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.
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.