The U. S. Department of Energy, Geothermal Technologies Office, announced early in 2014 that $3M would be available to spur geothermal energy development through the application of Play Fairway Analysis (PFA). This technique is used frequently by the oil and gas industry to reduce risk, but has been little used for geothermal exploration. The Energy & Geoscience Institute at the University of Utah (EGI) received three of the PFA awards to test this technique in west-central Utah, Tularosa Basin in New Mexico and Texas, and in the central Cascade Range of Oregon.
Play Fairway Analysis
The petroleum industry has developed the play fairway analysis technique to reduce risk by defining local areas (plays) which have high potential for gas and/or oil production while rejecting larger areas having a higher potential for failure. The PFA technique relies on three factors including (1) the presence of source rock in the subsurface – rock known to produce hydrocarbons, (2) reservoir charge – oil/gas migration from source rocks to permeable rock, such as sandstone, which act as a storage reservoir, and (3) reservoir seal – a cap of low permeability rock on the reservoir preventing leakage. Where all three of these factors are present, plays are identified. More refined and costly exploration techniques can then be applied at these spatially reduced targets with higher expectations of success.
PFA can also be applied to geothermal energy exploration by simply substituting three geothermal system development parameters into the play fairway concept, replacing those used for oil and gas. These include:
Heat of the Earth – the Earth contains a tremendous amount of heat due to internal generative and convective processes accessible for human energy needs.
Accessible fluids – subsurface waters, where available at depth, are heated and can be transferred to the Earth’s surface via wells to be used for direct heating or to generate electricity.
Fracture permeability – open fracture networks must be present in the rock to facilitate the movement and heating of deep subsurface water which can be accessed through well drilling.
To develop the play fairway model, factors related to each of the above parameters are mapped and combined into three layers, known as common risk segments, using a geographic information system. Where all low risk polygons are spatially correlated, a geothermal play is identified (Fig. 1). Details for the EGI PFA projects are detailed below.
Figure 1. Composite risk segments, created from various related factors such as heat-flow and structural geometries known to create permeability, are overlain to create the final play fairway model. The final model is represented by the colors green (low risk), yellow (moderate risk), and red (high risk), Fraser, 2010.
Structurally Controlled Geothermal Systems in the Eastern Great Basin Extensional Regime, Utah
Principal Investigator: Philip E. Wannamaker, Ph.D., Univ. of Utah/EGI
Co-Investigators: Joseph N. Moore, Ph.D, Univ. of Utah/EGI; Kristine L. Pankow, Ph.D, Univ. of Utah/G&G; Gregory D. Nash, Ph.D, Univ. of Utah/EGI
The eastern Great Basin of western Utah contains a prospective play fairway for structurally controlled, hydrothermal high-T, low-T and EGS geothermal resources (Fig. 2). Here, active Basin and Range extension with a north-south strike is superimposed upon pre-existing east-west structural belts of plutonic rocks to set up numerous opportunities for large-scale dilatant structures and excellent reservoir rocks (Wannamaker et al., 2008). To prioritize possible plays, we will analyze available geophysical data and structural information to identify fluid and heat sources and critically stressed rock volumes, and corroborate those with existing geochemistry and well temperatures.
Of high significance to this project is the very large amount of MT data that has been acquired recently, much of which has only received cursory interpretation (Fig. 2). These will be analyzed for adequacy in resolving fluidized volumes and relevant structures at depth through new precise methods of 3D inversion. The structures will be corroborated with new methods of imaging earthquakes and fluid-related seismic swarms (advanced from Arabasz et al., 2007). Our recent experience combining MT, geochemistry and structural geology shows a diagnostic combination of indicators favoring the presence of high-enthalpy geothermal systems (Fig. 3).
The final goal is to define 2–3 areas of increased favorability ranked using a Common Risk Segment approach (Fraser, 2010). In addition, we will incorporate the risk assessment tool GEOFRAT developed for the NGDS by EGI. This project combines world leaders in the respective methodologies, invokes state-of-the-art technology, and supports graduate students and post-doctoral researchers to develop new professional capabilities.
Figure 2. Left: DEM map of western Utah showing extension-dominated physiography. Producing geothermal systems include Roosevelt Hot Springs (RHS), Cove Fort (CF) and Thermo (TH). Important Q volcanic extrusives are Crater Bench (CB), Pavant Butte (PB), Twin Peaks (TP) and Cinder Knolls (CK). Urban centers are Delta (DL) and Milford (MF). Black polygon represents play fairway of this proposal with 5 mgal gravity contours. The central coarser swatch denotes the 3D MT coverage of the Sevier Basin interpreted by Wannamaker et al. (2013a). Upper right: Middle Cenozoic tectonism in Utah is dominated by voluminous plutonism in E-W belts: Tuscarora-Bingham (T-B), Eureka-Tintic (E-T) and Reno-Pioche-Marysvale-San Juan (R-M-S). The latter two are separated by the mid-Utah magmatic gap (MUMG). Lower right: Mid-Miocene to present Basin and Range extension overprints previous tectonic and plutonic episodes creating numerous intersecting trends and magmatic heat sources.
Figure 3. Integrative example (Wannamaker et al., 2013b) – regional MT indicators of high-T upwelling in the western Great Basin confirmed by follow-up examination at prospect scale. ST-DV(PP) is Stillwater-Dixie Valley and power plant. McGinness Hills (MH) was essentially a blind system when reconnaissance work was undertaken. Follow-up structural mapping indicated the setting is one of an accommodation zone between oppositely-dipping normal faults. Well fluid sampling showed anomalous 3He concentrations confirming lower crustal magmatic input. System production is from wells at B, but a large resource may be represented by A.
The Convergence of Heat, Groundwater & Fracture Permeability: Innovative Play Fairway Modelling Applied to the Tularosa Basin
Principal Investigator: Gregory D. Nash, Ph.D., EGI
Co-Investigators: Rasoul Sorkhabi, Ph.D. and Joseph N. Moore, Ph.D., EGI
Business Point of Contact: Carlon R. Bennett, Ruby Mountain, Inc.
The goal of this project is to identify areas with high potential for both low and moderate temperature geothermal resources in the Tularosa Basin in New Mexico and Texas (Fig. 4) through the development of play fairway models, using two different techniques, covering the entire basin. The Tularosa Basin is located in an area of anomalous heat-flow/temperature gradients within the Rio Grande Rift and is home to several military installations that would benefit if new geothermal resources were discovered.
The play fairway models will be developed using only existing data. The first model, a hybrid, will be based on both a data driven technique (weights-of-evidence) and expert knowledge. This will help overcome the possible lack of training sites needed to complete a full weights-of-evidence model. Where appropriate to do so, training sites outside of the study area may be used as well. Second, we will complete a knowledge-based model, which is based solely upon traditional play fairway analysis logic, that will be completed and compared with the hybrid model (Fig. 5). The results will be coupled with a novel reward/failure ratio map and ancillary data to support play identification and entry. EGS potential will also be considered through mapping minerals that affect well stimulation.
Where most of the geothermal exploration models created to date have been raster based, the results of both of our final models will be in a GIS vector format retaining legacy information in an associated table. The most critical success factors will be adequate data collection and the compilation of a spatially accurate and properly processed dataset. An intensive effort has already resulted in the collection of relevant data spatially covering from two-thirds to the entire basin. Additionally, ASTER multispectral satellite imagery has been used to map relative abundance of minerals, including quartz, calcite, and clay to help identify rock units that may be more readily stimulated.
Figure 4. Study area showing military reserves that could be positively impacted by geothermal energy development.
Figure 5. Two modeling methods will be tested (1) a hybrid weights-of-evidence/knowledge based model and a knowledge based model that is more closely aligned with traditional play fairway analysis.
The results of these play fairway analysis studies will help identify new geothermal resources over a significant part of the western United States. This includes a large area of military lands including Fort Bliss, Holloman Air Force Base, and White Sands Missile Range. According to a recent Pew Research report, energy related costs for the U.S. Military total an estimated $4 billion annually. To address these costs, the Pentagon has developed a comprehensive and strategic Master Energy Performance Plan (MEPP) which among other measures, encourages reducing energy demand through conservation and efficiency. The plan also calls for each facility to increase on-site electricity generation with renewable energy with a goal of deploying 3 GWe of renewable energy on military installations by 2025 as a way of reducing costs. The development of new geothermal energy can help the military meet this goal.
Additionally, the benefits to the general public from new geothermal development will include (1) creation of new jobs, (2) increased use of renewable energy, and (3) reduction of pollution which is a growing problem, especially in areas prone to winter inversion such as the Wasatch Front.
Structurally Controlled Geothermal Systems in the Central Cascadia Arc-BackArc Regime, Oregon
Principal Investigator: Philip E. Wannamaker, Ph.D., Univ. of Utah/EGI
Co-Investigators: Joseph N. Moore, Ph.D, Univ. of Utah/EGI; Andrew J. Meigs, Ph.D, Oregon State Univ.; B. Mack Kennedy, Ph.D, E. Sonnenthal, Ph.D, Lawrence Berkeley National Laboratory
Subduction zone volcanic arcs and continental extensional provinces represent the two most productive settings of exploitable geothermal resources. Central Cascadia is a singularly unique region for geothermal exploration because active Basin and Range (B&R) extension is superimposed upon and contemporaneous with subduction arc magmatism (Fig. 6) (Ingebritson and Mariner, 2010; Wannamaker et al., 2014). Thus, the volcanic arc segment and its near back-arc constitute a prospective play fairway for structurally controlled, hydrothermal high temperature, low-temperature and EGS geothermal resources.
Figure 6. Location maps showing proposed project area (cyan insets) (see Wannamaker et al., 2014, for abbreviations . Left (a): GPS geodetic motion estimate arrows are yellow, and faults and rhyolitic volcanism are after Hildreth (2007). Pink bands enclose active terranes Basin and Range and Modoc Plateau. Mt Jefferson is JF. Volcanic chain segments include Central segment of Oregon (CL). Right (b): Enlargement shows faulting in more detail, plus Newberry (NB), Mt Mazama (MZ) and Medicine Lake (ML) volcanic centers. The EMSLAB MT transect is yellow circles and blue squares, and Earthscope MT transportable array (TA) stations are inverted green triangles (Meqbel et al., 2014).
Figure 7. (A) Structural settings of extensional geothermal systems (after Faulds et al., 2013) and examples of structural control on fluid migration in the Central Cascadia fairway. (B) LiDAR topography image over the Belknap and Deer Creek Hot Springs areas. (C) Black Butte (BB), the Metolius Spring, and a series of cinder cones (red arrow) occur where active strands of the Sister’s fault zone truncate the earlier-formed Green Ridge (GR) fault and related splays.
To prioritize possible plays, we will analyze available structural information to identify critically stressed rock volumes and other evidence of fault-controlled fluid migration (hydrothermal and magmatic), and integrate those data with existing geochemistry and conceptual models of volcanic-hosted resources. An especially pertinent available data set is the substantial LiDAR coverage of the area at Oregon State Univ (Fig. 7). Copious spring and well fluid chemistries, including isotopes, will be modeled using new TOUGHREACT capability. Geophysical data (particularly MT) will be collated and analyzed for adequacy in resolving fluidized volumes and relevant structures at depth. In particular, we will draw on recent experience combining MT, geochemistry and structural geology that shows a diagnostic combination of indicators favoring the presence of high enthalpy geothermal systems.
The final goal is to define 2–3 areas of increased favorability deserving of follow-up investigation using a Common Risk Segment approach (Fraser, 2010). In addition, we will incorporate the risk assessment tool GEOFRAT developed for the NGDS by EGI. This project combines world leaders in the respective methodologies, invokes state-of-the-art technology, and supports graduate students and post-doctoral researchers to develop new professional capability.
References
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