THE DEPOSITION OF CALCIUM-CARBONATE IN GEOTHERMAL SYSTEMS (CALCITE, PRECIPITATION, SCALING)

The deposition of calcium carbonate inside the production well at geothermal facilities is a problem which afflicts a variety of fields throughout the world. The deposits which form are hard to remove and can severely degrade the usefulness of a given facility. Many different techniques have been proposed for dealing with this problem and it would seem quite useful to develop a relatively fundamental model of the process which could be used to evaluate the merits of these various techniques.;The model was also derived for the more specific case of deposition onto a rotating disk in order to facilitate an experimental evaluation of the requisite transport parameters and establish the validity of the model. Such an apparatus was constructed and operated at 1 atm CO(,2) partial pressure near 100(DEGREES)C. The results of the experiments indicate that the model can correcty predict calcium carbonate deposition rates using values for the various transport and kinetic parameters within the range one would expect from extant work.;The present work describes the development of a mathematical model for calcite deposition from solutions containing a variety of liquid-phase species at elevated temperatures. All significant homogeneous reactions are included as are diffusion effects and a heterogeneous surface reaction. In addition CO(,2) absorption into the bulk liquid is considered. The model is initially developed for the case where the convective diffusion process may be described by the product of a mass transfer coefficient and a concentration driving force. Such a model can be used to evaluate the relative importance of diffusion and surface reaction effects for a variety of systems. The model predicts that at most laboratory conditions, deposition is controlled by surface reaction at ambient temperatures, and that as the temperature is increased above 50(DEGREES)C diffusion plays an increasingly important role until it completely dominates at 100(DEGREES)C. This model also predicts that surface reaction could continue to play an important role in deposition in the geothermal well at temperatures to 200(DEGREES)C as a result of the extremely turbulent hydrodynamic conditions normally encountered, and the resulting high mass transfer rates.