John R. Muir, Sr. VP Business Development, GreenFire Energy
Despite its potential, geothermal power generation has always been a tough business. Finding the three components necessary for conventional geothermal projects – heat, water, and subsurface permeability – in the right proportions at the same site is comparatively rare. But even then, developing a project has involved high risk, long lead times, a lot of money, and dogged patience. The heart of the problem is the unpredictable nature of subsurface rock formations that resist easy characterization or modification. Consequently, every project, and every well, is a custom effort.
To address this complexity, geothermal developers have tried three general strategies. First, they have tried to reduce risk and improve production through continuous improvements in geophysical analytical tools. Second, they have tried to alter the resource itself by creating engineered fracture systems, a process known as EGS. While improvements in tools and EGS are still ongoing, the third approach, frequently referred to as “closed loop” well systems, has yet to be investigated at field scale.
The conceptual simplicity of closed loop systems is inherently appealing because a sealed downhole heat exchanger obviates the need to deal with the full complexity of subsurface formations. Drilling risk is reduced because intersecting natural fractures with flowing water is not essential; indeed, the problems associated with maintaining water circulation through rock are avoided. Finally, having a “closed loop” system to circulate the working fluid also reduces the problems of corrosion, non- condensable gases, other chemical reactions, and dealing with particulates. But, as has been asserted for a long time, closed loop systems have a major drawback – with limited surface area they can’t absorb enough heat to be economically viable. Or can they?
GreenFire Energy (GFE) has performed extensive modeling of the thermodynamics and costs of closed loop systems. This research suggests that closed loop geothermal production can be both technically viable and commercially attractive, particularly in two environments:
- Where very hot geothermal resources are available for greenfield projects and drilling can go “hot and deep,” such as at many locations around the Ring of Fire; in these cases, power is principally generated from conduction of heat from the resource (Full Scale ECO2G™), and
- Where conventional hydrothermal wells have become unproductive despite hot bottom hole temperatures. Many of these wells can be retrofit with appropriate technologies (typically involving a closed loop system but with proprietary enhancements) to make such wells productive. Substantial power is generated by heat transfer from, and coproduction of, geothermal brine (Retrofit ECO2G™).
In April 2019 GreenFire Energy will conduct the first field-scale test of a closed loop system using an idle well in the Coso geothermal field. First phase testing will compare the performance of water and supercritical CO2 (sCO2) as the production fluid in a closed loop system. Test results will be compared with the models that GreenFire has developed as a result of four years of research with the US DOE, national laboratories, and a variety of expert consulting firms. GreenFire will test at various flow rates of fluid in the closed loop, while also varying the brine flow conditions (including non-flowing conditions). This will be used to support and verify both Full Scale ECO2G and Retrofit ECO2G modeling.
The physical aspect of the project is conceptually straightforward. GreenFire will insert a tube-in-tube assembly into the well to a depth of 1000 feet. For testing with water, the water will flow down the outer tube to collect heat and then rise to the surface through the inner, vacuum insulated tube (VIT). For sCO2 the flow path will be reversed. Additional testing will involve the coproduction of brine to the surface to assess the potential for additional energy to be extracted. Careful measurements will be taken to determine the amount of power that could be generated using the conventional steam turbine equipment in operation at the site, or that could be generated by a wellhead generator designed for use of sCO2. Importantly, a thermosiphon will be created and measured in the system with both water and sCO2.
The GFE project involves a series of technical challenges relating to testing both water and supercritical CO2 in the same apparatus, particularly because of the high pressure involved with sCO2. The design also needed to include pumps and makeup tanks suitable for both types of fluids. Engineering for the project was provided by Veizades & Associates working closely with GreenFire and Coso Operating Company.
The Coso project was made possible via a major grant from the California Energy Commission from the GRDA fund. Additional funding was obtained from the Shell Oil “GameChanger” program and from the Electric Power Research Institute and its member J-Power, a major Japanese utility that operates its own geothermal fields. Even with this level of funding, the project could not have been successfully completed without the extraordinary cooperation, expertise, and support of Coso Operating Company, operator of the site.
All major equipment and materials have been delivered to the site and construction is in process. It is anticipated that active testing for the first phase will begin in late April of this year and will be completed in May. A final report of testing results will be made available to the California Energy Commission and project participants later this year.