James W. Johnson¹, Carl I. Steefel¹, John J. Nitao¹, Kevin G. Knauss¹

(1) Lawrence Livermore National Laboratory, Livermore, CA

ABSTRACT: Reactive Transport Modeling of Geologic CO2 Sequestration to Identify Optimal Target Formations: Quantifying the Relative Effectiveness of Migration and Sequestration Processes as a Function of Reservoir Properties

Geologic sequestration represents a promising strategy for isolating CO2 waste streams from the atmosphere. Scientific viability of this approach hinges on the relative effectiveness of CO2 migration and sequestration processes in the subsurface; its successful implementation relies on our ability to predict sensitivity of this migration/sequestration balance to key physical and chemical characteristics of potential target reservoirs. Quantification of this sensitivity reveals geochemical, hydrologic, and structural constraints on maximizing sequestration performance that can be used to identify geologic formations most likely to provide optimal storage capacity and isolation security. We are integrating kinetically-controlled reactive-transport and multiphase-flow simulators (GIMRT, NUFT), supporting geochemical software and thermodynamic/kinetic databases (SUPCRT92, GEMBOCHS), and recent equation-of-state and viscosity formulations for CO2 (Span and Wagner, 1996; Fenghour et al., 1998) to develop a unique modeling capability for identifying optimal target formations in this manner.

Initial benchmark modeling has focused on simulating CO2 injection at the unique Sleipner facility, where properties of the target aquifer and its bounding cap rock are well constrained and location of the migrating CO2 plume after three years of injection has been established. Preliminary results suggest that local permeability structure of the target formation controls CO2 movement by all migration processes (immiscible displacement, gravity segregation, and viscous fingering) and the potential effectiveness of all sequestration processes (structural, solubility, and mineral trapping). For typical sandstone aquifers, at least 90% of the injected CO2 migrates as an immiscible plume; hence, potential structural trapping represents the dominant sequestration mechanism. Intra-aquifer shales retard vertical and promote horizontal CO2 migration, thus expanding the volumetric extent of CO2-aquifer interaction, and thereby increasing the potential effectiveness of solubility and mineral trapping. Relative impermeability of clay-rich inter-bedded and cap-rock shales may be enhanced by carbonate precipitation at the CO2-shale interface, thus improving cap-rock integrity-the most important constraint on long-term isolation performance.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado