INL/LTD-16-38121 R1 DE-EE0007159 Geologic Conceptual Model April 2016 DISCLAIMER This information was prepared as an account of work sponsored by an agency of the U.S. Government. Neither the U.S. Government nor any agency thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness, of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. References herein to any specific commercial product, process, or service by trade name, trade mark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the U.S. Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the U.S. Government or any agency thereof. INL/LTD-16-38121 R1 Geologic Conceptual Model James St. Clair, University of Idaho Colleen Barton, Baker Hughes William Hackett, WRH Associates, Inc. Thomas Wood, University of Idaho Mike McCurry, Idaho State University Michael Janis, University of Oklahoma Robert Podgorney, Idaho National Laboratory Mitch Plummer, Idaho National Laboratory Travis McLing, Idaho National Laboratory Richard Smith, Smith Geologic and Photographic Services, LLC Lee Liberty, Boise State University Ahmad Ghassemi, University of Oklahoma Suzette Payne, Idaho National Laboratory John Wellhan, Idaho State University April 2016 Snake River Geothermal Consortium Hosted by Idaho National Laboratory Idaho Falls, Idaho www.snakerivergeothermal.org Prepared for the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Under DOE Idaho Operations Office Contract DE-AC07-05ID14517 EXECUTIVE SUMMARY This document provides a geologic model of the proposed site for the Frontier Observatory for Research in Geothermal Energy (FORGE) on the northern margin of the Eastern Snake River Plain (ESRP). FORGE marks the U.S. Department of Energy’s (DOE’s) largest effort to advance the deployment of enhanced geothermal systems (EGS). EGS has the potential to tap into the earth’s heat through engineered geothermal reservoirs, transforming the energy future for the United States and the world. The FORGE initiative aims to develop methodologies and technologies that will bring this resource into the nation’s energy portfolio. This project is being conducted by the Snake River Geothermal Consortium at the 110-km2 (42.6-mi2) Geothermal Resource Research Area (GRRA) on the Idaho National Laboratory (INL) Site. The goals of our conceptual modeling effort and this report are to (1) develop a set of conceptual model scenarios of the subsurface beneath the GRRA that honor the existing data, (2) demonstrate that the subsurface beneath the GRRA meet the criteria for FORGE, (3) develop a preliminary geomechanical model to assist in well and reservoir development planning, (4) develop a characterization plan to reduce uncertainty in the geologic model and reduce the risks for establishing FORGE, and (5) argue that our proposed site is representative of the EGS potential throughout the ESRP, demonstrating that lessons learned at FORGE may be applicable to a much larger EGS resource. The INL Site, one of DOE’s largest laboratories (2,300 km2 [890 mi2]), is located within the track of the Yellowstone Hotspot. Over the past 17 Ma, as the North American plate has moved southwest over the stationary hotspot, mantle-derived magmas were injected into the upper crust, melting the silicic upper crust and producing numerous rhyolitic eruptions that formed calderas similar to those observed in and around Yellowstone National Park. Today, these calderas in the ESRP and the associated rhyolitic deposits are buried under 1 to 2 km (0.6 to 1.5 mi) of interbedded basalt flows and sediments. Deep boreholes throughout the ESRP document the widespread occurrence of low-permeability hydrothermally altered rhyolites and high heat flows (>110 mW/m2) beneath the basalt layer. These potential EGS reservoir rocks are encountered in every deep well that penetrates through the overlying basalts and sediments on the ESRP. Selection criteria for the FORGE site are a permeability of less than 10−16 m2 (100 μD) and a temperature between 175 and 225°C (347 and 437°F) at a depth between 1.5 and 4 km (4,900 and 13,100 ft). Our target reservoir rocks for EGS at the GRRA include intra- or extra-caldera rhyolite flows or extra-caldera ignimbrite deposits. Permeability measurements have been completed on samples from representative lithologies in deep boreholes near the GRRA. Intra-caldera facies sampled from depths of 1.485 and 3.15 km (4,872 and 10,335 ft) depth have permeabilities of 7 × 10−20 to 1.8 × 10−17 m2 (0.068 to 18.2 μD), and an extra-caldera sample from 1.34 km (4,396 ft) depth has a permeability of 2 × 10−18 m2 (2 μD). Conductive temperature gradients measured in deep boreholes nearest the GRRA range between 44.4 and 76.6°C/km (2.4 and 4.2°F/100 ft) and predict that the 175°C (347°F) isotherm will be encountered between 2.4 and 3.8 km (7,874 and 12,460 ft) depth. Based on all available data and under every potential conceptual scenario, our site meets the temperature, depth, and permeability criteria defined by DOE. We present models of the subsurface that synthesize existing geologic, deep-borehole, and geophysical data. The GRRA sits on the northern margin of the ESRP. As a risk mitigation measure, we developed two geologic structural models that differ in how the geometry of the subsurface is interpreted. One end-member structural model is based on published estimates of caldera boundaries and eruptive volumes. It depicts a system of four nested calderas butted up against the mountains to the north of the GRRA, with rhyolites persisting to a depth of 6.25 km (20,500 ft). The other end-member structural model is based on the attitude of Mesozoic fold hinges north of the GRRA, which record subsidence of the ESRP due to the emplacement of a dense, ~10-km (32,808-ft)-thick, mid-crustal sill. This structural model depicts a uniformly dipping flexural surface defining the lower boundary of the ESRP rhyolites iii and shows extra-caldera rhyolite units thickening to the south. Overlaying the thermal gradients discussed above with the structural scenarios, in all geologic and thermal scenarios, we find an acceptable reservoir volume for FORGE at our site. We developed a preliminary one-dimensional geomechanical model to assess suitability of the site for fracture stimulation by fluid injection and present a workflow for extending this model into three dimensions. The preliminary model is based on rock strength measurements and well logs from a deep borehole known as INEL-1 located ~2.1 km (1.3 mi) east of the GRRA. These data indicate that the relative stress magnitudes are Sv ≥ SHmax > SHmin consistent with a transition between a strike-slip and normal faulting regime. Fracture analysis of ca. 1990 borehole televiewer data indicate that there are multiple fracture populations that can provide a base reservoir volume for stimulation. Finally, we present our characterization plans for Phase 2 of FORGE. These plans are broken into three subphases. Phase 2A is a 4-month period of permitting, planning, and design. Phase 2B is a 12-month period, during which we will characterize our site using a variety of geophysical methods, drill one corehole, and reexamine an existing deep borehole near our proposed FORGE site; the data collected in Phase 2B will be used to refine our conceptual and geomechanical models. Phase 2C is a 9-month period of extensive characterization and testing, during which we will drill a 3- to 4-km (9,842- to 13,123-ft)-deep characterization hole (the “pilot well”) and a shallow 250-m (820-ft) groundwater-supply well. Based on the available data, we conclude that the GRRA meets the DOE requirements for a FORGE site. We find evidence for a low-permeability rhyolite reservoir with temperatures greater than 175°C (347°F) within the target depth range of 1.5 to 4 km (4,921 to 13,123 ft). The available data indicate a well-developed fracture network, providing a base volume for stimulation, and our preliminary geomechanical model indicates favorable conditions for fracture stimulation. Our site has additional favorable characteristics, including an abundance of onsite water from the ESRP aquifer and easy access in all seasons due to its proximity to U.S. Highway 20. iv CONTENTS EXECUTIVE SUMMARY ......................................................................................................................... iii ACRONYMS ............................................................................................................................................... ix 1. INTRODUCTION .............................................................................................................................. 1 2. GEOLOGIC SETTING ...................................................................................................................... 3 2.1 Calderas in the Eastern Snake River Plain ............................................................................... 7 2.2 Geology of the Geothermal Resource Research Area ............................................................ 13 3. SITE SUITABILITY ........................................................................................................................ 15 3.1 Thermal Regime at Target Depth