CO2–brine–rock interactions altering the mineralogical, physical, and mechanical properties of carbonate-rich shale oil reservoirs

The introduction of CO2 in deep formations triggers complex geochemical/geophysical processes, affecting CO2 geological storage safety and enhancing oil/gas recovery. Previous studies have mainly focused on the long-term geochemical reactions in reservoirs with low carbonate content (<30%). However, the short-term CO2–brine–rock interaction in carbonate-rich formations has been ignored. In this study, soaking experiments were conducted, followed by a series of multi-scale tests, to systematically quantify the mineralogical, physical, and mechanical properties of carbonate-rich shale reservoirs after CO2 fracturing and to reveal the alteration mechanisms. The results indicate that the CO2–brine–rock interaction shows a high preference for dissolving carbonate, followed by nonpure feldspars, and then pure feldspars. Carbonate may impede the dissolution of pure feldspars but has little impact on nonpure ones. With carbonate content and reaction time increases, the porosity and permeability were improved by as high as 105% and 382.1%, respectively, while the hardness, tensile strength, and toughness were weakened remarkably. Fracture closure may occur at severely softened fracture areas with a surface hardness lower than 0.07 GPa, when the reaction time lasts more than 168 h. This study implies that a long shut-in stage after CO2 fracturing in carbonate-rich shale reservoirs should be avoided.