A Novel Multi-Phase Strategy for Optimizing CO 2 Utilization and Storage in an Oil Reservoir

In this paper, an innovative multi-phase strategy is developed and numerically tested to optimize CO 2 utilization and storage in an oil reservoir to support low carbon transition. In the first phase, the water-alternating-gas (WAG) injection is conducted to simultaneously store CO 2 and produce crude oil in the reservoir from the respective injection and production wells. In the second phase, the injection and production wells are both shut in for some time to allow CO 2 and water to be stratigraphically separated. In the third phase, CO 2 is injected from the upper part of the reservoir above the separated water layer to displace water downwards, while fluids continue to be produced in the water-dominated zone from the lower part of the production well. Lastly, the production well is finally shut in when the produced gas–water ratio (GWR) reaches 95%, but CO 2 injection is kept until the reservoir pressure is close to the fracture pressure of its caprocks. The numerical simulations show that implementing the proposed multi-phase strategy doubles CO 2 storage in comparison to applying the WAG injection alone. In particular, 80% of the increased CO 2 is stored in the third phase due to the optimized perforation. In addition, the CO 2 injection rate in the last phase does not appear to affect the amount of CO 2 storage, while a higher CO 2 injection rate can reduce the CO 2 injection time and accelerate the CO 2 storage process. In the proposed strategy, we assume that the geothermal energy resources from the produced fluids can be utilized to offset some energy needs for the operation. The analysis of energy gain and consumption from the simulation found that at the early stage of the CO 2 -WAG phase, the energy gain mostly comes from the produced oil. At the late stage of the CO 2 -WAG phase and the subsequent phases, there is very little or even no energy gain from the produced oil. However, the geothermal energy of the produced water and CO 2 substantially compensate for the energy loss due to decreasing oil production. As a result, a net energy gain can be achieved from the proposed multi-phase strategy when geothermal energy extraction is incorporated. The new multi-phase strategy and numerical simulation provide insights for practical energy transition and CO 2 storage by converting a “to be depleted” oil reservoir to a CO 2 storage site and a geothermal energy producer while enhancing oil recovery.