Interplay Between Permeability Retardation and Capillary Trapping of Rising Carbon Dioxide in Storage Reservoirs

Abstract

Summary The main objective of this work is to understand, by analytical and numerical study, how permeability retardation interacts with capillary-barrier trapping to cause accumulation as carbon dioxide (CO2) migrates upward in saline aquifers during geological sequestration. The study is of one-dimensional (1D) two-phase (CO2 and water) countercurrent flow. The analytical model describes CO2 buoyant migration and accumulation at a “flow-barrier zone” (low permeability) above a “flow-path zone” (high permeability). The relative importance of permeability retardation and capillary trapping is examined under different magnitudes of buoyant-source fluxes and porous-media properties. In the limiting case of zero capillary pressure, the model equation is solved using the method of characteristics (MOC). Permeability-retarded accumulation, induced by the permeability difference between the flow path and the barrier zone, is illustrated through CO2-saturation profiles and time/distance diagrams. Capillary trapping is subsequently accounted for by graphically incorporating a capillary pressure curve and capillary-threshold effect. Results demonstrate that the accumulation contributions from both the permeability hindrance and capillary trapping are convolved at sufficiently large fluxes. At a given time, the total CO2 accumulated by permeability hindrance is greater than that accumulated by capillary trapping, but the former approaches the latter at large time. The low-permeability zone need not be completely impermeable for accumulation to occur. We demonstrate that considering only capillary trapping understates the amount of CO2 accumulated beneath low-permeability structures during significant periods of a sequestration operation.