| Abstract |
Petroleum exploration wells confirm that the high permeability and high flow rates needed from geothermal production supporting large-scale power development can be found in deep stratigraphic reservoirs (> 3 km depth). Data from drilling in the Rocky Mountains and Great Basin of western U.S. show carbonate reservoirs at depths of 3 ? 5 km have slightly better average permeability than siliciclastic reservoirs (75 versus 30 mDarcies). These values are sufficient for high-flow-rate geothermal production wells. Deep wells in two Rocky Mountain basins also show that carbonate reservoirs, possibly dolomitic, can preserve high permeability when the temperatures are 220 - 240°C at more than 5 km depth. There may be a relationship between widespread, good stratigraphic permeability, and reservoirs being at hydrostatic pressure. If true, this may imply that over-pressure is a negative indicator for a large geothermal reservoir. Conventional oil well production flow rates are usually significantly lower than that required for geothermal power production, but this is due to oil viscosity being at least ten times higher than hot water, rather than low permeability reservoirs. The target conditions for stratigraphic geothermal reservoirs are temperatures of 175 - 200°C and depths of 3 ? 4 km. These conditions can be found within basins where the heat flow is about 90 mW/m2, the average heat flow for the Great Basin. The eastern Great Basin is underlain by a lower Paleozoic carbonate section that ranges up to 3 km in thickness and is known to have good permeability. Numerous reservoir targets where temperatures are 175 - 200°C at depths of 3 ? 4 km, and good stratigraphic permeability is known or inferred have been identified in the Great Basin. The large areas of these reservoirs (~ 102 to 103 km2) can each support power plants of more than 100 MWe. |