Fazhi Yan

Variation in the Pore Structure of Coal after Hydraulic Slotting and Gas Drainage

The integration of hydraulic slotting and gas drainage techniques has become a mainstream technique for enhancing permeability in coal seams with low permeability. However, the mechanism of action of this process is unclear. In this paper, field experiment and laboratory tests are described that aim at elucidating this process. Given the sensitivity and accuracy of test methods and their corresponding determination principles, a combination of mercury intrusion porosimetry and nitrogen gas adsorption was proposed as a complementary technique and the pore-size distribution (PSD) was obtained. It is shown that the proportion of minipores decreases remarkably, whereas that of the macropores gradually increases with the decrease in the distance from the slotted borehole. By contrast, the mesopores and micropores present insignificant changes. Meanwhile, the adsorption pore and the seepage pore show a similar variation in tendency with the minipores and macropores, respectively. Moreover, the specific surface area decreases substantially with the decrease in borehole distances. The integration of hydraulic slotting and gas drainage can lower the gas-adsorption properties and enhance the gas-seepage capacity within the disturbed zone significantly. The paper highlights the guiding factors for improving the enhanced coal bed methane recovery.

Cracking Process and Stress Field Evolution in Specimen Containing Combined Flaw Under Uniaxial Compression

Hydraulic slotting, an efficient technique for underground enhanced coal bed methane (ECBM) recovery, has been widely used in China. However, its pressure relief mechanism is unclear. Thus far, only limited research has been conducted on the relationships among the mechanical properties, flaw parameters, and crack propagation patterns of coal after hydraulic slotting. In addition, because of the limitations of test methods, an in-depth information is not available for this purpose. In this work, numerical models of specimens containing combined flaws are established based on particle flow code method. Our results provide insights into the effects of flaw inclination angle on the mechanical properties, crack propagation patterns, and temporal and spatial evolution rules of stress field in specimens containing combined flaws during the loading process. Besides, based on the initiation position and underlying mechanism, three types of crack initiation modes are identified from the failure processes of specimens. Finally, the crack propagation pattern is quantitatively described by the fractal dimension, which is found to be inversely proportional to the uniaxial compressive strength and elastic modulus of the specimen. To verify the rationality of the numerical simulation results, laboratory tests were conducted and their results match well with those obtained from the numerical simulation.

Experimental study on the microscopic characteristics affecting methane adsorption on anthracite coal treated with high-voltage electrical pulses

The low gas permeability of coal formations with limited coal pores and fractures leads to difficulty in coalbed methane exploration. High-voltage electrical pulse has a potential application in enhanced coalbed methane recovery. In this study, we discuss the microscopic characteristics of anthracite coals treated by high-voltage electrical pulse. We find that C, O, and other coal elements constituting oxygenic groups, which mainly account for gas adsorption, decreased slightly after high-voltage electrical pulse treatment, indicating that elemental variation may have little influence on gas adsorption. The scanning electron microscopy and low-pressure nitrogen gas adsorption (LP-N2GA) results show that the cumulative micropore volumes of high-voltage electrical pulse-treated coals were much larger than those of original coals. The mercury intrusion porosimetry results show that the cumulative macropore volumes, which act as gas migration channels in coal increased. Additionally, high-voltage electrical pulse-treated coals were found to have smaller entrapment areas, indicating that gas migration was enhanced.