The precipitation of solids from potential heat exchange fluids for use in a hdr geothermal system in granite

Abstract

An important consideration in the development and operation of a Hot Dry Rock geothermal system is the selection of a heat transfer fluid and the chemical composition of this fluid during circulation. The chemical reaction of the circulation fluid with the reservoir rock may lead to the undesirable corrosion or scaling of the reservoir itself, or associated engineering structures. Two potential circulation fluids for use in a high temperature (200°C) HDR system in granite in SW England, are a dilute ( TDS < 120 mg/1 ) groundwater, and a modified seawater composition. The reaction of these fluids with granite has been evaluated experimentally, with particular emphasis upon the characterisation of solid precipitates. Secondary solids associated with the reaction of groundwater with granite consist of clay and Ca-zeolite. Product fluids were alkaline (pH 9.1), of low salinity ( TDS < 600 mg/1 ) and were relatively benign for heat exchange purposes. The amount of clay precipitated may be linked to the amount of Mg in the fluid, but is less than 0.5 wt percent of the initial solid starting material. Chemical analysis of precipitated clay by analytical transmission electron microscopy reveal a range of composition between illite and smectite. Secondary solids associated with seawater-granite reaction include anhydrite, magnesium hydroxide sulphate hydrate (MHSH) and clay. The precipitation of MHSH and clay is instrumental in governing the low pH (pH 3.5) of the product fluid, which would pose problems concerning the corrosion of pumps, heat exchangers, etc in a possible HDR geothermal system. The suitability of each of the potential heat exchange fluids may be linked to their initial Mg contents which govern the acidity of the reacted fluid and the amount of precipitated clay.