Through the history of geothermal exploration and development a number of techniques have been applied to the problem of characterizing geothermal resources. USGS research in the 1970s established the volume method as a consistent and accurate approach to geothermal resource assessments, provided adequate data are available on the size and thermal state of potential geothermal reservoirs. Recent papers have noted a rough proportionality between the rate of natural heat loss (both advective and conductive) and electric power production capacity of geothermal reservoirs and suggested that the proportionality can be applied in geothermal resource assessments.

This study addresses the question of whether heat flow measurements alone can be used to estimate the approximate production potential of geothermal systems or at least provide a basis for assessing the magnitude of the resource. Results from a suite of numerical models for heat transport from reservoirs of varying shapes, sizes and depth extents support the observation that the recoverable thermal energy of a reservoir can vary over a wide range with little impact on the corresponding surface heat flux. The heat flow approach does not provide information equivalent to the volume method, as the rate of heat transport from a geothermal reservoir depends primarily on the temperature, lateral extent, and depth to the top of the reservoir.

By contrast, the thermal energy and production potential of a reservoir depends primarily on the temperature, lateral extent and vertical thickness. The depth extent of a reservoir does not necessarily have a strong influence on the magnitude of the resulting surface heat flow anomaly, and the deeper the top of a reservoir the smaller is the thermal contrast with the surrounding conductive crust. To some degree the apparent proportionality between heat loss and production capacity reflects the limited range of the depth to the top of producing reservoirs. Transient effects due to cooling or warming over geologic time, as wells as thermal anomalies arising from heat sources unrelated to fluid circulation, such as the shallow emplacement of magma, further complicate the relationship.

As noted by others, for those geothermal reservoirs at or near a thermal steady state, near-surface heat flow measurements provide a direct measurement of the natural heat flux required to maintain the hydrothermal system and thus provide an approximate estimate of the potential sustainable or renewable level of production. However, heat flow is best applied as a tool to estimate the thermal energy of a reservoir if the spatial distribution of the observed heat flow anomaly, or complementary geological, geochemical or geophysical data, can be used to constrain the reservoirís temperature and subsurface geometry.

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