Multi-Scale Investigation of CO₂-WAG Injection Mechanisms in Water-Sensitive Tight Oil Reservoirs

The CO₂-WAG injection method proposed for unconventional tight oil reservoirs has been successfully applied in field. However, for water-sensitive tight oil reservoirs, there is a lack of research focusing on the displacement mechanisms and effects of WAG injection at the pore scale. And the gap between nuclear magnetic resonance (NMR) and computerized tomography (CT) scanning technologies is not well explored. This study integrated deuterium oxide-shielded NMR and contrast agent (n-butane iodide and lead iso-octanoate mixture) enhanced micro-CT to study the immiscible CO₂-WAG mechanisms for water-sensitive tight oil. The aim is to investigate the microscopic distribution and displacement mechanisms of residual oil after CO₂ flooding and WAG injection in water-sensitive cores. Experimental results showed that NMR experiments and CT experiments identified the microscopic distribution of oil at different injection stages successfully. The contrast agent, despite its similar viscosity ratio, had poor representativeness in other properties, leading to significant differences in the microscopic fluid distribution results compared to NMR experiments. The primary WAG flood mechanism observed was that CO₂ dispersed oil droplets, with most being displaced out of the pores, while water played a mobility control role by blocking large pores. However, due to water sensitivity, medium and small pores also experienced blockage, making the effect of WAG injection after CO₂ flooding not significant. In the NMR experiment, the final recovery of core 6–1 after WAG injection was 64.71%. In the CT scan experiment, due to the smaller volume of the microcore, the water sensitivity effect was relatively weaker, and under the stronger influence of the capillary end effect, the final recovery reached 76.91%. In the five sets of long-core experiments, by controlling the gas–water ratio and slug size, the recovery ranged from 56.51 to 64.04%. The core flood experiments validated the representativeness of the microscopic experiments. And for water-sensitive tight oil reservoirs, controlling the volumes of water and CO₂ during the WAG injection process is crucial.