The Qinshui Basin is the main production base of high-rank coalbed methane in China. High-rank coal reservoirs in this region exhibit diverse conditions for coal formation and reservoir development, complex geological structures, low permeability, pronounced reservoir heterogeneity, and significant challenges in reservoir stimulation, which led to early issues such as a low effective resource utilization rate, low gas production per well, and low development profits. By analyzing the characteristics of high-rank coal reservoirs and the development patterns of coalbed methane, this study identifies three key constraints to the efficient development of high-rank coalbed methane: (1) poor precision in selecting areas for efficient development; (2) limited adaptability of development technologies; (3) a mismatch between stimulation processes and coal reservoirs. Investigations into microstructures, coal body structures, in-situ stresses, and fractures—combined with an evaluation of various geological factors’ impact on production—enabled a multidimensional division of development units to identify the geological features of each unit. Consequently, a “five-element” evaluation index system for production potential in efficient development areas was established, and an optimization method for selecting efficient development areas for high-rank coalbed methane was formulated. Analysis suggests that due to the low permeability and strong heterogeneity of high-rank coal, horizontal wells can connect more coal seam fractures, thereby expanding the drainage and pressure-relief areas and reducing the flow resistance of gas and water. This possesses advantages such as high per-well gas production and improved economic benefits. For different geological zones and development stages, in accordance with the principle of “maximizing controlled reserves, maximizing gas production rate, and optimizing economic benefits”, an optimized horizontal well layout technology for high-rank coalbed methane was developed. On this basis, with the objective of “initiating a fracture network, creating new fractures, and controlling reserves”, key technologies were devised—primarily including energy-focused directional perforation, stepwise hydraulic fracturing for incremental production enhancement, a combined application of fine-powder sand, and synchronous well-group interference. At the same time, the process technologies of bridge-plug-and-perforation using active water as the main body and well-group synchronous interference operations were refined, leading to the establishment of a linear fracture network system conducive to gas production, achieving efficient hydraulic fracturing. The application of these research outcomes in the Qinshui Basin has enabled the efficient development of high-rank coal, with daily gas production per horizontal well doubling, the ultimate recoverable reserve per well increasing by 50%, and the productivity attainment rate of newly-built blocks surpassing 90%. When extended to other high-rank coalbed methane blocks in China, these advantages provide technical support and a demonstrative model for strengthening the coalbed methane industry.

Based on the analysis of the coal-forming environment of the Carboniferous Taiyuan Formation in Xunyi exploration area, southern Ordos Basin, and combined with paleogeomorphological analysis and sedimentary facies research, the coal accumulation characteristics of Taiyuan Formation were identified. By using experimental analysis methods such as industrial analysis, scanning electron microscopy, and isothermal adsorption, along with well-logging modeling evaluation, the characteristics of coal petrology and coal quality, reservoir physical properties, and gas content were investigated. The key controlling factors for deep coalbed methane accumulation and reservoir formation characteristics were analyzed and summarized, identifying favorable zones for further exploration. Integrated with coalbed methane exploration practice, it was confirmed that deep coalbed methane had promising exploration potential. Research showed that: (1) the development of coal seams in the Carboniferous Taiyuan Formation in the Xunyi exploration area was influenced by two coal-forming environments: tidal flat peat bogs and lagoon peat bogs. Due to the influence of coal-forming environments and sedimentary paleogeomorphology, the coal seam distribution exhibited a “thin in the west and thick in the east” coal accumulation pattern. (2) The coal lithotypes were primarily bright coal and semi-bright coal, with the coal body structures mainly characterized by primary and fractured types. The types of reservoir space included plant cell lumen pores, intercrystalline pores of pyrite and clay minerals, and cleat fractures. (3) The gas content of coal seams ranged from 15.8 to 25.6 m³/t, indicating moderate to good gas-bearing properties. The enrichment of coalbed methane was controlled by factors such as the coal-forming environment, tectonic evolution, and preservation conditions. (4) The northwest slope area was characterized by underdeveloped faults, normal formation pressure, weak formation hydrodynamics, and large coal seam burial depth, making it a favorable zone for deep normal-pressure coalbed methane exploration. The southeast fault slope area had relatively developed faults, low formation pressure, strong formation hydrodynamics, and moderate coal seam burial depth, making it a favorable zone for medium-to-deep low-pressure coalbed methane exploration. The PZ1 well, located in the southeast fault slope area, produced a low gas flow during coal seam fracturing tests, demonstrating the promising exploration potential of deep coalbed methane in the structurally complex margin of the Ordos Basin.

The Sichuan Basin is abundant in coal resources and has achieved breakthroughs in coalbed methane exploration wells in recent years. To explore the feasibility of establishing a coalbed methane production base in the Sichuan Basin, this study reviews the stratigraphic development of coalbed methane reservoirs in the Sichuan Basin, as well as the geological and dynamic characteristics of coalbed methane development blocks in the southeastern and southern regions of Sichuan. The first coalbed methane production base in the Sichuan Basin, the Junlian-Mu’ai mining area, has more than 450 surface extraction wells, with an annual gas production of over 1.00×10⁸ m³ for five consecutive years. In the Shunan mining area, there are 328 production wells, with an annual gas production of 0.79×10⁸ m³. However, in the basin, the average daily production of coalbed methane wells targeting coal seams is less than 700 m³, while pilot exploration wells that apply general fracturing to coal seams and adjacent sandstone layers can achieve production rates of 5 000 to 8 000 m³/day. This indicates that the production dynamics of coalbed methane in the Sichuan Basin differ significantly from those in other domestic coalbed methane production bases. This difference is attributed to the widespread development of thin coal seams and the structural coal layers interbedded with tight sandstone. Consequently, it is not appropriate to apply the “sweet spot” evaluation and development methods used for typical thick coal seams in basins such as the Qinshui Basin and Ordos Basin. There is an urgent need to shift away from the approach of considering only coal seams as the sole target layer for coalbed methane wells. Practice shows that coal seams in the Sichuan Basin are primarily developed in transitional marine-continental strata. Although the lateral development of coal seams is not stable, a stable combination of “coal, sandstone, and mudstone” has formed. This special lithologic combination can create “coal-sand-coal” hydrocarbon source storage boxes, which is of significant importance for the development and production capacity of thin interbedded coal seams in the Sichuan Basin. Moreover, a large number of coal mines in the Sichuan Basin have been shut down in recent years, and the coalbed methane accumulated in the abandoned mines urgently requires secondary development using surface drilling techniques. In conclusion, based on the geological resources and existing extraction technologies in the Sichuan Basin, it is feasible to establish the third coalbed methane industry base, following the Qinshui Basin and the eastern edge of the Ordos Basin.

The Sichuan Basin is rich in coalbed methane resources, and shallow coalbed methane in the Junlian area has been commercially developed. In the adjacent Changning area, multiple drilling wells have tested and obtained gas in the Permian coal seam, revealing significant potential for coalbed methane in the Sichuan Basin. With significant breakthrough in the exploration and development of deep coalbed methane in China, and drawing from experiences in the Qinshui Basin and the Ordos Basin, a comprehensive study was conducted to assess the resource potential of deep coalbed methane in the Changning area. This study utilized data from logging, coal seam coring, and experimental testing to analyze the geology of the coal seams, evaluate gas-bearing properties, and investigate the main factors influencing reservoir formation and favorable zones. The study found that the 7th and 8th coal seams in the study area are thick, regionally stable, and high-quality, mainly composed of primary structural coals with high rank and high fixed carbon content. These seams are at the peak of pyrolysis gas generation, suggesting significant hydrocarbon generation potential. Coal seams have characteristics of high porosity, high permeability, and high cleat density, providing ample storage space, while the coal seam roof and floor—predominantly mudstone—offer excellent preservation conditions. Compared to the shallow coalbed methane in Junlian, the deep coalbed methane in the Changning area features a more stable structure, a higher proportion of free gas, and a more intact coal matrix. Coalbed methane reservoirs are formed in the target area far from erosion boundary and Class Ⅰ fault zones. The abundant free gas is more conducive to subsequent development. Based on geo-engineering conditions, a dual geo-engineering “sweet spot” index system for evaluating coalbed methane in the Changning area has been established. A favorable deep coalbed methane development area of 1 300 km² has been identified, with a calculated resource volume of 1 700×10⁸ m³, primarily located in the Luochang and Jianwu synclines. The research results have effectively guided the deployment of coalbed methane wells in the region, contributing to the high-quality development of unconventional natural gas in the Sichuan Basin.

In recent years, the Ordos Basin has achieved significant breakthroughs in the exploration and development of coal measures symbiotic gas reservoirs, whereas the Qinshui Basin continues to focus primarily on coalbed gas methane reservoirs. Notably, in the northern Qinshui Basin’s Shouyang block, Late Carboniferous to Early Permian the 3rd coal seam of the Shanxi Formation and the 9th and 15th coal seams of the Taiyuan Formation exhibit a high degree of thermal evolution, providing an excellent hydrocarbon source. The marine-continental transitional facies sedimentary environment promotes the development of various reservoir-cover combinations, demonstrating that the area has the essential geological conditions for the formation of coal measures symbiotic gas reservoirs. Drawing on data from 108 CBM exploration wells and previous studies on the depositional environment, the sedimentary characteristics of the Late Carboniferous-Early Permian coal measures have been meticulously investigated. Using a “point-line-surface” approach, the Taiyuan Formation is divided into three depositional units: the first unit (hereafter referred to as Tai-1) represents delta front deposition, the second unit (Tai-2) represents lagoon and tide flat deposition, and the third unit (Tai-3) again represents delta front deposition. The Shanxi Formation is classified into one depositional unit, namely, delta plain deposition. Based on these studies, eight typical lithofacies associations were identified, and a comprehensive analysis yielded three types with four categories of coal-measure gas reservoir covers. The study finds that in the northern part of the study area, only a single coalbed methane reservoir is developed, whereas the eastern part exhibits a combination of coalbed methane and conventional limestone fractured gas reservoir. In the central region, coalbed methane combined with tight sandstone gas predominates, while the northern-central zone is characterized by a mix of tight sandstone gas, coalbed methane, and shale gas.

The deep coalbed methane resources in the eastern Ordos Basin are abundant, and comprehensive development of coal measure gas can enhance resource utilization and improve single well gas production. To precisely identify the “sweet spot layer,” this study compares the pore development characteristics of coal measure mudstone, coal rock and tight sandstone in the Shan 2 Section of the Shanxi Formation in the eastern Ordos Basin using organic geochemical analysis, dual-beam scanning electron microscopy, high-pressure mercury intrusion, low-temperature N₂ adsorption, and low-temperature CO₂ adsorption tests. The results show that clay mineral content is the main factor influencing pore development in coal measure mudstone and tight sandstone. The microscopic pore structure of coal measure reservoirs exhibits significant variations: mudstones and tight sandstones are characterized by mesopores (2-50 nm) within clay minerals, with their mesopore-specific surface area and pore volume being roughly equal. Coal develops abundant micropores (<2 nm) in organic nanopores, with a micropore-specific surface area far exceeding the mesopore-specific surface area of mudstone and tight sandstone. Tight sandstone also develops numerous macropores (>50 nm) in clay mineral pores and microfractures, exhibiting better connectivity than mudstone. Tight sandstone provides substantial storage space for free gas, while the pores in mudstone and coal can adsorb a large amount of natural gas. The sand-mud-coal and sand-coal combinations are the main exploration targets for coal measure strata.

The differences in pore-fracture structures between deep and shallow coal reservoirs significantly affect coalbed methane extraction. Research on these structural differences provides theoretical support for exploring their physical properties and identifying favorable zones for coalbed methane exploration and development. This study analyzed coal samples from deep and shallow coal reservoirs in the Junggar Basin. These samples were tested using scanning electron microscopy, low-temperature N₂ adsorption, high-pressure mercury injection, and CT scan. The results showed that, from shallow to deep coal samples, the permeability, total pore volume, and distribution frequency of micropores and macropores gradually decreased. The shallow coal samples exhibited well-developed pores and fractures, with low fractal dimensions in the mesopore and macropore stages, strong homogeneity in pore development, and interconnection between macropores and microfractures. In contrast, the deep coal samples showed relatively isolated pore-fracture development, more complex pore development in the mesopore and macropore stages, and significant mineral filling within pores and fractures. A pore network model for the samples was established using the maximal sphere algorithm to analyze the distribution pattern, morphology, and three-dimensional structural development of the connected pores and fractures. The equivalent pores, throat parameters, and other structural parameters, along with the connectivity, were statistically analyzed. The results revealed that shallow coal samples showed higher connectivity and total porosity compared to the deep samples. The shallow samples exhibited more pores and fractures, with a dominance at the microfracture scale. Additionally, they exhibited shorter throats, larger pore-throat radii, denser pore development, higher coordination numbers, and better connectivity, which facilitated gas flow in the reservoir. The research findings provide experimental data support for the development of deep and shallow coalbed methane in the Junggar Basin using adaptive technologies, and offer valuable guidance for on-site development.

In order to establish a systematic evaluation of deep tight coal reservoirs and ensure the efficient, economic and safe implementation of moderate in-situ gasification projects in deep tight coal reservoirs, a feasibility evaluation method based on fuzzy analytic hierarchy process was developed. The methodology comprises: (1) the establishment of an evaluation index set of 3 first-level indicators, including resource conditions, reservoir conditions and preservation conditions, and 18 second-level indicators, including parameters such as coal rank, coal rock reservoir thickness, and coal rock reservoir pressure coefficient, along with a graded comment set categorizing outcomes as “feasible”, “basically feasible”, and “infeasible”; (2) the determination of indicator weights through the analytic hierarchy process; (3) the calculation of each indicator’s membership degree using a trapezoidal membership function to construct an evaluation matrix; and (4) the synthesis of the evaluation and weight matrices to ascertain the membership degrees corresponding to “feasible”, “basically feasible”, and “infeasible” for candidate areas, thus determining the feasibility based on the principle of maximum membership degree. The evaluation method was applied to the feasibility evaluation of moderate in-situ gasification for the deep No. 8 tight coal reservoir in the M block of the Ordos Basin. The evaluation results show that the membership degrees of “feasible”, “basically feasible” and “infeasible” for moderate in-situ gasification of No. 8 coal reservoir in the deep part of M block are 0.413, 0.425 and 0.162 respectively, with the maximum being 0.425, thus determining the feasibility as “basically feasible”. The comprehensive quantitative feasibility evaluation method of moderate in-situ gasification of deep tight coal reservoir, which places particular emphasis on the evaluation of preservation conditions, provides scientific guidance for the implementation of moderate in-situ gasification projects in deep tight coal reservoirs.