The University of Pittsburgh team was recently awarded US$1.26 million from the U.S. Department of Energy for two collaborative projects that will characterise the stresses in the rock formation targeted by Utah FORGE.

Andrew Bunger, associate professor of civil and environmental engineering and Pitt's principal investigator, described the importance of stress in the Earth with a comparison to a clay pot in a vice: "The force and direction of the pressure that the vice puts on the clay will impact where and how much it might crack. The same is true of the rock formations that comprise geothermal targets.

"Because geothermal wells are deep and expensive, you need to circulate large volumes of water for the energy produced to be economically viable," Bunger explained. "Designing these systems is difficult if you don't know how the rock will react as the wells are drilled and you can't simply dig a hole to collect a sample."

Another challenge is that rocks hot enough to make good geothermal wells are also too hot for the underground sensors that would typically be used to collect data. At around two miles below the Earth's surface, the targeted rocks are under enormous stress. They are also hot—exceeding 200C. 

Bunger's team is focused then on developing lab-based estimates of stresses to enable better analysis of subsurface measurements.

"We have to explore a variety of complementary and innovative approaches to estimate conditions in geothermal reservoirs and rely more heavily on measurements we can perform in the lab using rock cores," explained Bunger. "We also have to use more sophisticated analysis to extract all possible information from every precious bit of data we can get from the subsurface. When geothermal was first explored decades ago, the computational power wasn't there to simulate these systems, but today we can do it without expensive underground testing."

Creating a roadmap of geological stresses

One of the most popular methods of testing in situ stress—or stress on the rock that is still in the ground—involves injecting just enough water to begin to crack the rock. Traditionally, the fluid pressure record is analysed and is considered accurate as long as the crack pattern is very simple. But often at these depths the crack pattern is more complex yet could still provide critical information about the stress conditions.

Lab-based methods involve testing rock cores extracted from the formation targeted for geothermal development. There are several methods, but all rely on the microstructure of the rock ‘remembering' the stresses it was subjected to before being removed from the ground.

The Pitt team's core-based investigation will complement downhole methods being carried out by lead partner Battelle Memorial Institute. This project will use machine learning to estimate the stresses on FORGE wells based on both experimental data and stress tests in the well itself. The results will provide a profile of minimum and maximum stress and allow the researchers to compare existing stress estimates with other models. 

While lab-based tests offer advantages in terms of precision, subsurface measurements are essential for directly capturing observations of rock behaviour in its original environment. However, because fewer measurements can be taken at that depth, researchers must perform detailed analysis to complete the interpretation. Bunger is working with scientists at the Lawrence Livermore National Laboratory to create high-fidelity estimates of in situ stress based on these readings. Because of their specialised capability to run lab-scale experiments that simulate experiments run in the subsurface to estimate stresses, Bunger's group will be able to provide new insight previously unavailable.

"The work we're doing with Utah FORGE is really exciting because we're using the best technology available to help fill knowledge gaps. Innovation always requires iteration, so you get better as you go," said Bunger. "The prize is huge with geothermal, but we have to be much more sophisticated in our methods than ever before if we are going to develop it to the point we can rely on it to significantly contribute to our renewable energy portfolio. Funding like this gives us the space and resources to innovate so we can tap into this resource." 

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