Subsurface Characterization for

Evaluating Geothermal Resource

Potential from Existing Oil and Gas
Wells in Tuttle, Oklahoma
Preprint
Hyunjun Oh,1 Sertac Akar,1 Estefanny Davalos Elizondo,1
Cesar Vivas,2 and Saeed Salehi2
1 National Renewable Energy Laboratory
2 University of Oklahoma
Presented at 2023 Geothermal Rising Conference
Reno, Nevada
October 1-4, 2023

NREL is a national laboratory of the U.S. Department of Energy Conference Paper

Office of Energy Efficiency & Renewable Energy NREL/CP-5700-86947
Operated by the Alliance for Sustainable Energy, LLC October 2023
This report is available at no cost from the National Renewable Energy
Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

 Subsurface Characterization for
Evaluating Geothermal Resource
Potential from Existing Oil and Gas
Wells in Tuttle, Oklahoma
Preprint
Hyunjun Oh,1 Sertac Akar,1 Estefanny Davalos Elizondo,1
Cesar Vivas,2 and Saeed Salehi2
1 National Renewable Energy Laboratory
2 University of Oklahoma

Suggested Citation
Oh, Hyunjun, Sertac Akar, Estefanny Davalos Elizondo, Cesar Vivas, and Saeed Salehi.
2023. Subsurface Characterization for Evaluating Geothermal Resource Potential from
Existing Oil and Gas Wells in Tuttle, Oklahoma: Preprint. Golden, CO: National
Renewable Energy Laboratory. NREL/CP-5700-86947.
https://www.nrel.gov/docs/fy24osti/86947.pdf.

NREL is a national laboratory of the U.S. Department of Energy Conference Paper

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Laboratory (NREL) at www.nrel.gov/publications. 15013 Denver West Parkway
Golden, CO 80401
Contract No. DE-AC36-08GO28308 303-275-3000 • www.nrel.gov
 NOTICE

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provided the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Geothermal
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 Subsurface Characterization for Evaluating Geothermal
Resource Potential from Existing Oil and Gas Wells in
Tuttle, Oklahoma
Hyunjun Oh1, Sertaç Akar1, Estefanny Davalos Elizondo1, Cesar Vivas2, and Saeed Salehi2
1
National Renewable Energy Laboratory, Golden, Colorado
2
Mewbourne School of Petroleum and Geological Engineering, University of Oklahoma

Keywords
Geothermal energy, subsurface characterization, geothermal resource assessment, hydro
geochemistry, geothermometer, repurposing oil and gas wells

ABSTRACT

Oil and gas (O&G) wells often encounter co-produced hot water, possibly suitable for geothermal
direct-use applications. The City of Tuttle is located on the eastern part of the Anadarko
sedimentary basin in Oklahoma with high heat-in-place potential and recovery capability at depth.
This study aims at demonstrating the potential of geothermal energy production for direct-use
applications in two public schools and 250 nearby houses in Tuttle via repurposing existing O&G
wells. In this scope, geochemistry, geology, and borehole log data were collected and incorporated
into a 3D conceptual subsurface model. A digital elevation model (DEM) was used to represent
the study area topography with four O&G wells. In addition, hydrogeochemical characteristics of
the geothermal fluid and scaling potential were analyzed using ternary diagrams and chemical
ratios to develop mixing models. The subsurface geology model indicated that the study area
primarily consists of Permian to Mississippian Sandstone and Limestone formations, implying a
porosity ranging between 12% and 22%, and a permeability up to 3.90E-14 m2 in certain reservoir
levels. The reservoir temperature is expected to be ranging between 80°C to 95°C around 3 km
depth with an average temperature gradient of 22.8 °C/km. Chemical geothermometers also
estimated the reservoir temperature as 90°C. Findings of the chemical model demonstrated that
the geothermal fluid is Sodium-Potassium-Chloride-Sulfate type and possibly mixed with shallow
groundwater resulting in higher Ca and Mg concentrations and lower Na/K ratio implying lower
calcite scaling. These results comprehensively characterize the potential of geothermal resources
in the study area and imply that geothermal energy production by repurposing existing O&G wells
is suitable for low-temperature direct-use applications.

1. Introduction
According to the U.S. Energy Information Administration (EIA), the Oklahoma hydrocarbon field,
including the Anadarko basin, has been exploited for over a century to produce oil and gas (O&G),
and Oklahoma was the nation’s fifth-largest producer of marketed natural gas and the sixth-largest
producer of crude oil in 2021 (EIA, 2022). The EIA also reported about 40,000 natural gas
producing wells in Oklahoma in 2020. In addition to the EIA’s report, O&G Conservation at

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 Oklahoma Corporation Commission (OCC) identified that Oklahoma has more than 443,000 O&G
wells, including plugged, temporarily abandoned, and terminated wells, at a broad range of depths.
Due to the Earth’s internal heat (e.g., heat generated from decay of naturally occurring radioactive
elements, magma chamber, and latent heat from crystallization of molten outer core) and crustal
heat flow, ground temperature increases with depth, and thus the O&G wells often encounter co-
produced hot water possibly suitable for various geothermal direct-use applications, such as
heating and cooling in residential and commercial buildings, schools, and greenhouses. Previous
studies (e.g., Bu et al., 2012; Caulk and Tomac, 2017; Nian and Cheng, 2018; Kurnia et al., 2021)
described that repurposing of existing O&G wells for geothermal energy production is feasible,
without drillings, at lower cost and seismic risk than conventional enhanced geothermal system
(EGS) and borehole heat exchanger (BHE). Particularly, deep sedimentary layers at depths of 2.5
km to 4 km with normal geothermal temperature gradients (e.g., 30 °C/km) can be exploited for
low-temperature energy conversion systems without hydraulic fracturing (DiPippo, 2012).
As O&G wells have been drilled, regardless of the subsurface temperature, mostly into
sedimentary basins where geothermal resources may be limited (e.g., hot rocks at accessible
depths), certain conditions are needed to convert existing O&G wells to geothermal wells. For
example, the Department of Energy (DOE)’s GeoVision study described that the geothermal
reservoir requires a large volume with distributed fractures for the geothermal energy production
over long periods (i.e., relatively lower energy density of hot water), while the O&G reservoir
volume is limited around the boreholes for O&G production in relatively shorter periods (i.e., high
energy density of hydrocarbons) (DOE, 2019). In addition, the wellbores repurposed for
geothermal energy production should have sufficient depths, specifically with a minimum depth
of 2.4 km to 3 km depending on the geothermal gradient and geological formations (Bu et al.,
2012; Cheng et al., 2014). To ensure high outlet temperature from the O&G wells for heating
applications, additional top boiler and insulations are also suggested by Kujawa et al. (2005) and
Gharibi et al. (2018), respectively. Furthermore, the repurposed geothermal wells should be close
to the end users to minimize heat losses from the distribution pipes (Kurnia et al., 2021).
The OCC Oil and Gas Conservation demonstrated that there are more than 100 O&G wells in
Tuttle, Oklahoma, with bottomhole depths approximately from 1 km to 3.5 km. In other words,
the O&G wells in the Tuttle area may have the potential of geothermal energy production from
relatively deeper bottomhole depths to possible end-users at a close distance, as well as economic
benefits from ‘no drilling’. In this study, subsurface geochemistry, formation, and temperature
distribution in southern Tuttle were characterized to demonstrate the feasibility of geothermal
energy production from existing O&G wells for direct-use applications in nearby primary and
secondary schools and 250 houses. Four wells with bottomhole depths from about 3.3 km to 3.6
km and a close distance approximately one mile away from the elementary school were selected
for this study (Figure 1).

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 Figure 1. Study area with four oil and gas wells and two schools (modified from USGS topography map)

2. Literature Review for Subsurface Geology and Temperature in the Study Area
The study area, including the targeted four wells, two schools, and houses, is in the eastern part of
the Anadarko basin (Figure 2). Anadarko basin is a sedimentary basin, which extends along
Oklahoma, Texas, Kansas, and Colorado. Geological formations in the study area specifically
include Permian red shales and sandstones that are relatively younger formations around 1 km to
1.5 km depth and Pennsylvanian and Mississippian sandstones and limestones that are relatively
older formations around 1.5 km to 3 km depth and below 3 km, respectively. Crystalline basement
is expected below 5 km depth (Johnson and Luza 2008; Clement 1991). According to Clement
(1991), most of the oil and gas wells in the Anadarko basin had penetrated Lower Pennsylvanian
Springer or the underlying Mississippian Chester. Similarly, borehole logs obtained from the four
targeted wells demonstrated a wide range of formations from Tonkawa sandstone around 2 km in
depth to Hunton limestone around 3.5 km in depth where oil and gas have been mainly produced.

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 (a)

(b)

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 Figure 2. Geological characteristics in the study area: (a) geological map, (b) Anadarko basin cross-section
(modified from Johnson and Luza 2008), and (c) generalized stratigraphic column and producing fields
of Anadarko basin (Clement 1991). The depths of 1) Permian, 2) Pennsylvanian, 3) Mississippian,
Devonian, and Silurian, 4) Ordovician and Cambrian, and 5) pre-Cambrian systems in the study area
are approximately 3,000 ft (914.4 m), 10,000 ft (3,048 m), 13,000 ft (3962.4 m), and 17,000 ft (5181.6 m),
respectively.

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 Porro et al. (2012) estimated geothermal resources in 15 major sedimentary basins in the United
States, including the Anadarko basin, based on the volume of rocks for each 10 °C temperature
interval. The analysis results demonstrated that the Anadarko basin has a strong hydrothermal
recharge rate directly addressing the recovery capability of geothermal resources and has the
second highest total heat among the 15 sedimentary basins with a large rock volume of more than
20,000 km3. Moreover, Anadarko basin contains geothermal energy potential at temperatures
greater than 220 °C, while most of the sedimentary basins have relatively low thermal energy at
temperatures between 100 °C to 150 °C (Figure 3(a)). With a geothermal gradient of 34 °C/km,
Tuttle area is expected to have geothermal energy resources ranging from 150 °C to 200 °C
temperature around 5 km to 6 km below the ground surface (Figure 3(b)).

(a)

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 (b)
Figure 3. Estimated geothermal energy resource in Anadarko basin (modified from Porro et al. 2012): (a) total
heat in place for 15 major sedimentary basins in the United States and (b) Anadarko basin map where
ground temperatures are greater than 100 °C
Similarly, bottom hole temperatures (BHTs) can be used with ambient temperature to estimate the
geothermal gradient and subsurface temperature at depth. Southern Methodist University (SMU)
Geothermal Lab collected national-scale BHT database, and the database included 19 BHTs
approximately 2 to 4 miles (3 km to 6 km) away from Tuttle. The 19 BHTs were obtained at the
depth ranging from 3.75 km to 4.18 km (4.03 km on average) and the BHTs ranged from 72.8 °C
to 102.2 °C (92.5 °C on average), which is approximately aligned with Porro et al. (2012). The
corresponding geothermal gradient with an average ambient temperature of 15 °C ranged from
15.4 °C/km to 20.9 °C/km (19.2 °C/km on average).

3. Hydrogeochemical Characteristics of Geothermal Fluid

Geochemistry is important in the exploration, development, and utilization of geothermal
resources. In the exploration phase, for example, the geothermal reservoir temperature is estimated
using a chemical geothermometer characterizing chemical equilibrium, which is a function of
temperature (Ármannsson and Fridriksson, 2009; Flóvenz et al., 2012). The chemistry of
geothermal fluid (e.g., hot springs, fumaroles) is also analyzed to predict operational issues
including scaling of different components, by comparing the water chemistry to empirical database
for mineral solubility. In this section, the reservoir temperature and scaling potential in the study
area were evaluated through chemical geothermometer and piper diagram analysis, respectively.

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 The produced water chemistry was collected from USGS National produced waters geochemical
database (v2.3) and groundwater chemistry data was collected from Oklahoma Geological Survey.
Water chemistry in the targeted four wells was also incorporated in the analysis using the data
obtained from the operator, Blue Cedar Energy LLC.
Various geothermometers have been developed by previous researchers and particularly silica and
cation geothermometers have been widely used to estimate the reservoir temperature. Silica
geothermometers (e.g., quartz) are based on experimental measurements for silica solubility (e.g.,
Fournier, 1991) and relatively quickly respond to interactions between rock types and reservoir
conditions (Harvey, 2014). Similarly, cation geothermometers characterize cation ratios (e.g., Na-
K) at a chemical equilibrium between the geothermal solution and geochemistry as a function of
temperature. While the silica geothermometers can be invalidated due to mixing and dilution with
groundwater (near surface non-geothermal water), cation geothermometers are less affected by the
dilution (Harvey, 2014). The relatively slow equilibrium of cation geothermometers also can be
used as an indication of the cooling or heating history of the geothermal fluid (Flóvenz et al.,
2012). Fournier (1979), Giggenbach (1988) and Nieva and Nieva (1987) are some examples of the
well-known cation geothermometers. Another commonly used cation geothermometer is the Na-
K-Ca geothermometer of Fournier and Truesdell (1973), which is widely used and has frequently
provided excellent agreement with measured reservoir temperatures. The charts and
geothermometer equations for calculating reservoir temperature are based on the spreadsheet
which is described in Powell and Cumming (2010). The cation concentrations are in parts per
million (ppm).
Table 1 summarizes cation geothermometer results for three produced water samples (PW-1, PW-
2, and PW-3) in Grady County where Tuttle city is located and one water sample from the Tuttle
well. The geothermometer results showed that the reservoir temperature ranges from 61 °C to 109
°C with an average of 85 °C. The results also indicated that produced water samples from south
Grady County wells have much higher reservoir temperatures, up to 162°C with an average of 117
°C.
Table 1 Summary of cation geothermometer results for the Tuttle and Grady County samples (Temperature
units are in °C, PW: produced water, GW: ground water).
Sample Na-K-Ca1 Na/K2 Na/K3 Na/K4 K/Mg5

PW-1 145 105 125 94 117

PW-2 162 128 148 117 137
PW-3 121 82 103 72 103
Tuttle Well 109 70 92 61 91
1Fournier and Truesdell (1973), 2Fournier (1979), 3Giggenbach (1988), 4Nieva and Nieva (1987), 5Giggenbach (1986)

Figure 4(a) shows the triangular plot of Giggenbach (1988) for the selected water samples. The
sample from the Tuttle well fell very close to the equilibrium line indicating a medium water
temperature (over 90 °C). Samples from South Grady County wells (PW-1, PW-2, and PW-3) fell
within the partially equilibrated waters field and the equilibration temperature ranged between 120
°C and 140 °C. This is interpreted as cooling of thermal water upon migration toward the surface
and its Mg enrichment during water-rock interaction.

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 Relationship between log (K2/Mg) and log (K2/Ca) was also plotted for the five samples to estimate
the temperature at water-rock equilibrium and partial pressure of carbon dioxide (pCO2) of
geothermal liquids (Figure 4(b)). Tuttle-well sample was in a partial equilibrium condition at
temperature around 90°C. All produced water samples (PW-1, PW-2, and PW3) were in immature
conditions. Immature water is mainly controlled by water–rock interaction and requires short
residence time to gain temperature at depth (or more time to reach the surface). In such
hydrogeological conditions, it is unlikely that waters could attain chemical equilibria with host
rocks.

(a)

(b)
Figure 4. Cation geothermometers for estimating the geothermal resource temperature: (a) K-Mg-Na ternary
diagram (modified from Giggenbach, 1988) for the four selected water samples in the area Tuttle and
Grady County and (b) relationship between log (K2/Mg) and log (K2/Ca) for Tuttle well sample, selected
produced and ground water samples from South Grady County wells

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 In addition to the reservoir temperature estimation, geothermal fluid type and scaling potential
were analyzed using the fluid chemistry data. For produced water chemical analysis, 15 samples
from wellhead and 32 samples from separator were collected from oil and gas wells in Grady
County. Similarly, for chemical analysis of groundwater 22 samples were collected from nearby
water wells penetrating sandstone aquifers. The collected data was then plotted on a Piper diagram,
which graphically represents major cations and anions of water analyses expressed in percentage
of parts per million (% ppm) or milligrams per liter (mg/L) in three diagram panels shaped by a
mesh of equal-sized triangular cells, two triangular and one rhomboidal (Figure 5). Cations (Ca2+,
Mg2+, Na++K+) and anions (SO42−, CO32− + HCO3−, Cl−) were represented in the triangular panels
and then projected onto the central rhomboidal panel for cationic-anionic facies identification. The
results showed that the geothermal brine is expected to be Sodium-Potassium-Chloride-Sulfate
type. Such water may be a mixture of alkali chloride water and acid sulphate water, or it can arise
from the oxidation in alkali-chloride water or dissolution of S from rock followed by oxidation.
The chemistry of produced water samples from the separator was very similar to the groundwater
chemistry. As Bicarbonate (HCO3) concentrations were low (~200 mg/L) and the expected
production temperature was moderate (~ 70 °C), calcite scaling is not expected within the wellbore
and production pipeline. The chemistry results of samples taken from the separator were very
similar to the chemistry of groundwater sample; thus, mixing of groundwater and produced water
would not be expected to change the chemical characteristics of the geothermal brine as a heat
transfer fluid.

Figure 5. Piper diagram representing the geochemical characteristics and brine types of produced water and
groundwater in Grady County, Oklahoma

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 4. 3D Subsurface Geology and Temperature Distribution Modeling
In addition to the reservoir characterization using geochemistry, subsurface geology and
temperature distribution are important for demonstrating geothermal resource potential. The
temperature distribution directly addresses the potential of geothermal energy at desired depth,
while subsurface formations and lithologies are more closely related to fundamental backgrounds
on performance and efficiency of the geothermal system in terms of the resources’ hydraulic and
thermal properties (e.g., permeable rock or impermeable rock). An accurate model of geological
and geothermal variables such as lithology, temperature, pressure, porosity, and permeability, is
important to understand the geothermal resource potential (Akar et al, 2011). In this section,
subsurface geology and temperature distribution in southern Tuttle area were three-dimensionally
modeled using Leapfrog Geothermal, which is a commercial software for building and analyzing
conceptual models in 3D.
The Oklahoma Geological Survey has been compiling an interpreted fault map based on oil and
gas industry data and published literature (Johnson and Luza 2008; Marsh and Holland 2016).
Although Johnson and Luza (2008) indicated no major fault exists in the study area (Figure 2(a)),
the Oklahoma Geological Survey’s comprehensive fault database (Marsh and Holland 2016)
demonstrated there are two faults near the study area (Figure 6(a)). However, due to limited
information (e.g., fault type, strike, and dip), the faults were excluded in the modeling assuming
lateral continuity. Instead, four boreholes were added to the four targeted wells (total eight wells)
to extend the modeling region from the geothermal energy production area (i.e., four targeted
wells) to the end users (i.e., two schools and nearby houses). For the eight wells, well logging that
includes location, elevation, bottomhole depth, formations, and lithologies was collected from
OCC Oil and Gas Conservation database and the operator of the four targeted wells. Since the
eight O&G wells have been operated mainly in relatively older geological formations around 3 km
in depth, the formation and lithology information were limited for younger formation between 0
km and 1.5 km (e.g., Permian in Figure 2(c)). For a full range of subsurface modeling from 0 km
to 3.5 km, the eight borehole logs were thus combined with the information additionally obtained
from nearby O&G wells (within 1 mile distance) where the information on young formations is
available as well as previous studies (e.g., stratigraphic column in Figure 2(c)), assuming the lateral
continuity.
For the surface topography, digital elevation model (DEM), which is a 3D graphical representation
of ground topography, was generated for the study area using the National Geospatial Program
tool of United States Geological Survey (USGS) (Figure 6(b)). Then, the DEM was processed
(e.g., resizing, coordinates) in QGIS, which is a geographic information system (GIS) software,
and imported as 3D topography combined with USGS Topo surface map in Leapfrog Geothermal
(Figure 6(b)). For temperature distribution modeling, geothermal gradient near the study area,
approximately 2.7 miles away from the targeted wells, was also incorporated with ambient
temperature and the borehole logs.

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 (a)

(b)
Figure 6 Study area used in the subsurface conceptual modeling: (a) location of four additional wells and two
faults in the study area, (b) four targeted wells in digital elevation model (DEM), and (c) USGS Topo
map overlaid on the DEM

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 The 3D conceptual subsurface model is representing the geological formations tops and
temperature profiles incorporated with the eight borehole logs (Figure 7). Similar to the regional
formation and lithology described in the literature review section, the 3D geological modeling
results represented the study area consists of sedimentary formations and lithologies, including
Tonkawa, Oswego, Layton, and Cottage Grove sandstones represented by dark blue, green, hot
pink, and blue colors, respectively (relatively younger formations) and Mississippian and Hunton
limestones represented by orange and light blue colors, respectively (relatively older formations).
This result implies that the reservoir has relatively higher porosity approximately ranging between
12% and 22% and higher permeability approximately ranging from 7.63E-20 m2 for fine sandstone
to 3.90E-14 m2 for coarse sandstone (Wang and Park 2002; Tanikawa and Shimamoto 2009; Zhang
et al. 2016). That is, conventional hydrothermal geothermal systems may be thus suitable in the
study area, instead of enhanced geothermal system (EGS) or closed-loop geothermal system.

Although the subsurface temperature was estimated using cation geothermometers and regional
geothermal gradient without actual temperature measurements in the targeted wells, the subsurface
temperature distribution visually showed geothermal energy is available at around 90 °C
temperature at 3 km depth where geothermal energy production is targeted (Figure 7(c)). This
result implies that the geothermal resources may be exploited for direct-use applications, including
heating and cooling systems in the targeted two schools and nearby houses. For example, the
geothermal fluid around 80 °C can be used for both space heating and cooling using radiators and
absorption chillers where the geothermal energy can be used to drive the cooling cycle.

(a) (b)

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 (c)
Figure 7. 3D subsurface modeling: (a) lithology and formation logs in eight boreholes, (b) subsurface geological
modeling, and (c) deposit contact surfaces with the wellbore temperature distribution

5. Summary and Conclusion

In this study, subsurface geochemistry, geology, and temperature were characterized to
demonstrate the potential of geothermal energy production from four existing oil and gas wells in
Tuttle, Oklahoma. The subsurface geology model indicated that the study area primarily consists
of Permian to Mississippian sandstone and limestone formations, implying a porosity up to ranging
between 12% and 22% and a permeability up to 3.90E-14 m2 (≈ 40 millidarcy) in certain levels of
the reservoir. The reservoir temperature is expected between 80 °C to 95 °C at 3.3 km depth, which
was aligned with the regional temperature gradient of 22.8 °C/km. Chemical geothermometers
also estimated the reservoir temperature at 3 km depth around 90 °C. The geochemistry analysis
indicated the geothermal fluid mainly consists of Sodium-Potassium-Chloride-Sulfate with higher
Ca and Mg concentrations and lower Na/K ratio possibly due to mixing and intrusion of
groundwater, implying lower calcite scaling expected within the wellbore and production pipeline.
The 3D subsurface geological model represented the expected geological formation and
temperature at depth. Investigation of subsurface geology and geochemistry provided essential
information for the geothermal energy production potential in the study area. By repurposing
existing oil and gas wells for the geothermal energy production, the new system will bring
environmental and economic benefits to the community, including new job opportunities. Detailed
techno-economic performance of the repurposed energy system will be further discussed in a

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 separate article at 47th Geothermal Rising Conference. This study will be also extended with the
system techno-economic analysis for a full feasibility study.

Disclosure

This work was authored by the National Renewable Energy Laboratory, managed and operated by
Alliance for Sustainable Energy, LLC for the U.S. Department of Energy (DOE) under Contract
No. DE-AC36-08GO28308. Funding provided by the U.S. Department of Energy Geothermal
Technologies Office under Contract No. DE-EE0009962. The views expressed in the article do
not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government
retains and the publisher, by accepting the article for publication, acknowledges that the U.S.
Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or
reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.

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