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Abstract The deep coalbed methane (CBM) resources are widely developed in the Linxing Block. However, the evaluation of CBM geological areas suitable for CBM exploitation remains unexplored, hindering further development. This research optimizes the key geological parameters that influence the development of deep CBM from the perspectives of resource and development conditions. The evaluation system for deep CBM recoverability has been established, and the multi-fuzzy evaluation method has been used to perform the quantitative evaluation of recoverability. The results indicate that the resource conditions of No.8 + 9 coal seam are superior to those of No.4 + 5 coal seam. Favorable resource conditions are predominantly concentrated in the northeast and specific southern portions of the research area. Favorable development conditions for both coal seams are mostly concentrated in the northeastern area. Based on the classification standard of recoverable favorable areas, the Level II area is crucial for the development of No.4 + 5 coal seam. This area is primarily distributed in the northeast of the research area., Both Level I and Level II areas for the No. 8 + 9 coal seam are situated in the northeast. The Level III area is earmarked for deep CBM production and shows potential for exploration. Further analysis reveals that the resource conditions in the favorable area are generally superior to the development conditions. These areas are classified as Class A, including categories such as I-A, II-A, and III-A, indicating relatively complex reservoir transformation.

The Erlian Basin may be one of the most promising areas for low-rank coalbed methane (CBM) in China. The Cretaceous coal-bearing strata in the Bayanhua sag contain abundant low-rank CBM resources with suitable depths and large coal thicknesses (1.00–78.85 m, avg.22.53 m). However, the research on CBM geology and exploration potential is still lacking, which severely hinders its exploration and development. This paper presents an integrated study of CBM geology in terms of gas reservoir properties, gas-bearing characteristics and controlling factors. The results of the analysis show that lignite and subbituminous coals are the main coal types present in the study area, with medium porosity (7.3–27.8%, avg.20.77%) and low permeability (0.05–21.8 mD, avg.5.54 mD) being dominant. The pores are dominantly transition pores with inkpot shapes, which is beneficial to the adsorption of CBM. The gas content of the coal seams is 1.66–4.45 m 3 /t and most coal reservoirs are oversaturated. The CBM is primarily of secondary microbial origin, mostly carbon dioxide reduction. The accumulation of CBM is mainly influenced by the structure type, roof lithology and hydrogeology in the study area. According to the integrated analysis of the vertical changes in the physical properties and gas-bearing characteristics, the dominant horizon tends to be the No. 6-7 coal seam, followed by the No. 6-4 coal seam. Based upon the CBM geology and the factors controlling enrichment, the prospecting potential in the Bayanhua sag was appraised, and two favorable areas are proposed for CBM exploitation.

Abstract The Qinshui basin is the most successful area for coalbed methane (CBM) development in China. Since 2008, 45 multibranched horizontal wells (MBHWs) have been put into production in the Fanzhuang block, of which 10 wells have an average daily gas rate (DGR) of >10 000 m3/d. Well 4‐5 with the total gas production of 1.054 × 108 m3 achieved the China's highest DGR of 63 744 m3/d. Compared with vertical wells, MBHW is an effective way to develop CBM in low permeability reservoirs in China. However, there are still 23 wells have an average DGR of 16 m3/t) and saturation (>80%), long coal seam interval (>3000 m), reasonable branch spread, and stable wellbore are the basis for achieving high production. The encountered faults connected to the aquifer will cause a high water production and an extremely low gas rate (

The reservoir physical properties, pore types, diagenetic characteristics and reservoir quality controlling effect of the Xishanyao formation coal measure sandstone in the southern margin of the Junggar basin were discussed in this study based on thin section observation, high pressure mercury injection, low-temperature nitrogen adsorption and scanning electron microscope observation. The result shows that the porosity and permeability of the sandstone are generally low with a medium-high texture maturity and low compositional maturity. The sandstone storage space is mainly composed of residual intergranular pores, secondary dissolution pores, inter-crystalline pores and micro-fractures. The diagenetic stage of coal measure sandstone is in the mesodiagenesis A1-A2 stage, and their diagenetic interaction types mainly include compaction, cementation and dissolution. The reservoir quality of the coal measure sandstone deteriorates by compaction due to high matrix content and plastic debris content. Because of the large amounts of organic acids generated during the thermal evolution of the coal measure source rock, the coal measure sandstone suffers from strong dissolution. The secondary dissolution pores formed by the massive dissolution of feldspar, lithic fragments and early carbonate cementation in the sandstone significantly improved the reservoir quality. In the coal measure sandstone, clay mineral cementation is the most developed cementation form, followed by quartz cementation and carbonate cementation. Although kaolinite cementation and dolomite cementation can generate a small number of inter-crystalline pores, cementation deteriorates the reservoir quality. The Xishanyao formation coal measure sandstone formed in a lacustrine-delta environment, and its composition and texture make it susceptible to the influence of compaction and dissolution during diagenesis.

Macrolithotypes control the pore-fracture distribution heterogeneity in coal, which impacts stimulation via hydrofracturing and coalbed methane (CBM) production in the reservoir. Here, the hydraulic fracture was evaluated using the microseismic signal behavior for each macrolithotype with microfracture imaging technology, and the impact of the macrolithotype on hydraulic fracture initiation and propagation was investigated systematically. The result showed that the propagation types of hydraulic fractures are controlled by the macrolithotype. Due to the well-developed natural fracture network, the fracture in the bright coal is more likely to form the “complex fracture network”, and the “simple” case often happens in the dull coal. The hydraulic fracture differences are likely to impact the permeability pathways and the well productivity appears to vary when developing different coal macrolithtypes. Thus, considering the difference of hydraulic fracture and permeability, the CBM productivity characteristics controlled by coal petrology were simulated by numerical simulation software, and the rationality of well pattern optimization factors for each coal macrolithotype was demonstrated. The results showed the square well pattern is more suitable for dull coal and semi-dull coal with undeveloped natural fractures, while diamond and rectangular well pattern is more suitable for semi-bright coal and bright coal with more developed natural fractures and more complex fracturing fracture network; the optimum wells spacing of bright coal and semi-bright coal is 300 m and 250 m, while that of semi-dull coal and dull coal is just 200 m.

Permeability is one of the important factors that affect the production efficiency of coalbed methane, and it is mainly controlled by in situ stress. Therefore, it is very essential to study the in situ stress and permeability for the extraction of coalbed methane. Based on the injection/falloff well test and in situ stress measurement of 35 coalbed methane wells in the Liulin area in the east of the Ordos basin, the correlations between initial reservoir pressure, in situ stress, lateral stress coefficient, permeability, and burial depth were determined. Finally, the distribution characteristics of in situ stress and its influence on permeability were analyzed systematically. The results show that with the increase of burial depth, the initial reservoir pressure and in situ stress both increase, while the lateral stress coefficient decreases. The permeability variation is related to the type of stress field in different burial depths, and its essence is the deformation and destruction of coal pore structures caused by stress. The distribution characteristics of in situ stress at different depths and its effect on permeability are as follows: at depthsσh) and the permeability is a simple decreasing process with the increase of the depth; at depths>800 m, the vertical stress is dominant (σv≥σH>σh). The permeability of most coal is very small due to the large in situ stresses in this depth zone. However, because of the stress release at the syncline axis, coal with high permeability is still possible at this depth zone. Due to the existence of high permeability data points at burial depth (>800 m) and the fitting relationship between permeability and vertical stress, the maximum and minimum horizontal principal stress is poor. However, the coal permeability and lateral stress coefficient show a good negative exponential relationship. This indicates that the lateral stress coefficient can be used to predict permeability better.

The Hancheng block in the southeastern Ordos Basin is one of the earliest and the most important areas for coalbed methane exploration and development in China. However, there are significant production variations in different wells or even some adjacent wells in the Hancheng block. To reveal the reasons of production differences in such a small scale, further detailed studies of coalbed methane productivity in the Hancheng pilot test area, a precursor trial area in Hancheng block with mature, well-characterized coalbed methane reservoirs and long-term production database, were conducted. The influence of nine factors (including engineering and geological factors) on gas production was analyzed. By introducing the rough set theory, which is applicable to the vague, imprecise, and incomplete information system, this paper presents a method for quantitative evaluation of the influencing factors on gas production. The results indicate that there are certain distribution characteristics of productivity in Hancheng pilot test area, which can be partitioned into four zones. The degressive order of the influencing degree of these nine factors is (i) the distance between the well and the fault, (ii) the structure curvature of the coal seams, (iii) the gas content, (iv) the critical reservoir ratio, (v) the volume of the fracturing liquids per meter, (vi) the volume of the fracturing sand per meter, (vii) the dynamic liquid level drop rate, (viii) the depth, and (ix) the thickness. Geological factors, especially the influence of fault, structural curvature of the coal seams and gas content, play a major role in controlling long-term gas production. Engineering factors (effect of fracturing and dynamic liquid level drop rate) have always been integral parts of coalbed methane development.

The Songhe mine field located in western Guizhou Province of China is the most typical region of multiple coal seam development. However, the coalbed methane development practices of commingled drainage in recent years have been proved to be less ideal. In this work, on the basis of dividing gas system, the correlations between geological and engineering factors and gas production data were studied to determine the controlling factors of coalbed methane well productivity. The results indicated that the gas production had a positive correlation with cumulative thickness and gas content of fractured coal seams overall. However, the large cumulative thickness is generally resulted from the increase of coal seam number, and thus increases the reservoir heterogeneity and interlayer difference. The coalbed methane well productivity performance for this type region was a result of the strong interaction of cumulative thickness, burial depth, gas content, permeability, and reservoir pressure of fractured coal seam. However, the interlayer interference was the most direct factors restricting the productivity of commingle drainage by affecting the speed of dewatering and lowering of pressure and gas desorption time of gas-bearing systems. Additionally, the correlations between gas production and interlayer difference and interference were quantitatively analyzed, and the results showed that the wells with large interlayer difference and interference tend to have a poor productivity performance. As a result, the drainage method was particularly an important factor controlling the well productivity for multiple coalbed methane systems, because a proper combination of gas-bearing system and drainage and dewatering sequence are the keys to decrease the interlayer interference. Finally, the commingled drainage in this field was suggested to be conducted in steps according to the reservoir pressure, critical desorption pressure, and gas production pressure of each coalbed methane system.

To quantitatively analyze the effect of desorption characteristics on productivity, a division method of coalbed methane (CBM) desorption stages was established based on the Langmuir adsorption isothermal theory, and the indicating significance of the method was analyzed with cases of CBM production. The desorbed efficiency was used to quantitatively characterize the CBM desorbed rate under different reservoir pressures. On the basis of key nodes on mathematical curves, the starting pressure, turning pressure and sensitive pressure were defined. For different coal samples, the corresponding desorbed efficiencies of the three nodes were always constants, therefore the CBM desorption process was divided into four stages, i.e., inefficient, slow, fast and sensitive desorption stages. Studies reveal that, fast and sensitive desorption stages contribute a lot to CBM productivity, while inefficient and slow desorption stages have very little contribution; absorption capacity, gas-bearing property and reservoir pressure are the key factors for the desorption characteristics. The production status of southern Qinshui Basin and Liulin area in Ordos Basin in Northwest China demonstrates that the method can guide the CBM development effectively. Key words: coalbed methane, Langmuir equation, desorbed efficiency, desorption stages, southern Qinshui Basin, Liulin area

Aiming at a serious obstacle for building a log interpretation model to accurately interpret density and sonic speed value of each coal component which cannot be measured directly in lab scale, a log interpretation method of coal reservoir parameters was studied using the Hancheng mining area as an example. The enumeration method was used to calculate density and sonic speed value of fixed carbon, ash, and volatile according to experimental determination of industrial components, pore, gas content, density and sonic speed value of many coal samples in the Hancheng mining area. On the basis of the relationships of each coal component and its density and sonic speed value, logging response relationships between each component and its compensated density and acoustic travel-time differences were studied using numerical fitting. As fitted carbon content increases and ash content decreases, density decrease and sonic speed difference increase. By combining statistical analysis method and theory model calculation method, interpretation models of industrial components, gas content and pore for coal reservoirs were built and a logging integrated interpretation software was programmed, which have got good effect in actual application. Key words: coal reservoir, log interpretation, fixed carbon content, ash content, gas content, coalbed methane, Hancheng mining area

The Panguan Syncline contains abundant coal resources, which may be a potential source of coalbed methane. In order to evaluate the coalbed methane production potential in this area, we investigated the pore-fracture system of coalbed methane reservoirs, and analyzed the gas sorption and seepage capacities by using various analytical methods, including scanning electron microscopy (SEM), optical microscopy, mercury-injection test, low-temperature N2 isotherm adsorption/desorption analyses, low-field nuclear magnetic resonance and methane isothermal adsorption measurements. The results show that the samples of the coal reservoirs in the Panguan Syncline have moderate gas sorption capacity. However, the coals in the study area have favorable seepage capacities, and are conductive for the coalbed methane production. The physical properties of the coalbed methane reservoirs in the Panguan Syncline are generally controlled by coal metamorphism: the low rank coal usually has low methane sorption capacity and its pore and microfractures are poorly developed; while the medium rank coal has better methane sorption capacity, and its seepage pores and microfractures are well developed, which are sufficient for the coalbed methane’s gathering and exploration. Therefore, the medium rank coals in the Panguan Syncline are the most prospective targets for the coalbed methane exploration and production.

Based on the study of low-pressure distribution characteristics, the main factors that control the formation of low-pressure reservoirs in the Sulige gas field are analyzed from several aspects, such as structural evolution, sedimentary characteristics and formation fluid features. It is suggested that the gas pool pressure is low and affected greatly by buried depth. Its roof and floor developed several layers of uncompacted mudstone, indicative of good property and pressure sealing conditions. The formation water also indicates favorable preservation conditions. The reservoir pressure of the Sulige gas field experienced 4 major evolutionary stages: the normal pressure stage of the Late Triassic-early Jurassic, the pressure rising stage of the Middle Jurassic-Late Jurassic, the sustainable pressure increasing stage of the Early Cretaceous, and the pressure decreasing stage of the late Early Cretaceous. The pore rebound and temperature decrease caused by intense tectonic uplift after the late period of Early Cretaceous, caused the reservoir pressure of the Sulige gas field to reduce by 0.673 MPa and 23.08% of the original strata pressure, respectively. The low pressure of the Sulige gas field is related to sedimentary assemblage, tectonic evolution and hydrocarbon accumulation. Key words: low pressure gas pool, formation mechanism, tectonic uplift, Sulige gas field, Ordos Basin