Sinking CO2 in Supercritical Reservoirs

Geologic carbon storage is required for achieving negative CO2 emissions to deal with the climate crisis. The classical concept of CO2 storage consists in injecting CO2 in geological formations at depths greater than 800 m, where CO2 becomes a dense fluid, minimizing storage volume. Yet CO2 has a density lower than the resident brine and tends to float, challenging the widespread deployment of geologic carbon storage. Here, we propose for the first time to store CO2 in supercritical reservoirs to reduce the buoyancy-driven leakage risk. Supercritical reservoirs are found at drilling-reachable depth in volcanic areas, where high pressure (p > 21.8 MPa) and temperature (T > 374 degrees C) imply CO2 is denser than water. We estimate that a CO2 storage capacity in the range of 50-500 Mt yr(-1) could be achieved for every 100 injection wells. Carbon storage in supercritical reservoirs is an appealing alternative to the traditional approach. Plain Language Summary Geologic carbon storage, which consists in returning carbon deep underground, should be part of the solution to effectively reach carbon neutrality by the middle of the century to mitigate climate change. CO2 has been traditionally proposed to be stored in sedimentary rock at depths below 800 m, where CO2 becomes a dense fluid, minimizing the required storage volume. Nevertheless, CO2 is lighter than brine in the traditional concept, so a rock with sufficient sealing capacity should be present above the storage formation to prevent leakage. Indeed, one of the main hurdles to deploy geologic carbon storage is the risk of CO2 leakage. To reduce this risk, we propose a novel storage concept that consists in injecting CO2 in reservoirs where the pore water stays in supercritical conditions (pressure and temperature higher than 21.8 MPa and 374 degrees C, respectively) because at these conditions, CO2 becomes denser than water. Consequently, CO2 sinks, leading to a safe long-term storage. This concept, which could store a significant portion of the total requirements to decarbonize the economy, should start being implemented in deep volcanic areas, given that supercritical reservoirs are found at relatively shallow depths between 3 and 5 km. Key Points We propose a novel geologic carbon storage concept that reduces the buoyancy-driven CO2 leakage risk By injecting CO2 in reservoirs where the resident water stays in supercritical conditions, CO2 sinks because it is denser than pore water Supercritical reservoirs are found at relatively shallow depths between 3 and 5 km in deep volcanic areas