The risk associated with storage of carbon dioxide in the subsurface can be reduced by removal of a comparable volume of existing brines (e.g., Buscheck et al., 2011). In order to avoid high costs for disposal, the brines should be processed into useful forms such as fresh and low-hardness water. We have carried out a cost analysis of treatment of typical subsurface saline waters found in sedimentary basins, compared with conventional seawater desalination. We have also accounted for some cost savings by utilization of potential well-head pressures at brine production wells, which may be present in some fields due to CO 2 injection, to drive desalination using reverse osmosis. Predicted desalination costs for brines having salinities equal to seawater are about half the cost of conventional seawater desalination when we assume the energy can be obtained from excess pressure at the well head. These costs range from 32 to 40¢ per cubic meter permeate produced. Without well-head energy recovery, the costs are from 60 to 80¢ per cubic meter permeate. These costs do not include the cost of any brine production or brine reinjection wells, or pipelines to the well field, or other sitedependent factors.Keywords: carbon capture and storage, desalination, brines, reverse osmosis, osmotic pressure, produced waters 3
IntroductionA major risk associated with geologic sequestration of carbon dioxide is the buildup of pressure in the subsurface due to injection. The maximum sustainable pressure is limited by the ability of the cap-rock to contain the CO 2 as a low permeability barrier, and also the potential for the overpressure to create new fractures, or to reactivate existing fractures, which would then open new flow paths for CO 2 escape (Rutqvist et al., 2007). Moreover, emplacement of CO 2 could drive the existing brine into useful aquifers where potable water would be contaminated, and could also increase the likelihood of induced seismicity (Bachu, 2008).A method to avoid these potential problems is to withdraw the saline fluid existing in the subsurface and thus reduce the amount of overpressure. However, the outstanding problem with this approach is the disposition of the withdrawn fluids. Because of their high salinities (>10,000 mg/L TDS), these fluids are generally not useful for either domestic or agricultural use. The large volume of CO 2 to be emplaced implies a similarly large fluid volume must be produced. Such a large volume of brine cannot be disposed of as-is without significant cost.Desalinating the produced brines using membrane-based technologies is a potential solution that would allow brine withdrawal, with disposal partly accounted for by production of useful low salinity water. Moreover, some of the energy needed to drive desalination could be obtained directly from the overpressure present in the subsurface generated by the emplacement of CO 2 for some sequestration sites. This would allow a re-capture of some fraction of the energy used to pressurize and inject CO 2 into the subsurface to...
