REACTIVE TRANSPORT MODELING OF GEOLOGIC CO2 SEQUESTRATION

Geologic sequestration represents a promising strategy for isolating CO₂ waste streams from the atmosphere. Successful implementation of this approach hinges on our ability to predict the relative effectiveness of subsurface CO₂ migration and sequestration processes as a function of key reservoir and cap-rock properties, which will enable us to identify optimal target formations and evaluate their long-term isolation performance. Quantifying this functional relationship requires a modeling capability that explicitly couples multiphase flow and kinetically controlled geochemical processes. We have developed a unique computational package that meets these criteria by integrating a state-of-the-art reactive transport simulator (NUFT) and supporting geochemical software and databases (SUPCRT92 and GEMBOCHS). We have used this capability to model saline-aquifer disposal at Statoil's North-Sea Sleipner facility during both prograde (active-injection) and retrograde (post-injection) phases of the process.

Simulation results suggest that in the near-field environment of Sleipner-like settings roughly 85% by mass of injected CO₂ remains and migrates as an immiscible fluid phase (subject to hydrodynamic and structural trapping), about 15% dissolves into formation waters (solubility trapping), and <1% precipitates as carbonate minerals (mineral trapping). Although seemingly negligible, mineral trapping has enormous strategic significance. We have identified four distinct mechanisms: siderite-magnesite precipitates within inter-bedded and cap-rock shales, dawsonite cementation occurs throughout the intra-aquifer plume, and siderite-magnesite-calcite rind forms along both lateral and upper plume boundaries. Local porosity and permeability are reduced by each of these mechanisms; however, such reduction is most extreme in clay-rich shales, owing to their relatively large Fe and Mg concentrations. In our 20-year simulations, initial porosity and permeability of the basal cap-rock shale have been reduced by 8% and 22%, respectively. Hence, the strategic significance of mineral trapping: it continuously improves seal integrity and therefore containment of the voluminous immiscible plume and solubility-trapped CO₂.