pH and silica scaling control in geothermal field development

Abstract

Due to the increase of amorphous silica solubility as significant silicate ion forms in the pH range 7 to 8.5, the potential for the deposition of silica scale during reticulation (pipeline transmission) of waste geothermal waters to disposal sites is dependent on steam separation temperature, silica concentration and the pH of the residual fluid. For low salinity geothermal fluids the latter is related to the composition of the fluid, particularly the concentration of carbon dioxide remaining after the removal of steam for power generation using conventional separators. In general terms the higher the gas content of the initial deep aquifer fluid, the higher will be the pH of the waste waters, so that in many fields, where reservoir temperatures fall in the range 250 to 280°C, careful choice of separation pressures to maximise gas removal and hence elevate the pH of residual waters may obviate the problems of silica deposition, or the need to install costly chemical treatments such as acid or alkali addition. In designing and operating separators or flash plants it is important to avoid carryover of steam (containing CO2 and H2S) into the water reticulation lines, where, after heat loss, condensation occurs with resultant lower pH and higher scaling potential. Where flash plants are designed to receive a mixture of the two-phase discharges from a number of wells, consideration of the pH of the residual water resulting from steam separation may provide design constraints on the reticulation network such that silica scaling potential is minimised or entirely avoided. The Wairakei and Broadlands (Ohaaki) fields are used as examples of these design considerations in relation to the reservoir chemistry of the field and possible changes during extensive exploitation. Show more