3D Analysis of the Proposed “EGS Collab” Circulation Experiments in Sanford Underground Research Facility, SD
- A. Kamali (The University of Oklahoma) | A. Ghassemi (The University of Oklahoma)
- Document ID
- American Rock Mechanics Association
- 52nd U.S. Rock Mechanics/Geomechanics Symposium, 17-20 June, Seattle, Washington
- Publication Date
- Document Type
- Conference Paper
- 2018. American Rock Mechanics Association
- 2 in the last 30 days
- 133 since 2007
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ABSTRACT: Geothermal energy from naturally or hydraulically fractured reservoirs is extracted by cold water injection and circulation using injection and production wells. Well spacing, well pattern, and injection-extraction rate schedule are among the most important factors that control the efficiency of heat extraction. Aside from the heat transfer aspect of the process, the hydromechanics of the circulation and fracture stability also needs be considered. Previous work and analysis has been mostly restricted, assuming stationary fractures without the possibility of fracture propagation during the circulation period. This paper addresses these issues by focusing on the hydro-mechanical aspect of the circulation and the possibility of natural fracture propagation in geothermal systems with reference to the planned “EGS Collab” project’s circulation experiment. A fully coupled 3-dimensional model is developed for this study using the displacement discontinuity method along with a finite difference scheme. Fracture propagation is incorporated into the model by enforcing the critical stress intensity condition at the fracture boundary. Simulation results indicate that natural fracture propagation is likely to occur during circulation. Furthermore, fracture opening variation by propagation, in turn, controls the hydro-mechanics of the circulation as it affects the fracture conductivity and the production rate.
Extracting heat from geothermal reservoirs relies on injection of water into deep hot rocks and extracting the heated water via production wells. The injected water flows through natural or hydraulic fractures and extracts heat upon contact with the reservoir’s matrix. The design aspect of circulation has evolved over the past few decades owing to the extensive research on this multiphysics process. Simultaneous injection and extraction of water in geothermal applications involves heat transfer, fluid flow, fracture deformation, leakoff, and possibly fracture propagation. The coupling between these phenomena complicates the study of geothermal reservoirs.
Water circulation is conducted in two general ways: a) simultaneous injection and extraction of water (using multiple wells), b) huff and puff method in which injection and extraction are performed at different time intervals. The pressure transient of the huff & puff method is discussed in several studies using analytical models and numerical models including finite element and boundary elements (Wessling et al., 2009; Mathias et al., 2010; Safari and Ghassemi, 2011; Kamali and Ghassemi, 2018). While Mathias et al. (2010) took a simple uncoupled approach toward the hydromechanical modeling, others (Wessling et al., 2009; Kamali and Ghassemi, 2018; Safari and Ghassemi, 2011) used fully coupled 2- and 3-dimensional hydromechanical models to study the pressure transient in huff and puff tests. The latter used integral equation solution of heat transfer and thermo-poroelasticity within the framework of the formulations developed in Ghassemi et al. (2003), Ghassemi et al. (2005), Cheng et al. (2001), and Zhou et al. (2009).
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