Experimental Investigation of Enhanced Oil Recovery Mechanism of CO2 Huff and Puff in Saturated Heavy Oil Reservoirs

Due to the significance of carbon utilization and storage, CO2 huff and puff is increasingly receiving attention. However, the mechanisms and effects of CO2 huff and puff extraction in medium to deep saturated heavy oil reservoirs remain unclear. Therefore, in this study, by targeting the medium to deep saturated heavy oil reservoirs in the block Xia of the Xinjiang oil field, measurements of physical properties were conducted through PVT analysis and viscosity measurement to explore the dissolution and diffusion characteristics of CO2-degassed and CO2-saturated oil systems. Multiple sets of physical simulation of CO2 huff and puff in medium to deep saturated heavy oil reservoirs were conducted using a one-dimensional core holder to evaluate the EOR mechanism of CO2 huff and puff. The results demonstrate that the solubility of CO2 in degassed crude oil is linearly correlated with pressure. Higher pressure effectively increases the solubility of CO2, reaching 49.1 m3/m3 at a saturation pressure of 10.0 MPa, thus facilitating oil expansion and viscosity reduction. Meanwhile, crude oil saturated with CH4 still retains the capacity to further dissolve additional CO2, reaching 24.5 m3/m3 of incremental CO2 solubilization at 10.0 MPa, and the hybrid effect of CO2 and CH4 reduces oil viscosity to 1161 mPa·s, which is slightly lower than the pure CO2 dissolution case. Temperature increases suppress solubility but promote molecular diffusion, allowing CH4 and CO2 to maintain a certain solubility at high temperatures. In terms of dynamic dissolution and diffusion, the initial CO2 dissolution rate is high, reaching 0.009 m3/(m3·min), the mid-term dissolution rate stabilizes at approximately 0.002 m3/(m3·min), and the dissolution capability significantly decreases later on. CO2 exhibits high molecular diffusion capability in gas-saturated crude oil, with a diffusion coefficient of 8.62 × 10−7 m2/s. For CO2 huff and puff, oil production is positively correlated with the CO2 injection rate and the cycle injection volume; it initially increases with the extension of the soak time but eventually decreases. Therefore, the optimal injection speed, injection volume, and soak time should be determined in conjunction with reservoir characteristics. During the huff and puff process, the bottom hole pressure should be higher than the bubble point pressure of the crude oil to prevent gas escape. Moreover, as the huff and puff cycles increase, the content of saturates in the oil rises, while those of aromatic, resin, and asphaltene decrease, leading to a gradual deterioration of the huff and puff effect. This study provides a comprehensive reference method and conclusions for studying the fluid property changes and enhanced recovery mechanisms in medium to deep heavy oil reservoirs with CO2 huff and puff.