The T+CO2H Code for the Analysis of Coupled Processes, Including CO<sub>2</sub> Hydrate Formation and Behavior, during Sequestration in Saline Geological Formations
George J. Moridis
Abstract
This paper discusses the development of T+CO2H, a numerical simulator that describes all the coupled processes involved in CO 2 sequestration in saline aquifers. An impetus for this study was the investigation of the benefits of hydrate-assisted sequestration (HAS): (i) large CO 2 amounts can be trapped as immobile solid hydrates, which (ii) deteriorate very slowly and (iii) reduce the permeability of the media they impregnate, thus (iv) restricting─and possibly completely eliminating by creating a hydrate caprock─the escape of the underlying CO 2 . The mathematical and numerical basis for the Jacobian-based, fully implicit, multiphase, multicomponent simulator was developed first. All available laboratory data were then used to develop new constitutive equations describing the entire CO 2 –H 2 O phase diagram and the effect of salt on CO 2 hydration. It accounts for heat and up to four mass components (water, CO 2, hydrate, and water-soluble inhibitors such as salts or alcohols) that are partitioned among six possible phases: gas, aqueous, liquid CO 2, ice, CO 2 hydrate, and halite. The phase diagrams identified 40 possible states and the criteria for state changes. The code can handle any combination of the possible hydrate formation and dissociation mechanisms under either equilibrium or kinetic conditions. The code was tested in validation problems that included (a) comparison against the results of an earlier study involving CO 2 sequestration in a saline aquifer and (b) a series of relatively small problems that involved system changes covering most regions of the CO 2 –H 2 O phase diagram. Application tests included large-scale, long-term studies ranging from gaseous CO 2 injection into an onshore saline aquifer to liquid CO 2 injection into an oceanic aquifer. The study also analyzed the feasibility of HAS in onshore and offshore aquifers, and defined the envelope of the rather narrow, geology-imposed depth and temperature range that is conducive for oceanic hydrate formation.