Effects of CO2-brine-rock interaction on porosity/permeability and mechanical properties during supercritical-CO2 fracturing in shale reservoirs

A series of static soaking experiments, combined with the bulk-rock X-ray diffraction and scanning electron microscopy analyses, was conducted to investigate the influences of supercritical CO2 fracturing on the physical- chemical properties of shales. CO2-brine-rock reactions under different conditions, which result in the changes of porosity/permeability and mechanical properties, were investigated in detail. CO2-brine-rock reaction was confirmed to occur rapidly (maybe less than 0.5 h) during supercritical-CO2 fracturing in shale reservoir with high temperature and pressure. Minerals, such as calcite, dolomite, K-feldspar, and albite, were variably dissolved after the soaking of CO2-saturated brine. Mineral composition, reaction time, temperature, pressure, porosity, and permeability evidently influenced rock reactivity. Stronger reaction likely occurs in carbonate-rich shales of relatively high porosity and permeability. Mineral dissolutions resulted in numerous large etched pores, which eventually caused the significant increase in porosity and permeability (up to one order of magnitude), and decline in tensile strength (up to 71.3%) and surface friction coefficient (9.8%). Calcite-filled natural fracture can be corroded more deeply than the matrix, which has the potential to affect the growth behavior of supercritical-CO2-induced fracture when it intersects with the natural fracture.