THERMAL ENERGY EXTRACTION FROM A GEOTHERMAL RESERVOIR: NUMERICAL AND ANALYTICAL MODELING ANALYSIS

Understanding and predicting the temperature evolution of geothermal reservoirs under normal operating conditions is a priority for geothermal power plant operators, which requires knowledge of both the thermal energy and fluid flow pathways within the reservoir. Geothermal reservoirs in the deep subsurface often rely heavily on geologic media with a transmissive fracture network to serve as conduits for heat and water transport. As such, reservoir temperature evolution is highly constrained by heat transport processes occurring within and immediately surrounding fractures throughout the reservoir. As part of the integrated “PoroTomo” experiment at the Brady geothermal site, near Reno, Nevada, borehole pressure data were collected at two observation wells for a period of 30 days under various operational conditions, and borehole temperature data was collected in one observation well for a period of five days. Borehole pressure data indicate 2-D flow with rapid pressure responses to changes in pumping / injection rates, likely indicating fracture-dominated flow within a complex fault system. The temperature data show borehole temperature recovery following a 15 m³ cold water slug injection. Late time temperature vertical profiles show an average geothermal gradient of 0.23°C to a depth of 265 meters, where a temperature inversion is observed and the average geothermal gradient changes to -0.78°C to a depth of 370 m (the end of the DTS cable). The vertical temperature profile likely indicates production-related thermal drawdown associated with a transmissive fracture intersecting the borehole. A numerical heat transfer model is developed and used in conjunction with the early-time temperature data to determine the thermal diffusivity of the reservoir rock immediately surrounding the borehole. Using other analytical solutions, we assess to what extent observed data are consistent with historical energy extraction at the site, and comment on issues such as predicting future reservoir temperature evolution.

The work presented herein was funded in part by the Office of Energy Efficiency and Renewable Energy (EERE), U.S. Department of Energy, under Award Number DE-EE0006760.