A laboratory study of geomechanical characteristics of black shales after sub-critical/super-critical CO2 + brine saturation

Abstract

This paper presents an experimental study of the impact of CO2–brine–rock interaction on the mechanical properties of shales. Several black shale samples were soaked in sub-critical/super-critical dissolved brine solutions with different adsorption time. Uniaxial compressive strength (UCS) tests together with the AE and SEM technology are used to test the variations of shale’s mechanical properties, crack propagations and microstructures. According to the experimental results, CO2–brine–rock interaction has a great effect on shale’s strength, Young’s modulus and brittleness index which decrease with increasing saturation time. With a 30 days’ adsorption, the UCS and Young’s modulus show large reductions of 60.56 and 52.68% for sub-critical condition and 70.47 and 62.45% for super-critical condition, respectively. AE analysis presents that the total cumulative AE energy increases dramatically after adsorption. Longer saturation time creates more AE energy. Super-critical condition has a higher effect on shale’s mechanical values and acoustic emission energy than the sub-critical condition. Some new micron scale pores appear on SEM images after saturation. Based on the EDS analysis, CO2–brine–rock interaction also causes the increase of C, Na and Fe contents, and the decrease of O, Si, Al and K contents, which prove the occurrence of the dissolution and precipitation.