The Use of Calcite Antiscalants at Rotorua Geothermal Field

Abstract

Both calcite and anhydrite deposition occurs in geothermal wells in the Philippines. In an attempt to predict calcite deposition and deternine a deposition mechanism for anhydrite the solubilities of these minerals have been calculated using a computer algorithm which calculates chemical speciation in saline fluids at high temperatures and which can model the processes of flashing and mixing. Calcite and anhydrite are both close to saturation in preflashed geothermal fluid s in the Philippine geothermal fields studied. After flashing however, calcite is computed to become significantly oversaturated in the majority of wells studied whereas anhydrite is computed to become significantly undersaturated. Calcite scaling problems, however, have been restricted to low temperature wells . Mixing of high sulphate waters with moderately calcium rich reservoir fluids is concluded to be a strong depositional mechanism for anhydrite deposition in wells and may provide a technique to control high sulphate inflows.