SC-CO₂ and Brine Exposure Altering the Mineralogy, Microstructure, and Micro- and Macromechanical Properties of Shale

Abstract

Understanding the mechanism of the fluid−shale interaction, which is induced by SC-CO 2 injection, is critical to the safety of shale oil exploitation and the geological sequestration of CO 2 . In this paper, the effects of different soaking fluids (SC-CO 2 , brine, and SC-CO 2 + brine) and soaking time (7, 14, 21, and 28 days) on the physical properties of shale were investigated by performing static soaking experiments. A series of experimental methods (XRD, SEM, Nanoindentation, UCT, and Brazilian splitting) were used to analyze the variation of shale physical properties before and after soaking. The results showed that SC-CO 2 soaking had a limited effect on shale's physical properties. After soaking in brine, the degree of alteration in the physical properties of the shale increased. Furthermore, under the condition of SC-CO 2 + brine soaking, the mineral content and microstructure of shale changed most obviously. Following 28 days' soaking period, the microelastic modulus experienced an ultimate degradation extent of 44.99%, while the hardness experienced an ultimate degradation extent of 65.20%. The UCS, macroelastic modulus, and tensile strength were reduced by 47.23, 35.93, and 39.08%, respectively. Under the condition of SC-CO 2 + brine soaking, as the duration of soaking increased, the degree of variation of the shale mineral content intensified, the microstructure of shale altered more significantly, and the phenomenon of mineral dissolution and precipitation increased. Hence, as the soaking time increases, the deterioration of the micro-/macromechanical characteristics of the shale becomes more apparent, and the interaction between the soaking fluid and the shale becomes more obvious.