Analysis of Sedimentary Geothermal Systems Using an Analytical Reservoir Model

Abstract

An analytic reservoir model of a sedimentary geothermal well doublet system was developed based on the work of Gringarten (1978). This model describes the minimum well spacing and reservoir transmissivity requirements as a function of reservoir lifetime, well flow rate, and well productivity index. The reservoir model assumes flow of fluids with constant properties through homogeneous porous media between an injection and production well doublet. The reservoir model was incorporated into a parametric analysis of the main factors controlling the capital costs of developing sedimentary systems using a modified version of the Geothermal Electricity Technology Evaluation Model (GETEM). Reservoir temperature, depth, well productivity index, and well flow rate were studied. It was found that for a given productivity index, there is an optimum flow rate that minimizes total overnight capital costs, and that this flow rate increases with productivity index. A second scenario showed that optimum flow rates range from just over 100 kg/s for a productivity index of 2 L/s/bar to over 275 kg/s for a productivity index of 30 L/s/bar. The reservoir model can be applied to these results to determine the required reservoir properties, such as well spacing and transmissivity, to realize the proposed cases.