Multiscale investigation of CO₂ hydrate self-sealing potential for carbon geo-sequestration

Storage of liquid CO ₂ in shallow geological formations is a recently proposed concept that can facilitate increased storage capacity and improved mobility control. If stored below the gas hydrate stability zone (GHSZ), unwanted vertical migration of CO ₂ can be effectively inhibited by the formation of solid hydrate layers. Lowering the risks of CO ₂ leakage to the atmosphere is instrumental to accelerate the implementation of full-scale carbon sequestration in the North Sea and elsewhere. In the laboratory, we have successfully visualized CO ₂ trapping phenomena, measured CO ₂ leakage rates, and demonstrated that the integrity of the hydrate seal strongly depends on fluid-rock interactions and initial water distribution. CO ₂ propagation in water-filled core samples has been monitored over a total of 140 days inside the GHSZ. Solid CO ₂ hydrate formed and sealed the pore space in both homogeneous sandstone and heterogeneous limestone cores. However, the physical flow barrier developed considerably faster in sandstone (after 1.8 pore volumes – PV) compared to limestone (after 7.4 PV), with a factor ten reduced CO ₂ leakage rate through the seal in favor of sandstone. Furthermore, pore-scale images of upward CO ₂ migration verified trapping of CO ₂ both as solid hydrate precipitation and as liquid CO ₂ clusters made discontinuous and stabilized by capillary forces. Small-scale hydrate rearrangement followed initial formation, and caused temporarily dissociation of local hydrate structures without affecting the overall integrity of the seal. Our study suggests that a homogeneous, water-filled GHSZ directly above a CO ₂ storage site can provide a secondary safety mechanism and significantly reduce the risk of CO ₂ leakage.