Matched-pressure coreflooding and mechanistic validation of active carbonated water–gas alternating injection in ultra-tight Tarim sandstone

Ultra-tight sandstones suffer from severe capillary trapping and limited connectivity, making it difficult for conventional floods to achieve meaningful sweep and microscopic displacement. This study benchmarks waterflooding, continuous CO₂ injection, alternating gas–water processes (WAG/GAW), chemical flooding (ASP), and carbonated-water–assisted alternation (CWAG) and introduces an Active Carbonated Water Alternating Gas (ACWAG) sequence using core flood tests on Tarim Basin ultra-tight cores at 60 °C under matched operating conditions. Waterflooding and continuous CO₂ injection recovered 39.1% and 51.0% OOIP, respectively, while alternation improved recovery (WAG 58.4% > GAW 55.5%) and ASP reached 65.5% OOIP. Carbonated-water assistance produced the largest gains, with CWAG1 and the optimized CWAG2 achieving 67.6% and 77.4% OOIP, and ACWAG delivering the highest recovery (81.0% OOIP) and the lowest residual oil saturation (Sor = 14.9%), accompanied by the strongest wettability shift (contact angle reduced to 56°). Differential-pressure trends (0.72–2.34 MPa across modes) and mechanistic tests (CO₂ solubility, IFT, rheology, and contact angle) support the conclusion that the improvements arise from enhanced mobility control, increased capillary number, and altered wettability rather than injectivity impairment. Slug-size sensitivity identified 0.6 PV as the optimal operational point for the leading designs. These results demonstrate that optimized chemical alternation of carbonated water can substantially improve oil recovery in ultra-tight formations.