**1. Introduction**

The exploration and development lead time of a geothermal prospect directly affects its profitability because of the yearly interest factored into the cost. Sufficient profits purely produced by geothermal resources without financial support from other budgets are essential to enhance the development of environmentally friendly geothermal resources. This is why we cannot avoid trade-offs between exhaustively studying a geothermal reservoir and rapidly advancing a geothermal project.

**Figure 1** shows an example of a fiscal annual schedule during an explorational and developmental project in which the author was involved as a reservoir engineer. The project was conducted in a snowy and mountainous area in northeastern Japan, where several geothermal projects have been conducted over the last decade [1]. In such areas, the schedules of drilling, well testing, and any other work at a site are

#### **Figure 1.**

*Example of an annual schedule during an explorational and developmental project.*

strictly limited by the snowy season. In addition, mountainous conditions limit the number of site locations that can satisfy the following conditions:


Under these severe limitations, project personnel usually identify a small number of possible locations following a large amount of effort, rather than easily selecting a location from a large number of options.

Geoscientists and engineers have only a few weeks to update a reservoir model by analyzing, considering, discussing, and updating their understanding of a geothermal system after collecting all the new data from a given year (**Figure 1**). Under such severe conditions, it is essential to account for the updated reservoir model when planning for the subsequent fiscal year. This chapter describes a concept and techniques to construct and update a reservoir model, as well as to estimate the reservoir productivity, by making the most of the highly limited time available during an active explorational and developmental project. The concept and techniques are based on the author's experience as a reservoir engineer in a real project even though specific information regarding the project, including the observational data, cannot be shown because of confidentiality reasons. The techniques described in this chapter have been partially reviewed and published in several articles and proceedings. This chapter focuses on practical procedures to construct a reservoir model by assembling these techniques.
