Examination of the Change in Returning Molecular Weight Obtained During Inhibitor Squeeze Treatments Using Polyacrylate Based Inhibitors

Abstract

Abstract Scale inhibitors based on small polyelectrolytes are often employed in oilfield scale prevention treatments. These materials are injected into the near-well formation of producers in a scale inhibitor "squeeze" treatment. When the well is brought back on production, the objective is for the return concentration level of the inhibitor in the produced brine to be at or above a certain threshold level, Ct. This threshold level is the minimum inhibitor concentration required to prevent the formation of mineral carbonate or sulphate scales in that well. The squeeze lifetime depends strongly on the nature of the interaction between the inhibitor and the formation either through an adsorption or "precipitation" mechanism. Both adsorption and precipitation processes depend on the molecular weight of the scale inhibitor, as well as on a range of other factors. However, polymeric inhibitor species always display some degree of polydispersity (spread of molecular weight). In this paper, we examine the effects of molecular weight on adsorption/desorption phenomena for polyacrylate based inhibitor species. This work shows that, in the inhibitor effluent after a squeeze treatment, the molecular weight of the returning inhibitor may be different from that which was injected. For commercially available polymeric inhibitor species, we demonstrate using core floods that preferential retention of higher molecular weight components occurs and preferential desorption of lower molecular weight components is observed. This leads to a gradation in molecular weight in the return profile, which can lead to increased molecular weight components returning as the inhibitor concentration approaches the threshold level. The significance of this observation to field application of polymeric inhibitor species is discussed.