Numerical Simulation Study of CO2 Injection for Enhanced Recovery in Depleted Gas Reservoirs

The Sichuan Basin is rich in gas reservoirs, most of which are in the middle to late stages of development and have significant potential for CCUS (Carbon Capture, Utilization, and Storage). To reduce CO₂ emissions and assess the feasibility of CCUS-EGR, Wells 47 and 67 in the Maokou Formation of the Wolonghe Gas Field were selected for CO₂ injection tests. Drawing on gas storage operation and reservoir development experiences, numerical simulation was used to design parameters such as CO₂ injection mode, timing, and injection-production well pattern. Simulations were conducted on the CO₂ storage mechanism in the depleted gas reservoir of Well 47 and the numerical model study of CO₂ injection and storage at Well 67. Firstly, Well 47 was analyzed to assess changes in reservoir temperature and pressure under varying CO₂ injection rates, timings, and temperatures, revealing the CO₂ migration distribution within the formation. Then, a comparative analysis of Well 67 evaluated cumulative natural gas production under different injection gases, production rates, and injection-production ratios, involving Wells 67, 83, and 47. Finally, the optimal injection-production parameters and CO₂ injection scheme were determined: (1) Well 067-C1 and Well 067-C2 were injected with CO₂ at 150,000 m³/day; (2) Well 47, Well 67, and Well 83 produce gas at 100,000 m³/day; (3) Well 83 and Well 47 were closed immediately after the breakthrough. The scheme can cumulatively recover 530 million m³ of natural gas, accounting for 9.64% of dynamic reserves, accounting for 8.03% of geological reserves, and can store 3.686 million tons of CO₂. This simulation test is the first new energy project of Southwest Oil & Gas Field Company on CO₂ injection to enhance oil recovery. This simulation test provides support for the gas field company to explore the scientific development path of low-carbon new energy through the pilot test of CCUS-EGR.