Experimental study on tensile strength and acoustic emission characteristics of shale exposure to supercritical CO_2

摘要

The effect of supercritical CO2 on the mechanical properties of shale is a long-term process. To study the influence of supercritical CO2 on the tensile strength and acoustic emission characteristics of shale, the Brazilian splitting experiment equipped with a real-time monitoring of acoustic emission were conducted on the shale collected from Longmaxi formation in Sichuan Basin before and after supercritical CO2 treatment at different exposure times (0 day, 15 days, and 30 days) and a constant pressure (8 MPa) and temperature (40 degrees C). The results show that the tensile strength of shale decreases and the failure deformation increases after supercritical CO2 treatment. The peak acoustic emission energy generated by the treated shale specimens in the process of Brazilian splitting experiment was significantly lower than that of the untreated, indicating that supercritical CO2 deteriorated the failure strength of the shale. The percentage of C element content has no obvious change, and the percentage of H element content decreased after supercritical CO2 treatment, which indicates that the supercritical CO2 fluid has negligible influence on organic matter in shale, while the water content of shale reduced. After supercritical CO2 treatment, the pore structure of shale measured by Poremaster-33 shows an increasing trend, the most probable pore size is shifted from 100-150 mu m to 50-100 mu m, and many new micro-pores (20 mu m) are generated in the shale. In addition, the formed CO2-water weak acid system in the process of supercritical CO2 treatment dissolves the carbonate and silicate in the shale measured by SEM/EDS. The main reasons for the decrease of tensile strength of shale after supercritical CO2 treatment can be attributed to the increased pore structure/volume and the dissolution of carbonate and silicate in the shale. The research results can provide reference for supercritical CO2 fluid exploitation of shale gas.