Engineered Geothermal Systems
September 26, 2017 by Denis Pombriant
Conventional hydrothermal-heat mining relies on three major characteristics. Hydrothermal wells must be hot and their high heat gradients must be within the reach of conventional drilling technology; their rock formations must be porous enough to enable good flow of liquid water through the rock to absorb heat; and they must have enough fluids to replenish the reservoir as hot water is extracted. While conventional hydrothermal sites have all these elements, potential engineered geothermal systems (EGS) sites lack at least one of them, hence the need to enhance the existing resources through engineering.
In hydrothermal heat mining the hot water coming from the earth is replenished by natural sources (no additional water has to be injected into the well). But there are many more places where heat mining would work well but that lack an underground water supply. Mining heat in these locations requires injecting water (or some other fluid), into the subsurface rock formations to absorb energy and conduct it to the surface. Moreover, those formations often don’t have enough natural fissures and the rock may not be porous enough to enable a sufficient volume of water to circulate. This brings us to EGS.
All the noted deficiencies can be supplemented with the techniques of EGS. The needed drilling techniques are already in use in the oil business and they are highly successful and they can certainly be employed to create geothermal wells. Oilfield drilling also already employs directional drilling, in which a drill can turn sideways to develop several wells at one time.
Wells drilled into tight rock formations would need to be fractured to increase the porosity of the rock and enable flow. This fracturing (or fracking) is already used to enable hydrocarbons to flow in other rock formations and applying it to heat mining is a logical extension of the basic technology.18 This is very much in the tradition of western science and technology adapting a technology to different uses. For instance, James Watt used John Wilkinson’s boring technology originally developed for making cannon to build a better steam engine. Another example, silicon chip technology originally developed to make computers has now been converted to fabricate light absorbing solar collectors.
So, how do you mine heat? In some ways, the question sounds like a joke. After all, heat isn’t a liquid (although it behaves like one in at least one important respect—it flows—but unlike a fluid it can flow straight through rock). Also, heat is not a solid that you can mine (like iron or copper or coal) by removing it from a mine and sending to a refiner.
Heat is pure energy and in most cases, it’s a waste product. Your car gets hot when it runs because heat energy has been liberated from the fuel burned in the engine. If you can sense the heat then this energy is not doing any useful work, it’s just dissipating into the environment and eventually out to space. The same thing happens when you use your brakes—they heat up because they are taking kinetic energy out of your car, robbing it of momentum so that you can stop safely. Less than 20 percent of the energy in the gas you buy for your car actually goes to moving you around; the rest of it mostly radiates away in the form of waste heat.
Heat is the energy of now. It doesn’t store well and if you don’t use it, it’s gone. Even in the case of the Earth, which has been cooling since it was formed about 4.5 billion years ago, radioactivity replenishes heat in the core and it then flows to the surface and eventually it radiates into space. EGS tap heat between 10,000 and 30,000 feet below the surface, about the same depth as natural gas wells.
EGS has been well studied. According to “The Future of Geothermal Energy—Impact of Enhanced Geothermal Systems on the United States in the 21st Century,” a report prepared by MIT scientists, engineers, and industry experts, “Field studies conducted worldwide for more than 30 years have shown that EGS is technically feasible in terms of producing net thermal energy by circulating water through stimulated regions of rock at depths ranging from 3 to 5 km. We can now stimulate large rock volumes (more than 2 cubic km), drill into these stimulated regions to establish connected reservoirs, generate connectivity in a controlled way if needed, circulate fluid without large pressure losses at near commercial rates, and generate power using the thermal energy produced at the surface from the created EGS system.”19 So the essentials are ready, though there is still some engineering work to do to prove the concept—just as you’d expect in the early stages of a K-wave.
Creating a heat mine
The process of creating a thermal well and extracting heat for EGS is straightforward.
- Drill a very deep hole in the ground (actually two holes, for reasons that will be clear in a moment). This should be done boring into a rock formation that has temperatures of from 150 degrees to more than 400 degrees centigrade. For comparison, water boils at 100 C but amazingly, it doesn’t boil way down there because the pressure is so intense, a property that EGS uses to its advantage.
- Fracture the rock at or near the bottom of the well so that you have a pocket of porous rock to send water through. It’s important to understand the containment part of this step: Containment happens because the rock fracturing is limited by how much force is used. The formations we’re talking about drilling into are usually granite or other hard materials, which are formed from molten rock seeping from the Earth’s interior into the crust, where it cools into a mass. The pressure at those depths compresses the molten rock and turns it into granite. Tectonic shifts can force this granite up, forming mountain ranges like the Rockies. This granite was never part of a petroleum formation—petroleum is most often formed in sedimentary rock. So in this situation, there’s almost no chance of discovering large quantities of natural gas or oil.
- Pipe ordinary water into the geothermal well and pipe very hot water from the well into a more or less conventional steam power generator. The EGS resource is a closed system with hot, high-pressure water coming out of one side and cooler spent water going back into the well to be recharged. On the surface sits a more or less conventional steam turbine generating station that makes electricity, thanks to the steam.
This setup is non-intermittent; it runs day or night, no matter what, except for routine periodic maintenance, which is true of any generating station.
Let’s stop for a quick word about hydraulic fracturing, commonly known as fracking. Natural gas explorers have become adept at drilling deep holes and using fracking to release the natural gas stored in tight rock formations. Fracking has become controversial because it destabilizes the rock, enabling gas to sometimes get into surface water supplies.
Also worth noting, in Oklahoma, where fracking is commonplace for development of fossil-fuel resources, significant seismic activity—such as earthquakes—has increased thanks to fracking. The fracked rock deep below ground is unstable and settles during gas extraction and because drillers inject wastewater from their activities back into the Earth. In 2016, Oklahoma experienced 623 magnitude 3+ earthquakes (fewer than the 903 in 2015, but more than the 579 in 2014).
Seismic activity is also possible with heat mining, though likely to a lower extent. As we’ll see, some of the best places in the continental US for heat mining are in sparsely populated places in mountains or under deserts.
There are potential downside consequences to heat mining but they are manageable and do not pose significant roadblocks. In the interest of full disclosure we should look at them.
- It’s possible to temporarily cool an EGS site enough so that generating power there would become uneconomic. An EGS well is projected to have a lifecycle of approximately 30 years.22 However it is feasible to drill new wells and pipe hot water into the same plant, thus extending its life. Also, there is evidence that a relatively cooled area could be replenished after having been dormant for about 10 years. Of course, you could always drill multiple wells to draw from, which would greatly reduce the likelihood of a cool-off. But also, moving or rebuilding a plant in a different location after decades of service could be factored into the cost of electricity produced over the plant’s working life.
- It’s not free. MIT states that there’s still some applied research to be done to make this idea commercial, but the estimates are in 2004 dollars and are almost laughably small compared to other costs in the energy business—$800 million to $1 billion to prove out the concept. Compared with the cost of building a single refinery $200 million to $1 billion or an oil super-tanker at $120 million or more, the costs are feasible.
- Moving to an all-electric energy paradigm would likely require building a new and advanced power grid, as well—a true, nationwide electrical grid to distribute all this power. Building a new grid that spans the continent and has built in safety in these turbulent times makes a good deal of sense. Currently, we have a collection of regional grids that provide the semblance of a national system but it’s time for an upgrade.
There’s more to say about heat mining and EGS but this is getting long so let’s save it for another post.