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The good sealing caprocks are significant for the integrity of underground gas storage (UGS) in depleted natural gas reservoirs. The screening of parameters, weight assignment, and evaluation method are important for evaluating the sealing performance of caprocks. Many factors can affect the sealing performance of caprocks, including caprock thickness, lithology, brittleness, porosity and permeability, breakthrough pressure, etc. In this paper, the dominant factors in the sealing performance of caprocks in UGSs are systematically analyzed, and the weights of these factors are analyzed by the analytic hierarchy process (AHP). The fuzzy comprehensive evaluation method (FCEM) is applied in the sealing evaluation of caprocks in three typical underground gas reservoirs (i.e., Zhujiadun, Xu-2, and Xing-9) in China. The sandstone reservoir in the Zhujiadun gas field is only about 20 m, and the thickness of the overlying mudstone is about 600 m. The caprock of the Xu-2 gas reservoir in Zhongba gas field is well distributed and developed, and the breakthrough pressure is relatively large. The caprock of Xing-9 gas field is mudstone with a thickness of over 400 m. The results show that the breakthrough pressure and permeability are the key parameters affecting the sealing ability of caprocks, with weights of 0.4291 and 0.2157, respectively. Among these three examples of gas fields, the sealing performance of caprocks in Zhujiadun gas storage is the best. The evaluation procedure and methods proposed in this paper are valuable for the evaluation of the tightness of caprocks in depleted gas reservoirs. [ABSTRACT FROM AUTHOR]

The underground gas storage (UGS) in depleted sandstone reservoirs forms the largest proportion of the UGS market in China. Multiple cycles of natural gas injection and production in the sandstone cause the rapid increase and drawdown of pore pressure, which may induce damage to the rock skeleton structure, and cause complex fluid flow paths in the sandstone reservoir. In this paper, transverse relaxation time (T2), nuclear magnetism resonance imaging, and high-pressure mercury intrusion analysis are combined to evaluate the variation in pore structure of medium-grained sandstone. The results show that cyclic injection and production of fluid leads to a slight increase in total pore volume, indicating that weak damage to rocks occurs. The T2 spectrum at the low pore pressure (10 MPa) and high pore pressure (25 MPa) both show that the shrinkage of the medium-size pores occurs after multiple cycles of injection and production. The pore volume of large-size pores was not highly correlated with the number of cycles. With the increase in pore pressure, the pore volume ratio under high pore pressure increased with the number of cycles, while it fluctuated strongly under low pore pressure. [ABSTRACT FROM AUTHOR]

The seepage characteristics of granite are complicated due to flow exchange between fractures and micropores, which are strongly heterogeneous and anisotropic. The potential geothermal reservoirs of hot dry rocks are often related to granite. The key to geothermal production from hot dry rocks is to generate an efficient fracture network through enhanced geothermal system (EGS) technologies. Although hydraulic fracturing technology is applied in most EGS projects worldwide, it has been indicated that thermal and chemical stimulation methods are also efficient. In this paper, different thermal and chemical treatments are applied to Mesozoic granite samples obtained from geothermal region of Liaodong Peninsula in northeastern China, and their impacts on the seepage characteristics of the granite are carefully analyzed. The thermal and chemical damage mechanisms of granite, in terms of the mineral components, microstructures and physical properties, are also studied. The results show that cooling in cold water results in a much larger permeability increase than that achieved with cooling in air after the granite is heated to a specific temperature. Thermal treatments cause mechanical damage (e.g., compressive strength, elastic modulus, etc.) of granite. However, the differences between the damage factors of various granite samples are large, which are highly dependent on the original mineral composition and microstructures. Thermal cracking induced by either thermal fatigue or thermal shock causes changes in the porosity and permeability of granite. The permeability of the granite changes substantially, i.e., 3–4 orders of magnitude, after soaking in mud acid solutions, while it changes by less than 1 order of magnitude after soaking in alkaline solutions. The dissolution ability of the mud acid solution with the highest concentration (12% HCl + 3% HF) is the strongest among the three types of mud acid solutions tested (S1, S2 and S3). But it mainly acts on improving the proportion of small pores with the size of 0.01–10 μm. It has the least impact on the generation of large pores (10–50 μm), which may be caused by the heterogeneity of mineralogy among different granite samples. Different chemical solutions infiltrate the interior of the granite through various micropores and fissures, enhancing the chemical interaction through the dissolution and precipitation of minerals, the porosity, and the permeability, destroying not only the grains and minerals but also the microstructures of the granite. [ABSTRACT FROM AUTHOR]

The global warming induced by the emission of greenhouse gases, especially the carbon dioxide, has become the global climate and environmental issues. China has been working in the CO2 emission reduction and carbon sinks with the purpose of becoming the carbon-neutral country by 2060. The CO2 capture, utilization and storage (CCUS) technologies and the reforestation technology represented by the Conversion of Cropland to Forestland Program (CCFP) have great potential for sinking CO2 emission. However, the trade-off among CCFP, CCS/CCUS and Water-Energy-Food (WEF) nexus are not well evaluated. In this paper, the remote-sensing data are collected and used to evaluate the sustainability of CCFP by analyzing the variation of land use and land cover (LULC), crop production, etc. The results show that 13.29% of the cropland in 2001 vanished and converted to grassland (8.3%), mosaic cropland (3%) and urban land (0.98%) in 2019, demonstrating that the CCFP is successful in both WEF nexus and carbon sink. The total crop production has increased around 50% between 2001 and 2019, implying that the CCFP will not lead to the food risk during the conversion of croplands into other types of land in China. A sustainable implementation of CCFP and other environmental Payments for Ecosystem Services (PES) policies in 2019–2060 could reach an estimated total growth of 7.462 billion m3 in comparison of that in 2018 and the total plantation forest stock of about 10.852 billion m3 in 2060, with a corresponding minimum CO2 sink of 2.90 billion tons in 2060. The estimated peak of net equivalent CO2 emissions before 2030 is about 11.0 billion tons and could not be reduced to zero by 2060 without the large-scale application of the CCS/CCUS technologies as geological sequestration of CO2. Besides, the application of CCS/CCUS can be beneficial for WEF, e.g., through replacing the water by CO2 during energy production, especially in the shale gas production in the regions with high water risks in China. In one word, CCS/CCUS and CCFP are two decided pathways of carbon sequestration and should be systematically applied to achieve China's carbon neutrality by 2060. [ABSTRACT FROM AUTHOR]

Energy security is a global strategic issue that limits economic development and social stability. Improving the energy storage system is the key step and global solution for low-carbon energy transformation. Compared with above-surface energy storage, underground energy storage in geological spaces, especially in porous media, is significant and accounts for the largest proportion. This paper clarifies the framework of underground energy storage systems, including underground gas storage (UGS), underground oil storage (UOS), underground thermal storage (UTS) and compressed air energy storage (CAES), and the global development of underground energy storage systems in porous media is systematically reviewed. Then, the current state of art of underground energy storage engineering in porous media in China, including the construction status, policy environment, technical challenges and possible solutions, are comprehensively analyzed and discussed. In-depth theoretical research, novel methodology, comprehensive studies on site characterization and selection, sealing performance evolution, monitoring, etc., using integrated experiments, physical simulation experiments, and numerical simulations are the key to ensuring the safety, environmental friendliness, and economic efficiency of underground energy storage in porous media. In China, the development of both underground gas storage in depleted natural gas reservoirs and thermal energy storage in shallow aquifers is obvious and cost-effective. The UGS in depleted oil reservoirs and aquifers, UOS in porous media, UTS in deep aquifers, and CAES in porous media, however, are still at the stage of technological research and development, requiring strong policy support and government investment in the construction of demonstration projects before the future industrialization process. • Framework of underground energy systems is clarified based on storage space, stored materials, heat or electrical energy. • State of the art of the of global UESs in porous media is investigated. • UES engineering projects in porous media of China are summarized. • Policy environment, site characterization and selection, economic and safety risks associated with the UES in porous media are analyzed. [ABSTRACT FROM AUTHOR]

CO sequestration in deep saline formations has been proved to be an effective method for reducing greenhouse emissions into the atmosphere. However, pure sequestration of CO will add to the costs incurred by both industries and governments. A win-win method of CO injection and hot brine (water) extraction can become attractive to the investors, as it will not only increase the storage capacity of the injected CO, but also offset the costs by selling and using the produced hot water for industrial, agricultural or household purposes. For instance, water from very hot geothermal reservoirs ( T ≥ 150 °C) can be used for electricity generation in power plants and water from low-medium temperature reservoirs, the most predominant in natural systems, are more popular for direct use, e.g., in heating systems, household hot water, baths, aquaculture, etc. In this paper, low-medium geothermal reservoirs widely distributed in China, especially those in the Ordos Basin, were selected for the numerical case studies using TOUGH2MP with the ECO2N module for the simulations. Generally, simulation parameters were taken from the Ordos Basin, where the first full-integration CO sequestration project had been operated since 2010. The simulations in the base case study lasted 35 years, based on the lifespan of a normal geothermal project. Shallow re-injection systems were also considered to investigate the influence of thermal breakthrough, pressure perturbation, etc. Results show that injection of cold CO causes sharp decrease in temperature in the reservoir region near the injection well, which is enlarged with continuous injection. The region near the production well is dominated by different fluid phases during the CO driven process, including a single water phase, a two-phase fluid (water and CO) and a phase of almost pure CO. Results also show that the CO breakthrough lags far behind the pressure response in the geothermal production system. Before breakthrough, the injected CO pressurizes the reservoir, improving the overall performance of the geothermal reservoir. Furthermore, the heat extraction efficiency of CO-based system is obviously higher than HO-based system. [ABSTRACT FROM AUTHOR]

Because of the high costs involved in CO sequestration in deep saline formations, it has been difficult for its widespread application in the short term. Recently, however, a new technology called 'combined geothermal production-CO sequestration technology' has made the technique more attractive not only by increasing the CO storage capacity, but also by decreasing the operational costs, through the utilization of geothermal energy, either directly or for the purpose of producing electricity. A CO-aided (or CO-involved) geothermal extraction system (CO-AGES) is presented, based on existing technologies and the new ideas of 'combined geothermal production-CO sequestration' introduced in this paper. This system can be used to extract geothermal energy from sedimentary aquifer formations (low-medium temperature) at different stages. The purpose of this paper is to set up an evaluation system for selecting a suitable site for this CO-AGES system. There are both similarities and differences in the procedure needed for selecting a site for a conventional pure CO sequestration and for this CO-AGES system. Both are carried out at different scales, including basin, region and target formation scales. The biggest difference is the temperature gradient, which plays an opposite role in the two systems. By using the preliminary ranking and screening method presented in this paper, it can be shown that the Bohaiwan, Songliao and Qiangtang basins have the highest potential for the application of this CO-AGES technology. However, if CO sequestration is considered alone, the Ordos, Tarim and Bohaiwan basins should be selected as the best three operational sites in China. While preliminary evaluation methods can provide some useful information on the selection of the best sites for the 'combined geothermal production-CO sequestration technology', more detailed work is still required because of the strong uncertainties that exist in the determination of the upper and lower boundaries for each indicator. [ABSTRACT FROM AUTHOR]

Vertical cutoff walls have been widely used in the remediation of contaminated sites. However, determining the best method for evaluating the long-term barrier performance of vertical cutoff walls presents a major difficulty in actual projects. Here, a case study is presented for a typical electroplating, medical, and chemical industrial park in China. Based on the analysis of groundwater pollution characteristics at the site, pollutants included metals (Ni, Al), ammonia nitrogen, and 1,2-dichloroethane. Finite element model simulations of Ni transport at the site showed that a vertical cutoff wall with a thickness of 60 cm and a hydraulic conductivity of 1.0 × 10−8 cm/s could significantly attenuate pollutant transport in the horizontal direction. Compared with other methods such as reducing the hydraulic conductivity or increasing the adsorption retardation factor of the vertical cutoff wall, increasing the thickness was more effective in controlling pollutant transport at the study site. Doubling the thickness would cause the Ni leakage concentration to decrease by more than 98% and the breakthrough time to increase by more than 47 years. It is recommended that the thickness of cutoff walls be maximized to optimize their effects on pollutant transport. • The pollution characteristics of an industrial contaminated site were analyzed. • The pollutant migration processes at the site were simulated for nickel. • Increasing cutoff wall thickness was effective in controlling simulated pollution. [ABSTRACT FROM AUTHOR]

Most of the discovered high-temperature geothermal energy systems are often related with granite that is characterized by natural faults, fractures and cracks of different size. However, the porosity and permeability of the granite matrix is very low, greatly limiting the efficiency of heat extraction in granitic rock. Chemical stimulation, which is regarded as one of the most important means of reservoir stimulation, has consequently received more and more attention. In this paper, a Triassic granite obtained from the eastern region of Liaoning Province in China was reacted with three different concentration of mud acid solution (8% HCl + 1% HF, 10% HCl + 2% HF, 12% HCl + 3% HF) and the resulting microstructure changes studied by scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR). The experimental results show that the number of micropores in the granite increases after chemical corrosion by mud acid solution. A higher mud acid solution concentration results in a much higher pore volume. Triaxial compression tests on the granite before and after chemical corrosion were carried out to study the effect of acidification on the mechanical characteristics of granite, showing that the peak stress and elastic modulus of granite decreases 25.7% and 16.5%, respectively, after exposure to mud acid solution (12% HCl + 3% HF) corrosion for three weeks at room temperature. The particle flow program PFC2D based on discrete element method was used to investigate the mechanical response before and after the chemical corrosion. Considering that the granite is rich in microcracks, the study is simplified by considering them all grouped into one main closed fracture. The influences of main crack inclination angle, crack length, friction coefficient and confining pressure on the mechanical response were investigated. Under the triaxial compression loading state, wing cracks appear at the initial crack tip, then secondary cracks begin to appear. The sensitivity analysis shows that three characteristic strengths (crack initiation strength, damage strength and peak strength) are strongly correlated with crack length, crack inclination angle, crack surface friction coefficient and confining pressure. These three characteristic strengths decrease 60%, 59% and 53%, respectively, compared with their initial values with the increase of main crack length from 6 mm to 22 mm, while they present positive correlation with the fracture friction coefficient from 0 to 1.0 and confining pressure from 10 to 50 MPa. There is a critical inclination angle of the main crack (i.e., 45°), meaning that these three characteristic strengths of granite decrease with inclination angles smaller than 45°, while they increase with an inclination angle larger than 45°. After the corrosion effect of mud acid solution on granite, the pore structure was changed and mechanical properties was damaged, which further affect the failure mode and failure process of granite samples affected by mud acid solutions. This paper provides a theoretical reference for evaluating the effect of chemical stimulation technology on the mechanical characteristics of granite, serving for the continuous hydraulic stimulation design after the chemical stimulation. [ABSTRACT FROM AUTHOR]

Impure salt rock strata are extensively distributed in China, giving them great significance in the study of the physical properties of impure salt rock for the construction of underground gas storage in salt mines. To investigate the confining pressure and impurity effects on the mechanical properties and gas permeability characteristics of salt rock, permeability tests under hydrostatic confining pressure and conventional triaxial compression (CTC), on salt rock samples with different impurity contents, were carried out. The results demonstrate that the confining pressure effects cause an increase in triaxial compression strength, but a decrease in permeability. However, impurity enhances the bearing capacity and permeability of the salt rock; both rock strength and permeability increase with an increase in impurity content. Moreover, the broken salt rock specimens were analyzed after the CTC test using Computed Tomography (CT) equipment. To understand the relationships between pore volume and permeability, considering the confining pressure and impurity effects, the cracks were divided into four groups according to different crack diameter ranges: ~0.05 mm, 0.05–1 mm, 1–10 mm and ~10 mm. The CT analysis results show that while the pore volume of smaller cracks shows an "increasing–decreasing" trend by increasing confining pressure, the pore volume of large cracks gradually decreases, indicating that the rock permeability is highly related to macro-cracks. However, impurity has more complicated implications on rock permeability and cracks, and needs further investigation. [ABSTRACT FROM AUTHOR]

Large-scale underground oil storage has a great effect on national energy safety. China's oil dependency has exceeded 70% for four consecutive years, so it is necessary for China to build strategic oil storage. The salt cavern is a good medium to store oil, and it is widely used in foreign countries. China is rich in salt mines and has basic conditions for building salt cavern oil storage (SCOS). In this paper, the necessity and advantages of SCOS are analyzed, the salt rock mechanics experiments of SCOS are conducted, the geological feasibility evaluation model is built, the 3D numerical creep stability model in three working conditions (long-term, oil pressure dynamic, injection and production) is built, and the economic feasibility evaluation model is built. The results show that the implementation of SCOS in China is feasible, the earthquake (51%) and fault (28%) have a great effect on geological selection, the SCOS has good stability under three working conditions, the storage cost of SCOS is lower by 64.3% and 84.3% contrasted to rock cavern and storage tank in 2025. The study has a reference value for the development of SCOS in China. • A comprehensive feasibility evaluation of the salt cavern oil energy storage system is proposed. • The necessity and advantages of salt cavern oil storage are analyzed. • The geological evaluation model is built to select the best site. • A 3D stability numerical model under three working conditions is analyzed. • The economic evaluation model is built to select the cheapest oil storage methods. [ABSTRACT FROM AUTHOR]

Salt cavern gas storage (SCGS) is a mature energy storage method that is applied around the world. Insoluble sediment particle (ISP) accumulated at the bottom of the salt cavern seriously affect the storage capacity of salt caverns. The ISP has greatly restricted large-scale underground energy in salt caverns in China. However, if the void in the ISP can be used for gas storage (SVGS), the gas storage capacity of salt caverns will be greatly improved. The sediment void is related to the grain size distribution, accumulation morphology, mechanical properties, and falling movement. Therefore, four models (grading model, accumulating model, mechanics and motion model, and simulation model) of the SVGS are constructed by the four theories (grading theory, accumulation theory, contact force chain and motion theories, and experiment and numerical theories) to analyze the feasibility of SVGS. Then, the practical application, model validation, and prospects of SVGS are also discussed. The models and practical application show that the SVGS is feasible, and it has wide application prospects. The research provides a favorable reference for the implementation of SVGS in China. • A method of the salt cavern sediment void gas storage is proposed. • The grain model and accumulation model of sediment void gas storage are analyzed. • The mechanics, motion model, and simulation model of sediment void gas storage are analyzed. • The practical applications of sediment void gas storage are analyzed. • The evaluation methods, site problems, key points, prospects of sediment void gas storage are analyzed. [ABSTRACT FROM AUTHOR]

The safe and efficient development of ultra-deep shale gas deeper than 4500 m is closely related to the comprehensive evaluation of the mechanical behavior, brittleness and anisotropy characteristics of reservoir rocks under in-situ stress conditions. However, the above-mentioned information about ultra-deep shale is rarely reported. This study aims to fill this gap by systematically investigating the mechanical and anisotropic characteristics of cores, with 0° and 90° bedding orientations, from different layers in the vertical depth of 4900–4930 m, under confining pressure of 110 MPa. Furthermore, a new brittleness index with three parts related to brittle failure, i.e., the energy storage performance in the pre-peak stage, the performance of elastic energy used to maintain the post-peak failure, and the severity of the energy released during the post-peak stage, all of which are physically meaningful, was proposed. Subsequently, the index was validated and applied to the brittleness evaluation of ultra-deep reservoir rocks. The brittleness was evaluated in the following descending order: black shale of Longmaxi Formation ＞ gray-black shale of Wufeng Formation ＞ limestone of Baota Formation. Overall, the Wufeng- Longmaxi Formation shale still exhibits brittle failure under such high confining pressure, while the Baota Formation limestone exhibits quasi-ductile failure, which is consistent with the phenomenological behavior of cores. Further, various experimental techniques have been employed to study the brittle and quasi-ductile failure mechanisms of reservoir rocks. The anisotropy evaluation results show that the anisotropy degree of the mechanical and brittleness parameters corresponding to the deep formation is significantly lower than that of the shallow formation. The anisotropy degree of strength and deformation parameters (Young's modulus, Poisson's ratio) is closely related to the content of quartz and clay minerals, respectively. The above results are expected to provide data support and theoretical guidance for the development of deep and ultra-deep shale gas in the southern Sichuan region of China. • The mechanical characteristics of ultra-deep reservoir rocks in the vertical depth range of 4900–4933 m are reported. • The influence of the bedding effect on the mechanical behavior of reservoir rocks under high confining pressures is studied. • A new three-part brittleness index in which each part has a clear physical meaning related to brittle failure is proposed. • The brittle and quasi-ductile failure mechanisms of reservoir rocks under high confining pressures are revealed. [ABSTRACT FROM AUTHOR]