CO2-WAG Enhances Oil Recovery and Controls Gas Channeling in Low-Permeability Reverse-Rhythm Carbonates

Abstract

This study aims to reveal the oil displacement mechanisms and gas channeling behavior of continuous CO2 and CO2-water alternating gas (CO2-WAG) in low-permeability, reverse-rhythm carbonate reservoirs, thus providing a theoretical basis for optimizing development strategies. We designed reverse-rhythm physical core models using similarity criteria and conducted systematic displacement experiments under reservoir conditions. Nuclear magnetic resonance technology enabled quantitative characterization of in-situ oil mobilization across different layers and pore scales during displacement. The approach captured fluid distribution changes before and after gas channeling. Results show that the displacement process of continuous CO2 injection can be divided into four stages: gas-free oil production, initial gas breakthrough, gas channeling development, and complete gas channeling. CO2 flooding achieves a displacement efficiency of 50.4%. In contrast, CO2-WAG injection significantly suppresses gas channeling as water slugs block high-permeability channels, forcing gas into medium-low permeability layers. Consequently, ultimate recovery reaches 58.1%. NMR analysis further demonstrates WAG substantially improves oil mobilization in medium-low permeability layers and micropores (<0.1 μm). This research presents the pore-scale NMR clarification of gas channeling behavior and microscopic oil mobilization mechanisms for CO2 and CO2-WAG in low-permeability reverse-rhythm carbonates. The results show that the CO2-WAG process synergistically enhances oil recovery by improving both macroscopic sweep and microscopic displacement efficiencies. These findings provide new mechanistic insights for controlling gas channeling and enhancing oil recovery in analogous reservoirs.