Caldwell Ranch: Innovative Exploration Technologies Yield Geothermal Potential

July 11, 2013

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The Energy Department's Caldwell Ranch project is located at The Geysers field in northern California, the largest geothermal complex in the world.

As part of a geothermal exploration effort to search for geothermal resources nationwide, a $5 million U.S. Department of Energy investment to Calpine Corporation this year culminated in the confirmation of an initial 11.4 MW of equivalent steam — 50% more than early estimates — from three previously abandoned wells at The Geysers geothermal field in northern California. Situated in the largest operating geothermal complex in the world, Calpine Corporation's Caldwell Ranch Exploration Project is a first-ever achievement to re-open an abandoned steam field, validating universally applicable technologies that bear immediate implications for other geothermal-rich regions of California—Coso, Salton Sea, and Medicine Lake. The project faced down barriers to bring commercial increases at unproductive geothermal wells nationwide and created 43 full-time temporary jobs and tax revenues to Sonoma County and the state of California.

Two technical achievements secured the success at Caldwell Ranch. Key components of the demonstration were Calpine's dramatic improvement of reservoir pressure – and consequently, productivity of the wells – and the lowering of carbon dioxide concentrations and other deleterious gases in the steam by more than 65%. This project also took the innovative step of combining data sources, including temperature logs and isotopic measurements, to better define and model the 3-D volume of the geothermal reservoir. These innovative exploration technologies could significantly lower the costs of geothermal development by avoiding risks associated with drilling and stranded assets. While modeling is not a new concept to geothermal development, pinpointing the reservoir and accurately measuring volume in this replicable model will better target drilling and prevent overestimating reservoir reserves.

Calpine further estimates that, in combination with a portion of the presently undeveloped enhanced geothermal system (EGS) demonstration project to the west, Caldwell Ranch could produce up to 45 MWe equivalent of steam. A proposed power plant has been permitted, and construction is dependent upon Calpine's ability to secure a long-term power purchase agreement.