Kinetic study of scale inhibitor precipitation in squeeze treatment

Oilfield formation damage by scale formation can occur when two incompatible brine streams are mixed. A common method for preventing scale formation is the use of chemical scale inhibitors such as aminotri(methylene phosphonic acid) (ATMP). Scale inhibitors are injected and retained in the reservoir by adsorption and/or precipitation. The induction time, the period between the establishment of supersaturation and the detection of a new phase, is a measure of the ability of an inhibitor solution to remain in the metastable state. As a result, long induction times allow transport of inhibitor fluids into the near-wellbore regions without precipitation of the scale inhibitor and subsequent formation damage. In this study, an induction time model is applied to precipitation of the inhibitor (ATMP) with Ca2+ ions. The nucleation kinetics can be described by classical nucleation theory. Solution equilibrium was calculated by accounting for inhibitor dissociation and cation-inhibitor complexing as a function of ionic strength. Conditions such as the initial concentration of inhibitor, the solution pH, and the presence of soluble impurities significantly impact the precipitation kinetics of inhibitors. Long induction times were observed at low initial concentrations of inhibitor, at low values of the solution pH, and in the presence of impurities. Monovalent cation impurities (Li, Na, and K) inhibit the nucleation of Ca-ATMP to the same extent, indicating there is no effect on the different types of monovalent cations. Divalent cation impurities inhibit the nucleation of Ca-ATMP more than monovalent cations, and different divalent cations have different induction times. The reduction of nucleation rate is a result of increasing the surface free energy. This study provides an understanding of scale inhibitor precipitation kinetics which will be beneficial for delaying inhibitor precipitation in order to avoid reservoir permeability problems in near-wellbore region.