Carbon dioxide (CO2) injection into saline aquifers is one of the promising options to sequester large amounts of CO2 in geological formations. During as well as after injection of CO2 into an aquifer, CO2 migrates towards the top of the formation due to density differences between the formation brine and the injected CO2. The timescales of CO2 migration towards the top of an aquifer and the fraction of CO2 that is trapped as residual gas depends strongly on the driving forces acting on the injected CO2.

When CO2 migrates to the top of an aquifer, brine may be displaced downwards in a counter-current flow setting particularly during the injection period. A majority of the published work on counter-current flow settings have reported significant reductions in the associated relative permeability functions as compared to co-current measurements. However, this phenomenon has not yet been considered in the simulation of CO2 storage into saline aquifers.

In this paper we study the impact of changes in mobility for the two-phase brine/CO2 system as a result of transitions between co- and counter-current flow settings. We have included this effect in a simulator and studied the impact of the related mobility reduction on the saturation distribution and residual saturation of CO2 in aquifers over relevant time scales. We demonstrate that the reduction in relative permeability in the vertical direction changes the plume migration pattern and has an impact on the amount of gas that is trapped as a function of time. This is to our best knowledge the first attempt to integrate counter-current relative permeability into the simulation of injection and subsequent migration of CO2 in aquifers. The results and analysis presented in this paper are directly relevant to all ongoing activities related to the design of large-scale CO2 storage in saline aquifers.

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