Experimental Study on Co2–Brine–Rock Interaction Altering Mineralogical, Physical, and Mechanical Properties of Carbonate-Rich Shale Oil Reservoirs

The introduction of CO2 in deep formations will trigger complex geochemical/geophysical processes, which affects CO2 geological storage safety and enhanced oil/gas recovery. Previous studies mainly focused on the long-term geochemical reactions in reservoir rocks with a relatively low carbonate content (<30%). However, the short-term CO2–brine–rock interaction in carbonate-rich formations was ignored. In this study, soaking experiments were conducted followed by a series of tests on multiple scales to systematically quantify the mineralogical, physical, and mechanical properties of carbonate-rich shale oil reservoirs after CO2 fracturing and reveal the alteration mechanisms. The results indicate that CO2–brine–rock interaction shows a high preference to dissolve carbonate, followed by non-pure feldspars and then pure feldspars. Carbonate may impede the dissolution of pure feldspars but has little impact on the non-pure ones. With the increase of carbonate content and reaction time, the porosity and permeability were improved, while the hardness, tensile strength, and toughness were weakened remarkably after CO2–brine–rock interaction. Fracture closure may occur at the severely softened fracture areas when the reaction time lasts more than one week. This study implies that a long shut-in stage after CO2 fracturing in carbonate-rich shale oil reservoirs should be avoided