Early evaluation of geothermal steam reserves is a key step toward rapid and proper development of such reserves. The term "reserves" indicates the quantity of geothermal fluid which can be produced at an acceptable rate and for a period of time sufficient to make the development investment reasonable and profitable. Unfortunately, there has been a general belief among technologists that reservoir-engineering methods developed in oil and gas production are not applicable to geothermal fluid production. Although some modification of existing reservoir-engineering methods is necessary, basic principles are to a large extent applicable.

Many geothermal fluid reservoirs currently being produced are believed to be dominated by fracturing and to be in communication with aquifers recharged by surface water. Many such oil and gas reservoirs have been recognized, and methods for forecasting their behavior have been developed in the last two decades. One very useful body of literature exists in connection with oil and gas well test analysis. The effects of fractures, production limits, fluid sources, and other pertinent reservoir features have been thoroughly documented. Such tests and their interpretations can be utilized, much more than has been appreciated, in geothermal wells.

The heat transmission involved in geothermal fluid production is a complication not commonly involved in reservoir engineering. However, the development of processes involving the subsurface combustion of crude oil and the displacement of crude oil by steam and hot water injection led to important literature concerning reservoir and well-bore heat flow. Application of existing and new reservoir engineering techniques to geothermal fluid production should lead to better understanding of this class of reservoirs and to more rapid development of this significant resource.

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