Reactive Transport Modeling of Geohazards Associated with CO2 Injection for EOR and Geologic Sequestration
Authors: Johnson J.W. , Nitao J.J. , Morris J.P. , Blair S.C. 
Published: October 2008

Abstract
Reactive transport modeling and a constitutive law for effective stress are used to evaluate long-term cap rock integrity as a function of the concomitant geochemical and geomechanical processes triggered by CO2 injection for EOR and geologic sequestration. Shale microfractures are continuously narrowed by mineral dissolution/precipitation reactions, while at first rapidly widened then slowly constricted by pressure and dependent effective-stress evolution. The net effect of these initially opposing contributions to cap-rock permeability may be enhancement or degradation of seal integrity, depending on site-specific hydrological, compositional, geomechanical, and injection characteristics. Subsurface CO2 migration and sequestration processes are reviewed and potential geomechanical deformation of the cap rock is analyzed as a function of reservoir permeability and lateral continuity, two key parameters that distinguish typical EOR and puresequestration settings. A conceptual model that represents geochemical counterbalancing of geomechanical effects as a function of diffusion distance and reaction progress is introduced, providing a theoretical framework for assessing geohazard potential.

Introduction
Offshore injection of CO2 for EOR and geologic sequestration leads to geochemical alteration and geomechanical deformation of the cap rock, enhancing or degrading its seal integrity depending on the relative effectiveness of these interdependent processes. In situations where geomechanical deformation dominates, cap-rock integrity may be significantly compromised, potentially triggering an environmental geohazard in the form of large-scale CO2 release from the target reservoir and ultimately the seabed (worst-case scenario). The evolution of cap-rock permeability during and after CO2 injection can be assessed through reactive transport modeling, an advanced computational method based on mathematical models of the coupled physical and chemical processes catalyzed by the injection event.
Using our reactive transport simulator (NUFT [1]) and supporting geochemical databases and software (SUPCRT92 [2]), we have previously modeled the integrated hydrological and geochemical processes that govern CO2 migration and sequestration during saline-aquifer disposal [3-6]. This work has shown that within shale-capped sandstone reservoirs (e.g., at Statoil's North Sea Sleipner facility) CO2(g)-shale interaction along water-wet fracture walls converts clay-rich assemblages to those dominated by carbonate minerals. A volume increase of 15-20% attends this kinetic process; hence, geochemical alteration tends to reduce the apertures of shale microfractures, thereby reducing permeability and improving seal integrity.
In the present study, we review the reactive transport modeling approach, parametric dependencies of cap-rock integrity, and subsurface CO2 migration and sequestration processes, then address geomechanical deformation of the cap rock, which tends to initially increase then subsequently decrease microfracture apertures. Specifically, we interface the injection-induced pressure evolution at and above the reservoir/shale interface-as predicted from a new series of NUFT simulations-with a simplified constitutive effective stress law, yielding a first-order assessment of dependent aperture evolution due to geomechanical displacement. We then introduce a conceptual model that represents geochemical counterbalancing of this geomechanical effect, which can be used to predict long-term enhancement or degradation of caprock integrity for specific offshore CO2 injection scenarios. Finally, we outline our c