Wellbore Simulation to Model Calcite Deposition on the Ngatamariki Geothermal Field, NZ

Abstract

Calcite scaling in geothermal production wells is a major cause of decline in many geothermal fields worldwide. Reservoir fluid, saturated with respect to calcite, flows up the wellbore and begins to flash to two-phase. During flashing, dissolved CO2 migrates into the vapour phase, and calcite in the liquid phase becomes oversaturated. This mechanism has been observed in the Ngatamariki reservoir in New Zealand, and has resulted in production decline in several wells. Anti-scalant injection, mechanical cleaning, and chemical cleaning have all been used to manage wellbore calcite scaling at Ngatamariki. Previous modelling work on calcite deposition has focused on the geochemical mechanism behind the formation of scale in a wellbore or reservoir formation. Based on this work, wellbore simulation has been used at Ngatamariki to predict the location and extent of deposition in wells to assist clean-out operations, forecast fuel capacity, and manage production wells effectively. This paper aims to summarise the observations at Ngatamariki and describe the methods in which wellbore simulation has helped field operation.