Advanced Decline Curve Analysis in Vapor-Dominated Geothermal Reservoirs
Authors: Faulder D.D. 
Published: December 2007

Abstract
The Fetkovich decline curve analysis method was extended for application to vapor-dominated geothermal reservoirs to estimate the permeability-thickness product (kh) from a well's transient production response. The analytic dimensionless terms for pressure, steam flowrate, decline rate, and decline time were derived for saturated steam using the real gas pseudopressure and customary geothermal production units of pounds-mass per hour. The derived terms were numerically validated using "Geysers-like" reservoir properties at permeabilities of 1, 10, and 100 mD and at a range of initial matrix liquid saturations from 0 to 100%. The rate-time responses collapsed onto a single set of curves using the derived dimensionless terms, validating the derived dimensionless equations. This collapse was accomplished by including the effective compressibility of a boiling liquid or by an alternative formulation considering an apparent time. This technique was applied to actual field production data at The Geysers, California, the world's largest developed geothermal, vapor-dominated reservoir. The production data for over 100 wells in the southeast Geysers were analyzed and the permeability-thickness products determined by application of the derived analytic dimensionless terms and the Fetkovich production decline type curve.

Introduction
Reservoir engineering studies of The Geysers are hampered by a lack of formation evaluation techniques typically used to characterize hydrocarbon reservoirs. An improved method is needed for an understanding of the reservoir permeability distribution and to develop a better conceptual reservoir description and numerical model.
The goal of this study is to extend the Fetkovich production decline method to vapor-dominated geothermal reservoirs using customary geothermal production mass units with particular emphasis on the transient production response for the determination of the kh. Analytic expressions for dimensionless pressure, dimensionless production rate, dimensionless decline time, and dimensionless decline rate are derived for saturated steam in a porous media. A "Geysers-like" numerical model is used to validate the analytic terms. The derived dimensionless expressions are applied to a set of wells located in the southeast Geysers to demonstrate the practical utility of this approach for estimating permeability-thickness from the transient production response and the identification and quantification of interference effects.

Literature Review
Production decline curve analysis has been used at The Geysers since 1969 when Ramey demonstrated through the use of material balance calculations, the p/z method modified for geothermal reservoirs, and production decline curve analysis, that The Geysers shallow steam reservoir was undergoing depletion. Empirical rate-time semi-log analysis using Arps method is the standard method used to forecast remaining steam reserves for individual wells and leases at The Geysers.
Fetkovich noted that the concepts of dimensionless pressure and dimensionless time, used in pressure transient analysis, could be used to analyze the transient production response of a well. A dimensionless production rate was defined as the reciprocal of dimensionless pressure. Two additional terms were defined; the dimensionless decline time and the dimensionless decline rate. These two terms were combined to completely describe the transient production response and the depletion or pseudo-steady state period of a well producing at a constant pressure. P. 139^