An introduction to the next clean energy frontier: Superhot rock and the challenges in developing enhanced geothermal systems
This blog is part of a series exploring and explaining the science behind next-generation geothermal energy, with a special focus on superhot rock geothermal, through a curated tour of influential technical and academic papers. This edition highlights key features of the 2021 World Geothermal Congress Proceeding What Are the Challenges in Developing Enhanced Geothermal Systems (EGS)? Observations from 64 EGS Sites by Pollack et al.
Next-generation geothermal is having its moment in the spotlight. With continued technology improvements and reductions in project costs, the Internation Energy Agency suggests that geothermal has the potential to meet up to 15% of global electricity demand growth through 2050. However, the transition from commercial geothermal to next-generation geothermal hasn’t been without its hurdles, which have included difficulties with site characterization, issues with maintaining well integrity, and insufficient connectivity between the injection and production wells. It is important to remember the early lessons learned in order to avoid similar challenges in the future – and early projects, like the first enhanced geothermal system (EGS) demonstration at the U.S. Department of Energy’s (DOE’s) Fenton Hill project in New Mexico, can provide insights into how to scale next-generation geothermal with a higher rate of success and at a lower cost.
Pollack et al. (2021) provide a comprehensive review of past EGS projects, highlighting common challenges in drilling, well completion and integrity, site characterization, and reservoir creation – issues that have led to delays, equipment failures, and even project terminations. Their findings offer valuable insights into the risks and opportunities associated with developing EGS projects and, by extension, SHR projects, which are capable of generating 5-10 times the MW output of a typical commercial geothermal well. Unlocking the potential of SHR geothermal is the focus on CATF’s Superhot Rock Geothermal program, which you can learn more about here. Given the even greater technical demands of SHR conditions, these lessons are crucial for ensuring future success. This blog highlights some of the key challenges identified in the Pollack et al. (2021) conference proceeding from the World Geothermal Congress and discusses progress made since the publication of their 2021 paper.
There are two key lessons to learn from early next-generation geothermal projects:
1. Problems with well integrity can lead to cost increases, project delays, and project failures
Well integrity is a critical factor in the success of geothermal projects, influencing costs, timelines, and overall project viability. Failures in well design and construction have led to significant setbacks, making this a key area for improvement as geothermal technologies advance.
Of the 64 sites looked at in Pollack et al. (2021), 24 of them suffered from issues with drilling, well completion, and well integrity. Multiple sites suffered from issues attributed to well integrity failures during or soon after the stimulation phase. Other projects were delayed or terminated due to equipment breaking or getting lost or stuck in the wellbore. Some projects were unsuccessful due to an underestimation of the pressures needed for stimulation or overpressure of the well. Pollack et al. (2021) also detail project failures as a result of well failures caused by corrosive fluids and clogged tubing due to mineral scaling. In one project, a drill string was cemented into the borehole due to the cement setting too quickly.
These issues can be exacerbated due to the high cost of drilling and stimulation, which Pollack et al. (2021) identify as being one of the barriers to commercialization. This barrier is already being lowered, as the U.S. National Renewable Energy Lab (NREL) recently decreased the U.S. DOE’s estimated drilling costs by up to 24% for vertical wells and 26% for horizontal wells.
Many of these same issues will be exacerbated in SHR wells. As is detailed in CATF’s Bridging the Gaps report on Well Design and Construction, well design has been the most common point of failure for drilling programs in SHR conditions due to the unique challenges posed by high temperatures and pressures, temperature variation, and corrosive hydrothermal environments. Specifically, temperature-dependent changes to the material properties of cements, cement alternatives, and cement additives need to be quantified for SHR systems.
Lessons learned: Challenges with well integrity are one of the most common points of failure in development and operation of an EGS project. Prior planning, understanding the geology and expected pressures, and developing equipment that can withstand the high and variable temperatures and pressures of these facilities are essential for continued project success.
2. Thorough characterization of the local geology is essential for successful reservoir creation
Successful EGS projects create a reservoir that allows for high flow rates between the injection and production wells, minimal thermal drawdown, and minimal water loss. The three main challenges in reservoir creation and successful circulation identified by Pollack et al. (2021) include: 1) injectivity increasing away from the production well; 2) insufficient increase in injectivity; 3) injectivity increasing along a single preferential flow path that leads to early thermal breakthrough.
Careful site selection, thorough characterization of the subsurface, prediction of subsurface response to stimulation, and project planning can help to ensure successful reservoir creation. Two primary challenges in characterizing the subsurface detailed by Pollack et al. (2021) include: 1) estimating the bottom hole temperature while drilling (e.g., mud circulation during drilling decreases the temperature inside the wellbore and it can take days or longer for the wellbore to return to its normal temperature), and 2) the location of microseismicity can inaccurately indicate the vicinity of increased fracture networks (e.g., the cloud of seismicity can extend further than the area where permeability has been enhanced). They note that numerical modelling, which is a computational tool that simulates underground conditions to predict reservoir behavior and optimize well placement, based on a thorough understanding of the subsurface is necessary to be predict the reservoir’s response to stimulation. Model predictions can be highly sensitive to several site parameters – with some parameters having a greater than 10% impact on model outputs with small changes to a single parameter.
Careful siting and characterization of the subsurface will also be essential for SHR projects. As is detailed in CATF’s Bridging the Gaps report on Siting and Characterization, there isn’t enough existing data to draw strong connections between geophysical signals (e.g., seismic velocity and electrical conductivity), and rock conditions (e.g., temperature, stress, and permeability) in SHR environments.
Lessons learned: Careful planning, learning by doing, and thoroughly understanding the local geology at higher temperatures are essential to achieving successful higher temperature EGS projects.
What’s next?
Although Pollack et al. (2021) highlight the challenges faced by EGS projects, they also noted that 29 of the 64 projects reviewed are still active. Furthermore, many of the challenges detailed in their publication have been improved upon in the last few years. More recently, stimulation and circulation tests at the Utah Frontier Observatory for Research in Geothermal Energy (FORGE) site successfully completed their stimulation and circulation tests in 2024, with more than 90% of the fluid being returned. From 2022-2023, Fervo Energy successfully drilled, stimulated, and completed a 30-day well test at their Project Red site and began producing electricity in late 2023.
Many challenges faced by lower temperature EGS projects are shared in the development of SHR projects – lessons learned by these early EGS projects are essential for the future success of SHR projects. We are already seeing progress in the success rates and cost decreases at lower temperature EGS projects like at Utah FORGE and Fervo Energy.
Stay tuned for more as CATF continues to push forward bold ideas on how to scale SHR and realize its full potential as a reliable electricity source.
This blog is part of a series exploring and explaining the science behind next-generation geothermal energy, with a special focus on superhot rock geothermal:
- Superhot rock geothermal and a vision for firm, global clean energy
- Superhot rock geothermal and pathways to commercial liftoff
- Superhot rock and the future of geothermal
Through a curated tour of influential technical and academic papers, the series aims to provide a fresh perspective from a geoscientist entering the geothermal industry. The goal is to share my learning journey and encourage collaboration around these groundbreaking solutions, which are critical to achieving a clean energy future. Whether you’re new to geothermal or looking to deepen your knowledge, I hope this series offers valuable insights into this fast-evolving field.
