Treating wells with self-precipitating scale inhibitor

A process for emplacing a scale inhibitor within a subterranean reservoir comprising:

• dissolving at least one component each of (1) an anionic scale inhibitor that is a nitrogen-containing phosphonate which is (a) capable of preventing scaling by fluid produced from the reservoir at the reservoir temperature when present in the aqueous phase of the produced fluid in a relatively low but effective concentration, and (b) forms multivalent cation salts of multivalent cations which salts have a water solubility that is significantly less at a pH below about 10 than at a pH of from about 10 to 14, (2) a multivalent cation salt and (3) an alkaline compound in an aqueous liquid to provide a scale inhibitor solution containing scale inhibiting anions and multivalent cations which are mutually soluble at a relatively high pH but which, at a lower pH and the reservoir temperature, are precipitated as a scale inhibiting compound of relatively low water solubility;
• also dissolving within the scale inhibitor solution at least one water-soluble ester or amide of a water-soluble acid which hydrolyzes at a relatively slow rate to reduce the pH of the solution;
• wherein the rate at which the pH of the inhibitor solution is reduced, is controlled by selection of the particular pH reducing agent and scale inhibiting agent or the concentration of said pH reducing agent or scale inhibiting agent and/or by further dissolving in said scale inhibiting solution a buffering system;
• said rate of the pH reduction -being correlated- with the temperature and injectivity properties of the well and reservoir so that (a) a selected volume of the inhibitor-containing solution can be injected into the reservoir before the occurrence of any significant amount of precipitation and (b) a significant amount of precipitation will occur within a few hours after that injection;
• and,
• injecting the inhibitor solution into the reservoir before the occurrence of a significant amount of precipitation at a rate such that a significant amount of said scale inhibiting compound of relatively low water solubility is precipitated near the borehole of the well.