Thermodynamic stability of residually trapped carbon dioxide in geological carbon sequestration
Keywords: Energy Resources Engineering.

Geological CO2 sequestration is an effective approach to mitigate greenhouse gas emissions by permanently trapping CO2 in the subsurface. In addition to being trapped beneath a seal that prevents upward migration of CO2, 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, nonconvective transport can play an important role in redistributing residually trapped CO2, driven by thermodynamic gradients other than the pressure gradient that causes the bulk flow. However, the mechanisms and time scales for redistribution have yet to be explored thoroughly. In this dissertation, we explore multiple transport mechanisms for nonconvective transport and their impacts on the long-term entrapment for CO2. First, we explore the impact of Ostwald ripening. Ostwald ripening is a pore-scale phenomenon that coarsens a dispersed phase until thermodynamic equilibrium is established. In porous media, past studies using pore-scale modeling of trapped CO2, show that that multibubble equilibrium is possible and likely in complex porous media. Here we develop a new continuum-scale model for Ostwald ripening in heterogeneous porous media. In this model, porous media with two different capillary pressure curves are put into contact, allowing only diffusive flow through the aqueous phase to redistribute a trapped gas phase. Results show that Ostwald ripening can increase the gas saturation in one medium while decreasing the gas saturation in the other, even when the gas phase is trapped in pore spaces by capillary forces. We develop a retardation factor to show that the characteristic time for Ostwald ripening in the CO2-brine system is about 100000 times slower than single-phase diffusion due to the fact that separate-phase gas requires a much larger amount of mass transfer before equilibrium is established. An approximate solution has been developed to predict the timescale for the saturation redistribution between the two media. The model has been validated by numerical simulation over a wide range of physical parameters. Millimeter to centimeter-scale systems come into equilibrium in years, ranging up to 10,000 years and longer for meter-scale systems. These findings are particularly relevant for geological CO2 storage, where residual trapping is an important mechanism for immobilizing CO2. Our work demonstrates that Ostwald ripening due to heterogeneity in porous media is slow and on a similar time scale compared to other processes that redistribute trapped CO2 such as convective mixing. Furthermore, we show that other natural gradients in geologic formations -- hydrostatic pressure, geothermal gradients and capillary heterogeneity -- can also redistribute CO2 by nonconvective transport. Mechanisms for resulting nonconvective 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 time scale for gas phase redistribution is 100000 years/m; for a 100 m thick formation, redistribution is complete after about 10000000 years. Although nonconvective transport is an extremely slow process, it causes local accumulation of the gas phase and in some settings may remobilize the trapped gas phase. Finally, we show that in regions where the gas saturation increases due to nonconvective transport, the residual gas saturation can be exceeded, resulting in remobilization of the gas phase. Mobilized gas accumulates in regions with low capillary pressure. The gas phase in regions with high capillary pressure remains immobile but is depleted through nonconvective transport. In reservoirs with top seals or local capillary barriers, these layers confine CO2 within the low capillary pressure layers to prevent upward migration of the gas phase to caprock. However, in a dipping reservoir, the gas phase that accumulates underneath capillary barriers will migrate up dip, driven by buoyancy. The greater the number of layers in the reservoir, the faster the rate of redistribution and mobilization. In reservoirs completely lacking a top seal or sealing fault, eventually the CO2 would migrate upward until it is completely dissolved or discharged into the vadose zone. In highly heterogeneous reservoirs characterized by short spatial correlation lengths in the horizontal direction, redistribution is largely dominated by nonconvection redistribution processes