Experimental Investigation on Pore-Permeability Evolution of Tight Carbonate Rocks in SC-CO2 Acidizing

The Lower Paleozoic carbonate reservoirs in the Yan’an Gas Field are characterized by low porosity, low permeability, and strong heterogeneity, which make effective stimulation difficult. Although supercritical CO2 (SC-CO2) acidizing has shown promising results in field applications, the effects of SC-CO2 concentration and acid type on the pore structure evolution of tight carbonate rocks remain poorly understood. In this study, matrix acidizing experiments were conducted using SC-CO2+acid multi-component fluids, and nuclear magnetic resonance (NMR) together with porosity–permeability measurements were employed to characterize the changes in pore structure before and after acidizing. The results indicate that compared with a single acid injection, SC-CO2+acid combined acidizing significantly improves core porosity and permeability. It promotes the dissolution and expansion of small and medium pores, enhances the connectivity of isolated pores, and increases the proportion of medium and large pores along with pore connectivity. SC-CO2 concentration is a key influencing factor: higher concentrations result in stronger dissolution and greater improvements in porosity and permeability. The acidizing effect of gelled acid mixed with SC-CO2 is superior to that of regular acid, attributed to the higher viscosity of gelled acid, which prolongs the acid–rock reaction time, enabling more uniform and sufficient dissolution and thus more significant improvements in pore structure and acid flow capacity. This research provides valuable guidance for the design of SC-CO2 acidizing treatments in carbonate reservoirs.