Surface dissolution: minimizing groundwater impact and leakage risk simultaneously

Any substantive campaign to mitigate greenhouse gas emissions must involve sequestration of CO2 in geologic formations. For stakeholders to accept this technology, the risk of leakage from the storage formation must be balanced against the economics of capture and injection. The standard approach to geologic sequestration assumes that CO2 will be injected as a bulk phase into a saline aquifer. The primary driver for leakage in this approach is the buoyancy of CO2 relative to native brine under typical deep reservoir conditions. If no leakage occurs, the primary impact of storage will be the displacement of large volumes of groundwater, equal to the volume of CO2 injected at reservoir conditions. Here we investigate an alternative storage approach that alleviates these concerns. The incremental cost of this approach over the standard approach therefore sets an upper bound on reasonable costs for monitoring and verification of the standard storage scheme and for avoiding groundwater contamination. We analyze a prototypical process in which the brine to be used as solvent is extracted from the target aquifer. Captured CO2 is dissolved into this brine in a surface facility prior to injection into the aquifer. Wells are arranged in a line-drive pattern, i.e. a row of injectors separated from a parallel row of extraction wells. The CO2-laden brine is slightly denser than brine containing no CO2, so ensuring the complete dissolution of all CO2 at the surface eliminates the risk of buoyancy-driven leakage. The pairing of extraction/injection wells controls the plume movement and reduces the likelihood of exceeding pressure limits during injection. The volume occupied by saturated brine is about 5% greater than the volume of the original brine. This incremental volume results in a net displacement volume 40% smaller than the volume of water displaced in when bulk-phase CO2 is injected. Surface dissolution thus reduces the overall footprint of a storage project and reduces impact on groundwater resources. The process can be implemented at a greater range of depths than the standard approach. The operational costs of surface dissolution are estimated to be 20% greater than for the standard process if CO2 capture is accomplished with monoethanolamine. The capital costs are 50% greater, mainly because of the larger number of wells needed. (C) 2008 Elsevier Ltd. All rights reserved