Although the chemistry of scale formation and inhibition downhole and in surface equipment for oilfield and geothermal applications continues to be studied extensively, the mechanism by which it occurs has not been given sufficient attention. In this paper, the chemistry of precipitation and scale formation in geological systems is discussed in terms of precipitation (solubility) domains for the sodium silicate system alone and in the presence of salt and hardness ions. The regions which delineate the conditions under which precipitates form are very sensitive to time, hydrogen ion and salt concentrations, and hardness ions present. Calcium and magnesium ions at concentration levels of only 100 ppm each change the shape and position of the silicate domains by several pH units and several orders of magnitude in the concentration of SiO2 needed for precipitation to occur. In an alkaline flood there are a number of mechanisms contributing to the presence or absence of various ions in solution whose composition is constantly changing as it moves through the formation. Ions can go into solution by several mechanisms, including the dissolution of and ion exchange with clays and other minerals. A theoretical model has been developed which takes into account the ion exchange reactions of Ca+2/Na+ and Na+/H+ that can occur in strata formations during an alkaline flood. These reactions can deplete the alkaline slug and change its composition and pH value to levels where precipitation and scale formation occur.
Abstract:The properties of colloidal dispersions of "Positive Sol 130M" (E. I. du Pont de Nemours and Co.), which contains silica particles coated with alumina, have been studied in detail. Drifts of pH, leaching of aluminum species, and changes of electrokinetic mobility occurred during aging at room temperature for periods of up to a month, and were attributed to surface reactions. Despite its name, the sol has a point of zero charge (PZC) at pH 9, and consequently the particles carry a positive charge only below this pH. The alumina of the coating dissolves in acidic environments; the extent of leaching of aluminum species is dependent upon pH and length of aging at room temperature. The effect of different anions on this process was examined by using various inorganic acids ; it was shown that phosphate and sulfate ions exhibit the strongest.effects. The removal of alumina changed the surface characteristics as well as the stability of the Sol 130M. The mobilities of the particles were found to be directly related to the composition of the particle surface, regardless of the time of aging or the pH.
Batch and flow leaching studies were carried out using a lime-precipitated plating waste sludge to study the influence of soluble silicates on metal ion leachability. A reduction in metal ion leaching was observed when soluble silicates were used either alone or with a portland cement/fly ash combination. These results suggest that sodium silicate reduces the solubility of metal ions in the waste matrix in addition to protecting the matrix from mild acidic breakdown.
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