Experimental investigation on the mechanical properties of a low-clay shale with different adsorption times in sub-/super-critical CO2

Knowledge of the effect of carbon dioxide (CO2) on the mechanical properties of low-clay shales is essential to shale gas production and CO2 sequestration. In this paper, a series of uniaxial compressive strength (UCS) variable-time experiments were performed on low-clay shale samples saturated in sub-/super-critical CO2. The crack propagation process and micro scale variations were recorded by acoustic emission (AE) sensors with 3D ARAMIS technology and SEM tests together with EDS analysis. According to the experimental results, sub-/super-critical CO2 adsorption weakens the strength and increases the ductility of the shale. The UCS and Young's modulus decrease with the increase of saturation time. Compared to samples saturated in sub-critical CO2, samples saturated in super-critical CO2 present lower strength and Young's modulus. AE results show that samples saturated at a longer time in sub-/super-critical CO2 present a higher number of peak cumulative AE energy. Super-critical CO2 saturation creates more AE energy than sub-critical CO2 saturation. Based on the SEM results, sub-/super-critical CO2 adsorption creates some new pores in shale samples which lead to the strength decreasing. EDS analysis presents that CO2 adsorption increase the C content of the shale which demonstrates the occurrence of chemical reactions in the shale.