Geothermal Steam Well Testing: State of the Art

Abstract

Summary Well test methods established in petroleum engineering have been applied successfully to geothermal wells. Modifications have been necessary because of property differences and distinctive geometries of geothermal fields. This paper presents a comprehensive state of the art in pressure transient analysis of geothermal steam wells. The techniques encompass drawdown and conventional buildup as well as the newer fractured parallelepiped models. The latter have been used successfully in the analysis of field data from Larderello, Italy, and The Geysers in California. Field examples follow the presentation of each technique. Introduction Two publications - by Homer and Miller et al. - have formed what generally is recognized as the basis of modem well test analysis. Significant contributions to the understanding of fundamental concepts were made by van Everdingen and Hursts and Matthews et al. The methods described in these papers have been applied successfully to geothermal wells. Notable are the publications by Ramey, Ramey and Gringarten, and Barelli et al. Recently, considerable efforts have been made to describe geothermal pressure transient analysis assuming distinct geometries penetrated by fractures. These configurations have been used successfully to describe the geothermal reservoirs at Larderello and The Geysers. The results were presented by Barelli et al., Cinco-Ley et al., and Economides et al. Drawdown testing also has generated attention. New techniques have been developed to contend with the frequent shut-ins and flow rate fluctuations. Methods that use effluent chemistry may offer an assessment of the liquid reserves. Methods outlined in this paper assume a vapor-dominated zone within the reservoir. "Immobile" liquid water throughout the reservoir has been proved experimentally by Hsieh. He discovered that absorbed water may account for as much as 15 times the amount of fluid in the vapor phase for typical conditions found in geothermal steam reservoirs. Work is now under way at the U. of Alaska on the effect of a "source term" in the solution of the diffusivity equation. Developement of Fundamental Concepts Well test analysis techniques and basic equations were derived from the solutions of partial differential equations describing fluid flow through porous media. The most familiar form of the flow equation is (1) This equation-the result of the continuity principle, Darcy's law, and an equation of state-presumesradial flow,homogeneous (constant beta and isotropic (constant k) medium,uniform thickness (constant h),fluid of small and constant compressibility (c),constant viscosity (mu),no gravity effects, andsingle-phase flow. Although some of these assumptions often are violated, they have been proved flexible and several solutions have represented real cases sufficiently. Traditionally, three major categories of "drainage area" have been considered:infinitely acting,no flow outer boundary, andconstant-pressure outer boundary. The solution most commonly encountered in the classic references of Carslaw and Jaeger and Muskat is for an infinite reservoir. The "line-source solution" in particular describes the pressure history at the wellbore. JPT P. 976^