Quantitative assessment of calcite scaling of a high temperature geothermal well in the Kangding geothermal field of Eastern Himalayan Syntax

Abstract

High temperature geothermal systems in China are mainly located along the Himalayan belt and one of the main problems during production is calcite scaling. This paper presents a quantitative assessment of calcite scaling and possible removal and prevention methods for the Kangding geothermal field in the Western Sichuan Plateau, as an example. Reservoir fluid composition is reconstructed based on geochemical processes that may take place from reservoir to surface. Results show that the fluid is HCO3⋅Cl-Na or Cl⋅HCO3-Na type with a temperature of 259−283 °C. It’s over-saturated with respect to calcite in both surface and reservoir conditions but under-saturated to quartz and amorphous silica, indicating that the calcite scaling will be a problem. For well BH6, the fluid pH is 5.63 at reservoir conditions and the steam fraction at the wellhead is about 6.0 %. Adiabatic boiling calculation indicates that from reservoir to surface conditions, CO32− and CaCO3 concentrations in the fluid keep increasing and the fluid evolves to become over-saturated with respect to calcite and the saturation index is higher than 0.5 and calcite precipitates in the pipeline. The boiling depth is estimated to be about 150 m from the wellhead which can provide a guide for scaling depth determination. The calcite scale quantity is calculated to be 151−300 kg or a thickness of 0–2.94 cm according to the pumping test, consistent with what has been observed. Calcite scale can be removed mechanically or prevented by injecting chemical inhibitor as well as thermodynamic methods (including injecting acid, CO2, cold water and putting the feeding pump below the boiling depth). Counter-measures should be chosen based on the mode of utilization and its cost. For well BH6 that is planned to be used for power generation, chemical inhibition may be the choice.