A Desktop Review of Calculation Equations for Geothermal Volumetric Assessment

Abstract

The volumetric method has been used for geothermal resource assessment since it was proposed by USGS in 1970s. While the originally proposed calculation method has been in practice, two other calculation equation sets have also been used under the name of volumetric method or heat-in-place. Different ranges of numerical value for three variables of reference temperature, heat-electricity converting efficiency and/or recovery factor are selected for those methods, most of them without clear explanations on why or how those are selected. As calculation results of geothermal electricity capacity largely depend on these three variables, resource assessments without describing clear explanations may sometimes have resulted in unclear conclusions; with what conditions could an investor get this much electricity energy? Or is this feasible for our field? This paper reviews the origins of reference temperatures, heat-converting efficiency and recovery factor being adopted by the three different calculation methods, thereby, clarifies their applicability and limitations for use. Through the review, we pointed out a small but theoretically fundamental modification should be necessary to the original USGS method. We also found that recovery factor being used by the three equation sets should be used as a temperature dependent variable in theory, although it was originally defined as an independent of reservoir temperate.