Analysis of a time dependent injection strategy to accelerate the residual trapping of sequestered CO2 in the geologic subsurface

A time dependent injection strategy for greatly accelerating the immobilization of geologically sequestered CO2 is proposed and analyzed. The injection of high density CO2 into a brine aquifer is followed by brine flooding facilitating residual trapping and dissolution of the CO2 on time scales much shorter than those that would occur by natural processes. One-dimensional kinematic wave equations are derived for the two-phase flow of brine and CO2 and for the transport of dissolved CO2. A solution of these equations using the method of characteristics reveals that brine flooding is most effective when the kinematic wave speed of CO2 saturation is higher than the propagation velocity of a shock wave separating the two-phase flow from the native brine. Finite volume simulation using the reservoir simulator TOUGH2 with PetraSIM interface are generally in good agreement with the one-dimensional model, but show that gravitational overriding of the CO2 can become important if the duration of the injection process is too long. Both methods show that brine flooding is able to reduce the mass fraction of mobile CO2 to less than 10% using a volume ratio brine:CO2 of less than 2.75 on time scales comparable to that of the CO2 injection.