Mechanism and Potential of CO2 Injection to Enhance Recovery Rate of Gas Reservoir

Abstract This paper aims to clarify the mechanism and feasibility of carbon dioxide (CO2) injection into carbonate gas reservoirs to enhance recovery and evaluate its potential. Based on this, a theoretical basis for large-scale field tests is provided, and parameter optimization for mine tests is supported. The synergistic application of CO2 capture, utilization, and storage (CCUS) technology and CO2-enhanced gas recovery (CO2-EGR) technology is also explored. With ongoing global warming, the synergistic application of CCUS and CO2-EGR technologies can contribute to global climate governance. Most gas reservoirs in the Sichuan Basin are in the late stages of development. PetroChina has selected the WL gas reservoir to conduct CO2 injection tests to enhance recovery. However, the mechanism and potential of CO2 gas injection into the carbonate gas reservoirs to enhance recovery remain unclear. Through extensive experimental and numerical simulation studies, this research aims to demonstrate the feasibility and clarify the influencing factors of CCUS-EGR, paving the way for practical applications in the field and contributing to the global effort to reduce CO2 emissions. First, we conducted experiments to observe the phase behavior of CO2 displacing methane (CH4) under different temperature and pressure conditions. The experiments were performed under high-temperature- and high-pressure-resistant containers, which could be observed from outside, using high-definition cameras to capture the entire process of gas phase changes. Subsequently, long-core displacement experiments were carried out in a simulated reservoir environment. These experiments involved the displacement of CH4 by CO2 through various injection methods and under different production conditions. The objective was to elucidate the potential and influencing factors of CO2 injection for enhancing recovery under laboratory conditions. Finally, based on the experimental results, comprehensive numerical simulation studies were performed with different injection and production parameters on the mechanisms of CO2 injection to improve recovery and storage in the subject WL gas reservoir. The most reasonable injection and production plans were chosen, and the potential for CO2 injection to enhance recovery and storage in the field-scale WL gas reservoir was clarified. Phase behavior experiments indicate that when CO2 reaches a supercritical state, the CO2-CH4 mixed system presents a vertical distribution of pure CO2, a transition zone, and CH4. However, as the temperature increases, the transition zone continues to expand. The long-core experiment results show that low injection pressure is favorable for natural gas development but increases the length of the transition zone between CO2 and CH4. A higher reservoir pressure during CO2 displacement results in a lower flow ratio that is closer to piston displacement, achieving a higher recovery rate. High injection rates can lead to the rapid accumulation of inlet pressure, thus achieving high-pressure injection effects. Numerical simulation studies can help choose the best CO2 injection and production plan for the WL gas reservoir, which could cumulatively recover about 500 million cubic meters (m3) of natural gas, accounting for approximately 9% of dynamic reserves and about 8% of geological reserves, while storing about 3.6 million tons of CO2.