A BETTER KNOWLEDGE OF THE FORMATION OF CARBONATE SCALING AND ITS PREVENTION WILL REDUCE THE NUMBER OF WORKOVERS OF A WELL AND WILL THEREFORE MAKE THE HEAT PRODUCTION MORE RELIABLE AND MORE ECONOMIC.
The precipitation of calcium carbonate in supersaturated solutions has been investigated over a range of pH between 8.00 and 9.50 and at various temperatures between 25 and 80 C. The investigation involved both stable supersaturated solutions and unstable supersaturated solutions. The experimental conditions in the latter case were selected so that spontaneous precipitation occurred following the lapse of well defined, highly reproducible induction periods. At all temperatures it was found that vaterite precipitated was kinetically favoured. The presence of phosphonate compounds both retarded the calcium carbonate precipitation and stabilized vaterite formation. At higher temperatures, the transformation of the intially forming vaterite into aragonite has also been observed.
The crystallization of calcium carbonate in the stable supersaturated solutions was investigated by seeded growth experiments using calcite seed crystals. In all cases calcite was the only phase forming, as predicted by thermodynamics, following a surface controlled spiral growth mechanism, with an apparent growth order of 2.
The presence of additives such as oxalate favoured the stabilization of the unstable calcium carbonate monohydrate, which was rapidly converted into calcite. The presence of inorganic orthophosphate retarded the precipitation by adsorption onto the calcite surface which resulted in surface charge reversal.
Finally, the inert electrolyte effect appeared to influence the rates of formation of calcium carbonate without affecting the second order kinetics of the seeded growth of calcite. In concentrated electrolyte media, it has been shown that it is possible to apply the constant supersaturation approach.
STUDY OF THE KINETICS OF CALCIUM CARBONATE FORMATION FROM SUPERSATURATED SOLUTIONS AT ELEVATED TEMPERATURES. STUDY OF THE STABILITY OF THESE SOLUTIONS AS WELL AS THE NATURE OF PHASES THAT ARE FORMED DURING PRECIPITATION. THE EFFECT OF THE CO2 PRESSURE, THE PRESENCE OF MAGNESIUM, STRONTIUM, PHOSPHATE AND SULFATE IONS AS WELL AS CARBONATE SCALE INHIBITORS WILL BE STUDIED.