Raft River geoscience case study

Abstract

The Raft River Geothermal Site has been evaluated over the past eight years by the United States Geological Survey and the Idaho National Engineering Laboratory as a moderate-temperature geothermal resource. The geoscience data gathered in the drilling and testing of seven geothermal wells suggest that the Raft River thermal reservoir is: (a) produced from fractures found at the contact metamorphic zone, apparently the base of detached normal faulting from the Bridge and Horse We11 Fault zones of the Jim Sage Mountains; (b) anisotropic, with the major axis of hydraulic conductivity coincident to the Bridge Fault Zone; (c) hydraulically connected to the shallow thermal fluid of the Crook and BLM wells based upon both geochemistry and pressure response; (d) controlled by a mixture of diluted meteoric water recharging from the northwest and a saline sodium chloride water entering from the southwest. Although the hydrogeologic environment of the Raft River geothermal area is very complex and unique, it is typical of many Basin and Range systems. 1. bj ii ACKNOWLEDGMENTS This report contains information collected over seven years. Although many people contributed to making this effort successful, it is impossible to acknowledge everyone. We do wish to thank the Depart

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

“…The primary liquid‐dominated geothermal reservoir at the site resides in the approximately 150‐m‐thick layer of Elba Quartzite in the Precambrian basement underlying the Raft River Basin (Dolenc et al, ). The base of the basin is the Salt Lake Formation, a thick (1,200 m on average) layer consisting of Miocene‐Pliocene lacustrine deposits, and volcanic tuffs and flows.…”

“…Gravity and magnetic survey data suggest that the southern end of the Bridge Fault terminates against another major structure, the Narrows Zone (also termed as the Narrows Structure), a northeast‐southwest trending structure within the Precambrian rocks (Bradford et al, ). The deep upflow zone of the geothermal system is speculated to be controlled by the intersection of the Bridge Fault with the Narrows Structure (Dolenc et al, ; Mabey et al, ). The Narrows Structure has been interpreted to be a basement shear, and it appears as an important structural discontinuity that divides the geothermal system into two major compartments (Bradford et al, ) as corroborated by geochemical investigations (Ayling & Moore, ).…”

“…In U.S. Geothermal Inc.'s original development plan, production was to be from the deep Elba Quartzite aquifer, at a depth of approximately 1,100–1,500 m, via four production wells (i.e., RRG‐1, RRG‐2, RRG‐3, and RRG‐5), whereas injection would be to the intermediate aquifer at the depth of approximately 500–700 m, via three injection wells (i.e., RRG‐4, RRG‐6, and RRG‐7) to avoid temperature drawdown of the deeper hydrothermal resource (Covington, ; Dolenc et al, ; Millar, ). Since U.S. Geothermal Inc. acquired the lease, the deep wells were deepened or sidetracked and two additional wells were drilled.…”

“…We simplify the actual geological profile by leveling the strata, as the strata are gently dipping and detailed topography have only minimal effects on ground deformation. As summarized in Table , six horizontal strata of various thicknesses are determined based on data in the literature (Covington, ; Dolenc et al, ; Millar, ; Williams et al, ). The mesh, in the WGS 84/UTM Zone 12T coordinate system, covers an area of 30 km by 30 km to a depth of 2 km.…”

“…The Narrows Structure at the Raft River geothermal field is a geologic feature that has attracted substantial research attention (Bradford et al, ; Dolenc et al, ; Mabey et al, ). It was originally only inferred from a compilation of anomalous data from geophysical surveys that suggest major changes occurring in a northeast trend (Dolenc et al, ).…”