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1 Department of Earth Sciences, University of Liverpool, 4 Brownlow Street, Liverpool L69 3GP, UK r.worden{at}liv.ac.uk
2 IHS Energy Group, 5333 Westheimer Road, Suite 100, Houston, Texas 77056, USA
In this paper production geochemical data from oil fields where CO2 has been injected to enhance oil recovery (CO2-EOR) and experimental simulations of this process are reviewed. These data show that over the timescale of days to many years, CO2 injected into the subsurface typically results in the bulk dissolution of carbonate minerals. There is little evidence for the sequestration of the injected greenhouse gas as a solid phase carbonate mineral on the timescale of the CO2-EOR projects or experiments. There is extensive aqueous geochemical, petrographic and core analysis evidence that supports the conclusion that CO2, injected into oil fields to enhance secondary recovery, leads to the bulk dissolution of calcite, dolomite and siderite. Although carbonate dissolution leads to enhanced porosity, the expected commensurate increase in permeability may be offset by the migration of clays, liberated by the action of the acidic water on the rock, with consequent blocking of pore throats. Additionally, injection of CO2 into oil fields can result in asphaltene deposition on mineral surfaces. Such a bitumen coat could ultimately isolate the mineral matrix from injected fluids and insulate the rock to the injected greenhouse gas. Localized precipitation of calcite scale has been reported in the topside facilities of CO2-EOR projects and in the low-pressure region of experimental simulations.
