Each young geothermal energy well can be like a prodigy, brimming with potential to outshine their peers. However, like people, most things decline with age. In California, for example, the amount of electricity produced by geothermal power plants fell 15% between 2001 and 2018, in part due to the deterioration of existing wells.
“The history of geothermal has been this concept of degradation,” said Josh Prueher, CEO of geothermal startup XGS Energy. “This is happening all over the world.”
Many geothermal power plants inject water underground, which flows through cracks in the rock and absorbs heat from deep within the Earth. Once brought to the surface, it produces steam, which is used to drive turbines and generate electricity. However, as geothermal wells age, cracks may absorb too much water or begin to close, cutting the well from some of the hot rock it was previously in contact with.
Prueher's company believes there are many old geothermal wells around the world that could be restored, as well as many other previously neglected locations that could provide heat if the right technology were utilized. The technology is basically a special mud that fills the cracks around the well with minerals that are very effective at conducting heat, he said.
As a result, wells are not only more productive but also more predictable. In terms of power generation, “you can know in 30 days what you will be like in 30 years,” Prueher said.
XGS Energy's technology can operate in both existing and new wells. The new borehole itself can be a simple vertical hole that does not require expensive drilling equipment. Whether old or new, the startup places a metal casing in each borehole. We add a water slurry between the casing and the rock and a proprietary additive that deposits into the cracks around the borehole up to several feet away from the casing.
A slurry is a mixture of water and minerals suspended by a surfactant, Prueher said. (Surfactants are also known as emulsifiers and keep insoluble substances suspended in water, like mustard for oil in salad dressing or soap for dirt.) Once the slurry reaches a certain depth (and therefore temperature), the surfactant becomes “broken”. It releases minerals and allows them to settle in cracks. Prueher didn't provide specifics, but the patent filing suggests the conductive component could be a mix of materials like graphite and some minerals like silica.
XGS Energy reduces drilling costs by requiring only one borehole to produce heat. Other companies may inject water into one hole, let it flow through cracks in the rock, and then pump the water up to another hole further away. During the process, these rocks can absorb some of the water, forcing the company to add more water. The water rising to the surface has probably picked up minerals and some bacteria, which can settle the equipment that transfers heat from the water to the steam turbine. All of this makes well operations more expensive. XGS Energy, on the other hand, ensures that water does not leave the metal-cased borehole, providing another cost savings.
Because the water is separated, XGS Energy can extract the heat using less expensive equipment. You don't have to worry about minerals or bacteria in the surrounding rocks. “We can turn the page and have access to a whole range of very inexpensive and efficient heat exchangers that historically have been inaccessible with geothermal,” Prueher said.
XGS Energy will begin construction of a commercial-scale prototype at an existing geothermal field in California in July. To fund the project, it recently raised an additional $20 million in Series A funding led by Val Ventures and VoLo Earth Ventures, with participation from B Current Impact Investment, MIH Capital, and Thin Line Capital, the company told TechCrunch exclusively .
In the near future, XGS Energy plans to develop a version of the technology that will operate in older oil and gas wells. Most boreholes are not suitable for all as they try to avoid too much heat. However, there are many areas in the United States with oil and gas wells that are hot enough to support geothermal power generation.
“This is a huge opportunity because we can very effectively avoid blockages and disposal costs,” Prueher said. “This is a very suitable technology to re-energize the well.”