(149f) Predicting Thermal Conductivity of Geothermal Reservoir Rocks

Understanding rock temperatures and stress fields will be important in predicting fracture behavior during enhanced geothermal reservoir development. For many geothermal reservoirs, thermophysical property data (coefficients of thermal expansion, thermal conductivity and specific heat) are limited. Thermal conductivity, diffusivity and specific heat are relevant for quantifying the power potential and forecast for prospective and operating geothermal plants and govern the way temperature gradients move through the reservoir. The coefficient of thermal expansion and certain other mechanical parameters of the reservoir such as Young's modulus and Poisson's ratio intrinsically contribute to thermal alteration of in-situ stresses in response to temperature perturbations during reservoir development by hydraulic fracturing and during heat production.

Recognizing the lack of a substantial database of thermophysical properties, standard geophysical well logs and mineralogic analysis of core and cuttings from the Raft River Geothermal Field have been used to predict thermal conductivity. Predictions are compared with the experimentally-determined values from core samples collected from Raft River Geothermal well RRG-3C.

The work will support ongoing investigations to thermally and hydraulically stimulate well RRG-9 under a US DOE sponsored grant. RRG-9 was drilled to a 6089 feet measured depth (MD) in the Raft River Geothermal Field located in southern Idaho, near the Idaho-Utah state line. The field lies within the Basin and Range province. In the 1970s to early 1980s, the field was the site of intense study by the US Department of Energy. During that period a significant amount of geologic and reservoir data were collected. The reservoir is developed in Precambrian metamorphic and granitic rocks beneath approximately 5000 ft of Tertiary volcaniclastic deposits. U.S. Geothermal, Inc. operates a 13-MW plant at the site.