Study on the Evolution Mechanism and Compression Effect of Coal Pores Subjected to Water Injection
Yiyu Lu, Maolin Ye, Zhaolong Ge, Zhe Zhou, Shihui Gong, Zepeng Wang, Shan Huang, Yarui Guan
Abstract
Hydraulic fracturing for enhancing the permeability in deep coal seams may damage reservoirs, which can result in changes in the pore structure and seepage channels. Therefore, in this study, nuclear magnetic resonance-based water injection tests were conducted to investigate the effects of hydraulic fracturing water pressure and ground stress on the microscopic pore structure of coal. Based on the difference in the response of T2 distribution corresponding to different scales of pores at variable injection pressures (IPs) and confining pressures (CPs), we deduced the coal pores’ connectivity and categorized the pores into closed pores (CLPs, T2 ≤ 0.45 ms), transition pores (TRPs, 0.45 ms < T2 ≤ 3 ms), and connected pores (COPs, T2 ≥ 3ms). When a larger CP was applied, total pores space is reduced by CP compression. The average pore compression coefficient (APC) of the transition pores and connected pores was positive, and the effect on the complexity of connected pores was weak. In contrast, the APC of closed pores on the nanoscale was negative. This result was mainly owing to some of the connected pores’ transformation into closed pores by compression. When the water IP increased, Total pores space is increased.The injected high pressure water exerted an expansive effect on connected and transition pores. At the same time, since it cannot enter the tiny closed pores, there is a significant compression effect on the closed pores. Overall, compression effect was more pronounced for closed pores when increasing the IP than when loading the CP. . We modeled the effective stress coefficient under water IP. The effective stress coefficient of the coal samples increased with increasing IP and gradually approached 1. This study theoretically explains the permeability enhancement of coal seams under partial high-pressure hydraulic fracturing. It provides some technical reference for determining fracturing water pressure for hydraulic fracturing of coal seams.