CO2 Transport Mechanisms in CO2/Brine Coreflooding

Abstract

Abstract Concerns for the possible consequences of increased greenhouse gas emissions will undoubtedly increase carbon dioxide (CO2) injection into oil reservoirs for enhanced oil recovery (EOR) and probably into other geological formations for sequestration.1 The development of CO2 plumes and subsequent dissolution into formation brine are essential mechanisms in most sequestration scenarios and could aid in understanding long-term injectivity reduction in EOR WAG (water alternating with gas injection) projects. This paper describes laboratory tests on sandstone and carbonate core samples. Two types of displacement tests were performed; gas injection to a residual brine saturation with respect to gas, followed by brine injection to a residual gas with respect to brine. The level of CO2 saturation in the injected brine at reservoir pressure and temperature was varied from zero to over 90% saturation. Sandstone and limestone rock types were tested. This variation in CO2 saturation in the injected brine was to determine the effect on the CO2 saturation or plume size in the core. This information can be used in CO2-EOR-WAG projects and carbon sequestration into geological formations. By injecting CO2 into brine-saturated Frio sandstone the brine saturation was reduced to 68–71% in each of five tests. In each test, over 90% of the reduction occurred with less than 0.33PV of CO2 injected, with very little additional brine production after 0.5PV of CO2 injected. During brine injection, CO2 production was equivalent to the rate expected from brine saturated with CO2 at the reservoir conditions. This indicates that for Frio core at high irreducible brine saturation at the tested flow rate (~6 ft/day), the CO2 plume was reduced through dissolution, not displacement. With increasing CO2 saturation in the injected brine, the brine volume required to remove (dissolve) the CO2-plume increased proportionally. Results will be used to aid in predicting injectivity in CO2-EOR-WAG operations and CO2 plume migration and dissolution in EOR and sequestration. The principle mechanisms for CO2 saturation reduction are identified.