Modeling CaCO3 Deposition in Geothermal Wellbores

Abstract

The capacity of a geothermal liquid to carry calcium varies mainly with the concentrations of CO, and HCO,, temperature and ionic strength, of which the CO, concentration (pressure) changes most in the wellbore. Wellbore models that carry accurate computations for CO, and other gas pressures might be adapted to compute profile thicknesses of CaCO, scale. A general model for carbonate scale deposition in a wellbore must make a simultaneous accounting for pressures of H,O, CO,, and two or three other gases plus salt effects on those pressures. In addition, the elevation of flash initiation must be accurately identified and combined with profiles of temperature, etc., in the 2-phase zone. Such a model has been developed and its principle features are described here, including calibration of some factors with measured scale deposits. The model provides insight about the scale deposition processes through parametric studies. Tactics and strategies for confronting the effects of CaCO, deposition in wells and wellfields can be explored with the model. Modelling of specific wellbores/wellfluids can help quantify risks and benefits concerning scale inhibition, wellfluid monitoring, timing of consequences relating to failure of scale inhibiting apparatus, urgency of remedial actions, and other aspects.