Characterization of Smectite Scale and Scale Inhibition Test by Ph Control at the Mori Geothermal Power Plant, Japan

Abstract

Smectite scaling became a problem in 1988 at the Mori geothermal power plant, in the well discharges due to an incursion of Mg-rich and low enthalpy fluids. Bulk scale is mainly composed of smectite, plus lesser amorphous silica and calcite. This smectite is identified as stevensite by X-ray diffraction and chemical analysis (mainly MgO and SiO2, and selectively concentrated MnO). The characteristics and amount of scale of each production line is different depending on its fluid chemistry. Smectite scale is widely distributed in surface pipelines and installations, but its crystallinity decreases toward the re-injection well. Mineral-water equilibrium showed that major factors controlling smectite saturation were an increase of pH by degassing of CO2 and H2S, and concentration of solutes, as a result of boiling in the well. Field tests from 1994 to 1996 have been conducted to evaluate pH control methods for inhibiting smectite scaling. As a result, a smectite scale was inhibited by keeping pH at less than 6.5 at the Mori geothermal power plant.