Pore Structure Evolution of Carbonate Rocks During VES Self-Diverting Acid Treatment

To address the problems of preferential acid channeling, limited treatment coverage, and nonuniform stimulation during acidizing of fractured carbonate reservoirs, a viscoelastic surfactant (VES) self-diverting acid system was investigated with emphasis on the relationship between its microstructure and rheological properties, as well as its effectiveness in core acidizing. The rheological properties and microstructure of the VES self-diverting acid were characterized by rheological measurements and cryogenic scanning electron microscopy. Core-flooding experiments, nuclear magnetic resonance (NMR) T2 spectroscopy, and computed tomography (CT) scanning were conducted to investigate the evolution of pore structure before and after acidizing. The results show that the acid breakthrough volume exhibits a V-shaped trend with increasing injection rate, initially decreasing and then increasing. The optimal injection-rate range under the investigated reservoir conditions was determined to be 1.5–2.0 mL/min. After acid flooding at the optimal injection rates, the higher-T2 peak area increased by 11.97–23.64%, while the lower-T2 peak area increased by 25.72–28.97%. Quantitative CT analysis showed that the fracture volume of all tested cores increased by more than 120%, and dissolution-enlarged zones of different sizes as well as small branching channels were observed in the CT slices. These results demonstrate that VES self-diverting acid can effectively enhance acid coverage and improve stimulation uniformity in fractured carbonate rocks, providing experimental evidence and theoretical support for acidizing-based reservoir stimulation.