Brine Films in Reservoir Pores During Geologic CO2 Sequestration

In deep reservoirs used for geologic carbon sequestration, brine films reside along surfaces of pores invaded by supercritical CO2 (scCO2). In these environments, brine films are retained on topographically complex surfaces of mineral grains and intragranular contact regions through the combined influences of capillarity and adsorption. Thus, brine films are important in aqueous phase reactions and chemical transport occurring during geological C sequestration. Moreover, conditions leading to destabilization of adsorbed brine films are important because they lead to dehydration of mineral surfaces and direct scCO2-mineral contact. Work summarized here includes capillary scaling calculations on conditions required for brine film coexistence with scCO2 in reservoir pores, and calculations of electric double layer capillary (disjoining) pressure relations to adsorbed film thicknesses. Capillary scaling was used to predict conditions under which scCO2 invades pores, and to predict upper limits of resulting brine film thicknesses. Developments from electric double layer models are being applied to obtain limits on thicknesses of brine films equilibrated with scCO2. The combination of capillary and electric double layer considerations indicate that adsorbed brine film thicknesses are at most a few tens of nm thick upon initial formation by scCO2 displacement. Higher ionic strength and lower pH in the aqueous phase contribute to thinning of films. On the other hand, capillary effects at grain contacts and on rough surfaces can support much thicker films. This research on the hydraulic properties of brine films is part of the U.S. Department of Energy's Energy Frontier Research Center on Nanoscale Control of Geological CO2 led by Lawrence Berkeley National Laboratory.