic operation of oxidation dissolution and acid fracturing may provide new ideas for the effective transformation of shale reservoirs. To this end, a comparative analysis was conducted on the synergistic effects of oxidation dissolution and acid fracturing operations in improving shale gas production capacity. The research results indicate that the dissolution effect of oxidants is more effective than acid solution in the transformation process of shale reservoirs. The use of acid only widens the crack width from the initial 4.4 mm to the final 5.1 mm. However, the use of oxidants will result in the final width of hydraulic fractures reaching 8.3 mm. Meanwhile, the effects of acid concentration and oxidant concentration on hydraulic fracture conductivity and shale gas production capacity were investigated. The results indicate that increasing the acid concentration below the low concentration range can significantly enhance the fracture conductivity, thereby promoting the production capacity of shale gas. However, within a higher concentration range, its effect on shale gas production is significantly limited. It is recommended to set the acid concentration design value at 0.5 wt% during the acidizing and fracturing reservoir transformation process of the shale gas reservoir in Changning block. In addition, an increase in the concentration of oxidants can widen the width of fractures and increase permeability, thereby promoting the migration and extraction of shale gas. To avoid the increase in development costs caused by high oxidant concentration in the working fluid, it is recommended to design the oxidant concentration at 3 wt%. [ABSTRACT FROM AUTHOR]

The inhibition of surface hydration is the most fundamental technical measure for controlling shale borehole stability, but little research has been conducted on the inhibition of surface hydration. In this study, the low-molecular-weight amine inhibitor NH-2 was prepared. The inhibition performance of the NH-2 inhibitor was studied by linear expansion experiment, shale rolling recovery, isothermal adsorption, X-ray diffraction and thermogravimetric analysis. The inhibition mechanism was studied by X-ray diffraction and X-ray photoelectron spectroscopy. The results showed that 1 wt% NH-2 had a lower linear expansion ratio than did 5 wt% KCl and 1 wt% polyamine. The shale rolling recovery rate of the drilling fluid significantly improved after the addition of the NH-2 inhibitor. After the inhibitor NH-2 was added, the unit adsorbed water content of sodium montmorillonite decreased from 0.1104 g/g to 0.0331 g/g. The basal spacing of dry sodium montmorillonite increased from 1.01 nm to 1.3 nm, while that of wet sodium montmorillonite decreased from 1.9 nm to 1.31 nm. The DTG curve has only one low-temperature peak, and the sodium ion content in sodium montmorillonite was greatly reduced. These results showed that the NH-2 inhibitor has good inhibition performance. The inhibitor NH-2 enters the interlayer space of sodium montmorillonite through intercalation, displaces exchangeable cations, and minimizes the base spacing of sodium montmorillonite. Thus, the surface hydration of sodium montmorillonite was completely inhibited. In addition, the decomposition temperature of the NH-2 inhibitor was 232°C, which indicates good thermal stability. This study provides an inhibitor with high temperature resistance and efficient inhibition of hydration. [ABSTRACT FROM AUTHOR]

Radial borehole fracturing is a production method that combines ultrashort radius radial boreholes with hydraulic fracturing, which is expected to effectively improve the drainage and transformation effect of tight oil and gas reservoirs. In view of the problem that radial boreholes cannot be fully fractured when high-pressure liquids are injected into them simultaneously, this paper proposes a new method of fracturing the radial boreholes step by step, and this method is verified by the laboratory fracturing experiments. It designed and processed wellbores with both internal and external pipes, conducted 24 sets of radial boreholes step-by-step experiments of double-layer radial boreholes, analyzed the reproducibility of the experimental results, and compared the results of fracturing radial boreholes simultaneously and fracturing radial boreholes step-by-step experiments. The results show that this method can fracture two sets of radial boreholes distributed in different vertical planes. The results demonstrate that this method is feasible and can solve the problem that not all the radial boreholes are fractured when fracturing fluids are simultaneously injected into them. Besides, the azimuth is the key factor affecting fracture geometries. When the azimuths are 0° and 30°, the fractures can cut through radial boreholes in different layers within the rock samples. If the radial boreholes with larger azimuths are fractured, it will generate higher breakdown pressures. Fracturing radial boreholes with smaller azimuths first and then radial boreholes with larger azimuths should be selected. This study provides a theoretical basis for the field application of radial borehole fracturing technology. Highlights: A new method of fracturing the radial boreholes step by step is proposed. The azimuth is the key factor affecting fracture geometries. If the radial boreholes with larger azimuths are fractured, it will generate higher breakdown pressures. [ABSTRACT FROM AUTHOR]

Subcritical crack propagation in stressed carbonate rocks in a chemically reactive environment is a fundamental mechanism underlying many geomechanical processes frequently encountered in the engineering of geo-energy, including unconventional shale gas, geothermal energy, carbon sequestration and utilization. How a macroscopic Mode I crack propagates driven by a reactive fluid pressurizing on the crack surfaces with acidic agents diffusing into the rock matrix remains an open question. Here, the carbonate rock is modeled as an elasto-viscoplastic material with the mineral mass removal process affecting the rock properties in both elastic and plastic domains. A blunt-tip crack is considered to avoid any geometrically induced singularity problem and to allow a numerical analysis on the evolution of the chemical field being linked to the micro-cracking activities in front of the crack tip, affecting the delivery of acid. The model is capable of reproducing an archetypal three-region behavior of subcritical crack growth in a reactive environment. The crack propagation exhibits a prominent acceleration in Region III due to a two-way mutually enhancing feedback between mineral dissolution and the degradation process, which is most pronounced in front of the crack tip. With the consideration of initial imperfections in the rock, the macroscopic crack propagation is further accelerated with a secondary acceleration arising due to self-organization of micro-bands inside the chemically enabled plasticity zone. [ABSTRACT FROM AUTHOR]

Full-scale drilling for shale gas has been deepening, and the horizontal definitions have been increasing for drilling efficiency. Pressures and bending stresses are generated in the joints of gas-cored pipes, which seriously affect their durability. Premium casing connections are primarily forced at the interfaces of two parts, so the design of the threads is crucial, and the choice of optimal parameters is precisely related to their resistance to stresses. This article proposes a novel premium connection design, and its performance is validated through simulations to demonstrate good noise, vibration, and harshness (NVH) and durability performance. A parametric study is especially performed with the change of lower corner radius to observe maximum stress changes of the system and it is advised that a parameter be optimized. This study demonstrates that it is possible to design a premium connection capable of reducing the stress concentrated on the stab flank contact portion when drilling deep gas wells in response to parameter changes. [ABSTRACT FROM AUTHOR]

The Silurian palaeontology and stratigraphy of the Qinghai-Tibetan Plateau have been studied for more than 100 years. With the launch of the Second Tibetan Plateau Scientific Expedition and Research, it is necessary to update the summaries of the Silurian stratigraphy and fossil assemblages of the Qinghai-Tibetan Plateau and their correlation with its surrounding areas. In this study, we have selected 33 sections from the Qinghai-Tibetan Plateau and its surroundings areas, summarised the available data in terms of stratigraphic regionalisation, lithostratigraphy, chronostratigraphy, and biostratigraphy, and divided the study area into four stratigraphic provinces: the Xizang-Western Yunnan-Western Sichuan Province, the Kunlun-Qilian Province, the South China Province, and the Tarim Province. In general, the Silurian strata in the Qinghai-Tibetan Plateau is more complete than those in other areas of China. Palaeobiogeographical studies of the vertebrate faunas, sporomorph assemblages, and brachiopod faunas of the study area indicate a closer geographical relationship between the South China, Tarim, and Indochina palaeoplates, than previously considered. Compared with the surrounding areas where Silurian strata have been extensively studied, the study of Silurian strata in the main part of the Qinghai-Tibetan Plateau is still in the preliminary stage. In the future, it is necessary to conduct research on the age, distribution, and resource potential of basal Silurian black shales to provide new directions for shale gas exploration and development in China. The shallow marine strata of the middle to upper Silurian require further subdivision. Considering that the Silurian System in the Qinghai-Tibetan Plateau is more complete, it has the potential to supplement and improve the study of the carbon isotope stratigraphy of the Silurian in China. [ABSTRACT FROM AUTHOR]

We review empirical data and theoretical relationships to derive equations which relate the upper boundary of maximum earthquake magnitude to the volume of fluids injected by pumping water and supercritical СО2, as well as to magmatic activity preceding volcanic eruptions. The equations can be used to predict triggered seismicity occurring during the extraction of shale gas and oil, to develop systems for extraction of geothermal energy and for burial of supercritical СО2, as well as to assess the volumes of magma injected before eruptions of volcanoes. [ABSTRACT FROM AUTHOR]

The change of fracture propagation direction caused by stress interference between fractures is one of the main reasons that affect shale gas productivity. Natural fractures will be damaged by the induced stress, and the induced stress field produced by natural fracture damage will in turn affect the propagation of hydraulic fractures. Previous studies usually ignored the impact of stress field variation caused by natural fracture damage, leading to inaccurate fracture propagation simulation results. A new model for simulating hydraulic fracturing-induced stress field is established with consideration of the influence of natural fracture damage. Then, the natural fracture-induced stress is analyzed in open and closed fracture states. Through superposition of stress fields of natural fractures and hydraulic fractures, the interaction among open and closed natural fractures, reservoir and hydraulic fracture, and the corresponding fracture property evolution are studied. The results demonstrate that induced stress is not affected by elastic modulus and Poisson's ratio of the rock. The induced stress difference at the fracture tip is proportional to the net pressure and the length of hydraulic fractures. The results provide guidance for the optimal design of fracturing. When the deflection angle of hydraulic fracture is less than 90°, the maximum induced stress difference at the fracture tip decreases with the reduction in the deflection angle. When the deflection angle of hydraulic fracture is smaller than 90° or the deflection angle of natural fracture is smaller than 45°, the steering of hydraulic fracture is less hindered, which is beneficial to the formation of complex fracture network. [ABSTRACT FROM AUTHOR]

Water production from petroleum (oil and natural gas) wells is a topic of increasing environmental and economic importance, yet quantification efforts have been limited to date, and patterns between and within petroleum plays are largely unscrutinized. Additionally, classification of reservoirs as "unconventional" (also known as "continuous") carries scientific and regulatory importance, but in some cases the distinction from "conventional" wells is unclear. Using water, oil, and gas production data, we calculated a set of quantitative metrics that elucidate trends in the water-to-petroleum ratio over the life of each producing well. The percent growth of the water-to-petroleum ratio quantifies the degree to which a well "waters out" over time; values calculated for 153,900 wells in 18 oil and gas plays show generally much higher values for conventional wells than for continuous/unconventional wells. Analysis of the percent growth along with the slope and median metrics reveals greater variation between conventional plays and between continuous (unconventional) plays than previously recognized. Further, an example from the Bakken Formation in the Williston Basin, USA, illustrates that, within a single play, the metrics provide insight into spatial variation of water production trends, as influenced by geology and reservoir characteristics. By quantifying the variability of water production trends within individual plays and between plays, including differences between conventional and continuous (unconventional) plays, these results provide a more nuanced view of water production from oil and gas wells than has previously been possible and they illustrate the degree to which water management considerations vary spatially and temporally. [ABSTRACT FROM AUTHOR]

Studies on the assessment of heavy metal(oid) contamination in soils near shale gas development areas are scarce. Studying the distribution characteristics of heavy metal(oid)s in the soil of shale gas extraction areas, evaluating ecological risks and health risks, and analyzing the sources of heavy metal(oid)s are important for environmental protection and public health in the shale gas development process. Forty-six surface soil samples were collected from a platform in the Shale Gas Development Zone, Sichuan Province, China, and analyzed for As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn by inductively coupled plasma mass spectrometry and atomic fluorescence spectrometry. The results showed that the average concentration of As exceeded the screening value of China's agricultural soil environmental quality standard, and the content of all heavy metal(oid)s except Hg was higher than the background value. As and Cd had Nemerow indices of 3.25 and 6.62, respectively, and their mean Geo-accumulation indices were 0.38 and 0.52 each, both of which were noticeably greater than the contamination levels of other heavy metal(oid)s. It indicates that As and Cd contamination existed in the soil and the contamination was concentrated in the south of the study area. The results of the potential ecological risk evaluation indicated the existence of an ecological threat of Cd. There is no adult or child carcinogenic risk in the study area, but the non-carcinogenic hazard index for children is 2.71, posing a non-carcinogenic health risk to children (HI > 1), with As as the main risk factor. CV, Pearson correlation analysis, and PCA analysis indicate that As is mainly influenced by natural factors and may also originate from deep well strata during shale gas drilling. Cd is mainly derived from anthropogenic sources such as agricultural activities and shale gas development activities. [ABSTRACT FROM AUTHOR]

Deep shale gas development has great potential, but the frequent occurrence of shale gas well casing change problems triggered by geological factors seriously restricts deep shale gas development. In order to investigate the influence of geological factors on the casing stress of the cemented casing in the sleeve, a model of formation-cement sheath-double casing assemblage was established, and the influence of three-way geostress and fault slip on the casing stress of the casing-in-casing cementing was investigated by using finite element analysis. The results show the following: the smaller the difference between the vertical geostress and the maximum horizontal geostress is, the lower the equivalent force on the casing is, and when the difference gradually decreases from 20 to 7 MPa, the maximum equivalent force of the inner casing under fracturing condition decreases by 9.4%; the increase of the minimum horizontal geostress leads to the increase of the equivalent force of the inner and outer casing. When the minimum horizontal stress gradually increases from 80 to 90 MPa, the maximum equivalent force of the inner casing under fracturing condition increases by 5.9%. The larger the fault slip and the fault angle, the larger the equivalent force generated on the casing. The shear resistance of the double-layer casing is significantly greater than that of the single-layer casing, with an average increase in fault slip distance that can be withstood of about 45.25% and 40.2% in the no internal pressure and fracturing conditions. The larger the casing steel grade and the thicker the wall thickness, the higher the shear resistance. It is recommended to reduce placing of wells in areas where the difference between the vertical and maximum horizontal ground stresses is large, and at locations where the fault slip angle is large, and to use both higher steel grades and larger wall thicknesses of casing. This research result demonstrates the feasibility of "milling + casing-in-casing" technology in severe casing change wells and also provides useful guidance for the application of this technology in the field. [ABSTRACT FROM AUTHOR]

The micro-scale gas adsorption–desorption characteristics determine the macro-scale gas transport and production behavior. To reveal the three-dimensional stress state-induced gas adsorption–desorption characteristics in coal-bearing shale reservoirs from a micro-scale perspective, the coal-bearing shale samples from the Dongbaowei Coal Mine in the Shuangyashan Basin were chosen as the research subject. Isothermal adsorption–desorption experiments under three-dimensional stress state were conducted using the low field nuclear magnetic resonance (L-NMR) T2 spectrum method to simulate the in-situ coal-bearing shale gas adsorption–desorption process. The average effective stress was used as the equivalent stress indicator for coal-bearing shale, and the integral of nuclear magnetic resonance T2 spectrum amplitude was employed as the gas characterization indicator for coal-bearing shale. A quantitative analysis was performed to examine the relationship between gas adsorption in coal-bearing shale and the average effective stress. And a quantitative analysis was performed to examine the relationship between the macroscopic and microscopic gas quantities of coal-bearing shale. Experimental findings: (1) The adsorption–desorption process of coal-bearing shale gas follows the L-F function model and the D-A-d function model respectively with respect to the amount of gas and the average effective stress. (2) There is a logarithmic relationship between the macroscopic and microscopic gas quantities of coal-bearing shale during the adsorption–desorption process. This quantitatively characterizes the differences in the curves, which may be related to the elastic–plastic deformation, damage and fracture of the micropores in coal-bearing shale, as well as the hysteresis of gas desorption and the stress field of the gas occurrence state. [ABSTRACT FROM AUTHOR]

Shale gas reservoirs usually have well-developed fractures and joints, and the fracturing development process is prone to forming complex fracture networks near the wellbore, which is not conducive to the effective migration and efficient development of shale gas. The development of temporary plugging agents related to shale fracturing reservoir transformation is a key technology to solve this problem in the shale gas development process. Therefore, a water-soluble preformed gel temporary plugging agent was developed, and its expansion/swelling characteristics and temporary plugging performance were analyzed through experiments. At the same time, experimental evaluation was conducted on its unblocking performance. Research has found that although there are differences in the swelling rate of temporary plugging agents at different stages, they gradually increase during plugging operations. The volume of the temporary plugging agent expanded nearly 8.24 times throughout the entire experimental process. In addition, temporary plugging agents can temporarily seal micro fractures and rapidly expand hydraulic fractures, thereby significantly improving shale gas recovery rate. The shale gas recovery rate when temporary plugging agents are used in fracturing operations is about 12.3% higher than when temporary plugging agents are not used. In addition, the increase in working fluid temperature and the concentration of surfactants will mostly stimulate the activity of water molecules, thereby stimulating the hydrolysis of temporary blocking agents in microcracks. Taking into account the development cost and efficiency, it is reasonable to control the temperature of the working fluid and the concentration of surfactant during the unblocking process after the fracturing operation between 110-140°C and 0.75 g/m3, respectively. In order to provide technical support and design basis for the efficient development of shale gas through this study. [ABSTRACT FROM AUTHOR]

The Longmaxi Formation in the Sichuan Basin is a hotspot formation for shale gas exploration. In this paper, taking the Longmaxi Formation shale in Yuxi area as an example, a large number of TOC tests, X-diffraction whole-rock mineral fraction tests, electron microscope observations, physical properties, gas content, and CT scanning experiments were carried out, which in turn analyzed the characteristics of the shale gas reservoir and the factors influencing the gas content. The results of the study show that there are seven types of shale facies developed in Yuxi area, which are organic-rich siliceous shale facies (I), organic-rich carbonate-siliceous shale facies (II), organic-rich clayey shale facies (III), organic-poor clayey shale facies (IV), organic-poor carbonate-siliceous shale facies (V), organic-poor siliceous shale facies (VI), and organic-poor siliceous-carbonate shale facies (VII). The microstructural characteristics of the shale reflect that it was disturbed by multiple environmental factors at the time of deposition, and the sources of brittle minerals include both terrestrial clasts and biogenic siliciclastics. The bottom 30 m shale of Longmaxi Formation has good physical properties and gas content, which is a high-quality target section for shale gas exploration and development. TOC, Ro, permeability, quartz content and clay mineral content all have a significant effect on shale gas content and are important control factors for shale gas content. [ABSTRACT FROM AUTHOR]

Normal-pressure shale gas reservoirs are widely distributed in south-eastern Chongqing and show good potential for resource exploration. This paper reports the organic matter (OM), physical, and pore characteristics, mineral composition, and gas content of representative shale samples from the Upper Ordovician Wufeng Formation and Member 1 of the Lower Silurian Longmaxi Formation (Long 1 Member). Microscopic pores within different shale layers of the Long 1 Member were classified, quantitatively evaluated, and their development mechanisms were systematically studied. We found that OM characteristics, mineral composition, and pore type were the main factors affecting the enrichment and preservation of shale gas. The characteristics of the Long 1 Member are mainly controlled by changes in the sedimentary environment. There are evident differences in total organic carbon content and mineral composition vertically, leading to a variable distribution of pores across different layers. Organic matter abundance controls the degree of OM pore development, while clay minerals abundance control the development of clay mineral-related pores. Total organic carbon content generally controls the porosity of the Long 1 Member, but clay minerals also play a role in OM-poor layers. Pore connectivity and permeability are influenced by the development of pores associated with brittle minerals. We propose a microscopic pore development model for the different layers. Combining geochemical data and this pore development model, layers 1–4 are considered to be excellent shale gas preservation and enrichment reservoirs. Poor preservation conditions in layers 5–7 result in high levels of shale gas escape. Layers 8–9 possess a better sealing condition compared with layers 5'-7 and are conducive to the enrichment and preservation of shale gas, and can thus be used as future potential target strata. This research provides a theoretical basis for exploring and evaluating shale gas potential in the studied region or other complex normal-pressure shale blocks. [ABSTRACT FROM AUTHOR]

Waste oil-based drill cuttings contain dioxins and volatile organic compounds (VOCs), which have the potential to cause serious health effects in humans. Therefore, this paper took oil-based drill cuttings (OBDCs) as the research object and carried out the testing of VOCs and dioxins content by using GC–MS and HRGCS-HRMS and comprehensively evaluated the content, composition and distribution pattern of VOCs and dioxins and the risk to human health posed by the two pollutants in OBDCs. The results showed that the VOCs did not exceed the emission limits in ESPPI (GB 31571–2015), but it is vital to recognise that 1,2-dichloropropane has the potential to cause cancer risk, with soil and groundwater risk control values of 662.95 mg·kg−1 and 0.066 mg·kg−1, respectively. Benzene, 1,2-dichloropropane and 8 other VOCs pose a non-carcinogenic risk to humans. The levels of polychlorinated dibenzofurans (PCDFs) exceeded those of polychlorinated dibenzo-p-dioxins (PCDDs), which accounted for 95.76 percent of the total PCDD/Fs, 2,3,4,7,8-P5CDF (56.00%), 2,3,7,8-T4CDF (9.20%), 1,2,3,6,7,8-H6CDF (8.80%) and 1,2,3,7,8-P5CDF (8.00%) were the main contributing monomers. The findings of the assessment on exposure risk indicate that there is a respiratory risk to oil-based drill cuttings dioxins for adults and children exceeded the World Health Organisation (WHO) acceptable daily intake (ADI) (1–4 pgTEQ/kg/d). Finally, three aspects of solid waste pre-treatment prior to incineration, the incineration process and post incineration were used to reduce the environmental and human health risks from dioxins. [ABSTRACT FROM AUTHOR]

Currently, there is limited understanding of the structures and variabilities of bacterial communities in oil-contaminated soil within shale gas development. The Changning shale gas well site in Sichuan province was focused, and high-throughput sequencing was used to investigate the structures of bacterial communities and functions of bacteria in soil with different degrees of oil pollution. Furthermore, the influences of the environmental factors including pH, moisture content, organic matter, total nitrogen, total phosphorus, oil, and the biological toxicity of the soil on the structures of bacterial communities were analyzed. The results revealed that Proteobacteria and Firmicutes predominated in the oil-contaminated soil. α-Proteobacteria and γ‐Proteobacteria were the main classes under the Proteobacteria phylum. Bacilli was the main class in the Firmicutes phylum. Notably, more bacteria were only found in CN-5 which was the soil near the storage pond for abandoned drilling mud, including Marinobacter, Balneola, Novispirillum, Castellaniella, and Alishewanella. These bacteria exhibited resilience to higher toxicity and demonstrated proficiency in oil degradation. The functions including carbohydrate transport and metabolism, energy metabolism, replication, recombination and repair replication, signal transduction mechanisms, and amino acid transport and metabolism responded differently to varying concentrations of oil. The disparities in bacterial genus composition across samples stemmed from a complex play of pH, moisture content, organic matter, total nitrogen, total phosphorus, oil concentration, and biological toxicity. Notably, bacterial richness correlated positively with moisture content, while bacterial diversity showed a significant positive correlation with pH. Acidobacteria exhibited a significant positive correlation with moisture content. Litorivivens and Luteimonas displayed a significant negative correlation with pH, while Rhizobium exhibited a significant negative correlation with moisture content. Pseudomonas, Proteiniphilum, and Halomonas exhibited positive correlations not only with organic matter but also with oil concentration. Total nitrogen exhibited a significant positive correlation with Taonella and Sideroxydans. On the other hand, total phosphorus showed a significant negative correlation with Sphingomonas. Furthermore, Sphingomonas, Gp6, and Ramlibacter displayed significant negative correlations with biological toxicity. The differential functions exhibited no significant correlation with environmental factors but displayed a significant positive correlation with the Proteobacteria phylum. Aridibacter demonstrated a significant positive correlation with cell motility and cellular processes and signaling. Conversely, Pseudomonas, Proteiniphilum, and Halomonas were negatively correlated with differential functions, particularly in amino acid metabolism, carbohydrate metabolism, and membrane transport. Compared with previous research, more factors were considered in this research when studying structural changes in bacterial communities, such as physicochemical properties and biological toxicity of soil. In addition, the correlations of differential functions of communities with environmental factors, bacterial phyla, and genera were investigated. [ABSTRACT FROM AUTHOR]

Deep shale reservoirs (3500–4500 m) exhibit significantly different stress states than moderately deep shale reservoirs (2000–3500 m). As a result, the brittleness response mechanisms of deep shales are also different. It is urgent to investigate methods to evaluate the brittleness of deep shales to meet the increasingly urgent needs of deep shale gas development. In this paper, the quotient of Young's modulus divided by Poisson's ratio based on triaxial compression tests under in situ stress conditions is taken as SSBV (Static Standard Brittleness Value). A new and pragmatic technique is developed to determine the static brittleness index that considers elastic parameters, the mineral content, and the in situ stress conditions (BIEMS). The coefficient of determination between BIEMS and SSBV reaches 0.555 for experimental data and 0.805 for field data. This coefficient is higher than that of other brittleness indices when compared to SSBV. BIEMS can offer detailed insights into shale brittleness under various conditions, including different mineral compositions, depths, and stress states. This technique can provide a solid data-based foundation for the selection of 'sweet spots' for single-well engineering and the comparison of the brittleness of shale gas production layers in different areas. [ABSTRACT FROM AUTHOR]

Casing deformation occurs frequently in shale gas wells, resulting in the failure of some fracturing stages to complete normal operations, which seriously affects shale gas production. This paper firstly analyzes the casing deformation mechanism of shale gas wells and determines that the casing shear deformation caused by formation slip is the main factor of casing deformation. In order to solve the problem of casing shear deformation, a finite element numerical model including the formation, cement ring, and the casing is established. The casing shear deformation caused by formation slip is simulated, and the deformation results obtained are consistent with those measured in the actual wellbore. Based on the numerical model, a casing deformation governance method is proposed to optimize the wellbore trajectory and adjust the cement ring properties. It is calculated that reducing the dip angle á of the formation and casing vertical direction and reducing the formation slip is beneficial to reducing casing deformation. The use of low elastic modulus cement and uncementing operations can provide a buffer space for formation slip, effectively reduce casing deformation, and provide an effective treatment method for shear deformation of shale gas wells. [ABSTRACT FROM AUTHOR]