Practical Problems with Carbon Storage in Geological Formations─A Mini Review

To mitigate global warming, released CO2 must be sequestered in geological formations. This review mainly discusses the practical problems associated with CO2 storage in aquifers and hydrocarbon reservoirs. A very large open aquifer faces problems associated with reservoir characterization and monitoring, verification, and accountability or monitoring, reporting, and verification. A finite aquifer with semi-closed boundaries faces the challenges to ensure brine but not CO2 will flow through the boundaries. A finite aquifer with closed boundaries will have a pressure buildup problem. To relieve pressure buildup, some authors have used injection and production wells to simulate the process of CO2 displacing water, simulating the aquifer as a very large or infinitely large one. If brine is produced from aquifers, we will face problems of brine disposal and treatment of brine with high salts, heavy metals, and radioactive materials. The existing Class II wellbores in hydrocarbon reservoirs need to be retrofitted to Class VI. Sometimes retrofitting can cost more than drilling new Class VI wells, and permitting takes a longer time. High CO2 residual saturations reported in the literature may be due to experimental and modeling limitations. The correct residual saturation should not be the one in a swept zone; instead it should be the one within the swept zone where no movable CO2 exists. It is believed that residual trapping is permanent. However, if leak causes the pressure down, some trapped gas bubbles will expand and connect each other to form movable gas clusters, or connected clusters become disconnected. Storage of CO2 in hydrocarbon wells can occur only after tens of years of production when the hydrocarbon reservoir is depleted. CO2 may leak through cement, the interfaces of casing-cement and cement-formation rock, and intermediate formations that connect cement and the surface. Although CO2 will emit from burning oil produced by CO2-enhanced oil recovery (CO2 EOR), this CO2 should not be accounted for toward additional CO2 emitted because oil must be produced to meet our current need in one way or another. Therefore, the CO2 stored in a CO2 EOR process can be accounted for the CO2 storage capacity. Owing to these problems and financial limitations, the executed CO2 storage projects are far less (about 10–20%) than planned or needed for reaching the net-zero target.