EGI’s Geothermal Group presented in the 2016 Stanford University Geothermal Workshop February 22–24, where EGI scientists and associated research collaborators Joseph Moore, Stuart Simmons, Phil Wannamaker, Rick Allis, Stefan Kirby, Clay Jones, Christian Hardwick, and Mark Gwynn presented a combined five workshop sessions, including three sessions specific to the U.S. DOE FORGE Utah project.
The Stanford Geothermal Workshop brought together engineers, scientists and managers involved in geothermal reservoir studies and developments to provide a forum for the exchange of ideas on the exploration, development and use of geothermal resources, and to enable prompt and open reporting of progress. The workshop represented an impressive showing of scientists and engineers involved in geothermal reservoir technology today. Among those topics was Enhanced Geothermal Systems (EGS): current and future activities.
Readers may view and download the proceedings from selected workshops by selecting ‘PAPER’ at the end of each abstract.
EGI’s Geothermal Group presented the following (3 sessions):
A Comparison of Two Geothermal Play Fairway Modeling Methods as Applied to the Tularosa Basin, New Mexico and Texas
Gregory Nash, Carlon Bennett, Benjamin Barker, Joseph Moore, Adam Brandt, Stuart Simmons, Brigitte Swanson
Play fairway analysis (PFA) is a term derived from the petroleum industry, where datasets related to charge, reservoir, and seal are integrated into composite risk segment (CRS) layers, which are in turn integrated into a final deterministic PFA model which represents areas within a basin most likely to contain reservoirs, thus reducing exploration and development risk. The Tularosa Basin Geothermal PFA project replaces charge, reservoir, and seal with heat, fault related permeability, and ground water for geothermal exploration CRS development. Quantitative geothermal exploration models have also been developed in the past, but their use has been limited. One such model is created through the application of the weights of evidence (WoE) method, which considers the correlation of evidence layer values with those of training sites located at known geothermal systems and hot springs. This project tested the WoE method alongside the deterministic petroleum industry PFA method to compare the effectiveness of both techniques within the study area.
Supporting data for both PFA analyses consisted of heat flow, temperature gradients, and quartz geothermometers (heat CRS), Quaternary faults and zones of critical stress (fault related permeability CRS), and wells that penetrate ground water and springs (ground water CRS). For the petroleum industry logic PFA, these data were integrated into a final GIS vector based model which identified eight plays. WoE used raster input data and produced a probabilistic raster output, which identified ten plays. Of the 12 total identified plays, six were identified by both methods, two were unique to the deterministic method, and four were unique to the WoE method. Complimentary deterministic and probabilistic certainty maps were produced to help prioritize plays. The highest priority play was McGregor Range at Fort Bliss in Otero County, New Mexico, which was identified by both methods. This play contains the only known geothermal system in the basin. A medium-high priority play was gleaned from the WoE identified plays and a medium priority play from those produced by the deterministic method.
Both methods allowed the delineation of geothermal plays. However, most were low certainty, which was primarily due to data paucity. WoE identified the greatest number of plays; however, it is unknown if its apparent greater sensitivity is real. Play veracity will require additional work. Of the medium to high priority plays, economic analysis indicates that development could take place with reasonably low risk. PAPER
Play Fairway Analysis for Structurally Controlled Geothermal Systems in the Central Cascades Arc-Backarc Regime, Oregon
Philip E. Wannamaker, Andrew J. Meigs, John D. Trimble, Ellen A. Lamont, B. Mack Kennedy, Joseph N. Moore, Virginie Maris, Eric L. Sonnenthal, and Gregory D. Nash
A research team with membership from the University of Utah/EGI, the Oregon State University, and Lawrence Berkeley National Laboratory have carried out a play fairway analysis (PFA) for geothermal resources in the Central Cascades arc-backarc volcanotectonic regime of central Oregon. This is a unique region for geothermal exploration because active Basin and Range (B&R) extension is superimposed upon and contemporaneous with subduction arc magmatism. Cumulative heat flow along the N-S strike grows from negligible values at the north end to >300 MW in the space of ~100 km concentrated in the northern portion of the PFA area. Perhaps surprisingly then, no geothermal resources leading to construction of an electricity-producing power plant have yet been identified in the U.S. Cascades. Succinctly, our PFA adds to understanding of the potential sources of heat and permeability in the region, which are the two principal criteria for establishing a geothermal resource. A constraint of the PFA project was that only existing data could be analysed, albeit using new and state-of-the-art methods. A challenge in this PFA region is the paucity of existing data, presumably related to the lack of historical geothermal development as well as some non-trivial access.
Criteria selected for establishing heat potential include direct heat flow measurements in boreholes, magnetotelluric (MT) low resistivity anomalies, fluid geochemistry, and proximity to recent volcanic eruptions. Permeability is established by fault density, propensity of faulted sub-regions for slip or dilation, and MT low resistivity anomalies. Heat source and permeability potential are expressed in terms of their individual common risk segment maps, with a color scheme using green for most favorable (low risk) and red for least favorable (high risk). Due to data scarcity, we need to mix approaches based upon probability kriging and conceptual global models based on experience in other environments. For dominantly andesitic arc type resources, inferred heat sources and permeability as well as land and transmission access suggest that prospective areas may lie along a NW-SE fault trend passing from Breitenbush Hot Springs area through the Mt Jefferson volcanic edifice into the backarc area, as well possibly as areas nearby to the north of Mt Jefferson. For dominantly extensional type resources, new areas worth examining could lie along diffusely oriented normal faulting extending north of Newberry volcano and into the Warm Springs region. PAPER
Play Fairway Analysis for Structurally Controlled Geothermal Systems in the Eastern Great Basin Extensional Regime, Utah
Philip E. Wannamaker, Kristine L. Pankow, Joseph N. Moore, Gregory D. Nash,
A research team with membership from the University of Utah/Energy & Geoscience Institute, and the University of Utah/Dept. of Geology & Geophysics, have carried out a play fairway analysis (PFA) for geothermal resources in the Eastern Great Basin extensional tectonic regime of western Utah. Here, active Basin and Range (B&R) extension with volcanism having a N-S strike is superimposed upon pre-existing E-W belts of plutonic rocks and large-scale structural lineaments. Cumulative heat flow along the N-S strike of the state totals approximately 5 GW above background stable interior. Three electricity producing power plants currently exist with potential for greatly increasing this number. Our PFA adds to understanding of the potential sources of heat and permeability in the region, which are the two principal criteria for establishing a geothermal resource. An advantage of this PFA region is the relative abundance of existing data, related to a substantial history of geothermal exploration in the region.
Criteria selected for establishing heat potential include direct heat flow measurements in boreholes, magnetotelluric (MT) low resistivity anomalies, fluid geochemistry, and proximity to recent volcanic eruptions. Permeability is established by fault density, by identification of critically stress areas, and MT low resistivity anomalies. Heat source and permeability potential are expressed in terms of their individual common risk segment maps, with a color scheme using green for most favorable (low risk) and red for least favorable (high risk). Due to data quantity, statistical approaches to defining risk, together with good conceptual models of the area, lead to several followup recommendations. Two east-west conductive lineaments, one coincident with the Cove Fort transverse zone and one nearby to the north which we name the Twin Peaks-Meadow zone, appear to be controlling structures for several local fluid and heat upwellings such as Cove Fort, Twin Peaks, and several previously unnamed prospects. Priorities for followup include southward swath extension and detailed fill-in MT coverage, passive helium transect surveying, and detailed structural investigation. PAPER
The FORGE Utah project investigators presented the following (3 sessions):
Enhanced Geothermal System Concept Testing and Development at the Milford, Utah FORGE Site.
Rick Allis, Joe Moore, Stuart Simmons
The Milford FORGE site, located 16 km northeast of Milford City, Beaver County, and 350 km south of Salt Lake City, is ideal as an EGS field laboratory. The site covers an area of about 25 km2 and is underlain by large volumes of granite and gneiss at temperatures in the range of 175 – 225°C at 2 to 4 km depth. A deep exploration well, Acord-1, drilled in 1979 to 3.8 km depth 5 km from the proposed drill site, encountered granite and gneiss at 3.1 km depth and a temperature at total depth of 230°C. The well did not produce fluids and its conductive temperature profile indicates the crystalline rocks at depth are impermeable. Several deep wells are situated between the eastern edge of the FORGE drill site and the 36 MWe Roosevelt Hot Springs geothermal system, located about 4 km to the east.
These wells confirm the near-surface presence of granite and gneiss, high temperatures, and poor permeability at depth, and demonstrate the FORGE site is outside any active hydrothermal system. All of the wells have been extensively logged, with data and cuttings available for further analysis. The existing Acord-1 well, which was plugged and suspended, will be entered and cleaned out to provide open-hole access to hot crystalline rocks prior to the drilling of the FORGE wells. Considerable infrastructure is available near the FORGE site, including power and fiber-optic cable for real-time data streaming, a major paved road, airport, graded dirt roads, a rail line, and supportive private landowners. A motel and eating establishments are available in Milford City and in the larger community in Beaver, Utah, located 35 km further away. The project has secured sufficient groundwater for drilling, stimulation, and heat exchange testing. There are no anticipated environmental constraints with the site, which is adjacent to a 306 MW wind farm, a 240 MW solar PV plant under construction, and several large transmission lines. PAPER
Geophysical Signatures of the Milford Utah FORGE site
Christian Hardwick, Mark Gwynn, Rick Allis, and Phil Wannamaker
The Milford FORGE site is situated over Tertiary-Quaternary granitic intrusions that crop out in the Mineral Mountains, and are at 3.1 km depth in the Acord-1 well in the central Milford valley some 10 km west of the mountains. Modeling of the 20 mgal gravity low caused by the basin fill overlying the granite has been carried out using a decreasing density contrast with increasing depth based on Acord-1 geophysical logs. This shows the granite to dip gently down to the west beneath the surficial fan deposits on the western edge of the Mineral Mountains, but this dip increases at about 5 km forming a more localized basin beneath the center of the valley.
At the proposed deep drilling site, the granite is at about 0.5 km depth. A residual, reduced-to-pole aeromagnetic map is featureless near the proposed site due to the burial depth of the relatively magnetic granite. Long wavelength magnetic high anomalies west of the site may be originating from buried Tertiary andesite which occurs in the Acord-1 well immediately on top of the granite, and also outcrops in the Beaver Lake Mountains 10 miles to the west.
Several magnetotelluric soundings close to the site show the basin fill sediments have a resistivity of about 3 ohm-meters due to both their clay content and to the warm geothermal groundwater they host. This indicates that resistivity surveying may be a valuable technique for mapping the characteristics of the basin fill. A 3-D thermal model is being developed for the site using a combination of over 30 thermal gradient wells and 10 deep wells. Most of the shallow wells show thermally conductive gradients which can be converted to heat flows using characteristic thermal conductivities for the basin fill. The heat flow in the Acord-1 well is 120 ± 20 mW/m2, with heat flows derived from the thermal gradient wells increasing eastwards towards the Mineral Mountains. The Roosevelt hydrothermal system, with temperatures of at least 260°C, occurs along the Opal Mound fault adjacent to the Mineral Mountains, and it had a pre-development heat output of 60 – 70 MWth. Re-evaluation of the temperature data suggests anomalously hot granite occurs over an over an area of at least 100 km2 at 4 km depth. This thermal anomaly appears similar in area to seismic velocity and attenuation anomalies deeper in the crust. The stored heat that is potentially accessible in the granite, if technologies for enhancing and creating permeability can be developed, is gigawatts in magnitude. PAPER
The Geology, Geochemistry, and Hydrology of the EGS FORGE Site, Milford Utah
Stuart Simmons, Stefan Kirby, Clay Jones, Joe Moore, Rick Allis
The Milford FORGE site, located 16 km northeast of Milford City, Beaver County, and 350 km south of Salt Lake City, is ideal as an EGS field laboratory. The site covers an area of about 25 km2 and is underlain by large volumes of granite and gneiss at temperatures in the range of 175 – 225°C at 2 to 4 km depth. A deep exploration well, Acord-1, drilled in 1979 to 3.8 km depth 5 km from the proposed drill site, encountered granite and gneiss at 3.1 km depth and a temperature at total depth of 230°C.
Information about the geological setting of the FORGE site, water-rock interaction effects, and fluid chemistry come from investigations of the Acord-1 well and drill cores cuttings and production fluids from Blundell-Roosevelt steamfield, which is situated ~5 kms to the east. The shallow hydrology is known from a number of distributed ground water wells.
Preliminary results indicate the crystalline rocks that will host the EGS laboratory comprise plagioclase, hornblende, biotite, and magnetite which are weakly altered to illite, chlorite, calcite, epidote, and anhydrite. New analyses indicate the reservoir fluids at Blundell-Roosevelt are largely unchanged since production began in 1984, except for effects of steam-loss and partial inmixing of reinjected waters. The compositions of ground waters from wells, especially Cl/B, reveal a large but shallow outflow zone that extends across the FORGE site, down gradient from Roosevelt hot springs area. These data suggest that ground waters are a viable source of fluid required for stimulation experiments and that the resulting effects of water-rock interaction, notably mineral deposition reducing permeability, will be minimal. PAPER