Oil recovery from ultra-tight Tarim sandstone using active carbonated water within hybrid alternating-gas sequences: a laboratory study

Improving oil recovery from ultra-tight reservoirs remains a critical challenge due to their inherently low permeability, restricted pore connectivity, and poor sweep efficiency. This study presents a systematic experimental investigation of water-based enhanced oil recovery (EOR) techniques applied to core samples from the Tarim Basin. Coreflooding tests were conducted using conventional methods (waterflooding, CO₂ injection), hybrid strategies including Water Alternating Gas (WAG), Gas Alternating Water (GAW), and Alkali–Surfactant–Polymer (ASP), and advanced chemical-assisted schemes, including four Carbonated Water Alternating Gas (CWAG) variants and a novel Active Carbonated Water Alternating Gas (ACWAG) method. Results showed that ACWAG achieved the highest oil recovery (85.3%) and the lowest residual oil saturation (15.7%), outperforming all other techniques. Key performance indicators such as pressure behavior, wettability alteration, and slug size sensitivity confirmed the importance of injection sequencing and chemical formulation. This study provides new insights into the design of EOR strategies for ultra-tight formations and introduces a promising hybrid approach that could be scaled for field application.