Precipitation of scale inhibitors

#This invention relates to wellbore and near wellbore formation scale inhibitors.

#Slightly soluble inorganic salts, for example calcium carbonate, calcium sulfate, and barium sulfate, often precipitate from waters produced along with oil and gas. Such precipitation occurs because conditions that affect solubility, for example temperature and pressure, change, or because incompatible waters mix in, or near, producing well bores. Precipitates of slightly soluble salts are often called scale. Scale build up often occurs within the near wellbore formation and the wellbore equipment. Scale build-up within the formation decreases permeability and impairs well productivity. Scale build up within wellbore equipment shortens equipment useful life and can plug production tubing.

#To minimize scaling, scale inhibitors can be placed within the formation by "squeeze" methods. Squeezing involves injecting the inhibitor and, usually, overflushing the treated zone with water or brine. The well is then placed back into production. Inhibitor in the produced waters protects the wellbore and downhole equipment from scale build-up.

#FIG. 1 is a plot showing combination of calcium ion and pH that results in precipitation of a scale inhibitor.

#FIGS. 2a and 2b are plots of inhibitor concentration in brine produced from a core in which inhibitor had been placed without a base producing precursor, and according to the present invention, respectively.

#Precursors of the present invention include, but are not limited to amides.

#Urea, for example, hydrolyses to form an ammonium carbonate-bicarbonate-carbon dioxide buffer system according to the following reactions. ##STR1##

#Solutions buffered with hydrolyzed urea typically have pH's of about 7 to 8.