| Abstract |
Measurement of temperatures at multiple depths in shallow, 2-meter deep probes can be used to calculate temperature gradients and heat loss associated with shallow thermal outflow aquifers of geothermal systems. In the Great Basin, heat loss at shallow depths varies significantly over the course of a year in response to temperature changes at the earth?s surface; during summer months, heat flux in large portions of geothermal areas can be negative (i.e., directed downwards) due to ground heating from solar radiation. Conversely, heat loss during winter months is much greater. Multiple measurements of shallow temperatures over the course of a year were used in this study to calculate mean annual temperature gradients at a depth of 1.5 to 2.0 meters at the Desert Peak and Desert Queen geothermal areas in Churchill County, NV, USA, and these data were combined with thermal conductivity data to calculate seasonally adjusted annual heat loss associated with these shallow aquifers. Mean annual heat loss associated with the Desert Peak shallow aquifer is estimated at 9.5 MWt. If a nearby second aquifer is included, the total heat loss is estimated at 20 MWt. At the Desert Queen area, located 10 km northeast of Desert Peak, mean annual heat loss associated with another shallow thermal aquifer is estimated at 18 MWt, based on a correlation defined at Desert Peak between average 2-meter temperatures and average temperature gradients, and assuming that climatic conditions are similar at both areas. The shallow aquifer heat loss at Desert Peak of 9.5 MWt compares well with the initial electrical production capacity (name plate capacity of 12.5 MWt as of 1985) at the Desert Peak power plant. If the second shallow heat loss zone is included, the total heat loss of 20 MWt is similar to the expanded power plant capacity at Desert Peak of approximately 25 MWt. Of course, shallow thermal aquifers are not always present at geothermal systems, nor are they always detectable with shallow temperature surveys. Nevertheless, the similarity between the shallow heat loss and power plant production capacities suggests that shallow aquifer heat loss can be used to help assess the electricity generation potential of a geothermal system. As a subset of the total conductive heat loss of a geothermal system, heat loss associated with shallow thermal aquifers is of special significance because it is fed by convection of thermal fluids from depth. This in turn implies a degree of permeability connectivity of the type sought when drilling for geothermal reservoirs. The size and strength of shallow temperature anomalies at the Desert Peak and Desert Queen areas are similar to those found at other geothermal systems in Nevada, suggesting that heat losses of 10-20 MWt may be fairly typical of shallow thermal outflow aquifers associated with geothermal systems in the Great Basin. These heat losses are much greater than those typically associated with hot springs, and they comprise a significant fraction of the total heat loss associated with geothermal systems. A number of geothermal systems with such shallow thermal aquifers have not been tested with deep drilling, and this data suggests that they may be capable of sustaining electrical energy production. |