Molecular Weight Effects in Polymeric Scale Inhibitor Precipitation Squeeze Treatments

Abstract

Abstract The most common scale inhibitors (SI) in the oil industry are polymeric and phosphonate species which are widely used to prevent calcium carbonate and barium sulphate scale formation, and these SIs may be applied in the field in downhole “squeeze” treatments. This paper focuses on the underlying mechanisms associated with polymeric precipitation squeeze treatments, where the polymer is precipitated in situ to enhance its retention in the formation and thus extend squeeze lifetime. Three polymeric SIs of similar structure are studied in this work, PPCA, SPPCA and PFC (full names in paper). Inhibition efficiency (IE) results are presented and analysed in terms of the molecular weight distributions (MWD) of the these polymers. IE and MWD results are shown for each for these polymers “as supplied” (the stock solution) and also for the “precipitated” and “supernatant” fractions, as would occur in a precipitation squeeze treatment. For PPCA, this precipitate has been shown to be a calcium complex of the polymer with stoichiometry, PPCA-Can (n ≈ 30). It is shown that the precipitated forms of PPCA and SPPCA are more efficient at inhibiting barite than either the stock or the supernatant due to the fact that they preferentially contain the larger molecular weight species, which increased the IE. However, the corresponding inhibition efficiency results for the PFC copolymer are rather different to those for SPPCA and PPCA and this is described in the paper. Further studies of the solubility of the PPCA-Can complex in successive samples of fresh brine suggest a “stripping model” for the dissolution/solubility of this complex, in which the lower molecular weight (MW) species are preferentially dissolved into supernatant brine until some equilibrium is reached. This, in turn, results in the original precipitate being enriched in higher MW species which are less soluble than the lower MW species. This model of dissolution is very different from a “solubility product” model as would describe a sparingly soluble mineral salt. This should now be taken into account in the modeling of polymeric SI precipitation squeeze processes, and it also has implications for the laboratory testing of polymeric SIs for application in precipitation squeeze treatments.