Classification of Geothermal Systems - A Possible Scheme

Abstract

At the request of the United States Department of Energy, the author was asked by the Geothermal Energy Association to prepare a white paper on this topic. While many possible criteria can be used to classify geothermal resources most would agree that reservoir temperature should be the primary, if not the only, criterion. This paper offers a possible scheme in which geothermal resources are classified into seven categories based on temperature: non-electrical grade (<100?C), very-low temperature (100?C to <150?C), low temperature (150?C to 190?C), moderate temperature (190?C to <230?C), high temperature (230?C to <300?C), ultra-high temperature (>300?C), and steam fields (230?C to 240?C).

The scheme is based not only on temperature but also according to a set of additional attributes important for practical utilization of the energy: (a) type of power generation technology applicable, (b) fluid state in the reservoir (a controlling factor in reservoir performance), (c) production mechanism and the state of the fluid at the wellhead (which influence operational economics), (d) factors other than temperature that control well productivity (these factors affect the optimization of field development and operation), and (e) unusual operational problems that impact power cost (such as scaling, corrosion, high content of non-condensable gases, etc.). The paper discusses the rationale for this scheme and why some other possible schemes were not considered. The author invites comments on this scheme that may lead to a generally accepted one.