An Enhanced
Geothermal System also known as an Engineered Geothermal System (EGS) involves
creating a fracture system in rock via hydraulic fracturing and pumping water
into and out of that system to be used for heat and electricity. The rocks
chosen for EGS can vary significantly but the best areas to do EGS will be very
near geothermally active areas or geothermal hotspots. This is known as nearfield
geothermal. Utilizing a nearby reservoir that is artificially stimulated can
have several advantages including easier drilling, less corrosion, and less
seismic activity.
In late
September 2023, Fervo Energy announced the commencement of an exploratory drilling
program to produce geothermal energy via EGS in Utah, “… at Cape Station, a
next-generation geothermal energy project set to deliver 400 MW of 24/7
carbon-free electricity. Cape Station will begin delivering around-the-clock,
clean power to the grid in 2026 and reach full scale production in 2028.”
On November
28, 2023, Fervo announced that power was flowing to the local power grid to
Google’s data centers from their EGS project in Nevada, begun two years
previously. The project consists of “two horizontal wells and installed
fiber-optic cables to capture data that shows the flow, temperature and
performance of the geothermal system in real-time.” One well is an injector
well and the other is a production well.
Details of the
Nevada project were released in a July 2023 paper by Jack Norbert and Timothy
Latimer and the abstract is publicly available. The project is in north-central
Nevada adjacent to an existing geothermal power plant. According to the abstract
the stimulated and producing interval is a “metasedimentary and igneous
formation, comprised of phyllite, quartzite, diorite, and granodiorite,
representative of the geology across the most prospective geothermal areas
throughout the western US.” The hole sizes in the wells are a little larger
than typical oil & gas wells at 9 7/8” and 7” casing was run into them. The
wells are about 3250 ft in length and just over 7000 ft in depth. The maximum temperature
of the formation was recorded at 376 deg F. A vertical monitoring well was also
drilled further out along the lateral sections of the doublet wells. It should perhaps
be pointed out that there are sedimentary formations with temperatures nearly
that high in the Louisiana Salt Basin’s Haynesville and Bossier natural gas
plays at about 300 deg F. However, it would likely be more difficult to get
full hydraulic containment in those formations. Stimulation and flow testing
were described as follows: “A modern multistage, plug-and-perforate
stimulation treatment design with proppant was used to enhance the permeability
of both horizontal wells. A 37-day crossflow production test was performed in
April-May 2023, confirming that the EGS wells are connected hydraulically by a
highly conductive fracture network. During production testing, the system
achieved flow rates of up to 63 L/s, production temperatures of up to 336
degrees F and a peak power production of 3.5 MW electric power equivalent.”
This is the most productive EGS doublet in history in terms of flow rate and electric
power. As a first-of-a-kind project, it is thought that improvements will come
in the future. Fervo’s simulation modeling suggests that with innovation the
power capacity for these types of EGS projects can be increased to 8MW per production
well.
Some slides offering details of the Nevada project from a Geothermal workshop at Stanford University in February 2023 are included below. That paper gives some background: “The geologic setting at Blue Mountain is representative of many areas throughout the Basin and Range Province with high quality geothermal resource potential. The lateral sections of the wells targeted the Grass Valley formation, a Mesozoic metasedimentary formation comprised predominantly of interbedded phyllite and quartzite, as well as intrusive diorite and dikes and sills. The horizontal wells were placed in a southern reservoir compartment believed to have relatively few large-scale faults and low intrinsic permeability.” Incidentally, I did my undergraduate geology field camp in the Basin and Range province of Central Nevada, so I am somewhat familiar with some of those rocks. I remember the dikes and sills that outcropped where I was quite well. At one, I had a peculiar experience. Dikes are volcanic intrusions that cut across the horizontally positioned sedimentary rocks while sills are the part of the intrusives that were emplaced horizontally between the sedimentary layers. The sill in question was black with reddish stripes and as I looked at it from a bit of a distance, I noticed it appeared as if part of the rocks were moving. On closer inspection I noted that lizards were running along the rock that had coloration virtually identical to those outcropping volcanic rocks, likely evolving those colors for camouflage to increase survivability.
All three of the
Nevada wells were outfitted with reservoir diagnostics utilizing distributed
fiber optic sensing (DFOS). They deemed these reservoir diagnostic tools to be successful in determining the parameters of the hydraulic fracturing that were achieved.
According to a case study presented at the SPE/AAPG/SEG Unconventional
Resources Technology Conference in Denver in June 2023: “The recorded DFOS
data include in-well and cross-well distributed temperature (DTS), acoustic
(DAS), and strain (DSS) sensing data. We evaluated the adaptability of DFOS to
geothermal applications and showcased that DFOS is a beneficial tool for
optimizing multi-stage completions, characterizing the stimulated reservoir
volume, and determining well placement in geothermal reservoirs.”
“The 16-stage plug-and-perf stimulation treatment
described in this study was the first of its kind in a high-temperature mixed
metasedimentary and granitic formation in a fully horizontal geothermal well.
To our knowledge, we recorded the first cross-well strain data during the
stimulation of a geothermal well. The DFOS data acquired with three
fiber-instrumented wells prove the applicability of unconventional approaches
and their value for optimizing completion designs and well placement strategies
in geothermal development programs. The in-well DAS data indicate that all
clusters were opened during fracture initiation, and the treatment uniformity
was high. Also, we found that strain change signals from induced fractures can
be detected over large distances (> 1,500 ft). The DSS response recorded
during the injector injection test confirmed the hydraulic communication
between the injector and producer doublet before producer stimulation.”
Hot granitic
basement rock is considered ideal for EGS as a reservoir if it is not too deep.
This is due to its characteristic lack of natural fractures. It is ideal only
where faults are not present since injecting water into basement faults can
trigger induced seismicity, lubricating the faults and causing them to slip. After
these conditions are met the two keys to EGS are sufficient temperature and
sufficient flow rates of the water after the rocks are hydraulically fractured.
Granites and other basement rocks are very hard and slow to drill, often requiring
drill bit changes. That makes drilling them more expensive and more
time-consuming than drilling sedimentary rocks.
Fervo’s Utah
project will be adjacent to the DOE’s FORGE site that has been running for a
few years now testing a similar rock, temperature, and flow configuration at
about 8500 ft in vertical depth with the goal of de-risking EGS technology. This
will be a much larger project and Fervo’s CEO Timothy Latimer thinks that it
can be done at economic projections.
Advantages of an Engineered Geothermal Reservoir in
Meeting Power Demand and in Being a Self-Contained Artificial Hydrothermal
System
An EGS system
provides similar benefits to a conventional geothermal power plant in terms of
dispatchability and baseload power capabilities. These geothermal plants can be
run continuously or ramped up and down very quickly so that they can meet power
demand when it is needed. According to a May 2022 paper in Applied Energy - The
value of in-reservoir energy storage for flexible dispatch of geothermal power
by Wilson Ricks, Jack Norbeck, and Jesse Jenkins: “Across a range of
realistic subsurface and operational conditions, our modeling demonstrates that
confined, engineered geothermal reservoirs can provide large and effectively
free energy storage capacity, with round-trip storage efficiencies comparable to
those of leading grid-scale energy storage technologies. Optimized operational
strategies indicate that flexible geothermal plants can provide both short- and
long-duration energy storage, prioritizing output during periods of high
electricity prices. Sensitivity analysis assesses the variation in outcomes
across a range of subsurface conditions and cost scenarios.” They also
point out that the low-permeability rock within which an EGS system is typically
drilled can provide hydraulic confinement so that the engineered hydrothermal
system is contained without losses to the surrounding rock outside of the induced
fractures. This prevention of fluid leak-off is important to the stability and
reliability of the system. The goal is to provide high-conductivity flow paths
between an injection well and a production well and to prevent such leak-off
outside the system.
Future Potential, Some Limitations, and Optimization
of EGS Systems
Marc McClure of
ResFrac has modeled and written much about EGS systems. He is bullish for EGS which
utilizes convective heating vs. so-called advanced geothermal systems (AGS)
which utilize conductive heating. His ResFrac Blog is a valuable source of
information for reservoir stimulation of EGS systems. In a July 2023 post, he
offers the following key takeaways:
1. Thermoelastic
fracture opening and propagation can have a significant negative effect on the
uniformity of flow. On the other hand, interactions between fracture opening
and buoyancy-driven fluid circulation cause downward fracture propagation
during long-term circulation that greatly improves the thermal longevity of the
system.
2. Passive inflow control design can significantly
mitigate the negative effect of thermoelastic fracture opening on flow
uniformity, while maintaining the positive effects of thermoelastic fracture
opening and propagation on flow rate and thermal longevity.
3. Overall, simulations suggest that an EGS doublet with
8000 ft laterals at 475˚ F – using inflow control at the production well –
could sustain electricity generation rates of 8-10 MWe for more than 30 years.
Without inflow control, 6-8 MWe over 30 years is possible; however, there is
greater risk of uncontrolled thermal breakthrough.
He notes that EGS systems will eventually be tapped out as
temperature drops due to years of recycling fluids through the rocks. The
upfront costs need to be balanced with the longevity of the economic electricity production
of the projects. He thinks one of the keys to longevity is fracture stimulation
which provides enormous surface areas of contact between the rocks and the circulating
fluid. He notes: “To prevent thermal decline at the production well, heat
conduction into the fractures must ‘keep up’ with the rate of fluid flow through
the fractures.” Heat conduction through rock is not so great so a huge
surface area of fractured rock is required. His work involves simulating the long-term
circulation of fluid through the induced fracture system. He thinks the key to
success is inflow control in the production well: “Because of
buoyancy-driven convection and thermoelastic stress reduction and crack propagation,
the simulation with inflow control in the production well exhibits outstanding reservoir
performance – high flow rate and high produced temperature for more than 30 years.
Designs without inflow control show strong – albeit somewhat lower –performance,
but carry more risk of severe thermal breakthrough.”
It is yet to be
determined how much EGS geothermal will contribute to energy and electricity
production but in time it may provide a small boost. I do not believe, however,
that it will be any kind of panacea and both EGS, and particularly AGS will be
difficult to economize without significant and perhaps continued subsidization.
Perhaps new breakthroughs, more ideal rock, and better fracture stimulation with
higher surface areas stimulated will contribute to future improvements.
References:
Thermoelastic
fracturing and buoyancy-driven convection – Surprising sources of longevity for
EGS circulation. Mark McClure. ArXiv. July 24, 2023. 2308.02761.pdf (arxiv.org)
Commercial-Scale
Demonstration of a First-of-a-Kind Enhanced Geothermal System. Jack Norbeck and
Timothy Latimer. July 2023. Commercial-Scale Demonstration of a
First-of-a-Kind Enhanced Geothermal System (researchgate.net)
A New
Type of Geothermal Power Plant Just Made the Internet a Little Greener. Gregory
Barber. Wired. November 28, 2023. A New Type of Geothermal Power Plant
Just Made the Internet a Little Greener | WIRED
Case
Study: Completion and Well Placement Optimization Using Distributed Fiber Optic
Sensing in Next-Generation Geothermal Projects. Aleksei Titov; Jack Norbeck;
Sireesh Dadi; Katharine Voller; Mark Woitt; Steven Fercho; Emma McConville;
Camden Lang; Saurabh Agarwal; Christian Gradl; Timothy Latimer. Paper presented
at the SPE/AAPG/SEG Unconventional Resources Technology Conference, Denver,
Colorado, USA, June 2023. Paper Number: URTEC-3852680-MS. Case Study: Completion and Well
Placement Optimization Using Distributed Fiber Optic Sensing in Next-Generation
Geothermal Projects | SPE/AAPG/SEG Unconventional Resources Technology
Conference | OnePetro
The
value of in-reservoir energy storage for flexible dispatch of geothermal power.
Wilson Ricks, Jack Norbeck, Jesse Jenkins. Applied Energy. Volume 313, 1 May
2022, 118807. The value of in-reservoir energy
storage for flexible dispatch of geothermal power - ScienceDirect
A
Review of Drilling, Completion, and Stimulation of a Horizontal Geothermal Well
System in North-Central Nevada. Jack Norbeck, Timothy Latimer, Christian Gradl,
Saurabh Agarwal, Sireesh Dadi, Eric Eddy, Steven Fercho, Camden Lang, Emma
McConville, Aleksei Titov, Katharine Voller, and Mark Woitt. PROCEEDINGS, 48th
Workshop on Geothermal Reservoir Engineering. Stanford University, Stanford,
California, February 6-8, 2023. A Review of Drilling, Completion, and
Stimulation of a Horizontal Geothermal Well System in North-Central Nevada
(stanford.edu)
A
first-of-its-kind geothermal project is now operational. Michael Terrell.
Google. November 28, 2023. Google and Fervo launch
first-of-its-kind geothermal project (blog.google)
New
Google geothermal electricity project could be a milestone for clean energy. Jennifer
McDermott. AP News. November 28, 2023. New Google geothermal electricity
project could be a milestone for clean energy | AP News
Fervo
Energy Breaks Ground on the World’s Largest Next-gen Geothermal Project. Fervo
Energy. September 25, 2023. Fervo Energy Breaks Ground on the
World’s Largest Next-gen Geothermal Project - Fervo Energy
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