A potentially valuable technology in achieving carbon dioxide emission targets is the development of Carbon Capture and Storage (CCS) technologies, whereby carbon dioxide is injected into underground porous rocks. Porous rocks can potentially sequester CO2 at depth over long periods of time, contained by a low permeability caprock. CO2 storage has a number of benefits, including the extensive characterization of existing reservoirs and availability of pre-existing infrastructure (e.g. wells and pipelines) suitable for adaption to CO2 injection. Geomechanical alterations of an initially fractured reservoir, resulting from poro-thermo-elastic deformation, may cause propagation and reactivation of fractures and faults, affect fracture aperture variation, and lead to dynamic changes of the permeability of the reservoir, and more importantly, of the caprock.
We model fracture growth of discrete fractures based on the finite element-based computation of stress intensity factors at the tips. Fractures are treated as volumetric bodies with variable, geometrically-expressed apertures, and their growth accounts for effects of deformation, fluid pressure and temperature variations within the matrix. Of particular interest is the porothermo-elastic deformation of the reservoir and caprock, and its effect on fracture growth and reactivation of faults. In addition, we model the effects of deformation and reactive transport on contact stress-dissolution aperture variability, and on the connectivity of fracture networks, while quantifying the permeability tensor of the fractured porous medium, taking into account the evolution of fracture geometry, topology, and aperture distribution, as a function of deformation.
We find that relatively coarse tetrahedral meshes can be used to discretize fractured media, yielding modal stress intensity factors (SIF) that are accurate within 2-5%. This allows the combination of mechanical and fluid meshes in the modelling of coupled deformation processes. The growth approach, based on a failure criterion, a Paris-based propagation law, and an energy-based angle criterion, uses the mesh only as an instrument to compute deformation, and significantly reduces the impact of mesh structure and refinement on growth. Fracture growth can be modelled directly as a SIFinformed deformation of a geometry described by a parametric surface, a method that is compatible with yet-to-be-developed threedimensional isogeometric analysis of fractured porous media.
The reactivation of pre-existing fractures and faults in reservoir caprocks can threaten the long-term containment of large quantities of sequestered CO2, which in turn can result in uncontrolled episodes of leakage of greenhouse gases into the atmosphere. Our modeling tools can help mitigate the risk of this occurring.
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