Modeling Flexible-Mode Geothermal Energy Production in California: Comprehensive Physical-Chemical Modeling to Reduce Risks and Costs of Flexible Geothermal Energy Production

In this project, numerical modeling tools were developed and applied to study flexible-mode geothermal energy production from typical California geothermal systems. The project focused on the impact of conversion from steady baseload to flexible-mode production on technical challenges such as well integrity, reservoir performance, and mineral scaling and corrosion in production wells.

Mechanical well integrity modeling showed changes in temperature and pressure of trapped fluids that can potentially cause mechanical failure in casing, cement, and adjacent host rock. It was found that the biggest risk of mechanical failure occurs during the initial startup of production because of large and rapid temperature increases from initially cool temperatures near the ground surface. However, if production cycling is carefully controlled, e.g., ramping up production slowly and not completely shutting down production, these impacts on the well assembly can be minimized.

The reservoir modeling showed little impact of variable production on reservoir performance because of limited pressure and temperature perturbations in the reservoir. Reservoir performance is affected by cold-water injection into geothermal reservoirs since the fracturematrix heat exchange is a function of the shape and size of low-permeability matrix blocks and flow channels (e.g., fractures, fracture zones). The reservoir away from injection wells often acts like a single continuum with equilibrated temperature, which retards thermal breakthrough.

The reactive chemistry modeling showed that mineral scaling and corrosion can be controlled by keeping the wellhead pressure above the saturation pressure, while at the same time keeping the temperature above the silica saturation temperature. In a steam-dominated system, corrosion can be limited by avoiding condensation that can occur during production curtailment.

The modeling tools developed and demonstrated in this project can be applied to any new geothermal sites to develop site-specific operational strategies for safe variable geothermal production at reduced costs.