Dave Waters, Paetoro Consulting UK Ltd

The growth of geothermal in direct-use heat has been accelerating. It is dominated by ground source heat pumps but bathing, space (building) heating, and greenhouses also figure prominently. At depths <10-20m ground sourced heat is largely solar in origin, but below that starts to become truly geothermal in the strict sense. The growth in direct-use heating overall can reasonably be described as exponential. It is catching on fast.  Notable though, has been the relative absence of geothermal direct heat use deployment in industry. That is a notable slower-growth element which we shall return to. 

Geothermal power growth is different in character. It has been better than linear. But not by a huge amount. It has been steadily growing, but incremental is probably the best description.

The story of geothermal right now is unavoidably the story of the competition. Geothermal is coming more into the sights as the candidates for baseload decarbonatization of heat and power are examined. At scale, there aren’t that many options in the pure “no add-on” sense: hydroelectric, nuclear, geothermal. Add things like energy storage options or carbon capture and the surface renewables (wind & solar most importantly) and gas also come into play. 

How geothermal is faring right now is a function of the drive to decarbonise – but its rise is not as stellar as one might think, because it is also a function of how other surface renewables are rapidly expanding. 

Although intermittent and not baseload, there are clear attractions of wind and solar in the low-emissions arena. There is less resource risk. The sun and wind conditions of a place are very understandable, despite being daily and seasonally variable. There is a reasonable element of certainty and predictability, pre-construction, about the amount that will be delivered on an annual basis. There is no geological risk to speak of, and there is no drilling cost up front before you have even proven up a reliable source. 

Then of course gas still lingers in the background. It is cheap and for many it is easy. As long as there is no carbon penalty in its combustion, competing with it is a reality of geothermal for many regions, and it can be difficult. 

Drilling, scalability, chemistry, customer and volume.

These are my picks for the geothermal enablers in the rest of this decade. They are not new messages either for myself or others. They are drums that many have been banging for a long time. 

Drilling is the no-brainer thing to focus on with geothermal. It is an underground resource, drilling is what gets us there, and drilling is one of its dominant costs. Although different well designs no doubt have an important place in the evolution of future geothermal too, what I’m talking about here really is the drilling bit. The digging. How to drill hard, fast, deep, hot. Not necessarily to vast new frontiers, just to the depths we normally target, faster and cheaper, safely. Things like slim hole exploration drilling in the pre-production stages and down-the-hole hammer drilling to tackle hard rock quicker.  

Scalability – ramping up the size of the opportunity beyond individual projects - is one of the key features of my current geothermally directed work. Geothermal for direct use heat, by virtue of its time transient heat commodity, needs to be near to market and that makes each project uniquely local, even without all the additional local variations in the resource itself. Making something like that regionally scalable is not without challenges, but it can be done, and needs to be done to show the true investment potential. 

Water, and hot water in particular, is a unique chemical and an amazing solvent. That means it “converses” with the rocks where heat resides in each place differently. The chemistry of those rocks is partly, sometimes subtly, sometimes strongly, imparted to it. Harnessing geothermal energy involves changes in pressure and temperature and when that happens, those chemistries have something to say about it.  

This can be both an advantage in supplying new commodities, and it can be a challenge, in depositing solutes where we don’t want them to go, as pressures and temperatures change, including with reinjection of cooler fluids. That can happen in the wellbore, it can happen in the reservoir. Understanding these aspects has always been a critical feature of geothermal energy deployment, and it remains so. Ways of bypassing these water-borne complications? Perhaps there are things emerging, but not without compromises on other things, that remain to be fully tested in anger at a large-scale. They will be tested in demo-projects most likely, and that’s good, whatever the result. We will learn. 

The message is sinking in more fully now that geothermal success is fundamentally customer-centric. It’s not just about where the resource is, it’s more about where the demand is – and demand for particular styles of energy. There is not the luxury of oil and gas profit margins to leave contemplation of the precise customer to later. It’s not just geothermal that suffers this. It’s just about everything that isn’t gold or oil and gas these days.  

Knowing who you are going to sell to is the first thing to work out. As it happens, if my five years of “reading between the lines” is correct, the most commercially sensible, simple and most profitable geothermal projects seem to be those surrounding single large customers.  That brings the question of industry into focus. 

Sometimes the complication with industry is the shear scale of energy required, but maybe a key thing to keep in mind for the future is that geothermal doesn’t have to be the whole solution to be a useful part of it. If it gives a leg up to other energy sources, it is a useful contribution. If there is industry that has a degree of flexibility in location to follow the resource, that helps too. 

Volume is perhaps the recurrent theme that can’t be overstated enough with geothermal. If the commodity has lower margins, you need lots of it, lots of it, lots of it. Heat available is a function of mass delivered – itself a function of flow rate. Temperature on its own is not enough, and in many ways geothermal is much more able to deal with lower temperature than it can with lower volumes. 

Supercritical fluids pack an awful lot of punch and will continue to attract attention to see if we can’t make something more of that, but they are also much more chemically and physically aggressive and for all their punch we still need to find them at long-lived sustainable volume. This generic question of volume is why big diameter boreholes are always a help - where you can wangle it in geothermal. We are not looking to sip a hydrocarbon cocktail through a straw, we are looking to gulp like we’ve found an oasis in the desert.

Not impossibilities, but challenges.

So, a “big five”: drilling, scalability, chemistry, customer, and volume. These are my choices of things I perceive most pivotal to the next generation of geothermal – because they have been pivotal to the past generations of geothermal. They are not show-stopping obstacles – but they are things which are difficult, and which we always do well not to underestimate. Amidst all the excitement around new technologies, keeping an eye firmly on what the ongoing issues have been in the past is never a waste.

The challenges of chemical scale and corrosion on longer production time scales, and the sheer difficulty of drilling and maintaining deep holes in the ground, especially the effects on both at higher temperatures and pressures – these are never small challenges. Finding a customer that is happy to spend big now for a long-term benefit – that’s not always easy either. They exist though.

Not “can we do it” but “can we do it more commercially than all the competition, now and in the future?”

Not everything that can be done is commercially attractive. Not everything that can return a profit is commercially attractive. 

At the root of it all is a need for the commercial reality. The nature of geothermal is that it can’t be done out of petty cash. It usually requires significant investment, and that means it needs at least one significant investor. They need to have a good reason to get on board. Demonstrating that geothermal can deliver a commercial profit is not enough. Investors have a choice of investments, and they have a choice of timescales.  Rather, geothermal has to provide something that returns not just a profit, but which competes favourably with the best of the competition in a certain market corner (e.g. long term baseload heat supply) and enables an investor to develop a repeatable niche. One which they find attractive for a variety of reasons - always unique to each investor.  

Decarbonisation and land footprint is on geothermal’s side

The playing field is changing under the auspices of different carbon pricing regimes, which will steadily help geothermal relative to higher emission options – but it is a more complex relationship with other surface-based low-emission renewable options. Especially given they are evolving so fast. 

Changes in the technical foundations of things like wind, such as the shift from onshore to offshore, are big changes that investors will be watching, as are grid and energy storage developments that will work increasingly to address intermittency issues. Increasingly an attraction of geothermal will come to be the “geothermal battery” concept (Green et al 2021) where heat for power or direct-use on-demand, can be stored in subterranean reservoirs. 

Whatever the future holds, the focus perhaps needs to shift away from the costs and rewards of geothermal on its own, and to the decarbonatization and efficiency improvements it can provide to the wider renewable “grid”. It brings value not just on its own but in how it’s consistent, and flexible nature can optimise other renewable players.

The advantage of geothermal is its consistency, its baseload nature, and its much smaller land footprint - but the competition is not standing still.  The land use issue may also work in the favour of geothermal as more and more land is devoted to wind and solar and biomass, but that crunch has not really arrived yet, to be a significant driver in 2021. The amount of time it will take for land footprint to become a significant additional issue remains to be seen. It is likely to come, but how soon is difficult to say.

When it comes to a matter of low land footprint and low emissions baseload energy, then nuclear and geothermal are sort of the biggest players in town, and nuclear makes even geothermal look like a bargain. However, it has to be said, trends to incorporate solar and wind more clearly into existing land use and building construction are also likely. So too is a move offshore, so their land footprint reduction strategies are also unlikely to remain static. 

Energy Supermarket. No meerkats but lots of lemmings.

“Crafting” geothermal for the future is as much about understanding the future of wind and solar and nuclear and energy storage as it about understanding geothermal. The scope for hybrid projects where they can help each other is something that is increasingly recognised. That won’t work everywhere but the complementarity option is one that will be turned over more and more routinely.

If investing heavily in an energy supply and an industrial project already, then adding a geothermal element that can help other aspects to improve efficiency and consistency can make easy sense. That is where I perceive the real growth area over coming decades in geothermal, and that is what I am pursuing with clients in the geoscience, industrial, and geospatial arena.

The size of original project makes a difference

Drilling holes in the ground is not cheap, and it becomes more expensive the deeper the hole. The relationship is not linear – the deeper you go the more expensive per metre extra depth it becomes. Deep geothermal wells cost some millions. That means geothermal has more chance of being deployed in projects where the overall capital spend is big to start with. If getting some long-term baseload energy out of geothermal is 2% of a project’s initial capex cost, it is a much easier sell than if it is 20%. That should help us screen who to sell geothermal to. 

Time - it’s flowing like a borehole

One of the great advantages of geothermal is that once established, it gives a longer-term secure energy supply that is baseload and keeps on ticking on. There are some maintenance issues and costs, but typically these are relatively low, and the systems can keep delivering for decades. However, there are things that can change with time and which are not always certain. Wind and solar at a given site are highly variable on a short time scale but are relatively stable and predictable on long term. Geothermal is for the most part not very dissimilar but has the added bonus of short time-scale consistency too. 

That said, in most instances, taking warm out often means putting cool in – that has impact on longer time scales. Some geothermal reservoirs are naturally overpressured, and their heat travels a long distance like a constantly replenishing river. They are nice to find. It means the heat is being constantly rejuvenated and we don’t have to work as hard to extract the resource. The overpressure provides easy natural flow to surface as soon as we create a hole. Foreland basin settings are one example of this kind of advective regime. For these we don’t have to worry about heat depletion so much. It is a steady river of heat coming to us.

In other more typical situations though, taking fluid resource out of the ground for the heat it carries, means you also want to put fluid back in the ground to maintain pressures in the reservoir. That is true whether it is a big reservoir, or simply the closed loop of a well. If you don’t do that all sorts of things can happen and not usually good ones. The aim is to maintain chemical equilibrium in the reservoir. Unwanted precipitates arising out of changes in temperature or pressure can affect a reservoir’s permeability. Dissolution in contrast, can result in subsidence. Hydration in the wrong place can create swelling. These things mean we have to keep a careful eye on reservoir and fluid circulation chemistries, pressures and temperatures. 

The wells giveth, and the wells taketh away.

The energy we can extract from geothermal is always a function of the difference a fluid is produced at compared to the temperature we “discard” (and typically re-inject) it at. If we have a certain mass of water at 90 deg C, we get different amounts of heat out of that resource if we reinject it back into the ground at 30 deg C than if we do at 50 deg C. It stands to reason. The colder we reinject it at, the more energy we have extracted on the surface. That’s the good news, but the bad news is that injecting cool back into the ground, will to some degree cool the rocks. Sometimes it’s a trivial effect, sometimes not. It’s a function of flow rate too. The faster we pump cool in, the more we will cool. Remember that what we take out in terms of flow rate also directs our reinjection flow rate, if we want to stay in control.

Now in some situations this cooling effect might be totally infinitesimal compared to the huge volume of “heat catchment” we are drawing from. It comes back to the question of volume. If we have a huge volume of hot fluid in our reservoir, then reinjecting a little bit of cool won’t make much difference. The heat flow coming in might be enormous. Nevertheless, we rarely know the exact dynamics of fluid flow in the subsurface, and so there is an element of watching and seeing to observe the changes that occur with time due to these effects. Sometimes, while a reservoir might have big porosity volumes, permeability “highways” might mean we are accessing a smaller proportion of it than we thought. The smaller the volume we are drawing the heat from, the more likely that injecting cool in will have a difference. That is relevant to deep closed loop systems, because they provide their own permeability superhighway within a smaller wellbore volume. 

That’s not to say that it is a showstopper, but the moment we restrict our fluid heat extraction to that of a narrow borehole, as opposed to a large laterally extensive reservoir, then of course we are restricting the volume of rock we extract heat from, and we are also concentrating the effect of any cool we put back in, to the same the volume of rock. Fracking or other reservoir-stimulation and any advective flow might help, but, it’s still a downsize on the heat catchment volume.

That means there is an element of watching closely on a longer-term time scale to see what is happening. That’s why closed loop systems like to try and put lots of subparallel well strands down there, to increase the heat catchment. That might help. It still won’t be on the same scale as a laterally extensive reservoir. In compensation, what closed loop is giving is an assurance that you can still get something out of the well independent of a lithology porosity and permeability. Knowing whether it is enough though, takes some time. What do matter are the thermal conductivity and thermal diffusivity of the rocks involved, as they need to conduct heat to the well bore. The more and faster they do so the better. These things can be modelled based on what we know, so it’s not to exaggerate the issues, but models are models and rocks and their fluids are, well, quirky. 

Knowing the precise behaviour over time - and that trade-off between production flow rates and heat depletion due to injection - can impact economics, and once again, the investment performance relative to competing options. Whether it gives enough to be commercial on a project scale life span is something that takes time to assess, and meanwhile, the more strands you drill, the more drilling capex you spend. So it’s not trivial to know. More projects will happen, and we will learn more as time goes on. Suffice to say the time to crack open the champagne is not the first heat or power extraction, it’s a few years down the line when we are more confident that we understand the heat replenishment and well maintenance characteristics of the system.

But all power to those that are giving new technologies and well designs a go – including those which to some extent bypass the geological risks – [within reason] we never know until we give things a try. Just as long as we realise that anything which bypasses the interaction with the fluid poroperm of a geological reservoir, also largely bypasses most of the heat associated with that fluid. There is therefore a compromise that takes place on the fluid volume side - and as we’ve already observed, volume provides the heat and is a big part of the successful economics of geothermal. That may be academic if the heat reservoir is big and replenishing fast enough. A small portion of that heat might be enough for the customer purposes. 

The other thing to say is that closed loop - if truly closed - allows more experimentation with fluids other than steam and water - such as supercritical CO2. That, and the chemical bypassing of the reservoir fluid in such cases also has positive implications for scale issues. Lots of things to check out. Lots of trade-offs and balances. Time will tell. Always though, not can it be done, rather can it pay. Someone has to buy the steel.

The upside of sedimentary geology, even if it’s cooler

These complexities mean we do need to be measured, in a very literal sense of the word, in our expectations – at least while things are still in the “early days” stage for new technologies or approaches. As for me, well I’m a geologist, so my own personal interest is quite naturally in the things which do have geological risk, and the potential they can still offer us. I’m under no illusions as to the issues surrounding geological uncertainties, but I’m not afraid of taking them on either. That’s true of myself and a huge swathe of other experienced geoscientists out there. Yes, geology is tricky sometimes, but we are up for it. We should not abandon those routes while others are explored. Geological sedimentary reservoirs and aquifers are widespread. Many cities are built over them.

The thing is, if you get enough volume, depending on your usage, you can reduce the need for temperature, and utilise heat pumps to boost temperature, if a boost is needed. That means you can aim shallower, with lower drilling costs. If you aim shallower you are also likely to have higher porosities, hence more fluid volume. Sedimentary rocks compact with depth and so generally (with exceptions) the trend for a given lithology is of porosity decreasing with depth. If the drilling capex is also less, it also means you are entering the affordability windows of more customers. If you go shallower, you are getting to a place where various geophysical methods can tell you more too. 

So yes, there is always going to be some geological risk, but the scalability for lower cost is the attraction. Sedimentary reservoirs of this nature don’t need stimulation or fracking so there is far less induced seismicity risk also.   Remember if we are doing this under cities where the market is, there is a need to be conservative in the number of physical and chemical changes we introduce during exploitation. We want to keep subsurface reservoir conditions as close to their original as we can.

The uncertainty surrounding details of the local geology and reservoir character significantly decreases every time you put a borehole in a new area, so the first one is always the riskiest. That’s not to say surprises can’t happen down the line, but you are in a place where you know a whole lot more than you did before, every time you put another hole down in an area. So, the geological risk of failure doesn’t stay the same forever. The risk decreases as you understand more. This is why places like Paris Basin, Netherlands, and Bavaria are progressing with traditional geothermal doublet approaches. They have to work to earn that insight and understanding, but it is possible. Now, down the line with good geological understanding, they are reaping the benefits more widely.

Seeing underground

The ease of optimising a “marriage of customer to resource” is always greater onshore. The costs of infrastructure are also always less onshore. Maintenance is cheaper onshore. Geothermal is predominantly a resource for onshore application. Yes, producing oil and gas wells offshore can make more use of the geothermal resource implicit already in their deployment, but the commissioning of new geothermal projects without prior drilling is almost exclusively an onshore affair.

That is relevant because onshore and offshore geophysical imaging of the subsurface are very different affairs. This is especially true of seismic acquisition. The uniformity of a nice clean bubble implosion source and the interaction with relatively flat and homogenous near seafloor sediments enables the generation of the beautiful marine seismic images we have become so used to. Acquiring seismic onshore is a much more complicated process. The sources are different in character and the near surface variations and absorption of signal, very different and highly variable. Yet advances in onshore seismic acquisition and reprocessing continue, and the information we expect from seismic for utilisation in many ways has much lower expectations than that for hydrocarbon exploration. We might never fully replicate the beautiful seismic imaging we get in offshore regions. We might not need to.  We don't need AVO for geothermal.

Accompanying seismic, is the raft of other technologies that are emerging and/or developing to new resolutions to “see” the subsurface. Things like GPR, InSAR, various resistivity and conductivity methods including magnetotellurics, and increased resolution of gravity and magnetic effects. Not to mention increased sophistication of geochemical “sniffer” techniques and various remote sensing approaches including LIDAR. Each one of these things on their own is interesting, but the ability of increased computing power and various data analysis and artificial intelligence techniques to look at all the data sets simultaneously and see patterns, is taking these things to new places. Not without issues of their own, but nevertheless new places.

One of the key challenges is a not uncommon need to perform these geophysical investigations in built up areas where the geothermal market is located. That can be challenging for a variety of reasons, but many of these techniques are becoming less and less invasive or obtrusive with time. The nature of the ground surface in built up areas, or the presence of man-made electromagnetic noise, can pose challenges to some of these techniques, but noise reduction is a rapidly advancing topic in all geophysics. It is another topic assisted by increases in computing power and advances in data analysis.

Many look to new technologies to go deeper or to approaches like closed loop which bypass geological risk, and they may also provide dividends, yet it is worth noting that even as that happens, those branches of geothermal which still rely on imaging and understanding the subsurface have a growing arsenal to draw upon. In the shallow geothermal realm that is even more true.  

It can decarbonise. How quickly and how widely, depends on us.

Those of us working with geological uncertainties and risks hence recognise an underexploited realm of resource for decarbonisation. How much of it is economic right now, will be economic in ten years, or will be economic in fifty - are harder questions to predict – but the trend towards increased utilisation isn’t. 

To an extent it’s a case of we need to do this – so what do we have to do to our wider all-sector economic models (not cherry-picked subsidies) to damn well make it economic. To persuade others of that, quite reasonably takes some work first. Everybody is saying their energy is the best thing since sliced bread, so a convincing account is in order. The demonstration of scalability for regional scale repeatability needs to happen before that wider political acceptance can be achieved. That takes knuckling down to the data with ongoing graft in lots of different places. And then sharing what you’ve found. Telling the story as you find it, to leaders, to public, to investors, to anyone who wants to listen. But you know, for a geologist, that’s kinda fun.

Whatever our view on geothermal in 2021, the most important thing, always - in those immortal words of some of London’s greatest subsurface explorers and environmental and social governance advocates, is to “remember you’re a womble”.  Through all the ups and downs, keep at it. Cleaning our surroundings is a worthy goal.

KeyFacts Energy Industry Directory: Paetoro Consulting