Surface Dissolution: Addressing Technical Challenges of CO2 Injection and Storage in Brine Aquifers

Sequestration of CO2 in geologic formations will be part of any campaign to mitigate greenhouse gas emissions. The risk of leakage from the target formation must be weighed against economic feasibilities for this technology to gain stakeholder acceptance. The standard approach to large-scale 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 under typical deep reservoir conditions. This research investigates an alternative storage approach that eliminates the buoyant leakage concern and addresses other injection and storage concerns. The tradeoff for this added storage assurance is increased cost, which is quantified and compared to the standard approach. The prototypical surface dissolution process uses the brine extracted from the target aquifer as a solvent. Captured CO2 is dissolved into this brine in a surface facility prior to injection into the aquifer. The CO2-laden brine is slightly denser than brine containing no CO2, so ensuring the complete dissolution of all CO2 into brine at the surface eliminates the risk of buoyancy-driven leakage. The displacement volume is about 30% smaller than the volume of water displaced in the standard approach, so surface dissolution reduces the overall impact on groundwater resources. However, the operational costs of the surface dissolution process are about 10-18% greater than for the standard process. Also, the capital costs are about 25%-50% greater, mainly because of the larger number of wells needed. In terms of $/t-CO2, the surface dissolution process will be about 20% more than the standard approach to geologic sequestration. This alternative strategy is particularly attractive in shallow aquifers (2,000 to 4,000 ft) which are less likely targets for the standard approach. The potential risk of buoyant migration along faults and the complications of multi-phase flow are not concerns for surface dissolution. Two bulk phase storage and injection issues are discussed in conjunction with the surface dissolution. First, natural geologic uncertainty like fault conductivity poses important risk to buoyant mobile CO2. As time progresses, more fault-prone sites are likely to be selected as the need for storage space increases. In this research, methods currently used to determine fault sealing behavior and studies of petrophysical fault zone descriptions are presented and discussed in detail. Also, during bulk phase CO2 injection, salt precipitation and multi-phase flow effects can reduce the injectivity. An injectivity model was developed to help understand the injectivity mechanisms and determine the potential injectivity reduction because of near wellbore drying and phase interaction.Petroleum and Geosystems Engineerin