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Using Engineered Geothermal Systems to Meet our Energy Demand

In 1904, Prince Piero Ignore Conti generated electricity from a generator driven by steam from a natural geothermal system at Lardarello, Italy. That first experimental system was upgraded to a commercial power plant in 1913 that is still producing power 100 years later demonstrating the durability and practicality of using the earth’s heat to produce power commercially. It would be some 45 years before other commercial geothermal plants would be built at Wairaki Station in New Zealand and the Geysers in California in 1958 and 1960.

Today, the Geysers is largest geothermal field in the world producing close to a Gigawatt of power for the grid, and Wairaki Station is home to the world’s largest geothermal binary power plant. From the 9 MW produced at Lardarello by 1916, the world output of geothermal energy stands just under 14 Gigawatts. The U.S. is the world’s largest producer with about 3.5 GW of capacity.

While all of that sounds impressive, when held up next to the total U.S. energy production of about 1000 GW, the worlds’ total production of geothermal energy is only 1.4 percent of the US energy demand. Given that geothermal energy is abundant, renewable, and provides 24/7 baseload power, one has to ask why aren’t we utilizing it more? The answer is partly technology, partly politics, and partly the financial models we use to develop energy.

The lion’s share of all existing and planned geothermal power comes from conventional geothermal systems that rely on steam or hot water stored and circulating in naturally occurring cracks in the bedrock. Natural hydrothermal systems function as heat exchangers, concentrating heat from rock to water and then transporting the heat closer to the earth’s surface. Power is produced by drilling into these “natural” heat exchangers and producing steam and hot water to turn turbines. This works great, but the number of natural systems is limited, which in turn limits the scalability of geothermal power. In addition, locating a natural system that may be several thousand feet underground can be difficult. Drilling geothermal wells is expensive, and prospecting for new conventional systems has a 10-20 percent success rate. That is roughly on par with the early wildcatting days in the oil industry, but the oil industry has increased their success rate to 50 percent for new wells today. Add to that the massive infrastructure and established financial models for investment in oil and gas combined with cheap natural gas from the current shale gas boom, and it is not difficult to see why geothermal faces a steep slope attracting new capital investment.

What is needed is a way to decrease the risk associated with drilling new wells, reduce the cost of produced energy, and expand the area where geothermal energy can be utilized to increase the potential for large scale deployment of geothermal power. Across the western United States, there is hot rock within a few kilometers of the surface with a huge energy potential. Studies conducted in the last few years have estimated that there is from 500 to 5000 GW of recoverable geothermal energy in this hot rock if we can develop an economic means to extract the heat. Remember that the total energy production of the United States is about 1,000 GW, so this very significant. The challenge is that unlike a conventional geothermal resource, the hot rock resource has no naturally permeable cracks to allow heat exchange with circulating water. Enter Engineered Geothermal Systems (EGS).

Unlike conventional geothermal systems that require hot water circulating through naturally permeable rock, EGS only requires heat, and that heat is abundant across the western U.S. and even some areas in the eastern U.S. Moreover, the heat is well-mapped and easy to find compared to locating a natural system with water circulating through permeable rock. The success rate for EGS wells is over 80 percent which solves the risk problem for new geothermal development quite nicely. The major problem remaining is that EGS is seen as an unproven technology to energy investors already familiar with tried and true investments in oil and gas.

Creating an Engineered Geothermal System

Engineered Geothermal Systems create artificial reservoirs using hydraulic pressure to create a system of small fractures in the rock that acts a radiator transferring the heat from the rock to water that is circulated through the system. The hydraulic pressure is applied to “stimulate” existing cracks in the rock to slip and open slightly  resulting in increased permeability for the stimulated crack. The slip is caused by the tectonic forces that exist in the earth’s crust and are enabled by the lubrication from the water in the opened crack. This stimulation process is called hydroshearing.

At first glance, it appears to be similar to hydraulic fracturing (aka fracking) used in the oil and gas industry but there are key differences. Hydroshearing uses moderate surface pressures to open very small cracks (1-2 mm) with the goal of creating a network of thousands of permeable cracks for the efficient transfer of heat to the water. Fracking uses much higher pressures to initiate new tensile fractures, which propagate rapidly away from the well and result in wide fracks that require propents to keep open. Hydroshearing cracks self-prop from the slippage and uneven surfaces on each side of the crack. Furthermore, there are no fracking fluids used in hydroshearing, just water, so the problem of ground water contamination is eliminated.

This photo shows the main stimulation pumps at the Newberry EGS project in Central Oregon during the stimulation process in 2012. The pumps are on the concrete pad in the center with the electric drive motors directly behind, and three 1-MW diesel generators housed in the tractor trailers behind the motors.

The latest development in EGS technology is multi-zonal stimulation. What that means is the ability to create multiple stimulation zones on a single well. One way to think about it is stacking reservoirs on top of each other like a high rise building stacks office space. Just as the multiple floors in a high-rise office building allows dramatic increases in density on a single piece of real estate, multi-zone stimulation increases the amount of rock that can be stimulated, thus increasing the size of the reservoir and the amount of energy that can be produced from the well by factors of three or more.

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Creating multiple stimulation zones requires that one zone be sealed off before another zone can be stimulated. One approach for this is to use mechanical devices used in the oil and gas industry to block one zone before stimulating the second one. Another approach, pioneered by AltaRock, is a technique using thermally degrading polymers to block successive zones. This approach reduces the risk of equipment getting stuck in the wellbore, and eliminates the cost of having a drill rig on the site during the stimulation process.  Time and the market will tell which approach prevails, but the multi-zone approach to EGS offers the most promise for making geothermal energy a major player in our energy portfolio.

Multi-Zone EGS Lowers the Cost of Geothermal Power

Until this process was developed, EGS was very expensive, and often not commercially competitive. The problem was that single stimulation projects don’t produce enough power to pay for the capital investment to drill the wells and create the system. Increasing the power production 3, 4, or 5X with multi-zone stimulation leverages those capital costs, and can decrease the cost of power produced by well over 50 percent. Put in real world terms, a typical single-zone EGS with one injection well and two production wells might produce in the order of 1.5 MW of power with a single zone system. That is in line with existing EGS’s at Landau Germany and Soultz France. Using multi-zone stimulation on the same 3 well layout would result in 10-15 MW of power production. A typical 5 acre geothermal pad can support about three injections wells with 6 or more production wells. Single-zone EGS in this scenario, could produce about 5 MW of power on a 5 acre pad. Multi-zone EGS would produce 30-50 MW of power on the same pad. The capital investment for the single-zone and multiple-zone systems is similar, so the ramifications are dramatic and quite clear.

The graphic depicts an EGS with three stimulation zones and two production wells. Starting on the left, the sequence starts with an injection well, then the first stimulation zone, then pressure is reduced and diverter is pumper to block the fractures in the first zone, then pressure is increased to initiate hydroshearing on the second  zone, and the process is repeated until all stimulation zones ate created. The final step in the drilling process is to drill production wells that intersect the stimulation zones to return heated geothermal fluids and/or steam to a power plant on the surface to generate electricity.

So, problem solved, right? Unfortunately, it’s not that simple. Even with all of these great features and potential, EGS is still capital intensive on the front end with a long return interval for investors on Greenfield projects. Also, remember that investors look at EGS as a new technology, and combined with those long return intervals, it still adds up to a risky investment compared to oil and gas. One solution being looked at by the industry is to reduce the risk and capital investment even further by using the technology to increase the production and extend the life of existing conventional geothermal field. This is called commercial stimulation, and is the latest development in the geothermal energy field. Basically, stimulations are done on existing, idle or underperforming wells in the field to enhance permeability and increase the size of the reservoir. All of the infrastructure and agreements for the power plant, transmission lines, and power purchase agreements are already in place, and the wells are already drilled. Using this method, hundreds of megawatts of new geothermal energy can be created in the U.S. by expanding existing conventional systems.

Ormat has used this process to successfully increase the production of their Desert Peak facility by about 1.5 MW, and AltaRock is in contract negotiations with several conventional geothermal projects to expand their systems. Several other companies are also looking at using EGS to increase the production of existing systems. Most of these companies are looking at single zone stimulations, which are limited in the amount of capacity they can add to a system. The multi-zone approach definitely offers the most promise for increasing our geothermal power production.

Conclusion

Geothermal energy is our cleanest renewable energy source with a seemingly unlimited potential to meet our energy needs. EGS is relatively risk free, has a very small footprint on the land, produces 24/7 baseload power, and can be sited over vast areas where subsurface heat is close enough to the surface to be economically recovered. Using multi-zone EGS to increase the capacity of existing conventional systems will increase geothermal energy production significantly, and demonstrate the efficacy of this technology to the industry and the investor community.

With increased access to capital investment, the deployment of EGS in greenfield applications has an immense potential to supply the world with many gigawatts of clean, renewable, baseload power into the future.

Trenton Cladouhos is Senior Vice President of Research and Development at AltaRock Energy.  He holds a B.Sc. degree in Geology from Stanford University and a Ph.D. in Geological Sciences from Cornell University.  His research specialty, both in academia and industry, has been the mechanics and fluid flow in fractured and faulted rock through field work and modeling.

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