Experimental Investigation on Permeability Evolution of Limestone Caprock under Coupled THM Processes

To investigate the gas sealing capacity of limestone as cap rock for gas reservoir at different burial depths, triaxial compression coupled with permeability tests of limestone subjected to different confining pressures and temperatures were carried out using the improved MTS 815 test machine. During the tests, acoustic emission (AE) monitoring system was also employed to estimate the rock damage. Test results demonstrated a degradation of triaxial compression strength with the increase in temperature and decrease of confining pressure. An empirical equation for the peak stress was established considering confining pressure and temperature. Three different permeability evolution patterns of limestone samples can be proposed based on the experimental results under different coupled thermo-hydro-mechanical (THM) conditions. According to the spatial-temporal characteristics of AE signals, permeability evolution was highly related to the fracture growth and connection in limestone. Ultimately, the experimental results revealed that the permeability of limestone gradually decreased with the increasing confining pressure and temperature. In addition, an exponential relationship between initial permeability and confining pressure was established.