Liquid CO2 phase transition fracturing (LCPTF) is a kind of novel waterless fracturing technology for enhancing the coalbed methane (CBM) recovery. Relevant current studies are focused on exploring the transformed effect of LCPTF on the nanopore structure in coal. However, its influence on gas adsorption capacity has been rarely reported. This study addresses coal induced with LCPTF. The structure alterations of mesopores (2–50 nm) and macropores (>50 nm) and their effects on the gas adsorption capacity are evaluated with comprehensive measurements of a mercury intrusion porosimetry, low-temperature N2 adsorption, and isothermal adsorption. The results indicate that LCPTF has an enlarged effect on macropores, resulting in an increase in pore size and volume and a reduction of the pore specific surface area. The pore-enlarged-transformed effects of LCPTF cause an increase in pore size and a reduction of the pore volume and pore specific surface area of mesopores. The variations of the pore structure after LCPTF cause a reduction of the adsorption constant a and the increase in the adsorption constant b, indicating LCPTF’s reductive effect on adsorption capacity as well as its enhanced effect on desorption capacity. A novel effect evaluation method of LCPTF for improving CBM recoverability is proposed based on the variations of gas saturation and critical desorption pressure. This study examines, from the perspective of adsorption and desorption, the mechanism of LCPTF for enhancing CBM recovery, which provides theoretical guidance for LCPTF’s technical improvement and optimization of field application so as to secure a more reliable and efficient CBM recovery.

Clay minerals contain a massive amount of nanopores and play a significant role in gas adsorption in shale. Although the pore structure of clay minerals has been widely studied, the characteristic of pores with diameter < 1 nm remains unclear. To investigate the pore characteristics of different clay minerals, especially for micropores, and to reveal the effect of pore structure on the methane adsorption capacity, the isotherm types, pore size distribution, pore volume, and surface area, as well as the CH 4 adsorption capacity of pure clay mineral samples, including kaolinite, montmorillonite, illite, and illite−smectite mixed layer (I/S), were investigated based on low-pressure N 2 and CO 2 adsorption and CH 4 adsorption isotherm measurements. The results show that the isotherm types of the studied clay minerals based on N 2 adsorption are all type IV, characterized by the presence of hysteresis loops. According to the features of hysteresis loops, it can be inferred that kaolinite mainly has cylindrical pores and slit-shaped pores, while pores in montmorillonite, illite, and I/S are dominated by inkbottle-shaped pores, with a small amount of slit-shaped pores. The studied clay minerals all display pore width peaks around 0.56−0.66 nm and 0.82−0.87 nm. Pores with diameters < 1 nm in kaolinite, illite, and I/S are all interparticle pores. Montmorillonite has microporous interlayer pores in addition to the interparticle pores, leading to its relatively large micropore volume and surface area. The CH 4 sorption capacity on different clay minerals is mainly influenced by the surface area, and montmorillonite has the highest CH 4 adsorption capacity.

For the problem where numerous coalbed methane (CBM) stripper wells exist in China, this paper analyzes the genesis of the stripper wells from the aspects of geological conditions and development technologies combined with the CBM development of some typical blocks. A series of key secondary stimulation technologies for CBM stripper wells are put forward, including low-damage fracturing fluid for preventing reservoir damages, proppants with multigraded sizes for supporting multilevel fractures, large-scale fracture network stimulation (FNS) for improving reservoir permeability, and coal measure gas development for increasing the exploitable resources within a single well scope, as well as coordinated stimulation of parent–child wells for the overall production improvement of low-yield blocks. Also, it is pointed out that all types of stripper wells could adopt the low-damage fracturing fluid and multigraded proppant and optimize the drainage schedule to inhibit reservoir damage and promote the maintenance of fracture conductivity. For resource-controlled stripper wells, large-scale FNS of coal seams, coal measure gas development, and coordinated stimulation of parent–child wells could be adopted according to the differences in resource abundance and coal seam distribution. For the stripper wells controlled by the coal structure and ground stress, FNS of the surrounding rock could be conducted to construct stable and efficient channels for CBM migration. In addition, by conducting large-scale FNS, the stimulation effect of fracturing-controlled stripper wells improves, while after unblocking and reopening the existing reservoir fractures of the drainage-controlled stripper wells, an optimized drainage schedule could be adopted to prevent reservoir damages and promote the maintenance of fracture conductivity.

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To investigate the effect of analytical particle size on pore structure, mesopore (2-50 nm) and micropore (<2 nm) characteristics of high volatile bituminous coal and anthracite with different particle size were determined using low-pressure N2/CO2 adsorption analyses. Mesopore structure parameters in the two coals increase with decreasing particle size, which are attributed to the opening of closed mesopores during the pulverization process. The closed mesopores with different pore size ranges are opened with a certain percentage in high volatile bituminous coal, but opened irregularly in anthracite during pulverization. Micropore structure parameters of the two coals show different variations with decreasing particle size, which are not related to the reconstituted micropore structure. Mineral matter contributes more mesopores than organic matter in anthracite and exerts the negative effect on micropore in the two coals. An evolution model is established to elaborately describe the change of pore structure during the pulverization process, where mineral matter plays a mediating role in the effect of particle size on pore structure.