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. Introduction Residual CO2 saturation is suspected to be a significant factor for reducing injectivity during many water alternating with gas (WAG) processes for CO2 EOR projects.2 Also, there is increasing interest in minimizing CO2 (greenhouse gas) emissions by sequestrating CO2 in waterflooded oil reservoirs or in saline aquifers. Thus, this interest in increasing CO2 injection into geological formations requires a better understanding of mechanisms and extent of CO2 plume development and subsequent dissolution into formation brine. This paper describes laboratory tests on Frio sandstone and Indiana limestone. Several types of displacement tests were performed; gas injection to a pseudo-residual brine saturation with respect to gas injection, followed by brine injection to a pseudo-residual gas saturation with respect to brine injection. The level of CO2 saturation in the injected brine at reservoir pressure and temperature was varied from zero to over 90%. This variation of CO2 saturation in the injected brine was to determine the effect on the CO2 saturation or plume size in the core. Determination of CO2 saturation in a core was sought after injection of CO2 into a core that was originally saturated with brine. This was then followed by the injection of brine into the core while determining when brine displaced free CO2 versus producing CO2 dissolved in the brine. Currently, CO2 is being injected into reservoirs nearing their waterflood economic limit and into aquifers, thus injecting CO2 into geological formations containing high brine saturation. To aid in conformance control and reduce the amount of CO2 required for injection, CO2 and water are alternately injected into oil reservoirs. Also, it is being proposed to inject CO2 into innumerable aquifers for carbon sequestration. Thus tests are required for both the understanding of how brine and CO2 streams flow through porous media and how their mutual solubilities change their saturations with time.