Characterizing the Power Potential of Hot Stratigraphic Reservoirs in the Western U

Stratigraphic reservoirs with high permeability and temperature at economically accessible depths are attractive for power generation because of their large areal extent (> 100 km 2 ) compared to the fault controlled hydrothermal reservoirs (< 10 km 2 ) found throughout much of the western U.S. A preliminary screening of the geothermal power potential of sedimentary basins in the U.S., assuming present day drilling costs, a levelized cost of electricity over 30 years of ≤ 10c/kWh, and realistic reservoir permeabilities, indicates that basins with heat flows of more than about 80 mW/m 2 , reservoir temperatures of more than 175°C, and reservoir depths of less than 4 km are required. This puts the focus for future geothermal power generation on high heat flow regions of California (e.g., the Imperial Valley and regions adjacent to The Geysers), the Rio Grande rift system of New Mexico and Colorado (especially the Denver Basin), the Great Basin of the western U.S., and high heat flow parts of Hawaii and the Alaska volcanic arc. By far the largest area of high heat flow is within the Great Basin. Here the highest, regionally extensive temperatures at depths of less than 4 km exist beneath the late Tertiary to Recent basins. Basins with more than 2 km of unconsolidated sediments are the most attractive because of the insulating effects of these sediments. The challenge is to locate the hottest basins with potential reservoirs within the underlying bedrock units. Not all basins of the Great Basin have high heat flow, and adequate permeability for geothermal power production may not exist beneath some high heat flow basins. The lower Paleozoic carbonate units beneath the eastern Great Basin are known to be locally very thick (up to 5 km), and commonly have high permeability. A review of permeability measurements at 3 – 5 km depth from petroleum and groundwater wells for the Great Basin and adjacent Rocky Mountains shows carbonates have the highest permeabilities (median value of 75 mDarcy), followed by siliclastic units (30 mDarcy). These values are sufficient for geothermal reservoirs. Intrusive and volcanic rocks have much lower overall permeabilities at depth and unless fractures are induced artificially, can be expected to be relatively poor candidate reservoir rocks. In contrast to many oil and gas producing basins in the U.S., there is no evidence of over-pressures at depths of 3 – 4 km within the eastern Great Basin. The two major challenges to development are the identification of the hottest basins and characterizing the permeability at economically drillable depths. The latter must include the roles of low and high angle faults and mineral dissolution in locally enhancing stratigraphically controlled fluid flow. Large-scale power production from these reservoirs may require the application of enhanced permeability techniques such as acid treatment of carbonate reservoirs and hydrofracturing. Reducing or minimizing the drilling costs by repetitive drilling in these basins to 3 – 4 km depth may be important for the economics, as is the possibility of solar augmentation of power generation.