BACKGROUND
Geothermal energy represents an important and largely untapped resource for electricity and clean energy generation in the United States and abroad. Individual geothermal wells can cost $5 million to $20 million depending on depth, yet ~22% of geothermal wells drilled worldwide have been economically unsuccessful. Research to improve site selection and well planning can substantially improve economics of geothermal energy. For geothermal energy extraction to be economically viable, two fundamental subsurface components are required: heat and permeable flow pathways. Faults and fractures are essential fluid flow pathways in many geothermal fields, but their permeability is highly dependent on subsurface stress state. Understanding subsurface stress states and how they evolve during reservoir stimulation (“enhanced geothermal”) and geothermal production with associated reservoir cooling is critical for site selection, well planning and economic feasibility of geothermal projects. Fluid pressure change and heat extraction from geothermal fields are key drivers in stress state change over time that can modify the permeability of faults and fractures in the subsurface. This project leverages the team’s extensive expertise from modeling faulting and fracturing for oil and gas and seismic hazard investigations to implement finite-element geomechanical modeling and resolved stress analysis. This work is performed to explicitly understand key stress drivers and resulting spatial and temporal stress state variations in geothermal reservoirs. Our goal is to provide geothermal operators an approach that reduces risks of locating and designing geothermal wells.
APPROACH
This project aims to advance the understanding of subsurface stress states to reduce one of the key risks of geothermal energy exploration and production: site selection and well planning. This effort leverages our multi-decade experience in structural geology, mechanical stratigraphy, and stress and geomechanical analyses. Two- and three-dimensional geomechanical models are being used to characterize the spatial and temporal evolution of the stress state throughout the problem domain. Simulations use geologically realistic subsurface configurations with different lithologic domains, pre-existing discontinuities, and initial conditions and imposed temperature and pore pressure changes as the primary loads. Key outputs include modeled temporal and spatial evolution of stresses. Slip and dilation tendency analyses using SwRI-developed 3DStress® software provide a rigorous means for assessing the fluid flow potential of discontinuities under applied and dynamic stress conditions. Tracking slip and dilation tendency behavior of known and potential geologic fluid conduits (faults and fractures) during geothermal-field stress evolution derived from geomechanical models represents a major innovation.
ACCOMPLISHMENTS
Project efforts to date include: (i) assessment of the relative importance of key factors (e.g., mechanical and thermal rock properties, pre-existing discontinuities, initial stress state) that influence stress state evolution via a series of 2D and 3D simulations, (ii) slip and dilation tendency analysis of geomechanical model results to characterize the spatial and temporal evolution of subsurface stress states in response to temperature and pore pressure changes from geothermal energy production, and (iii) initial 2D and 3D geomechanical simulations of a geothermal reservoir with an actively producing geothermal power plant in the Basin & Range region of the western United States.