THEORETICAL FRACTURING AND POWER PRODUCTION FROM LOW PERMEABILITY TUFFS IN THE SNAKE RIVER PLAIN, SOUTHERN IDAHO

The potential for using geothermal reservoirs as a contribution to base-load energy is becoming increasingly feasible economically and technologically with the continuing development of enhanced, or engineered, geothermal systems (EGS). EGS augments natural thermal reservoirs within the crust that lack either the permeability or hydrothermal water necessary for power production using traditional geothermal power plants. Typically, high-temperature systems are found in regions of high strain rates and geothermal gradients, such as the basin and range province. EGS has the potential to access the stored heat in regions with lower strain rates and non-ideal primary or secondary permeability. Recent boreholes drilled to a depth of 2 km in the hotspot project have reached massive welded ashflow tuffs that potentially represent one flow unit. Borehole scans reveal very little fracturing, supporting the use of the tuff unit as an EGS test site. We present an analytical solution using production targets of ~63 l/s (1000 gpm) for 20 years that provides theoretical fracture spacing, apertures, flow path lengths, and production rates that an EGS system would need to produce power in the snake river plain (SRP) in southern Idaho. The solution does not consider the thermomechanical properties of the reservoir that control the propagation of fractures through the unit at a given depth, thus we consider prior testing performed on tuffs. Together, the model and tuff properties allow us to give a range of possible reservoir geometries to be expected when fracturing the unit, as well as an estimate of potential power output. The work presented here provides a low-cost and quick exploratory method of testing regions for their potential as enhanced geothermal fields using basic geologic information of the site.