A Full-Process Experimental Evaluation of Water-Blocking Mitigation in Tight Sandstones by Supercritical CO2 Shut-In Soaking Coupled with N2 Displacement

Abstract

Water blocking severely restricts gas mobility in tight sandstone reservoirs because of capillary-dominated retention of aqueous phases within micro–nanopore systems. This study establishes an integrated experimental framework to quantify the full-process evolution of water blocking─from water invasion and entrapment to gas displacement and supercritical CO2 (SC–CO2) treatment. Vacuum-assisted water saturation, pressure-controlled N2 displacement, and SC–CO2 shut-in soaking experiments were conducted on tight sandstone cores, and multiscale characterization methods, including nuclear magnetic resonance (NMR), microcomputed tomography (micro-CT), powder X-ray diffraction (XRD), and thin-section casting, were used. The results show that water invasion follows a three-stage temporal pattern, with rapid initial imbibition causing most of the water-blocking damage. Conventional N2 displacement effectively removes mobile water from pore domains with NMR-equivalent radii larger than approximately 300 nm, but it fails to mobilize capillary-trapped water in smaller pore domains, yielding displacement efficiencies less than 40%. SC–CO2 shut-in soaking significantly alters the pore network through mineral dissolution, possible wettability-related changes, and enlargement of micro–nanothroat structures, leading to enhanced pore connectivity and reduced capillary resistance. Following SC–CO2 pretreatment, the N2 displacement efficiency increases to 50–70% and the effective porosity shows a 10–20% improvement. The combined SC–CO2–N2 approach has a strong potential for mitigating water blocking in ultratight formations and may provide an experimental basis for future field-scale optimization of gas recovery and CO2-assisted reservoir treatment.