Thermochemical Modelling of Cooled Brine Injection into Low Enthalpy Sedimentary Reservoirs

Abstract

The present paper addresses the thermochemical impact of the injection of heat depleted brines into the source reservoir, exploited by means of the doublet concept of heat extraction widely used in geothermal district heating (GDH) undertakings. Maximizing heat production requires to increase heat depletion i.e. to minimize injection temperatures. This trend is often enhanced by addition of heat pumps, a design making it possible to lower injection temperatures down to 20°C compared to the usual 40°C practiced by most grid operators. The present work is aimed at investigating the consequences on well and reservoir performance of this strategy, whose benefits could be offset by induced thermochemical hortcomings, such as precipitation or, to the contrary, enhanced by dissolution of sensitive mineral species. Prediction of injection-induced thermochemical behaviour was achieved by modelling rock/water interaction using two solute reactive transport software packages, SHEMAT and TOUGHREACT, applied to two case studies representative of the sedimentary, carbonate and clastic, environments hosting most GDH-eligible reservoirs. Simulation of reactive transport kinetics tends to show that although porosity changes caused by either temperature-induced precipitation or dissolution of sensitive species (mostly calcite, anhydrite, silicates) did occur, their magnitude (hardly a few percent losses/gains) did not significantly affect porositiy neither permeability nor subsequent reservoir performance. This is likely a consequence of the fairly low temperature contrasts (? 50°C) in the contemplated solubility ranges.