Investigation on the physics structure and chemical properties of the shale treated by supercritical CO2

Abstract

Shale gas is the second largest source of unconventional fuel in the world. Supercritical carbon dioxide (SC-CO2) fracturing not only allows effectively breaking paper to the shale gas recovery but also could the replace of methane with carbon dioxide. In this method CO2 adsorption in the shale affects the physical and chemical structure of the shale formations. Therefore, the change of reservoir physical properties will affect the efficiency of fracturing. The main objective of this study is to investigate supercritical CO2 (SC-CO2) effects on the physical structure (porosity, deformation, and mechanical properties) and chemical structure (mineral components) of the shale. Four types of experiments were conducted: (1) pore characteristics by low-pressure nitrogen adsorption. (2) the mineral composition of the shale test. (3) The CO2-induced (adsorption-induced and pressure-induced) deformation of the shale teste. (4) The triaxial compressive strength and tensile strength of samples test. The results show that shale pore properties could be change by the SC-CO2. The specific surface of the shale decreased and the porosity and average pore size increased. The mineral content of the shale minerals (except quartz) decreased after SC-CO2 treatment. The samples exhibited swelling strains caused by SC-CO2. At low pressures, shale deformation was mainly caused by adsorption, while deformation at higher pressures was mainly caused by pressure. After the SC-CO2 treatment, the tensile strength, triaxial compressive strength, and elastic modulus of the shale decreased. The strength decreased as the treatment time increased. The changes in the strength of shale could be explained by two mechanisms: the dissolution effect of SC-CO2 and the adsorption/pressure-induced strain. It indicate that supercritical carbon dioxide can change the characteristics of shale reservoir, and it could reduce the fracturing pressure of supercritical carbon dioxide fracturing and enhance the efficiency of fracturing.