Abstract

Phosphino poly carboxylic acid (PPCA) is one of the most commonly used polymeric scale inhibitors (SI) for application in precipitation squeeze treatments. Precipitation processes with PPCA generally occur by forming the sparingly soluble SI_Ca complex. The SI return curve in this process is then governed by the solubility (Cs) of this complex and the kinetics of the dissolution. As in all squeeze treatments, the objective is to control the precipitation in a way that leads to optimum squeeze lifetime without causing any significant formation damage.

In this paper, the fundamental mechanism of the precipitation and dissolution of the PPCA_Ca complex is studied in order to help us to design more efficient precipitation squeeze treatments using PPCA and other polymeric scale inhibitors. At high temperature, the precipitated complex separates out and this precipitate is enriched in (less soluble) higher molecular weight (MW) species. The precipitate is then successively exposed to fresh brine samples in a "successive extraction" process in which we observe that the lower MW components are more available for dissolution. We refer to this as a "stripping" dissolution mechanism.

The bulk results from the successive extraction experiments on the PPCA_Ca complex are then compared with dynamic sand pack results. The objective of these floods was to investigate the dynamic precipitation /dissolution characteristics of the precipitate at various flow rates for the PPCA system. These floods can be shown to range from equilibrium to non-equilibrium dissolution conditions. Non-equilibrium experiments were conducted to analyse the effect of multiple flow rates on the precipitated SI effluent concentrations. The cation concentrations were also analysed to investigate their involvement in the retention mechanism.

This study establishes a hypothesis about the dissolution of SI_Ca complex which we believe can be generalised to any polymeric SI which is applied in a precipitation squeeze. These results clearly point to a "stripping" model of the polymer/Ca complex dissolution where the lower MW species are preferentially dissolved into supernatant brine until some equilibrium is reached. This model of dissolution is very different from a normal "solubility product" model as would apply to a sparingly soluble salt of a phosphonate, for example. This should now be taken into account in the modeling of polymeric SI precipitation squeeze processes.

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