Genesis of Smectite Scales in Mindanao Geothermal Production Field, Philippines

Abstract

Smectite [Al4Si8O20(OH)4n•H2O], a hydrated expanding clay mineral is present in significant amounts in the surface facilities of the Mindanao Geothermal Production Field (MGPF). It is the dominant constituent of total deposits in the Low-pressure (LP) Flash Vessel Station, and Separator Vessels (SV) 1A and 1B. It is found in minor quantities of total deposits downstream of these vessels. The fragmental clay deposits cause recurrent operational problems such as flooding of brine surge tank and flash vessel, clogging of pump at the LP station, and subsequent MGPF plant trip.

Petrologic and scanning electron microscopy (SEM) analyses of clay samples reveal micro-textural features and mineral association suggesting that they deposited directly from the brine in the surface facilities, and are not hydrothermal alteration minerals from production wells. Moreover, simulation using SV-1A/1B brine chemistry gives a high saturation index of smectite (log Q/K of 10-24) indicating high potential for depositing smectite. Very high saturation index is due to the brine’s elevated aluminum content of 640 ppb. Thus, abundant smectite scales will likely form in SV-1A, SV-1B, and LP station where separated brine from SV-1A, SV-1B and SV-2C is further flashed at 120°C. These soft clay deposits are easily removed from the vessels and transported as fragmental solids downstream of these facilities.

Since aluminum is the major parameter that controls smectite saturation index, clay scaling may be controlled by brine treatment with Al-sequestering or complexing agent (Gallup, 1997).