Phase behavior of light oil reservoirs in Tahe Oilfield during CO₂, CH₄, and composite gas injection

To clarify the phase behavior of light oil when injecting different media, systematic experiments were conducted on Tahe light oil, including high-pressure physical property tests, slim tube tests, and high-temperature high-pressure interfacial tension measurements. The phase behaviors during the injection of CO₂, CH₄, and composite gas (with CO₂/CH₄ volume ratios of 9∶1, 7∶3, and 1∶1, respectively) were comparatively analyzed. The results showed that CO₂ exhibited the strongest solubility in crude oil, promoting the greatest oil swelling, while CH4 showed the weakest swelling effect. Vaporizing miscibility is the key mechanism for achieving miscible displacement, which boosts the compositional convergence between oil and gas through two-way mass transfer. When oil–gas interfacial tension approaches zero, a characteristic “smoke-like” flow pattern emerges, which can serve as an indicator of miscibility. The corresponding pressure is consistent with the slim tube test results, confirming the reliability of IFT method for determining the minimum miscibility pressure(MMP). MMPs for the injected gases are ranked as follows∶ CH₄ (54.84 MPa) > composite gas (52.85 MPa at ratio of 1∶1) > CO₂ (44 MPa). As CH₄ proportion in the composite gas increases, the miscibility becomes increasingly difficult to achieve. In conclusion, CO₂ presents the lowest MMP and the strongest extraction capacity, making it the optimal option among this three media. In composite gas flooding, a CO₂ volume fraction above 70% is required to achieve effective miscible displacement. These results provide a theoretical basis for the development of light oil reservoirs in Tahe Oilfield by gas miscible displacement.