Visual Investigation of Phase Evolution and Component Migration in the CO2-Oil-Formation Water Three-Phase System

Abstract CO2-enhanced oil recovery is an important application pathway of carbon capture, utilization, and storage (CCUS). After injection into reservoirs, CO2 interacts simultaneously with crude oil and formation water, leading to complex phase evolution and component redistribution in a three-phase system. In this study, visual PVT experiments were conducted for the CO2-oil-formation water system under two representative configurations, namely direct oil–water contact and oil–water separation. Interface positions, oil phase volume, postexperiment fluid appearance, and oil composition were jointly analyzed to clarify the phase response and component migration behavior under different water contact modes. The phase evolution can be divided into three stages: CO2 dissolution, coexistence of CO2 dissolution and oil component migration, and strong mass transfer. In all systems, the oil phase volume exhibits initial swelling followed by extraction-induced shrinkage. The maximum swelling values are 22.29%, 22.14%, and 13.18%, with corresponding peak pressures of 9.20, 9.00, and 10.00 MPa for the CO2-oil system, direct oil–water contact system, and oil–water separation system, respectively. Formation water modifies the swelling response of the oil phase, and this effect depends on the water contact mode. Oil component redistribution shows a segmented pattern with C15 as the characteristic boundary: C4–C14 components decrease in residual oil but dominate the oil-derived components entering formation water, whereas C15–C36+ components become relatively enriched in residual oil. Direct oil–water contact produces a turbid water phase with dissolved and dispersed contributions, while oil–water separation results in a relatively clear orange-pink water phase dominated by dissolved-state migration. These findings provide experimental insight into multiphase fluid interactions and crude oil component redistribution during CO2 flooding and storage under reservoir conditions.