For those of you who don’t know, Engineered or Enhanced Geothermal Systems (EGS) is an idea that will allow geothermal energy projects to be built anywhere, with minimal risk and at an affordable price. A typical EGS project is conceived as a well pair where water is injected into one well, the injector, forced to travel through hot rock towards the other well (flow is enabled by fractures both natural and unnatural), and ultimately produced by the other well. The hot water or steam produced by the production well is then fed into a power plant which generates electricity. The hot water can also be used directly by industrial processes. The company or companies which figure out how to build these systems at a competitive price will unlock an enormous business opportunity. Whoever can prove out this technology will own the cheapest, lowest risk, renewable baseload power source. These resources will use far less land than wind and solar and be far more reliable. With many states and municipalities aiming for 100% renewable energy, this technology will be in unique position. The potential for growth is enormous.
I am writing this piece to engender questions that will push EGS forward. I want this idea to succeed. One of the things that concerns me is that I have seen many presentations on EGS projects which do not adequately address obvious first principle issues. Today I want to specifically challenge the idea that EGS projects should be conceived of as two horizontal wells. While I fully admit that I may be wrong, it is my contention that there are many issues which make horizontal wells problematic.
Beyond the technical issues, which I will list, this idea of horizontal wells appears to be the result of lazy thinking, and I think we as a community can do better. Why is the idea of horizontal wells lazy? Glad you asked. Long horizontal wells are a well configuration that was designed to optimize oil and gas extraction from organic rich shales. Why is this important? This is important because the nature of organic rich shales is quite different from geothermal resources. Organic rich shales are located along relatively thin, horizontal, and laterally extensive rock units. In the context of oil shale, the well configuration which gives you the highest energy output per linear foot drilled is a long horizontal well drilled through an organic rich shale horizon. Most EGS projects are going to be in areas with a conductive gradient, or where temperature increases with depth. This effectively means one will always find higher energy density rock the deeper one goes.
The next major difference between oil and gas and geothermal is that geothermal heat is used in a power plant. So what? you might say. Well, this fact has some large implications for the design of any EGS project. Here are some facts you need to know. The efficiency of power plants increases with higher inlet temperatures. With pressurized water at 150°C, we can build a power plant with an 8-14% second law efficiency. [We use second law efficiency, because measuring efficiency in terms of the total energy contained by the water does not make sense. We are never going to cool water to 0°K so why even use first law efficiency. What does it matter? You might say. Well it’s problematic because when people compare the first law efficiency of geothermal power plants with natural gas or coal, it needlessly makes geothermal power plants look as if they perform poorly.] Moving on. In contrast to the low enthalpy pressurized water well, a power plant using a steam well at 300-350°C can achieve efficiencies of up to 45%. However, not only are power plants which use higher temperature fluids more efficient, but each kg of water (steam phase in this case) contains far more energy. The implication of these two trends is that energy output of a resource will increase exponentially with increasing temperature.
There is another important constraint that power plants impose on the design of an EGS system, this is the design point of the power plant. A power plant is designed to operate at peak efficiency at a specific temperature and pressure. The power plant can still operate outside of this design point, but it will not run as efficiently. The power plant efficiency decreases exponentially the further one gets from the design point. This means that there is typically a band of temperatures where it is economic to run the power plant and outside of this band it is better to shut down the project. In the context of EGS, this means that we are not solely trying to maximize the heat extraction from a volume of rock. Rather, we are trying to maximize heat extraction from a volume of rock with a hard constraint that the produced fluid must be above a certain temperature. This slight variation in outlook is important because it means that the rate of cooling which occurs along a fractures surface really matters.
The last bit of context required for this discussion is the creation of fractures. The oil and gas industry has shown that drilling a horizontal well in the direction of SHmin allows for the greatest number of stimulated fractures per unit length of any well orientation. However, it is important to ask, what constrains the number of fractures which can exist along a producing interval? This question is answered by studies looking at optimal fracture spacing. In my understanding, oil and gas companies optimize for fracture formation and geometry (meaning they don’t want the fractures to curve over and propagate along the length of the wellbore) and for the extraction of oil or gas in the volume of rock between the set of fractures. Placing multiple fractures within the same depth interval and along the same axis of SHmin means that these fractures are facing one another. As one drops the pressure within two neighboring fractures which enclose a volume of rock (producing oil), the pressure at the midpoint between these two fractures will drop much faster than if this rock volume was not enclosed on either side by a plane of low pressure. While I am out of my area of expertise here, it appears that this enhanced pressure drawdown will help maximize EUR from that volume of rock. In other words, this allows them to maximize energy (oil) extraction. In the context of EGS, we are concerned with temperature drawdown as opposed to pressure drawdown, but we should expect similar results to the scenario which has been outlined (fluid flow through a media and heat flow through a media are governed by similar equations, meaning the effect we see in oil and gas with regards to pressure will be similar to what we see in EGS with respect to temperature). If we have a horizontal well with neighboring fractures enclosing a volume of rock, we should expect a higher cooling rate than if the two fracture were not enclosing a volume of rock. While a horizontal well configuration may be good at maximizing the heat mined from a volume of rock, it probably is not good at maximizing heat extraction with the constraint of a minimum operating temperature.
On to the list:
- In a horizontal injection well, how do you overcome the pressure loss which occurs as fluid travels along the entire length of a horizontal section of pipe? Wouldn’t fractures located on the toe of the lateral see significant pressure loss by the time the water reaches the end of the well? Doesn’t this inherently mean that fractures near the heel will end up taking most of the flow, causing a short circuit in one’s reservoir?
- Can’t the potential issues with pressure and short-circuiting caused by parallel horizontal wells be mitigated by drilling the wells at an angle and maybe by having the wells get closer near the toe?
- If the energy density of rock always increases with depth, wouldn’t it better to drill at an angel? This would increase the potential energy extraction per unit length drilled while balancing the need to create fractures.
- Won’t drilling at an angle allow you to drill along SHmin while mitigating some of the higher cooling rates one would expect from a horizontal well configuration?
- Isn’t drilling a horizontal well more difficult and costly than drilling an angled well?
- While we are incredibly adept at drilling horizontal wells in sediments, can the same really be said for drilling horizontal wells in igneous, metamorphic, or volcanic rocks at depth?
- Is a horizontal well configuration, which was specifically designed for oil and gas extraction out of thin and horizontal shales, optimized for geothermal energy production?
It is my contention that the optimal EGS configuration is an angled well pair specifically designed to address all the issues that I have outlined. The optimal drilling angle will be a function of temperature gradient, lithology, stress, and the length of the producing interval. I hope this gets everyone thinking. Also, I admit that some of my assumptions may be off, so please correct any errors you may have uncovered. Let’s learn together and present solutions that are optimized to our specific set of issues and not copied and pasted from a different industry which is addressing a different set of problems. It is my belief that the greatest strength of the geothermal community is creativity. Let’s use all our combined creativity and build an elegant solution.