Numerical analysis of CO2-water alternating injection effects on heat extraction and carbon storage in plume geothermal systems

In the context of the global energy transition and carbon neutrality, CO2 plume geothermal systems, as a CCUS technology that can synergistically achieve geothermal development and carbon storage, show broad application prospects. This study focuses on the Quantou Formation reservoir in the Songliao Basin. A three-dimensional heterogeneous model is constructed to compare four injection schemes: pure CO2 continuous injection, CO2-water alternating injection with 1-year cycles, CO2-water alternating injection with 2-year cycles, and CO2-water alternating injection with 3-year cycles. The impact of different injection rates on the system's heat extraction and storage performance is analyzed. The results indicate that the alternating injection strategy can significantly improve the overall system performance. At a flow rate of 40 kg/s, the system lifespan is extended to 26.8 years, an increase of 25.8% compared to pure CO2 injection; at a flow rate of 50 kg/s, the cumulative heat extraction reaches 2.83 × 1015 J. Thermal quality analysis shows that 30 kg/s is the optimal flow rate for intermittent injection, achieving a balance between thermal extraction and sustainability. In terms of CO2 storage, pure CO2 injection has a faster short-term storage rate, storing 1.5 × 1010 kg over 15 years at 50 kg/s, whereas alternating injections, although initially lagging, achieve a comparable long-term storage volume. However, alternating injection can cause periodic fluctuations in reservoir pressure, and the local high pressure formed during the water injection phase may affect the integrity of the caprock.