Decline curves are commonly used at The Geysers geothermal fleld to assess the generating capacity of a producing lease. It is generally assumed that wells will initially be drilled using 40-acre (400 m) spacing, with infill drilling used later to provide additional producing wells as needed. It is commonly believed that the final well spacing should not be less than 10 acres (200 m). Decline curves are.used with this approach to estimate the number of make-up wells during a project lifetime (up to 30 years), as well as the appropriate plant size (MWe). Problems arise when one must choose the proper decline curve method because there does not appear to be a sound basis for choosing between the standard types of curves (hyperbolic, harmonic or exponential). Budd (1972) published theoretical decline curves for The Geysers, but it is not clear how he derived these curves (Fig. 1). Dykstra (1981) developed an average decline curve based on production data available in the open literature. Lack of data made it necessary for Dykstra to average results from wells with different spacings to obtain a single decline curve; this limits the applicability of his result. Aside from these results, there are no published decline curves for The Geysers that take into account variability in the parameters controlling production decline, such as permeability, porosity, and fracture spacing.

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