Numerical Simulation of Chemically Induced Permeability Changes During Heat Mining in Hot Aquifers

Abstract

SHEMAT is a new finite difference code for simulating hydro-geothermal reservoirs. Flow, heat transfer, transport of dissolved substances, and geochemical reactions are mutually coupled. Dissolution and precipitationof matrix minerals are calculated by an extended version of the geochemical modeling code PHRQPITZ (Plummer et al., 1988). Chemical water-rock interactions change the porosity which influences flow and transport properties of the reservoir. In SHEMAT this is taken into account by a fractal model of the pore space case study of the injection well of a geothermal installation has been carried out, focussing on the near vicinity of the borehole. The injection of cool water exerts a great influence on the hydraulic conductivity of the aquifer indicating continuous head pressure increase at the well. Dissolution and precipitation of anhydrite induced by the temperature changes increase the permeability of the formation in a growing region around the borehole and decrease it at the temperature front. In the initial period of aquifer exploitation considered here the negative effect of the cold water on the injectivity is partially compensated by the relocation of anhydrite..