Coso Operating Company, LLC (COC), and the Energy and Geosciences Institute (EGI) at the University of Utah have been granted funding from
the Department of Energy to develop an enhanced geothermal system (EGS) at Coso. Coso is an operating geothermal plant that will provide an
excellent opportunity to experiment with methods for enhancing the geothermal reservoir through hydraulic, thermal, and chemical stimulation. Any additional energy produced at this plant can be used immediately. However, stresses to casing and cement during reservoir enhancement could result in the movement of steam around the outside of the casing
string if the cement fails, causing lost steam production and possible safety hazards.

COC and Halliburton, a partner in the study team for the EGS project, are using Halliburtonís advanced WellLifeô analysis software to predict stresses on casing and cement in a wellbore subjected to the temperature and pressure changes planned for the project. A number of cementing options were modeled, including foamed cement and cements
resistant to attack by wet CO2. Near-wellbore stresses and rock physical properties collected during early phases of the Coso EGS project provided input to the model. Data collected by Brookhaven National Laboratory and Halliburton on the physical properties of cements were also used in the model.

The modeling included pressure changes during fracture breakdown testing and thermal cycling in the production well, as well as hydraulic stimulation and thermal stimulation in the injection well. Results
indicate that tensional stresses are most likely to cause failure. Foamed cements, which are both resilient and non-shrinking, fared the best under both temperature-induced and pressure-induced stresses. Conventional nonshrinking cements also showed a reduced risk of failure.

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