Discussion of Techno‐Economic Challenges in Carbon Sequestration

To consume fossil energy and to mitigate global warming, carbon dioxide (CO2) released from burning fossil energy needs to be captured and stored, because it is believed that released carbon will lead to global temperature rise. Most of the CO2 captured is stored in geological formations like saline aquifers and oil or gas reservoirs. This article addresses the challenges in these areas. When an aquifer has a closed boundary, the storage capacity which is the compressibility‐limited incremental storage is about 1% of the pore volume or at most a few percent because of pressure buildup. Because of this limit, many authors argue that actual aquifer boundaries are semi‐closed or open so that the aquifer size is very large or infinite. Such a large aquifer faces the challenges of reservoir characterization, and monitoring, reporting, and verification (MRV), because the large aquifer needs to be characterized, tested, and monitored, and the semi‐closed boundaries also need to be tested for CO2 leakage. To relieve the pressure buildup, production wells are used to simulate the process of CO2 displacing water. Then another challenge emerges from the displacement process: separation of CO2 and water, treatment of a large volume of produced water, and reinjection of CO2. Water disposal can be even more problematic than CO2 emission. Although oil and gas reservoirs are well characterized, their storage capacity is limited. The wells used in oil and gas reservoirs are Class II which does not satisfy the specifications of Class VI used for permanent CO2 storage. CO2 may leak through cement and interfaces of cement with reservoirs and casings, and so forth. The leakage was widely observed in existing abandoned wells and the leak rates varied significantly depending on the well conditions. Although CO2 helps to produce oil and can be stored in a CO2‐enhanced oil recovery (EOR) process, CO2 emitted from capturing CO2 and burning the extra oil is not lower than the amount stored. Because of these challenges and economic hurdles, the executed CO2 storage projects are far fewer than what was planned and needed to achieve the net‐zero target, being about 6%–30%. This article suggests that more fundamental research is needed to study the techno‐economic feasibility to safely store CO2 in geological formations and to estimate the economic burden from CO2 storage.