Abstract

The apparent consequences of increased greenhouse gas emissions will encourage increased carbon dioxide (CO₂) injection into oil reservoirs for enhanced oil recovery (EOR) and encourage injection into other geological formations for sequestration.¹ The development of CO₂ plumes and their 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 carbonate rock samples were tested. This variation in CO₂ saturation in the injected brine determined the effect on the CO₂ saturation or plume size in the core. This information can be used in CO₂ EORWAG projects and for carbon sequestration into geological formations.

Injecting CO₂ into brine-saturated sandstone and carbonate core results in brine saturation reduction of 62 to 82% in the various tests. In each test, over 90% of the reduction occurred with less than 0.5 PV of CO₂ injected, with very little additional brine production after 0.5 PV of CO₂ injected. During brine injection, CO2 production was equivalent to the rate expected from brine saturated with CO₂ at reservoir conditions, except for the first ~0.1 PV of the Queen Sandstone CO₂ production. This indicates that in each core at high end-point brine saturation at the tested flow rate (~2 m/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 CO₂ dissolution in EOR and sequestration.