A Reexamination of USGS Volumetric Heat in Place Method

Abstract

The USGS volumetric estimation method together with Monte Carlo simulations is often used to provide estimates of the probable electrical generation capacity of a geothermal system. The methodology consists of combining probability density functions for uncertain estimates of the temperature, area, thickness, and thermal recovery factor of a geothermal reservoir to obtain the probability distribution function for the stored energy (“heat in place”) and the recoverable heat. The electrical capacity of the potential geothermal reservoir is then computed using a conversion (or utilization) efficiency. In a previous paper, we discussed the importance of choosing the probability distributions of the reservoir parameters based on actual field data. Herein, we examine the specification of the reference temperature and the conversion efficiency. We show that the conversion efficiency depends on both the reference temperature and the power cycle (flash, binary). A proper understanding of the latter relationship is essential for obtaining estimates of reservoir capacity for electrical generation.