Anhydrite deposition in Cawayan wells, Bacman geothermal field, Philippines : prediction and possible remedies

Abstract

The anhydrite mineral deposition inside the wellbores of Cawayan wells is investigated in order to predict its occurrence and come up with possible remedies. The chemistry of the fluids is thoroughly evaluated and some geochemical indicators of deposition such as Ca2+, SO42-, Mg2+, SiO2, and Cl- are established. The behaviour of various calcium and sulphate species during deposition, and the effects of temperature and pH are determined using the SOLVEQ and WATCH 2.1 chemical speciation programmes. Results show that with increasing temperature, the activity of sulphate and calcium species in solution decreases and consequently anhydrite supersaturation is increased. At low pH, sulphate activity is significantly reduced by the formation of bisulphate ions, and anhydrite undersaturation is attained; at high pH, undersaturation is also predicted because of decrease in calcium activity. However, the solution becomes supersaturated with respect to calcite at high pH. The CHILLER programme is utilized to simulate fluid-fluid mixing, and determine the effects of NaCl addition, Na2HPO4 addition, acid treatment, and CaCl2 injection on anhydrite deposition. Results show that NaCl, Na2HPO4 and HCl are possible inhibitors of anhydrite deposition. Increasing the salinity of production fluid before mixing with high-sulphate waters tends to retard deposition and increase the amount of mixing fluid necessary to initiate formation of anhydrite. Na2HPO4 prevents anhydrite deposition by significantly reducing activity of calcium through association with phosphate ions. However, the solution tends to become supersaturated with respect to apatite, which may be deposited. CaCl2 addition can induce deposition in acid zones and thus seal them off but this further lowers the pH of the high-sulphate acid fluid as bisulphate dissociates into sulphate ions and releases hydrogen ions.