Geothermal plants can change the earth around them, and not just while they’re active.
Such plants draw on heat in fractured underground reservoirs. They pump up heated water, which can be used for power generation, and replace it with cooler water that is, in turn, warmed by the natural forces within the earth.
But when the plants shut down, the pressures they introduce into surrounding rocks change. University of Wisconsin-Madison geological engineers are seeking to understand how that change in pressure and stress affects subsurface fluid flow.
Kurt Feigl, a professor of geological engineering and geoscience, is leading the effort to collect measurements from a geothermal field in San Emidio Valley in northwest Nevada. The U.S. Department of Energy is funding the project.
Sabrina Bradshaw, a geological engineering researcher and assistant project manager, says the plants reduce the amount of pressure in underground reservoirs when they’re pumping water out for energy production. When a geothermal plant shuts down production, water slowly fills back into the reservoir, which increases pressure on the rock throughout the geothermal field. That change in pressure could induce small vibrations in the earth’s crust that can only be detected with sensitive equipment.
In spring 2022, Feigl’s research team deployed a network of 450 densely placed seismographs around an industry partner’s geothermal plant in the San Emidio Valley. The seismographs, which are about the size of a coffee can or small potted plant, remained in place for a month to measure tiny vibrations. The researchers also are monitoring movement in the earth’s crust by comparing successive satellite images.
“We’re studying crustal deformation around this geothermal plant,” says civil and environmental engineering graduate Anya Wolterman. “We’re looking at how the plant’s operations, which alternate between normal pumping and shutting down for maintenance, affect the underlying fault.”
In addition, the team is monitoring pressure and flow rates from 13 observation wells around the site. The researchers also are gathering geologic data by looking at and imaging rocks along surface outcrops and from cuttings collected from the wells. Analyzing those samples in a lab will show the rocks’ elasticity, porosity and other geologic factors that can impact how the ground responds to stress.
“The point is to understand all of these mechanisms,” Bradshaw says. “We want to be able to model the stress, temperature, the hydrologic factors and all the other things that are occurring, based on how the different types of rocks experience stress. Throughout this project, we’ll be looking at a lot of different types of science that all interact together.”
Feigl’s team will create four-dimensional models of the earth’s crust around the plant by identifying when and where microseismic events happened. The models range in scale from a meter all the way up to the entire geothermal field, which is about six kilometers across, and they’ll focus on times around when the plant is shut down and the water flow in the reservoir changes.
Feigl says he hopes the project will increase understanding of how geothermal plants impact their surroundings. If researchers can create models to accurately predict how a plant will change the underground stresses in a geothermal field, they can work with geothermal industry partners to optimize plant operation and minimize their environmental impact.
“If we can model it, we can manage it,” he says. “We want to optimize the flow patterns of water in the subsurface — for example, how hard and fast do we reinject and how hard and fast we pump hot water out. By modeling it, we can manage and optimize for the lifetime of a geothermal field.”
Featured image caption: A UW-Madison geological engineering team plants a seismograph near the site of a geothermal plant in Nevada. The instruments gather data that will help researchers understand how geothermal plants affect the surrounding earth. Pictured, from left, are Anya Wolterman, DJ Bustos, Ben Jahnke, and Samantha Kleich.