Modeling of a Liquid Phase Geothermal Doublet System at Regina, Saskatchewan, Canada

Abstract

The paper describes a computer‐mathematical model used to analyze the nature of the subsurface temperature disturbance that results from extraction of hot liquid from a geothermal production well with concurrent injection of cooled tailings water in a nearby disposal well. The model was designed specifically to determine, at an early stage of development, an appropriate well spacing for the sedimentary‐basin geothermal project at Regina, Canada. Assumptions include an infinite horizontal confined aquifer with uniform thickness and a finely dispersed pore system. The mathematical model is based on an energy balance equation that combines Darcy's law, Fourier's law, and the change in heat content of an element within the active zone of the aquifer. The numerical model is developed by means of the integrated finite difference technique. Short computer run times are achieved through the use of symmetry, an expanding grid system, and the use of the Theis solution. Investigation of model sensitivity shows the model to be sensitive to such factors as interwell distance, pumping rates, porosity, and aquifer thickness, but quite insensitive to hydraulic conductivity and storativity. Sample results are given for Regin aquifers undergoing both continuous and seasonal pumping. Isotherm maps show that the temperature disturbance evolves with time from a cooled, circular area around the injection well to a strongly noncircular pattern with a protrusion of cooled water extending toward the production well. The model is used to predict time of thermal breakthrough for Regina aquifers using different pumping rates and interwell distances. Temperature histories for continuous pumping show that after thermal breakthrough, the produced water undergoes a slight decline of temperature over a considerable time span. For the withdrawal of equal volumes of fluid, the temperature disturbance around the disposal well is spread over a slightly larger area for the seasonal pumping case compared to continuous pumping. The thermal front is broader for the seasonal case because of the longer period of elapsed time during which thermal conduction occurs. For seasonal operations, thermal breakthrough will occur after less water has been pumped and there is a more gradual decline in the temperature of the production well after thermal breakthrough.