Calcite Inhibition Field Trials at the Mindanao Geothermal Production Field (MGPH), Philippines

Abstract

Wells APO-1D and SP-4D at the Mindanao Geothermal Production Field (MGPF) experience calcite deposition causing output declines of 0.6 MWe/month and a need for annual workovers. An attempt to inhibit calcite deposition at APO-1D by injection of a 10% solution Nalco 9354 inhibitor at 10 L/hr using relatively large diameter (ID of 2.4 cm and OD of 3.4 cm) carbon steel sucker rods to a depth of 950 meters was not successful. The sucker rods experienced corrosion and plugging of the injection nozzle and rods with debris and boiled inhibitor deposits. The relatively large diameter sucker rods may have reduced the output of the well. FT-IR spectra of the Nalco 9354 inhibitor and boiled deposits demonstrate that the inhibitor did not undergo any degradation at temperatures up to 240?C. Na-benzoate was successfully used as a proxy tracer to determine on-line concentrations of the inhibitor and for water flow measurements. The WATCH program successfully calculated the saturation index Q/KCaCO3.

A simpler, smaller diameter and lower cost delivery system using 0.635 cm OD SS316 capillary tubing and a teflon protected stinger in the wellhead was successfully installed and operated in well SP-4D. An inhibitor concentration of 5 ppm Nalco 9354 in the total mass flow of the well fully inhibits calcite formation. Due to the presence of a wellbore obstruction, the SP-4D well is now operated at reduced flow conditions to maintain the flash point at shallow depth where inhibition can be performed. Even at reduced flows it is more economical to operate this well continuosly with inhibition than to perform annual scale cleanouts.