Pore compaction and crack evolution of shale rock under high-speed impact loading and different confining pressures

High-speed impact loading has a crucial effect on pore compaction and microscopic crack evolution for shale at different confining pressure (P_c). In this study, we selected the shales from Jiangxi (JX) and Sichuan Province (SC) in China, and then performed Split Hopkinson pressure bar (SHPB) experiments at P_c = 0, 5, 15, and 25 MPa. Further, the impacted shales were tested by low temperature nitrogen adsorption (LTNA), mercury intrusion porosimetry (MIP) and environmental scanning electron microscope (ESEM). The results showed that confining pressure could cause different degrees of pore compaction and crack blockage: total pore volume (TPV) based on MIP of the JX and SC shales at different confining pressures decreased by 24.3 ~ 69.2% and 54.1 ~ 66.5%, respectively; total pore volume based on LTNA decreased by 36.2 ~ 49.0% and 49.9 ~ 58.2%, respectively; total specific surface area (TSSA) based on BET model decreased 68.3 ~ 75.8% and 39.5 ~ 60.0%, respectively. It was found that the confining pressure had a more significant effect on macropores relative to mesopores. Confining pressure also changed the failure patterns of shales. The shales at low confining pressure (P_c = 0, 5, and 15 MPa) showed a marked characteristic of brittle shear damage. However, the shales at high confining pressure (P_c = 25 MPa) showed a brittle-ductile transition (BDT) phenomenon. Additionally, high confining pressure resulted in a tiny ductile damage and a significant increase in elastic modulus. Besides, the effect of different mineral compositions and grain sizes on crack extension was discussed. This study is meaningful for understanding and controlling pore compaction and crack extension, thus improving production processes of dynamic shale gas production technology.