Influence of supercritical CO2-water on the micromechanical properties of sandstone

Abstract

The effects of CO2-brine-rock interaction on the physical and macro-mechanical properties of rock have been extensively studied in CO2 sequestration-related research. However, there are few studies focus on mechanochemical effects of the interaction of supercritical CO2 (SC−CO2), water, and rock and its effects on micromechanical properties of sandstone. In this work, we studied the micromechanical mechanism of crack initiation induced by SC−CO2-water saturated sandstone. A micromechanical model including parameters of fracture cohesive strength, friction coefficient, and fracture energy was proposed, which extended the “sliding surface” to include not only the friction, but also the cohesions on the surfaces and the tensile resistance at the crack-tips. To this end, tests of two saturation conditions, water and SC−CO2-water, were conducted on 25 mm diameter by 50 mm length Sichuan sandstone with a porosity of ∼15.57 % for 15 days and 30 days under temperature of 80 ℃ and pressure of 30 MPa. Afterward, samples were subjected to triaxial compression tests with confining pressure up to 24 MPa. The mineralogical alteration and induced crack morphology were examined to better understand the mechanism of mechanochemical coupling on compression failure induced by SC−CO2-water-rock interaction. Experimentally, mineralogical and microstructural changes induced by illite and kaolinite dissolution, weaken the quartz grain contacts in SC−CO2-water saturated sandstone. Compared to water-saturated sandstone, the SC−CO2-water saturated sandstone exhibits a maximum reduction by 18.82 % and 21.21 % in compressive strength and crack initiation stress respectively under unconfined condition. Additionally, reductions of 5%, 50 %, and 37.3 % were observed in friction coefficient, fracture energy, and cohesive strength respectively for SC−CO2-water saturated sandstone. The reductions of these three parameters, especially the fracture energy and cohesive strength, significantly weaken SC−CO2-water saturated sandstone. The results are representative for the partly saturated zone where SC−CO2 is displacing the in-situ pore fluid and could be used to analyze effects of CO2 injection on stability and integrity of storage formation under mechanochemical coupling effects of SC−CO2-water on sandstone.