Effect of Natural Acidity to Separated Brine with Very High Silica Supersaturation

Abstract

Mindanao-2 power plant is the second power station in Mindanao Geothermal Production Field operated by PNOC-EDC. Steam for the plant is provided by nine production wells. Two production wells KN2D and KN3B produce acid NaCl + SO4= fluids with pH at 25?C ranging from 2.7 to 5.3 and 3.6 to 5.9 respectively. Steam from these two wells contributes about 35% of total high-pressure steam production. Separated brine from the separators used by Mindanao-2 is also acidic with an average pH of 4.3 at 170?C.

The separated brine from pad C separators is highly over saturated with silica. The silica concentration of the brine average at 1084 mg/kg, which is about 45% over-saturated at the injection temperature. The excess silica is between 300 mg/kg to 400 mg/kg at 170?C. After injection for more than three years at this brine condition, the average thickness of silica scale recovered from the surface equipment and pipeline are on the average about 3 millimeters. This puts the scale rate of the brine at an average of about 1 mm/yr.

The scale recovered from the inspected equipment consists mostly of amorphous silica, corrosion products and rock fragments. The rock fragments were observed to be ubiquitous in all the equipment inspected with an estimated volume of at least 4 cubic meters. These rock fragments are responsible for the decline in the injection capacity of KN1RD and not silica scale deposition.