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The sequestration of supercritical carbon dioxide in saline aquifers has been proposed to mitigate global climate change. An important issue is whether it escapes to the atmosphere; chemical retention is permanent in well chosen aquifers. The effects of chemical reactions may take time, so short term containment techniques are needed. Taking advantage of CO2 retention due to permeability hysteresis during water imbibition has been suggested. We analyze supercritical CO2 retention in deep aquifers due to relative permeability hysteresis. This retention is predicted to occur in the capillary dominated regime, which predominates in large regions of the aquifer. We use recent steady state measurements of supercritical CO2 and brine relative permeabilities at high pressure and temperature. The usage of a CT scanner, constant flow rate and pressure boundary conditions contribute to the high quality of these measurements. The analysis is performed by a combination of exact analytic solutions based on fractional flow theory and the method of characteristics, as well as highly accurate modern simulation techniques adapted to take hysteresis into account. This combination is used to analyze the one-dimensional upward flow of fluids, first the injected supercritical CO2, then the injected brine that takes advantage of relative permeability hysteresis to immobilize the CO2. Our analysis finds it advantageous to inject supercritical CO2 at substantial rates before the injection of brine. The injected brine immobilizes the CO2 by means of a hysteretic shock that propagates very quickly, a peculiarity of supercritical CO2 revealed in the experiments, which has no counterpart in oil and is not contemplated in existing commercial simulators. Our analysis can be improved by taking into account supercritical CO2 compressibility, which could conceivably slow the hysteretic shock and reduce its immobilizing efficiency. We believe that considering radial flow away from a well would lead to analogous results through the use of an appropriate similarity variable, but this remains to be done. |
