Fresh Water Generation from Aquifer-Pressured Carbon Storage: Interim Progress Report (2009)

This project is establishing the potential for using brine pressurized by Carbon Capture and Storage (CCS) operations in saline formations as the feedstock for desalination and water treatment technologies including nanofiltration (NF) and reverse osmosis (RO). The aquifer pressure resulting from the energy required to inject the carbon dioxide provides all or part of the inlet pressure for the desalination system. Residual brine would be reinjected into the formation at net volume reduction. This process provides additional storage space (capacity) in the aquifer, reduces operational risks by relieving overpressure in the aquifer, and provides a source of low-cost fresh water to offset costs or operational water needs. Computer modeling and laboratory-scale experimentation are being used to examine mineral scaling and osmotic pressure limitations for brines typical of CCS sites. Computer modeling is being used to evaluate processes in the aquifer, including the evolution of the pressure field. This progress report deals mainly with our geochemical modeling of high-salinity brines and covers the first six months of project execution (September, 2008 to March, 2009). Costs and implementation results will be presented in the annual report. The brines typical of sequestration sites can be several times more concentrated than seawater, requiring specialized modeling codes typical of those developed for nuclear waste disposal calculations. The osmotic pressure developed as the brines are concentrated is of particular concern, as are precipitates that can cause fouling of reverse osmosis membranes and other types of membranes (e.g., NF). We have now completed the development associated with tasks (1) and (2) of the work plan. We now have a contract with Perlorica, Inc., to provide support to the cost analysis and nanofiltration evaluation. We have also conducted several preliminary analyses of the pressure effect in the reservoir in order to confirm that reservoir pressure can indeed be used to drive the reverse osmosis process. Our initial conclusions from the work to date are encouraging: (1) The concept of aquifer-pressured RO to provide fresh water associated with carbon dioxide storage appears feasible. (2) Concentrated brines such as those found in Wyoming are amenable to RO treatment. We have looked at sodium chloride brines from the Nugget Formation in Sublette County. 20-25% removal with conventional methods is realistic; higher removal appears achievable with NF. The less concentrated sulfate-rich brines from the Tensleep Formation in Sublette County would support >80% removal with conventional RO. (3) Brines from other proposed sequestration sites can now be analyzed readily. An osmotic pressure curve appropriate to these brines can be used to evaluate cost and equipment specifications. (4) We have examined a range of subsurface brine compositions that is potentially pertinent to carbon sequestration and noted the principal compositional trends pertinent to evaluating the feasibility of freshwater extraction. We have proposed a general categorization for the feasibility of the process based on total dissolved solids (TDS). (5) Withdrawing pressurized brine can have a very beneficial effect on reservoir pressure and total available storage capacity. Brine must be extracted from a deeper location in the aquifer than the point of CO{sub 2} injection to prevent CO{sub 2} from migrating to the brine extraction well.