Long-Term Redistribution of Residual Gas Due to Non-convective Transport in the Aqueous Phase

Abstract

Geological CO₂ sequestration is an effective approach to mitigate greenhouse gas emissions by permanently trapping CO₂ in the subsurface. A large portion of injected CO₂ is trapped by capillary forces in pores and eventually dissolves into the reservoir brine due to convective mixing to achieve permanent entrapment. In regions where convective mixing is slow, non-convective transport can play an important role in redistributing residually trapped CO₂, but the mechanisms and timescales for redistribution have yet to be explored thoroughly. In previous work, we have shown that capillary pressure difference among residually trapped gas ganglia can induce Ostwald ripening, thereby redistributing the separate-phase gas through diffusion despite the gas phase remaining trapped over the entire course of equilibration. In this study, we show from a thermodynamic point of view that other natural gradients in geologic formations— hydrostatic pressure, geothermal gradients and capillary heterogeneity—can also redistribute CO₂ by non-convective transport. Mechanisms for resulting non-convective transport include molecular diffusion, the sedimentation effect and potentially the Soret effect. Results show that hydrostatic pressure dominates redistribution such that the separate-phase gas is transported upward through molecular diffusion and accumulates under the seal at the steady state. A typical timescale for gas phase redistribution is 10⁵ years/m; for a 100-m-thick formation, redistribution is complete after ∼10⁷ years. Although non-convective transport is an extremely slow process, it causes local accumulation of the gas phase and in some settings may remobilize the trapped gas phase.