The situation we face with Hot Dry Rock (HDR) geothermal is a bit like the situation the industry faced with shale 20 years ago: We knew the shales were plentiful and had loads of hydrocarbons—we just didn’t know how to get it out of the ground and still make money. The key to unlocking affordable, reliable and plentiful shale was reducing cost and increasing output to the point that it could make economic sense. It wasn’t easy, but the industry did it.
This edition of Drilling Advances will continue to explore what would need to happen for HDR Geothermal to follow in the steps of shale and be economic in its own right—without any subsidies. The size of the potential prize is crazy big—IF (note the big ‘if’) the industry can reduce the capital needed to tap this virtually inexhaustible resource. With HDR Geothermal, plentiful and reliable aren’t the problem. Let’s explore just what drilling advances could make it affordable.
Past columns have discussed various flavors of geothermal energy and the promise of HDR geothermal, and explored the economics of one current, and admirable, HDR project being conducted by Geo Energie Suisse. We saw that in 140 ºC rock, that project needed to increase output by >50% and reduce capital cost by some 80% to become economic without any subsidy. With subsidies, HDR geothermal might have a place—but if the industry can make HDR geothermal economic in its own right, it would open a whole new baseload energy source and create a shale-like boom.
This article will show what I think are two advances that could help make similar HDR projects routine and economic. (As a reminder, HDR geothermal is where you construct a giant heat exchanger by fracing between two wells and extracting heat to make electricity).
Refer to my last columns Does geothermal 2024 = shale 2005? if you are interested in the complete story. I try to make each of these helpful as stand-alone missives, but there is a red thread running through them all and prior articles might be useful for new readers.
What does HOT COFFEE have to do with geothermal energy? Maybe not that much, but the title is catchy. But HOT and COFFEE do have two connections with drilling. First, for geothermal to be economic without subsidies, the laws of thermodynamics mean we are much better off going quite HOT. Spoiler alert: the usable energy you can extract from 450oC rock is about 28 times what you can get from 150 degC rock – more about how important that is below HOT is important.
What’s the COFFEE connection? Well, confining pressure has a major effect on apparent rock strength (i.e. drillability). That means penetration rate, bit life, and well cost would be much better if we could somehow reduce confining pressure seen by the bit by drilling hot dry rock on air. Drilling with air reduces the apparent rock strength by the weight of the mud column. What does that have to do with coffee you ask? Well, you’ve probably seen vacuum packed coffee bricks. You could almost build a house out of coffee – something with literally zero compressive strength – if you put enough confining pressure on it. Vacuum packed coffee bricks have a confining pressure of much less than 1 bar (~15psi). If HDR really is dry, maybe you could drill the HDR ‘pay zone’ on air. Drilling with air would reduce the confining pressure and make the rock really be ‘weaker’ – just like vacuum packet coffee loses its strength when the confining pressure is removed. If we can get HDR to lose its strength like coffee does when we open a vacuum-packed coffee brick then we might be able to build HDR wells more economically. Spoiler alert: removing the confining pressure that makes vacuum packed coffee bricks from the bottom of the hole by drilling with air instead of mud could reduce the apparent rock strength by a factor of 2 or more. More on this below.
How Hot for HDR?
Laws of Thermodynamics means that hotter rock is way better. First the heat content of anything is proportional to the difference between its temperature and the temperature of the sink which will extract the heat. If you have a heat sink of, say 100oC, then a 150oC rock will have half the heat content of a 200oC rock. A 100oC temperature difference is better than 50oC. That’s simple and intuitive.
But hotter is even better, because the laws of thermodynamics mean it’s not just the total heat content that matters, it’s more efficient to extract heat from hotter rock. In 1824, Nicolas Carnot found that the maximum possible efficiency with which we can extract energy from a heat source is equal to the difference in temperature between the heat source and sink, divided by the source’s absolute temperature (e.g. in degK).
Figure 1 summarizes the real effect of this somewhat unfortunate situation, as applied to the Geo Energie Suisse project introduced in the last column (insert link to that column here). That project is targeting 140oC rock, Carnot’s equation means that at most, 10% of that heat can be extracted, if the heat sink temperature is 100oC (e.g. using water as a working fluid). (Note: Working fluids other than water could increase that efficiency but would come at a cost). Since extraction efficiency is better with hotter rock, the maximum possible efficiency with 400oC source rock is about 45%—4x more efficient. You can extract more energy, more efficiently, out of hotter rocks, AND hotter rock has more energy in the first place. Figure 1 shows that the ratio of the extractable heat at 400oC is a phenomenal 22x that at 140oC.
Hotter is better, because there is just more heat to get, but more importantly, turning it into usable energy is easier. The blue curve in Fig. 1 shows that getting usable energy from 450oC rock is almost 30x more valuable than 150oC rock.
Getting hotter means going deeper, which costs more, so there is a tradeoff between the cost of drilling deeper in hot rock vs the benefit you get by going hotter. Building a 450 ºC HDR heat exchanger at depth means not just drilling deeper but building and controlling a pair of horizontal wells and fracing between them. Elastomers and electronics would be required, and these don’t do well at high temperatures.
The Geo Energie Suisse benchmark project we explored in the last column would be modestly economic (IRR = 6.3) without any subsidies, IF the target rock was 400ºC. It would produce 110MW instead of 5MW, even allowing for another $100 million for the increased power plant size.
Hypothesis: If HDR geothermal is going to go without subsidies, drilling will need to routinely utilize extended-reach horizontal wells at 350ºC plus—and be able to construct heat exchangers by fracing between two wells.
The vacuum-packed coffee lesson—confining pressure is no joke. We can easily see the effect of confining pressure at the grocery store by looking at vacuum-packed coffee. Less than 1 bar turns coffee grounds into a brick. Figure 2 is classic work by Warren Winters with Amoco in the 1980’s (ref: SPE 16696). It shows that confining pressure effect has a similar effect on important rock properties, too. Compressive strength and ductility are big drivers in rock drillability. The figure shows that adding 5,000 psi confining pressure can increase compressive strength and ductility by factors 2.3x and 2.8x, respectively. That means that adding 5,000 psi confining pressure to a rock reduces its “drillability” by a factor of two or more.
Figure 3 shows why 5,000 psi is a relevant number to talk about in the context of HDR geothermal. So, 5,000 psi is about the confining pressure reduction that could be achieved, if a 10,000-ft well was drilled with air instead of mud. The figure shows what a cartoon illustrating the bottom of the hole looks like when it’s “filled” with overburden (i.e. before the hole is drilled), and if it is filled with mud or air. Removing the overburden—by drilling the well—takes about 1psi per foot of pressure off the rock at the bottom of the hole.
When drilling with mud, if that overburden is replaced with 0.5 psi per foot gradient mud, there would be 5,000 psi LESS confining pressure on the bottom of the hole. Less pressure on the bottom of the hole would cause that bottom to tend to “pop up” (depending on the formation’s modulus of elasticity and to a certain extent its poisons ratio). In porous rock, this has some interesting effects that depend on the fluid in the pores, permeability, among other things. We don’t have the space to address all those issues here.
When drilling with air, the effect is even more pronounced. As the overburden pressure is removed and replaced with air instead of mud, the bottom of the hole will “pop up” even more. In this case, compared with drilling on mud, the bit will experience a rock with almost 5,000 psi lower confining pressure. The granite and gneiss, which we would target for HDR, can have compressive strengths of 20,000 to 30,000 psi (maybe more). Drilling those rocks economically is not easy. Removing 5,000 psi confining pressure could make them > 2x easier to drill. That would save drilling time and bit trips.
We know this “pop up” effect is real, because it is the same kind of phenomenon that can cause wellbore “breakout” in horizontal or directional wells. This “popped up” bottom of the hole in some cases can become much easier to drill.
The next column will continue this red thread, but to summarize for now: The industry needs to advance to make HDR geothermal economically attractive without subsidies—we need to drill very long laterals in very hot rock at rates approaching the technical limit. Next edition of Drilling Advances will explore just how cheaply we’ll need to drill and just how hot the rock will need to be and compare that with what the industry does everyday of the week with 15,000-ft laterals in shale wells. Maybe then, we can have a clear idea of the advances needed to make HDR an economic reality.
Until next time, I hope to start a conversation with any of you on how we can all help Drilling Advance. If you have any ideas, please email me at ford.brett@petroskills.com, and I promise I’ll respond. WO
FORD BRETT, P.E., is CEO of PetroSkills. He has consulted in over 45 countries, been granted >35 patents, authored >40 technical publications, and has served as an SPE Distinguished Lecturer, as well as on the SPE Board as Drilling and Completions Technical Director.