Mechanisms Of Mineral Scale Inhibition

Abstract

Abstract The adsorption and desorption potential of inhibitors to crystal nuclei has never been determined at the extremely low, threshold levels that are known to inhibit nucleation. An extensive study of adsorption and desorption isotherms of four phosphonates on barite or calcite are tested over a wide range of solution conditions (0 – 1 M NaCl, 0 – 0.1 M Ca, 0 – 0.33 M sulfate, 4.6–6.4 pH). From these adsorption/desorption observations, it is proposed that the primary driving force for adsorption is related to simple hydrophobic repulsion from solution of a macro neutral molecule and not, as is generally presumed some specific inhibitor-surface interaction. A single isotherm may be derived from these vastly different adsorbent, adsorbate combinations after the solid phase concentration is normalized to per unit surface area and the solution phase concentration is normalized to the concentration of metal phosphonate concentrations. From the nucleation study, it is observed that the inhibitor needed to completely inhibit barite formation is approximately equal to 16% surface coverage. Equation to predict minimum inhibitor need is proposed based upon this model and compared with field observations. Brine chemistry of a specific well dictates the solution speciation of scale inhibitors and adsorption potential, thereby, that will determine the efficiency of inhibitor. Surprisingly, the range of predicted inhibitor concentrations is quite similar to what is observed in the field as a minimum effective dose, even though it was arrived at by a completely independent method of calculation.