Inferring Geothermal Reservoir Processes at the Raft River Geothermal Field, Idaho, USA, Through Modeling InSAR‐Measured Surface Deformation

Abstract

Ground surface deformations detected with interferometric synthetic aperture radar provide valuable information for inferring subsurface reservoir processes that are difficult to observe directly. This study aims at building a reservoir model that honors the available geological, hydrological, and geomechanical data and also produces ground surface deformation consistent with interferometric synthetic aperture radar measurements at the Raft River Geothermal Field. In the thermo-hydro-mechanical coupled model developed, the reservoir deforms as a result of the rock's poroelastic response to changes in hydrologic pressure and thermal expansion/contraction. The results indicate that the observed deformation is the result of pressure decrease in the deep production reservoir and pressure increase in a shallower reservoir that accommodates the injected fluid (likely in the Salt Lake Formation). The combination of the uplift around injection wells with subsidence around the production wells, affected by the Bridge Fault as a flow barrier, creates a complex pattern of surface deformation in which the center of subtle subsidence significantly deviates from the location of the production wells. A parametric study suggests that (a) the Bridge Fault Zone is likely a flow barrier, (b) the surface deformation appears to be insensitive to the presence of the Narrows Structure, and (c) additional flow barriers likely exist to shape the flow system. This case study demonstrates the utility of a high-fidelity forward model that honors available known information and thermo-hydro-mechanical coupled processes in understanding geothermal reservoir characteristics.The inference of subsurface features via InSAR data analysis is typically accomplished through inverse analyses based on analytical continuum mechanics models or fully numerical models. Commonly used analytical models include inflation of a spherical source (Mogi, 1958), a planar dislocation source (Okada, 1992), and a

Surface Deformation and Seismicity Induced by Poroelastic Stress at the Raft River Geothermal Field, Idaho, USA

“…The best-fitting permeability for the injection reservoir is 5.9 ± 1.5 × 10 −13 m 2 in the direction of slow diffusion, which is trending N70°E, and 8.3 ± 2 × 10 −13 m 2 in the direction of fast diffusion, which is trending N20°W. The N70°E permeability is within error from the value of 4.7 × 10 −13 m 2 used by Liu et al (2018), while the N20°W permeability indicates faster diffusion than predicted in their study. While we are unable to directly determine the specific physical mechanism behind our observed anisotropy, we hypothesize that it arises from pre-existing fractures, which are oriented ∼N-S (Liu et al, 2018;Nash & Moore, 2012).…”

Surface Deformation and Seismicity Induced by Poroelastic Stress at the Raft River Geothermal Field, Idaho, USA

“…The number of wells is relatively small and the geological setting is simple and well documented (e.g., Bradford et al, 2013;Nash and Moore, 2012). In addition, a prominent geodetic signal has been previously observed using Synthetic Aperture Radar Interferometry, InSAR (Liu et al, 2018;Ali et al, 2018). Such measurements can indeed provide important constraints on the spatio-temporal evolution of the pressure field and fluid flow in the sub-surface reservoirs (Ali et al, 2018, Hoffmann et al, 2001Chaussard et al, 2014).…”

“…These injecting wells extend to the basement (~1550m depth) but are open at depths greater than 500m. The majority of the reinjected cold water flows into a "thief zone" consisting of ~300m thick permeable layer at 500m depth (Liu et al, 2018;Ahmed et al, 1979;Spencer, 1979;Bradford et al, 2013).…”

Spatio–Temporal Analysis of Deformation at San Emidio Geothermal Field, Nevada, USA Between 1992 and 2010

“…The availability of radar systems with diverse wavelengths and cadences increases the applicability of InSAR to a wide variety of geophysical studies, including monitoring geothermal processes. In particular, differential InSAR (DInSAR) has been widely used to monitor surface deformation at many geothermal sites in the Western U.S. Several studies have used observed subsidence to develop subsurface reservoir models (e.g., [3][4][5]). In addition, data sets of multiple InSAR pairs have been used to determine temporal variations in subsidence at geothermal fields (e.g., [6,7]).…”