Analytical and numerical modelling of cold water injection into horizontal reservoirs

Abstract

For reasons of environmental protection, reinjection of produced geothermal fluid after use is an important component of most geothermal projects in Ukraine. The geothermal reservoirs will cool down due to the reinjection. Therefore, the temperature of production wells may decrease. This problem can be avoided by a proper location of injection wells in order to minimise the cooling of production wells. For this purpose several analytical and numerical models of 1-, 2- and 3-D reservoirs are presented in this paper. The study shows that there is a good correlation between the analytical and the numerical models, obtained by applying the TOUGH2 simulator. Numerical dispersion was, however, of concern when simulating moving temperature fronts in the reservoir, but was minimised by increasing substantially the number of model elements. Tracer and thermal breakthrough times obtained by numerical modelling confirm the well known fact that chemical breakthrough occurs much earlier than thermal breakthrough. Finally, a safe distance between injection and production wells appears to be in the range 500-1000 m estimated by a 3-D numerical model. All the above modelling conclusions need to be recalculated when more reservoir data becomes available in Ukraine.