Method of inhibiting scale in wells

Abstract

Scale deposition is inhibited with a new scale inhibitor precipitation squeeze method wherein inhibitor solution is injected at a higher first pH into a subsurface brine producing formation. Thereafter, the solution is subjected to a fluid or substance which lowers the higher first pH of the scale inhibitor solution to a lower second pH, thereby causing precipitation of scale inhibitor in the formation. The scale inhibitor is characterized by the fact that the scale inhibitor exhibits a significant solubility decrease between the higher first pH and the lower second pH. The method of this invention, therefore, is based on decreasing the pH the scale inhibitor solution to deposit the scale inhibitor and does not use prior art divalent cation precipitation methods to precipitate inhibitors. This invention lessens the chances of formation damage or plugging. The preferred scale inhibitors are the amine phosphonates of a given general structure. The most preferred scale inhibitors are ethylenediaminetetra (methylenephosphonic acid) and hexamethylenediaminetetra (methylenephosphonic acid). Precipitation by pH may be accomplished by by keeping the well shut in and allowing low pH forming formation fluids to equilibrate with the injected scale inhibitor solution. Alternatively, the scale inhibitor may be precipitated by injecting a second solution having a pH lower than the second pH into the formation or a substance that lowers the pH of the inhibitor solution to the second pH.