The increased awareness and interest in renewable resources has raised the need to standardize the reporting requirements for geothermal resources that can be applied worldwide. As no agreed standards, guidelines or codes exist, there remains too much latitude in geothermal assessment, which leads to more resource uncertainty, more investment risk and less confidence in development. Standardizing the reporting of geothermal resources is a major challenge as it is difficult to define what the target actually is: the source, the reservoir, the fluids, the stored heat, the recoverable volume, the recoverable heat, the recoverable power, or the net profit. Formulating an agreed procedure to classify geothermal resources is further complicated by changing environmental, policy and regulatory constraints around the globe.

This paper provides a detailed review of published geothermal classifications, which contrasts and compares them with reporting requirements from the petroleum and mining industries. Present day techniques of computing geothermal resources provide only ballpark estimates at best. The classification and quantification of different geothermal occurrences will differ, depending on their static thermal energy in place, recoverable thermal energy, or net transformed energy.

An integrated approach to estimate geothermal resources is suggested that would couple the modeling of volumes and flows at subsurface and surface conditions, possibly allowing the future inclusion of technological advances to exploit geothermal resources that were previously reported to be sub-commercial.

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