Sequestering CO2 as CO2 hydrate in an offshore saline aquifer by reservoir pressure management

Abstract CO2 has been successfully sequestered in aquifers at shallow water depth as supercritical CO2. However, at a water depth larger than 630 m in tropical regions, there exists a hydrate stability zone (HSZ) extending below the seafloor where CO2 and water can exist as solid CO2 hydrate. It is generally believed that CO2 cannot be stored inside the HSZ as formation of CO2 hydrate will impair CO2 injectivity. In this study, we investigate the feasibility of storing CO2 inside this HSZ by reservoir pressure management vis-a-vis the CO2 hydrate formation pressure through the use of water producers and CO2 injectors. We carry out simulations to investigate CO2 storage in three aquifers in tropical waters each with an area of 94 km2 (9.7 km × 9.7 km), thickness of 50 m, porosity of 30% and permeability of 3,000 md. Three confined aquifers are compared. Aquifer 1 is a shallow water aquifer without a HSZ. It has a water depth of 300 m and buried depth of 830 mbsf. Aquifer 2, straddling (60% inside and 40% outside) the HSZ, has a water depth of 800 m and buried depth of 70 mbsf. Aquifer 3, residing inside the HSZ, has a water depth of 800 m and buried depth of 30 mbsf. The reservoir pressure is managed by four corner wells which function either as CO2 injectors or water producers. We simulate CO2 injection into these aquifers with the help of water production to manage the reservoir pressure to stay below either the reservoir fracture pressure or the hydrate formation pressure. For Aquifer 1, our study shows that by reducing the reservoir pressure to below the fracture pressure through water production, up to 6% PV or 61 Mt of CO2 can be stored. The amount of CO2 stored is limited by the amount of water produced and the breakthrough of CO2 in the water producers. On the other hand, by reducing the reservoir pressure to below the hydrate formation pressure through water production, up to 12% PV or 164 Mt and 21% PV or 13 Mt of CO2 can be stored in Aquifers 2 and 3, respectively. Results show that during CO2 injection, CO2 hydrate formation delays CO2 breakthrough and moderates reservoir pressure due to volume shrinkage thus allowing more CO2 to be stored. Furthermore, over half of the CO2 is stored as immobilized CO2 hydrate which also limits post-injection migration of free CO2 and leakage through the caprock. These results demonstrate the potential of storing CO2 in aquifers having a HSZ with important ramifications for CO2 geological storage.