Conventional geothermal systems are limited to specialized areas where heat, water, and porosity come together just so. But those areas are limited.
There’s plenty of heat stored down in all that normal, solid, nonporous rock, though. What if geothermal developers could make their own reservoirs? What if they could drill down into solid rock, inject water at high pressure through one well, fracture the rock to let the water pass through, and then collect the heated water through another well?
That, in a nutshell, is EGS: geothermal that makes its own reservoir.
To be clear, the line between a conventional hydrothermal resource and a resource that requires EGS is not sharp. There are many gradations and variations between wet/porous and dry/solid.
“What you really have is a supply curve, where the variables are temperature, depth, well permeability, and reservoir permeability,” says Tim Latimer, founder and CEO of the EGS company Fervo Energy. “Everything between the two extremes exists.”
To put it simply, as the resource gets deeper and the rock becomes hotter and less porous, the engineering difficulty of accessing it rises.
The basic idea has always been that EGS would start off within existing hydrothermal reservoirs, where fields are relatively well-characterized. Then, as it learned, honed its technology, and brought down costs, it would branch out from “in field” into “near field” resources — solid rock adjacent to reservoirs, at similar depth. Eventually it would be able to venture farther out into new fields and deeper into hotter rock. In theory, EGS could eventually be located almost anywhere in the world.
That’s been the game plan for a decade now, and it’s still the game plan, as laid out in the magisterial 2019 GeoVision study on geothermal from the Department of Energy. The EGS industry has had trouble, though, getting all the ducks in a row. There was a burst of activity around 2010, based on Obama stimulus money and binary power plants. But by the time the drilling technology from the shale gas revolution had begun making its way over to geothermal, around 2015, capital had dried up and attention had turned away.
It’s only been in 2020, Latimer says, that everything has finally lined up: strong public and investor interest, real market demand (thanks to ambitious state renewable energy goals), and a flood of new technologies borrowed from the oil and gas industry. EGS startups like Fervo are growing quickly and bigger, established companies are running profitable EGS projects today.