A Holistic Overview of the Technical Challenges of CO2 Sequestration in Depleted Hydrocarbon Reservoirs

Abstract While depleted hydrocarbon reservoirs (DHRs) may appear to be optimal for the purpose of geologic carbon (CO2) storage (GCS), their low abandonment pressure poses a substantial challenge. Injecting high-pressure, low-temperature CO2 into a low-pressure reservoir can induce various physical phenomena, particularly in the near wellbore region, including hydrate formation and thermal fracturing. These effects may be exacerbated by the Joule-Thomson (J-T) effect and are of critical importance for ensuring the sustainability of CO2 injectivity and maintaining the integrity of both wells and caprock. The near wellbore as well as integrity issues are crucial in using DHRs for GCS. Near the wellbore, the thermodynamic conflict between the warm low-pressure reservoir and the cold high-pressure CO2 can lead to J-T cooling, potentially forcing the system into the hydrate stability zone. On the other hand, salt precipitation near the wellbore can also act negatively on the CO2 injectivity. Cyclic CO2 injection is likely to exacerbate the situations. The fracturing of the formation as well as well components induced by both thermal and pressure effects can pause a serious threat to the containment of CO2 in the reservoir, especially in depleted gas reservoirs (DGRs). This paper aims to discuss the technical challenges of GCS in DHRs based on the laboratory and numerical works conducted in our Institute. We discuss the formation and equilibrium conditions of the metastable zone of CO2 hydrates formed in salt solutions in a specially designed setup. Dynamic experiments (coreflood) and numerical work on salt precipitation are also elaborated to identify the conditions that may aggravate the near wellbore issues. We report the results of the experiments carried out in a large-scale setup which mimics real wellbore environment in terms of cyclic pressure and temperature conditions. The overall discussion provides a holistic technical view and categorization of the challenges and their potential mitigations that may be useful for the certification and operation of GCS projects in DHRs.