Mechanism of CO2 Miscible Water-Alternating-Gas Flooding and Optimization of Injection Parameters in Tight Oil Reservoirs

Abstract

Against the backdrop of continuously growing domestic oil and gas consumption demand, achieving economically efficient development of ultra-low permeability reservoirs represents a major strategic requirement for ensuring national energy security and promoting the sustainable development of the oil and gas industry. To address the issues of low efficiency in water flooding alone for ultra-low permeability reservoirs and the insufficient understanding of the mechanisms behind CO2 miscible water-alternating-gas (WAG) enhanced oil recovery, this study takes the ultra-low permeability reservoir in the Qikeshu Oilfield as an example. Through slim-tube experiments, core flooding experiments combined with nuclear magnetic resonance (NMR) testing, and numerical simulation, research was conducted on the minimum miscibility pressure (MMP) of CO2 and crude oil, the microscopic characteristics of CO2 miscible WAG flooding, and the optimization of gas-water cooperative injection and production parameters at the core scale. The research indicates that the minimum miscibility pressure between CO2 and crude oil in the ultra-low permeability reservoir of the Qikeshu Oilfield is 20.6 MPa. The recovery factor of water flooding alone is relatively low, at only 18.36%. In contrast, pure miscible CO2 flooding achieved the highest recovery factor of 71.56%, as no gas channeling occurred due to the strong homogeneity of the core. Overall, the miscible CO2 WAG process with 8 slugs, involving gas injection followed by water injection, yielded the best oil recovery performance. The reason is that the initially injected CO2 slug forms an efficient miscible zone at the displacement front through miscible effects, maximizing microscopic displacement efficiency. Subsequently injected water slugs help control mobility and expand the sweep volume of CO2. Simulation results and experimental findings are mutually verified. The research outcomes can not only provide relevant technical guidance and theoretical basis for the practical field development of ultra-low permeability reservoirs, but also contribute to achieving China’s dual-carbon strategic goals.