Impact of Shut-In Time on Acid-Etched Fracture Morphology and Conductivity in SC-CO2 Acid Fracturing of Tight Dolomite

Supercritical CO2 (SC-CO2) acid fracturing injects an SC-CO2-acid mixture into carbonate formations to create high-conductivity channels. It retards acid-rock reaction at high temperature, creating long acid-etched fractures and facilitating CO2 transport and storage. Fracture conductivity is critical for fluid transport; however, the influence of shut-in time on this property remains poorly understood. In this study, we conducted SC-CO2 acid-etching experiments and fracture conductivity tests on tight dolomite samples at shut-in times of 0, 30, 60, and 120 min. The interval-averaged acid–rock reaction rates during the 0–30, 30–60, and 60–120 min shut-in periods were 2.23, 3.08, and 2.77 times those during acid injection, respectively. Because temperature increased while residual acid concentration decreased during shut-in, the reaction-rate evolution reflects their combined effects. Fracture morphology exhibited a non-monotonic response to shut-in time. After 30 min, pronounced flow-aligned channels developed along the acid-flow direction, accompanied by a marked increase in surface roughness. At 60 min, channel prominence and roughness amplitude decreased despite continued surface recession, whereas the 120-min fracture exhibited the highest roughness and stronger directional differentiation, with the morphology tending toward flow-perpendicular channels. Fracture conductivity was strongly dependent on closure stress. At 2.5 MPa, conductivity followed the order 30 min > 120 min > 0 min > 60 min, consistent with the ranking of mean and median fracture apertures. In the 25–60 MPa range, the ranking changed to 30 min > 0 min > 120 min > 60 min, indicating the increasing importance of persistent and connected residual flow pathways. Although the 120-min fracture exhibited the highest roughness, its high-stress conductivity remained lower than that of the 30-min fracture, demonstrating that roughness alone is insufficient to determine fracture conductivity. Among the investigated conditions, the 30-min shut-in exhibited the most favorable overall conductivity performance in the 25–60 MPa closure-stress range.