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18 pages, 6196 KiB

Open AccessArticle

Heterogeneity and Controlling Factors of Pore and Fracture Structure Collected from Coal Seam 10 in Xinjiang

by Benfeng Fan, Minghu Chai, Yunbing Hu, Xiao Liu, Zhengyuan Qin, Zhengguang Zhang and Yuqiang Guo

Processes 2025, 13(5), 1571; https://doi.org/10.3390/pr13051571 - 19 May 2025

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Abstract

Heterogeneity of pore and fracture structures has become an important factor affecting the migration of methane and water in coal reservoirs. However, controlling factors of pore and fracture structure collected from coal seam 10 in Taliqike Formation, Kubai Coalfield, Xinjiang need to be [...] Read more.

Heterogeneity of pore and fracture structures has become an important factor affecting the migration of methane and water in coal reservoirs. However, controlling factors of pore and fracture structure collected from coal seam 10 in Taliqike Formation, Kubai Coalfield, Xinjiang need to be studied. In this paper, carbon dioxide adsorption, cryogenic liquid nitrogen, and high-pressure mercury intrusion, as well as coal microscopic components, were used to study pore volumes and characterize pore diameter distribution heterogeneity. By the theory of single weight and multiple fractal formations, the heterogeneity of the pore fracture structure of coal reservoir is expressed, and the influencing factors of the heterogeneity of the pore fracture structure and the pore volume are also discussed. The results are as follows. (1) Micro-pore distribution presents a distinct bidirectional state, with the main peak at approximately 0.6 nm and 0.85 nm. R_o,max has an obvious influence on micro-pore volume. The single-fractal dimension of micro-pore is not affected by a micro-pore volume but is influenced by other factors such as R_o,max and microscopic composition. The heterogeneity of the low-value area controls the heterogeneity of micro-pore diameter distribution. (2) For lower R_o,max samples, mesopores of these samples are ink bottle-shaped pores, and the pore connectivity is poor. In contrast, meso-pore of higher thermal evolution coal samples are mostly simple pores, such as parallel plates. The main mesopores are 10–100 nm pores, accounting for 75% of the total meso-pore volume. For the single fractal dimension, D₁ is greater than D₂, which also shows that the heterogeneity of a pore structure greater than 4 nm is much stronger than that of a pore structure less than 4 nm in these samples. (3) For lower R_o,max samples, double S-shaped curves with distinct hysteresis loop are obtained, while samples of higher R_o,max samples show parallel curves, suggesting that macro-pore of this type of sample develops parallel plate-like pore. There is a positive relationship between D₋₁₀–D₀ and D₋₁₀–D₁₀, while D₀–D₁₀ and D₋₁₀–D₀ have a weak correlation. With the increase of 2–10 nm pore volume, pore distribution heterogeneity of lower value area (D₋₁₀–D₀) weakens. This indicates that pore volume is an important factor affecting the multifractal variation. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources, 2nd Edition)

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17 pages, 4932 KiB

Open AccessArticle

Numerical Simulation of Flow Characteristics in CO₂ Long-Term Storage in Bedded Salt Cavern

by Bo Cao, Xuehai Fu, Junqiang Kang, Pan Tang, Hui Xu and Yuanyuan Zhang

Processes 2025, 13(5), 1563; https://doi.org/10.3390/pr13051563 - 18 May 2025

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Abstract

The salt layer, characterized by its low permeability and excellent damage self-healing properties, is an ideal geological body for CO₂ geological storage. However, the relatively high permeability of mudstone interlayers may reduce the safety of CO₂ long-term storage in bedded salt [...] Read more.

The salt layer, characterized by its low permeability and excellent damage self-healing properties, is an ideal geological body for CO₂ geological storage. However, the relatively high permeability of mudstone interlayers may reduce the safety of CO₂ long-term storage in bedded salt caverns. This study establishes a thermal–hydraulic–mechanical (THM) coupled physical and mathematical model for CO₂ geological storage in the Huaian salt cavern, analyzes the factors affecting CO₂ flow behavior, and proposes measures to enhance the safety of CO₂ storage in salt caverns. The results indicate that the permeability of both salt layers and mudstone interlayers is influenced by stress-induced deformation within the salt cavern. From the salt cavern edge to the simulation boundary, the permeability and volume strain exhibit a trend of rapid decline, followed by a gradual increase, and an eventual stabilization or slight reduction. The seepage velocity, pore pressure, and flow distance of CO₂ in the mudstone interlayer are significantly higher than those in the salt layer, leading to CO₂ migration along the interfaces between the mudstone and salt layer. With the increase in storage time, the permeability of the mudstone interlayer gradually decreases, while the permeability of the salt layer shows a general tendency to increase. The elevated storage pressure reduces the permeability of the mudstone interlayer, while increasing the permeability of the salt layer, and enhances the seepage velocity in both the mudstone and salt layers. To enhance the safety of CO₂ long-term storage in bedded salt caverns, it is recommended to minimize the presence of mudstone interlayers during site selection and cavern construction, optimize the storage pressure, and strengthen monitoring systems for potential CO₂ leakage. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources, 2nd Edition)

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19 pages, 4593 KiB

Open AccessArticle

Applications of Advanced Presplitting Blasting Technology in the Thick and Hard Roofs of an Extra-Thick Coal Seam

by Shouguo Wang, Kai Zhang, Bin Qiao, Shaoze Liu, Junpeng An, Yingming Li and Shunjie Huang

Processes 2025, 13(5), 1539; https://doi.org/10.3390/pr13051539 - 16 May 2025

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Abstract

Based on the engineering conditions of the 1303 working face in Zhaoxian Coal Mine, this study investigates the characteristics of mine pressure behavior and the stress-relief mechanism of advanced presplit blasting in a working face with a thick and hard roof in an [...] Read more.

Based on the engineering conditions of the 1303 working face in Zhaoxian Coal Mine, this study investigates the characteristics of mine pressure behavior and the stress-relief mechanism of advanced presplit blasting in a working face with a thick and hard roof in an extra-thick coal seam. Through a combination of numerical simulations and field experiments, the effects of advanced presplit blasting on stress distribution, roadway stability, and microseismic activity are analyzed. Corresponding mitigation measures and optimization strategies are proposed. The results indicate that the primary cause of deformation in the gob-side roadway is the superposition of lateral abutment pressure from the goaf and the front abutment pressure of the advancing working face. Advanced presplit blasting effectively reduces the magnitude of front abutment stress, inhibits its transmission, decreases the hanging area of the goaf roof, and alleviates vertical stress on the roadway side adjacent to the goaf. Furthermore, both the daily average and peak microseismic energy levels decrease as the working face approaches the advanced blasting zone. The implementation of advanced presplit blasting technology in working faces with thick and hard roofs within extra-thick coal seams significantly mitigates rockburst hazards, enhances roadway stability, and improves overall mining safety. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources, 2nd Edition)

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21 pages, 11299 KiB

Open AccessArticle

Fracture System Characteristics and Their Control on Permeability Anisotropy in Bright and Dull Coal

by Liheng Bian, Yanxiang He, Rui Shi, Liang Ji, Wei Zhang, Zhuang Ma, Peng Wu and Jian Shen

Processes 2025, 13(5), 1509; https://doi.org/10.3390/pr13051509 - 14 May 2025

Viewed by 214

Abstract

Coal permeability, a key parameter influencing coalbed methane production and geological storage, is strongly governed by the dual-porosity nature of coal and the stress-dependent evolution of its fracture network. This study investigates the development characteristics of filled and unfilled fractures, and the resulting [...] Read more.

Coal permeability, a key parameter influencing coalbed methane production and geological storage, is strongly governed by the dual-porosity nature of coal and the stress-dependent evolution of its fracture network. This study investigates the development characteristics of filled and unfilled fractures, and the resulting permeability anisotropy, in typical bright and dull coals from the deep 8# coal seam of the Ordos Basin. Utilizing CT scanning and permeability anisotropy testing, we analyze how fracture development impacts coal permeability and its evolution under stress. Bright coal exhibits a grid-like distribution of mineral-filled fractures with good vertical connectivity, and a complex network of unfilled fractures. In contrast, dull coal displays a scattered distribution of mineral-filled fractures with poor vertical connectivity and a limited number of unfilled fractures. Results indicate an exponential decay trend in permeability with increasing confining pressure, strongly correlated with fracture system development. Permeability also demonstrates significant heterogeneity (face cleat > butt cleat > vertical). Bright coal exhibits a greater permeability decay rate than dull coal, indicating heightened stress sensitivity, while its permeability anisotropy is weaker, aligning with the observed fracture development patterns. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources, 2nd Edition)

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13 pages, 3915 KiB

Open AccessArticle

Mechanical Strength Degradation in Deep Coal Seams Due to Drilling Fluid Invasion

by Qin Zhang, Weiliang Wang, Mingming Zhu, Yanbing Zhang, Qingchen Wang, Huan Sun and Jiping She

Processes 2025, 13(4), 1222; https://doi.org/10.3390/pr13041222 - 17 Apr 2025

Viewed by 269

Abstract

With the rapid development of the coalbed methane (CBM) industry in China, coal seam No. 8 of the Benxi Formation in the Ordos Basin has emerged as a key target for CBM development due to its abundant deep reserves. However, wellbore instability during [...] Read more.

With the rapid development of the coalbed methane (CBM) industry in China, coal seam No. 8 of the Benxi Formation in the Ordos Basin has emerged as a key target for CBM development due to its abundant deep reserves. However, wellbore instability during deep CBM extraction has become increasingly problematic, with the degradation of coal mechanical strength caused by drilling fluid invasion being identified as a critical factor affecting drilling safety and operational efficiency. This study focuses on coal seam No. 8 of the Benxi Formation in the Sulige Gas Field, Ordos Basin. Through experimental analyses of the coal’s mineral composition, microstructure, hydration expansion properties, and mechanical strength variations, the mechanism underlying drilling fluid invasion-induced mechanical strength degradation is elucidated. The experimental results reveal that coal seam No. 8 of the Benxi Formation exhibits a high carbon content and a low absolute clay mineral content (approximately 6.11%), with minimal expansive minerals (e.g., mixed-layer illite–smectite accounts for 26.4%). Consequently, the coal demonstrates a low linear expansion rate and weak hydration dispersion properties, indicating that hydration expansion is not the dominant mechanism driving mechanical strength degradation. However, drilling fluid invasion significantly reduced coal’s Young’s modulus (from 1988.1 MPa to 1676.1 MPa, a 15.69% decrease) and compressive strength (from 7.9 MPa to 6.5 MPa, a 17.72% drop), while markedly affecting its internal friction angle. Friction coefficient tests further demonstrate that the synergistic action of water molecules and additives decreases microcrack sliding resistance by 19.22% with simulated formation water and by 25.00% with drilling fluid, thereby promoting microcrack propagation and failure. This process ultimately leads to a degradation in mechanical strength. Hence, the enhancement of sliding effects induced by drilling fluid invasion is identified as the primary factor contributing to coal mechanical strength degradation, whereas hydration expansion plays a secondary role. To mitigate these effects, optimizing the design of drilling fluid systems and selecting suitable anti-collapse additives to reduce sliding effects are critical for minimizing wellbore instability risks in coal seams. These measures will ensure safer and more efficient drilling operations for deep CBM extraction. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources, 2nd Edition)

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19 pages, 11287 KiB

Open AccessArticle

Differential Evolution of Reservoir Permeability Under Dip Angle Control During Coalbed Methane Production

by Chaochao Duan, Junqiang Kang, Xuehai Fu, Yibing Wang and Peng Lai

Processes 2025, 13(4), 1147; https://doi.org/10.3390/pr13041147 - 10 Apr 2025

Viewed by 216

Abstract

Permeability variations during coalbed methane (CBM) production remain a critical research focus. However, existing studies have primarily concentrated on nearly horizontal reservoirs, with limited in-depth analyses of the dynamic evolution of permeability in steeply inclined coal reservoirs (SICRs) (>45°). This study examined the [...] Read more.

Permeability variations during coalbed methane (CBM) production remain a critical research focus. However, existing studies have primarily concentrated on nearly horizontal reservoirs, with limited in-depth analyses of the dynamic evolution of permeability in steeply inclined coal reservoirs (SICRs) (>45°). This study examined the dynamic permeability characteristics across different reservoir dip angles, comparing variations in the up-dip (UD) and down-dip (DD) directions of SICRs and investigating their controlling mechanisms. The results indicate an asymmetry in permeability changes between UDs and DDs, with UDs generally exhibiting a greater amplitude of variation. The reservoir dip angle exerts a more pronounced influence on DD permeability changes, primarily through its effects on effective stress (ES) and matrix shrinkage (MS). Specifically, a reduction in the negative impact of ES in the UD enhances the overall reservoir permeability, whereas a reduction in the positive effect of MS in the DD leads to a more significant permeability decline. The comparatively smaller increase in DD permeability contributes to distinct evolutionary trends in the permeability between UDs and DDs. These dynamic permeability changes in SICRs have a more substantial impact on CBM production, particularly in low-strength and low-rank coal reservoirs. The findings of this study provide valuable insights for optimizing CBM production strategies in SICRs. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources, 2nd Edition)

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13 pages, 2079 KiB

Open AccessArticle

Mechanistic Analysis and Multi-Factor Coupling Optimization of Temporary Plugging Fracturing in Shale Oil Horizontal Wells: A Case Study from the Sichuan Basin, China

by Yang Wang, Jian Yang, Qingyun Yuan, Weihua Chen, Yiguo He, Zhe Liu, Zefei Lv, Zhengyong Li, Jinming Fan, Tao Wang, Wei Chen and Xinyuan Tang

Processes 2025, 13(4), 1134; https://doi.org/10.3390/pr13041134 - 9 Apr 2025

Viewed by 359

Abstract

Horizontal well fracturing is a pivotal technology for enhancing the efficiency of shale oil and gas development. Shale reservoirs exhibit significant heterogeneity and intricate fracture propagation patterns, often resulting in uneven multiple fractures caused by horizontal well fracturing. Temporary plugging technology plays a [...] Read more.

Horizontal well fracturing is a pivotal technology for enhancing the efficiency of shale oil and gas development. Shale reservoirs exhibit significant heterogeneity and intricate fracture propagation patterns, often resulting in uneven multiple fractures caused by horizontal well fracturing. Temporary plugging technology plays a critical role in optimizing fracture propagation patterns; however, there is currently limited research on its optimization. Based on a hydraulic fracturing fracture propagation simulation, an optimization study was conducted on temporary plugging technology for horizontal well fracturing in shale oil reservoirs. Numerical simulation results demonstrate that the uniformity of hydraulic fracture propagation during horizontal well fracturing in shale oil reservoirs is maximized when 30 perforations are plugged. The most uniform fracture propagation pattern is achieved by adding temporary plugging agents after pumping a total volume of 30% fracturing fluid. Furthermore, a comparison between one-time plugging with temporary plugging balls and multiple plugging was made to evaluate differences in fracture propagation. It was observed that performing temporary plugging once significantly improves the uniformity of fracture propagation compared to multiple temporary plugging. These research findings have been successfully validated through the practical application of hydraulic fracturing techniques, as indicated by substantial improvements in both the mode and uniformity of fracture propagation following temporary plugging. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources, 2nd Edition)

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26 pages, 13968 KiB

Open AccessArticle

Dynamic Evolution of Fractures in Overlying Rocks Caused by Coal Mining Based on Discrete Element Method

by Junyu Xu, Jienan Pan, Meng Li, Haoran Wang and Jiangfeng Chen

Processes 2025, 13(3), 806; https://doi.org/10.3390/pr13030806 - 10 Mar 2025

Viewed by 548

Abstract

Mining-induced fractures and overlying rock movement change rock layer porosity and permeability, raising water intrusion risks in the working face. This study explores fracture development in working face 31123-1 at Dongxia Coal Mine using UDEC 7.0 software and theoretical analysis. The overlying rock [...] Read more.

Mining-induced fractures and overlying rock movement change rock layer porosity and permeability, raising water intrusion risks in the working face. This study explores fracture development in working face 31123-1 at Dongxia Coal Mine using UDEC 7.0 software and theoretical analysis. The overlying rock movement is a dynamic, spatially evolving process. As the working face advances, the water-conducting fracture zone height (WFZH) increases stepwise, and their relationship follows an S-shaped curve. Numerical simulations give a WFZH of about 112 m and a fracture–mining ratio of 14.93. Empirical formulas suggest a WFZH of 85.43 to 106.3 m and a ratio of 11.39 to 14.17. Key stratum theory calculations show that mining-induced fractures reach the 16th coarse-sandstone layer, with a WFZH of 97 to 113 m and a ratio of 12.93 to 15.07. Simulations confirm trapezoidal fractures with bottom angles of 48° and 50°, consistent with rock mechanics theories. A fractal permeability model for the mined overburden, based on the K-C equation, shows that fracture permeability positively correlates with the fractal dimension. These results verify the reliability of simulations and analyses, guiding mining and water control in this and similar working faces. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources, 2nd Edition)

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15 pages, 8398 KiB

Open AccessArticle

Reservoir Characteristics and Regional Storage Potential Evaluation of Deep Well Injection and Storage of High-Salinity Water in Coal Mines in the Ordos Basin

by Yanjun Liu, Yidan Bu, Song Du, Qiaohui Che, Yinglin Fan, Yan Ding, Zhe Jiang and Xiang Li

Processes 2025, 13(2), 579; https://doi.org/10.3390/pr13020579 - 18 Feb 2025

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Abstract

Deep well injection and storage is an emerging technology for realizing the low-cost treatment of extremely large quantities of three types of waste in coal mines in China, while simultaneously supporting coordinated development that considers its impact on the ecological environment. There has [...] Read more.

Deep well injection and storage is an emerging technology for realizing the low-cost treatment of extremely large quantities of three types of waste in coal mines in China, while simultaneously supporting coordinated development that considers its impact on the ecological environment. There has been significant progress in research on the geological storage of carbon dioxide in China. However, the geological storage of fluids such as mine water and high-salinity water needs to be studied further. Based on a comprehensive analysis of the lithology, mineral composition, physical and mechanical characteristics, and spatial structure of the Liujiagou and Shiqianfeng formations in a mining area in the Ordos Basin, we determined the geological storage space for fluids, predicted the storage potential, and evaluated the feasibility of deep geological storage of high-salinity water in coal mines. In the study area, the Liujiagou Formation is dominated by fine sandstone and siltstone, while the Shiqianfeng Formation is dominated by medium sandstone and conglomerate. The main storage space comprises micro-cracks, as well as intergranular, dissolution, and intergranular pores. Among these, the intergranular pores are the most conducive to reservoir development. The burial depth intervals of 1820–1835 m, 1905–1920 m, and 2082–2098 m are favorable for storage and are characterized by high porosities, permeabilities, and storage capacities. The effective storage capacity within a 100 m radius of the storage well was estimated to be 33.15 × 10⁴ m³. The effective storage capacity in the favorable area is 27.69 × 10⁴ m³, accounting for 83.50% of the total storage capacity. The Liujiagou and Shiqianfeng formations thus can serve as favorable reservoirs for deep well injection and storage of high-salinity water in the Ordos Basin. This research provides new ideas for the treatment of high-salinity water in coal mines in the Ordos Basin and technical support for deep well injection and the storage of high-salinity water. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources, 2nd Edition)

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13 pages, 5465 KiB

Open AccessArticle

Monitoring-Based Study of Migration Characteristics of Highly Saline Mine Water During Deep Well Injection and Storage in the Ordos Basin

by Qiaohui Che, Song Du, Degao Zhang, Donglin Dong, Yinglin Fan, Xiang Li, Zhan Yang and Xiao Zhang

Processes 2025, 13(2), 494; https://doi.org/10.3390/pr13020494 - 10 Feb 2025

Viewed by 490

Abstract

Deep well injection and storage (DWIS) has recently been proposed and implemented to achieve zero mine water emissions. In 2023, DWIS for highly saline mine water was successfully applied to a local mine in the Ordos Basin for the first time with excellent [...] Read more.

Deep well injection and storage (DWIS) has recently been proposed and implemented to achieve zero mine water emissions. In 2023, DWIS for highly saline mine water was successfully applied to a local mine in the Ordos Basin for the first time with excellent performance. However, the storage characteristics of highly saline mine water in the storage layer during DWIS remain unclear. This study was conducted in situ with real-time, online monitoring of instantaneous flow and injection pressure, along with synchronous micro-seismic monitoring during the early stages of DWIS, based on the geological conditions and spatial structure of the storage layer. The results indicated that the early seepage characteristics of the fluid geological storage did not conform to Darcy’s law. Within a certain pressure range, as the water pressure increased, the flow also increased. However, beyond this range, further increases in pressure caused a gradual decline in the flow. During the initial phase of storage, the migration of high-salinity mine water within the storage layer occurred in two stages: breakthrough and stabilization. During the breakthrough stage, the water injection pressure propagated to the flooding front, overcoming the formation stress and expanding the storage space. At this stage, mine water primarily filled the pore microcracks within the flooding front. In the initial 10 days of storage, high-salinity mine water in the study area affected approximately 42,104 m² of the storage layer plane. The injection well affected an area nearly 200 m in depth, extending approximately 190 m northward and approximately 40 m upward. The predominant diffusion directions were northeast and east–southeast from the injection well. These findings could provide valuable insights into the treatment of highly saline mine water in the Ordos Basin, demonstrate the feasibility and safety of DWIS, and offer significant scientific contributions to the prevention and control of mine water pollution. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources, 2nd Edition)

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17 pages, 3863 KiB

Open AccessArticle

Adsorption Pore Volume Distribution Heterogeneity of Middle and High Rank Coal Reservoirs and Determination of Its Influencing Factors

by Kai Wang, Fangkai Quan, Shizhao Zhang, Yubo Zhao, He Shi, Tingting Yin and Zhenyuan Qin

Processes 2025, 13(2), 429; https://doi.org/10.3390/pr13020429 - 6 Feb 2025

Viewed by 547

Abstract

Heterogeneity of adsorption pore volume distribution affects desorption and diffusion processes of coal reservoirs. In this paper, N₂ and CO₂ adsorption and desorption experiment tests were used to study the pore structure of middle and high rank coal reservoirs in the [...] Read more.

Heterogeneity of adsorption pore volume distribution affects desorption and diffusion processes of coal reservoirs. In this paper, N₂ and CO₂ adsorption and desorption experiment tests were used to study the pore structure of middle and high rank coal reservoirs in the study area. The fractal theory of volume and surface area is used to achieve a full-scale fractal study of adsorption pores (pore diameter is less than 100 nm) in the study area. Firstly, adaptability and control factors of volume fractals and surface area fractals within the same aperture scale range are studied. Secondly, fractal characteristics of micro-pores and meso-pores are studied. Thirdly, fractal characteristics within different aperture scales and the influencing factors of fractal characteristics within different scale ranges are studied. The results are as follows. With the increase in coal rank, pore volume and specific surface area of pores less than 0.8 nm increase, and dominant pore size changes from 0.55~0.8 nm (middle coal rank) to 0.5~0.7 nm (high coal rank). As coal rank increases, TPV and average pore diameter (APD) decrease under the BJH model, while SSA changes are not significant under the BET model. Moreover, as the pore diameter decreases, the correlation between the integral dimension of pore volume and degree of coal metamorphism decreases. This result can provide a theoretical basis for the precise characterization of the target coal seam pore and fracture structure and support the optimization of favorable areas for coalbed methane. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources, 2nd Edition)

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15 pages, 4471 KiB

Open AccessArticle

Research and Application of Deep Profile Control Technology in Narrow Fluvial Sand Bodies

by Xu Zheng, Yu Wang, Yuan Lei, Dong Zhang, Wenbo Bao and Shijun Huang

Processes 2025, 13(1), 289; https://doi.org/10.3390/pr13010289 - 20 Jan 2025

Viewed by 1023

Abstract

Narrow Fluvial Sand Bodies are primarily developed along the river center, with horizontal wells for injection and production in some Bohai waterflooded oilfields. This results in a rapid increase in water cut due to a single injection–production direction. Over time, dominant water breakthrough [...] Read more.

Narrow Fluvial Sand Bodies are primarily developed along the river center, with horizontal wells for injection and production in some Bohai waterflooded oilfields. This results in a rapid increase in water cut due to a single injection–production direction. Over time, dominant water breakthrough channels form between wells, and the remaining oil moves to deeper regions, which makes conventional profile control measures less effective. We developed a quantitative method based on integrated dynamic and static big data to identify these breakthrough channels and measure the flow intensity between injection and production wells. To address deep remaining oil mobilization, we performed micro-analysis and physical simulations with heterogeneous core models, which led to the development of a deep profile control system using emulsion polymer gel and self-assembling particle flooding. Experiments show that the combined technology can reduce oil saturation in low-permeability layers to 45.3% and improve recovery by 30.2% compared to water flooding. Field trials proved to be completely effective, with a cumulative oil increase of over 23,200 cubic meters and a 12% reduction in water cut per well. This deep profile control technology offers significant water cut reduction and enhanced oil recovery. It can provide technical support for efficient water control and profile management in similar reservoirs. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources, 2nd Edition)

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14 pages, 7871 KiB

Open AccessArticle

Failure and Permeability Characteristics of Coal Pillar in Closely Coal Seams Gob Under Multiple Mining

by Hui Qiao, Song Liu, Lei Dong, Pinkun Guo and Ruifeng Gao

Processes 2024, 12(12), 2934; https://doi.org/10.3390/pr12122934 - 22 Dec 2024

Viewed by 598

Abstract

Coal pillars are loaded and unloaded repeatedly when mining, which lead to fractures in the coal close, open, generate and expand. As a result, the permeability of coal is changed. The high permeability fractures in coal and rock between the upper gobs and [...] Read more.

Coal pillars are loaded and unloaded repeatedly when mining, which lead to fractures in the coal close, open, generate and expand. As a result, the permeability of coal is changed. The high permeability fractures in coal and rock between the upper gobs and the lower working faces are the main channels for fresh air entering the upper gob, which could induce spontaneous combustion of coal in gob. To identifying the air leakage channels, multiple mining of closely coal seams was numerically conducted with three working face layouts. The failure and permeability characteristic of coal pillar in closely coal seams gob under multiple mining were obtained and analyzed. When the working faces are mined, the vertical stress and horizontal stress of the upper coal pillar in gob load and unload synchronously in all three working face layouts. The laterally directed horizontal stress could unload to zero due to no confine on the lateral side of coal pillar. The stress in the middle of upper coal pillar loads continuously until the lower working face is mined. When the lower coal seam working face is mined, the coal and rock between the upper and lower coal seams damage in shear and tension. When the lower coal seam working face is staggered from the upper coal seam working face, the permeability of the coal and rock pillar increases more than 22000 times due to tension damage of the coal and rock pillar. As a result, the coal and rock pillar is the main channel for fresh air flowing into the upper gob. The high permeability coal pillar provides favorable conditions for spontaneous combustion of coal in gob. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources, 2nd Edition)

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20 pages, 8161 KiB

Open AccessArticle

Research on Support Technology for Unstable Roof Roadway Under Abandoned Roadways in Ultra-Thick Coal Seam

by Xianyang Yu, Siyuan Lv, Yafei Luo, Pengchao Liu, Hao Fu and Yicai Zhou

Processes 2024, 12(12), 2886; https://doi.org/10.3390/pr12122886 - 17 Dec 2024

Cited by 1 | Viewed by 627

Abstract

Due to the impact of disordered mining activities in previous years, numerous abandoned roadways exist in the second mining district of the 13# coal seam in Chejiazhuang Coal Mine. The stability of the new roadway roof was analyzed under various distributions of abandoned [...] Read more.

Due to the impact of disordered mining activities in previous years, numerous abandoned roadways exist in the second mining district of the 13# coal seam in Chejiazhuang Coal Mine. The stability of the new roadway roof was analyzed under various distributions of abandoned roadways above. It was determined that the ultimate stable thickness of the coal layer between the new and abandoned roadways is 4.0 m. When the thickness between the two is less than 4.0 m, the roof becomes unstable after excavation, posing a risk of collapse. Advanced grouting reinforcement is required to enhance roof stability before installing U-shaped steel arches. Mechanical experiments were conducted on the polymer grouting consolidation of fractured coal, showing a significant increase in residual strength compared to intact coal. Furthermore, the uniaxial compressive strength of the polymer grouting consolidation partially recovered. On average, the consolidation coefficient and recovery coefficient were 5.28 and 85.51%, respectively. Grouting increased the ductility of the fractured surrounding rock, enhancing its resistance to deformation and plasticity. A polymer grouting consolidation technology for supporting fractured surrounding rock under the unstable roof of abandoned roadways is proposed, along with the design of corresponding support schemes and parameters. Monitoring the results of mine pressure indicated that the surrounding rock remained stable after roadway excavation, validating the effectiveness of the support schemes and parameters. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources, 2nd Edition)

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14 pages, 5465 KiB

Open AccessArticle

Methane Adsorption Energy Variation Affected by Industrial Components in Deep and Thick Coal Reservoirs

by Xiaogang Zhou, Kai Wang, Baozhen Yan, Zhengyuan Qin, Shi He, Fangkai Quan and Veerle Vandeginste

Processes 2024, 12(12), 2780; https://doi.org/10.3390/pr12122780 - 6 Dec 2024

Viewed by 650

Abstract

The relevant literature indicates that coal facies have a significant impact on the pore structure and adsorption properties of deep coal reservoirs. The content of submicroscopic components is used to calculate the parameters of coal facies. Based on traditional coal phase parameters and [...] Read more.

The relevant literature indicates that coal facies have a significant impact on the pore structure and adsorption properties of deep coal reservoirs. The content of submicroscopic components is used to calculate the parameters of coal facies. Based on traditional coal phase parameters and ash content, the coal phases of the coal samples in the study area were divided. Based on the adsorption potential theory, the differences in methane adsorption energy changes between different coal phases were compared. The results are as follows. The wet herbaceous swamp facies (type A) could be divided into two subtypes by using the ash yield: subtype A-1 (with an ash yield lower than 20% and a gel index (GI) lower than 5), and subtype A-2 (with an ash yield larger than 20% and a GI lower than 5). With the increase in micro-pore volume shown in A-1 samples, cumulative surface free energy increases linearly and the maximum rate of decline decreases linearly. Coal facies have an important effect on adsorption parameters: V_L increases and P_L decreases with higher structural preservation index (TPI). The effect of a low ash yield and different R_o,max on methane adsorption energy parameters is stronger. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources, 2nd Edition)

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26 pages, 20446 KiB

Open AccessArticle

Gas Content and Geological Control of Deep Jurassic Coalbed Methane in Baijiahai Uplift, Junggar Basin

by Bing Luo, Haichao Wang, Bin Sun, Zheyuan Ouyang, Mengmeng Yang, Yan Wang and Xiang Zhou

Processes 2024, 12(12), 2671; https://doi.org/10.3390/pr12122671 - 27 Nov 2024

Cited by 1 | Viewed by 957

Abstract

Deep coalbed methane (CBM) resources are abundant in China, and in the last few years, the country’s search for and extraction of CBM have intensified, progressively moving from shallow to deep strata and from high-rank coal to medium- and low-rank coal. On the [...] Read more.

Deep coalbed methane (CBM) resources are abundant in China, and in the last few years, the country’s search for and extraction of CBM have intensified, progressively moving from shallow to deep strata and from high-rank coal to medium- and low-rank coal. On the other hand, little is known about the gas content features of deep coal reservoirs in the eastern Junggar Basin, especially with regard to the gas content and the factors that affect it. Based on data from CBM drilling, logging, and seismic surveys, this study focuses on the gas content of Baijiahai Uplift’s primary Jurassic coal seams through experiments on the microscopic components of coal, industrial analysis, isothermal adsorption, low-temperature CO₂, low-temperature N₂, and high-pressure mercury injection. A systematic investigation of the controlling factors, including the depth, thickness, and quality of the coal seam and pore structure; tectonics; and lithology and thickness of the roof, was conducted. The results indicate that the Xishanyao Formation in the Baijiahai Uplift usually has a larger gas content than that in the Badaowan Formation, with the Xishanyao Formation showing that free gas and adsorbed gas coexist, while the Badaowan Formation primarily consists of adsorbed gas. The coal seams in the Baijiahai Uplift are generally deep and thick, and the coal samples from the Xishanyao and Badawan formations have a high vitrinite content, which contributes to their strong gas generation capacity. Additionally, low moisture and ash contents enhance the adsorption capacity of the coal seams, facilitating the storage of CBM. The pore-specific surface area of the coal samples is primarily provided by micropores, which is beneficial for CBM adsorption. Furthermore, a fault connecting the Carboniferous and Permian systems (C-P) developed in the northeastern part of the Baijiahai Uplift allows gas to migrate into the Xishanyao and Badaowan formations, resulting in a higher gas content in the coal seams. The roof lithology is predominantly mudstone with significant thickness, effectively reducing the dissipation of coalbed methane and promoting its accumulation. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources, 2nd Edition)

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19 pages, 3320 KiB

Open AccessArticle

Predicting Water Flowing Fracture Zone Height Using GRA and Optimized Neural Networks

by Haofu Dong, Genfa Yang, Keyin Guo, Junyu Xu, Deqiang Liu, Jin Han, Dongrui Shi and Jienan Pan

Processes 2024, 12(11), 2513; https://doi.org/10.3390/pr12112513 - 12 Nov 2024

Cited by 1 | Viewed by 735

Abstract

As coal mining depths continue to rise, consideration of WFFZ elevations is becoming increasingly important to mine safety. The goal was to accurately predict the height of the WFFZ to effectively prevent and manage possible roof water catastrophes and ensure the ongoing safety [...] Read more.

As coal mining depths continue to rise, consideration of WFFZ elevations is becoming increasingly important to mine safety. The goal was to accurately predict the height of the WFFZ to effectively prevent and manage possible roof water catastrophes and ensure the ongoing safety of the mine. To achieve this goal, we combined the particle swarm optimisation (PSO) algorithm with a backpropagation neural network (BPNN) in order to enhance the accuracy of the forecast. The present study draws upon the capacity of the PSO algorithm to conduct global searches and the nonlinear mapping capability of the BPNN. Through grey relational analysis (GRA), the order of the correlation degree was as follows: mining thickness > mining depth > overburden structure > mining width > mining dip. GRA has identified the degree of correlation between five influencing factors and the height of the WFFZ, among these, mining thickness, mining depth, overburden structure and mining width all show strong correlations, and the mining dip of the coal seam shows a good correlation. The weight ranking obtained by the PSO-BPNN method was the same as that obtained by the GRA method. Based on two actual cases, the relative errors of the obtained prediction results after PSO implementation were 2.97% and 3.47%, while the relative errors of the BPNN before optimisation were 18.46% and 4.34%, respectively, indicating that the PSO-BPNN method provides satisfactory prediction results and demonstrating that the PSO-optimised BPNN is easy to use and yields reliable results. In this paper, the height of the WFFZ model under the influence of five factors is only established for the Northwest Mining Area. With the continuous progress of technology and research, the neural network can consider more factors affecting the height of hydraulic fracturing development zones in the future to improve the comprehensiveness and accuracy of prediction. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources, 2nd Edition)

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13 pages, 2366 KiB

Open AccessArticle

Numerical Simulation of the Coal Measure Gas Accumulation Process in Well Z-7 in Qinshui Basin

by Gaoyuan Yan, Yu Song, Fangkai Quan, Qiangqiang Cheng and Peng Wu

Processes 2024, 12(11), 2491; https://doi.org/10.3390/pr12112491 - 9 Nov 2024

Viewed by 799

Abstract

The process of coal measure gas accumulation is relatively complex, involving multiple physicochemical processes such as migration, adsorption, desorption, and seepage of multiphase fluids (e.g., methane and water) in coal measure strata. This process is constrained by multiple factors, including geological structure, reservoir [...] Read more.

The process of coal measure gas accumulation is relatively complex, involving multiple physicochemical processes such as migration, adsorption, desorption, and seepage of multiphase fluids (e.g., methane and water) in coal measure strata. This process is constrained by multiple factors, including geological structure, reservoir physical properties, fluid pressure, and temperature. This study used Well Z-7 in the Qinshui Basin as the research object as well as numerical simulations to reveal the processes of methane generation, migration, accumulation, and dissipation in the geological history. The results indicate that the gas content of the reservoir was basically zero in the early stage (before 25 Ma), and the gas content peaks all appeared after the peak of hydrocarbon generation (after 208 Ma). During the peak gas generation stage, the gas content increased sharply in the early stages. In the later stage, because of the pressurization of the hydrocarbon generation, the caprock broke through and was lost, and the gas content decreased in a zigzag manner. The reservoirs in the middle and upper parts of the coal measure were easily charged, which was consistent with the upward trend of diffusion and dissipation and had a certain relationship with the cumulative breakout and seepage dissipation. The gas contents of coal, shale, and tight sandstone reservoirs were positively correlated with the mature hydrocarbon generation of organic matter in coal seams, with the differences between different reservoirs gradually narrowing over time. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources, 2nd Edition)

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16 pages, 1099 KiB

Open AccessArticle

Geophysical Monitoring Technologies for the Entire Life Cycle of CO₂ Geological Sequestration

by Chenyang Li and Xiaoli Zhang

Processes 2024, 12(10), 2258; https://doi.org/10.3390/pr12102258 - 16 Oct 2024

Viewed by 1756

Abstract

Geophysical monitoring of CO₂ geological sequestration represents a critical technology for ensuring the long-term safe storage of CO₂ while verifying its characteristics and dynamic changes. Currently, the primary geophysical monitoring methods employed in CO₂ geological sequestration include seismic, fiber optic, [...] Read more.

Geophysical monitoring of CO₂ geological sequestration represents a critical technology for ensuring the long-term safe storage of CO₂ while verifying its characteristics and dynamic changes. Currently, the primary geophysical monitoring methods employed in CO₂ geological sequestration include seismic, fiber optic, and logging technologies. Among these methods, seismic monitoring techniques encompass high-resolution P-Cable three-dimensional seismic systems, delayed vertical seismic profiling technology, and four-dimensional distributed acoustic sensing (DAS). These methods are utilized to monitor interlayer strain induced by CO₂ injection, thereby indirectly determining the injection volume, distribution range, and potential diffusion pathways of the CO₂ plume. In contrast, fiber optic monitoring primarily involves distributed fiber optic sensing (DFOS), which can be further classified into distributed acoustic sensing (DAS) and distributed temperature sensing (DTS). This technology serves to complement seismic monitoring in observing interlayer strain resulting from CO₂ injection. The logging techniques utilized for monitoring CO₂ geological sequestration include neutron logging methods, such as thermal neutron imaging and pulsed neutron gamma-ray spectroscopy, which are primarily employed to assess the sequestration volume and state of CO₂ plumes within a reservoir. Seismic monitoring technology provides a broader monitoring scale (ranging from dozens of meters to kilometers), while logging techniques operate at centimeter to meter scales; however, their results can be significantly affected by the heterogeneity of a reservoir. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources, 2nd Edition)

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13 pages, 19727 KiB

Open AccessArticle

Oolitic Sedimentary Characteristics of the Upper Paleozoic Bauxite Series in the Eastern Ordos Basin and Its Significance for Oil and Gas Reservoirs

by Fengyu Sun, Changling Qu, Gaoshe Cao, Liqin Xie, Xiaohu Shi, Shengtao Luo, Zhuang Liu, Ling Zhang, Xiaochen Ma, Xinhang Zhou, Sen Zhu and Zhenzhi Wang

Processes 2024, 12(10), 2123; https://doi.org/10.3390/pr12102123 - 29 Sep 2024

Cited by 2 | Viewed by 1309

Abstract

In recent years, great breakthroughs have been made in gas explorations of the Upper Paleozoic bauxite series in the Longdong area of the Ordos Basin, challenging the understanding that bauxite is not an effective reservoir. Moreover, studying the reservoir characteristics of bauxite is [...] Read more.

In recent years, great breakthroughs have been made in gas explorations of the Upper Paleozoic bauxite series in the Longdong area of the Ordos Basin, challenging the understanding that bauxite is not an effective reservoir. Moreover, studying the reservoir characteristics of bauxite is crucial for oil and gas exploration. Taking the bauxite series in the Longdong area as an example, this study systematically collects data from previous publications and analyzes the petrology, mineralogy, oolitic micro-morphology, chemical composition, and other sedimentary characteristics of the bauxite series in the study area using field outcrops, core observations, rock slices, cast slices, X-ray diffraction analysis, scanning electron microscopy and energy spectra, and so on. In this study, the oolitic microscopic characteristics of the bauxite reservoir and the significance of oil and gas reservoirs are described. The results show that the main minerals in the bauxite reservoir are boehmite and clay minerals composed of 73.5–96.5% boehmite, with an average of 90.82%. The rocks are mainly bauxitic mudstone and bauxite. A large number of oolites are observable in the bauxite series, and corrosion pores and intercrystalline pores about 8–20 μm in size have generally developed. These pores are important storage spaces in the reservoir. The brittleness index of the bauxite series was found to be as high as 99.3%, which is conducive to subsequent mining and fracturing. The main gas source rocks of oolitic bauxite rock and the Paleozoic gas series are the coal measure source rocks of the Upper Paleozoic. The oolitic bauxite reservoirs in the study area generally have obvious gas content, but the continuity of the planar distribution of the bauxite reservoirs is poor, providing a scientific basis for studying bauxite reservoirs and improving the exploratory effects of bauxite gas reservoirs. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources, 2nd Edition)

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