Effect of Supercritical CO2-Water/Brine-Rock Interaction on Microstructures and Mechanical Properties of Tight Sandstone

Supercritical CO2 (SC-CO2), with many particular properties, has been considered a fracturing fluid for exploiting unconventional hydrocarbon recently, which can also contribute to achieving partly geological sequestration of carbon. However, understanding the changes in mechanical attributes and microstructure of formation rock during CO2 injection is essential for the processes of fracture propagation, carbon storage, and hydrocarbon flow in porous media. In this study, a series of soaking experiments between tight sandstone and dry/water/brine-SC-CO2 are conducted. Several quantification techniques, such as XRD, SEM, three-dimensional (3D) laser scanning confocal microscope, etc., are adopted to investigate the variation of mineral components, mechanical properties, and rock microstructure. The XRD results indicate that after the treatment of SC-CO2, the relative contents of carbonate and feldspar minerals in the tight sandstone decrease, while the quartz and clay minerals increase. Through the microstructure analysis, three main mechanisms in the interaction between rocks and brine/SC-CO2 are obtained: strong dissolution of rock structure, new mineral precipitation generation, and CO2 adsorption causing expansion. Moreover, the quantitative topographic results show that the maximum height, root mean square, and fractal dimension of rock surface are decreased to different degrees after SC-CO2 interactions. The SC-CO2 has a deterioration effect on the mechanical properties of rock, and it will further intensify the rock damage with the introduction of water and brine. The porosity of rocks increased slightly by 2.15% under the interaction of dry/SC-CO2, while it decreased, respectively, by 5.4% and 11.64% under the interaction of water/SC-CO2 and brine/SC-CO2. The permeability increases gradually after the interaction of dry/water/brine-SC-CO2. The reduction of mechanical strength induced by SC-CO2 would be conductive to lowering the initiation pressure of rock during the fracturing operation, but it will affect the geological stability of reservoir rock.