The effect of molecular weight distribution on the inhibition efficiency performance of polymeric scale inhibitors during retention

Polymeric scale inhibitors (SI) used for downhole oilfield squeeze treatments must perform the dual role of inhibiting scale formation at low concentration levels (∼2-20ppm) whilst giving acceptably long return curves at the wellbore; i.e. good [SI] vs. time or produced water volume behavior with [SI]>MIC, Minimum Inhibitor Concentration). Both the inhibition efficiency (IE) and retention design aspects of polymeric SI treatments are thought to be functions of the molecular weight (Mw) of the polymeric species. However, this has not yet been fully demonstrated experimentally. In this paper, we studied three polymeric SI, viz. PPCA, SPPCA and PFC in some detail. This paper describes the effects of the molecular weight distribution (MWD) on the IE performance of the polymeric SI. Our findings reveal that the precipitated PPCA and SPPCA are more efficient at inhibiting barite than either the stock or supernatant. We demonstrate that this is due to the fact that the precipitate preferentially contains larger Mw species which have an increased IE. We believe that the ideas proposed in this work for PPCA precipitation and inhibition is generally applicable to all polymeric SIs. Thus, this work will help us to develop a model for polymer precipitation (and adsorption) which can be used for designing squeeze precipitation treatments for polymeric SIs.