Retardation of CO2 Caused by Capillary Pressure Hysteresis: A New CO2 Trapping Mechanism

Abstract

Summary Containment security of geologically stored CO2 is improved substantially through trapping mechanisms. Therefore, to simulate the potential viability of a storage site, it is necessary to account for immobilization processes. In this paper, we focus on a quantitative measure for the contribution of hysteresis in reducing plume transport, with particular emphasis on capillary-pressure-induced migration retardation. Rocks with large pore-body-to-throat-size ratio, or a low permeability, are the best candidates for this mechanism to be operative. In the present work, a self-consistent relative permeability and capillary pressure hysteresis model is incorporated within a simulator. With this model, it is possible to compare and contrast hysteresis-induced retardation to other mechanisms of trapping. The self-consistent parameterization of all of the transport properties is used to quantify sensitivity compactly. The sensitivity of the CO2-plume shape and the amount of CO2 trapped to the strength of the capillary pressure hysteresis is also described. Simulated results show that the CO2-plume shapes with and without capillary pressure hysteresis are significantly different. As expected, capillary pressure hysteresis retards the buoyant transport of the CO2 plume. Although a portion of the CO2 is connected, and therefore not residual, the plume remains immobile for all practical purposes. Also, because of the decreased driving potential, gravity tonguing below the caprock is reduced in comparison to the case without capillary pressure hysteresis, thus suggesting enhanced storage efficiency. However, the total dissolution of CO2 in saline water is reduced because of the reduced contact area with the brine. Thus, one mechanism of containment is offset by the other. Inclusion of accurate hysteresis models is important for qualifying storage sites constrained by spatial-domain limits. It is anticipated that site-acceptability criteria would change as a result of this study, thereby impacting risk evaluation.