Abstract

The prediction and optimisation of scale inhibitor squeeze treatments demands careful attention to both aqueous phase behaviour and the resulting retention mechanisms in consolidated core material. Phase behaviour patterns for single and binary systems of acrylate and phosphonate scale inhibitors in sea water have been generated over a range of pH values and concentrations appropriate to flooding and field conditions. The phase maps were utilised to define coreflooding strategies that would yield separate adsorption and precipitation retention mechanisms for detailed study under a controlled pH environment.

Retention and desorption characteristics of the single component inhibitor agents were performed at 80 deg. C on Clashach sandstone core specimens in both adsorption and precipitation regimes where possible. Return curve analyses precipitation regimes where possible. Return curve analyses were quantified using inductively coupled plasma spectroscopy and an analytical colourimetric method for the phosphonate and acrylate respectively. Desorption profiles phosphonate and acrylate respectively. Desorption profiles have been fitted to mathematical expressions and the physical significance of the models used have been interrogated.

Binary inhibitor systems capable of yielding both adsorption and precipitation phase behaviour at 80 deg. C were selected for test purposes to establish the prevailing retention mechanisms of both agents in competition. Separate desorption profiles for each inhibitor following a binary system shut-in were generated. The role and suitability of simple mathematical models to aid the interpretation and impact for future formulations has been addressed.

Introduction

Scale inhibitor molecules are believed to function by adsorbing onto crystal surfaces and blocking active growth sites, although the actual inhibiting mechanism is poorly understood. Inhibitors not only retard growth, but can also delay nucleation; standard test routines have been identified to specify threshold values for performance. Whilst the inhibitive properties of such molecules is essential, the threshold ppm level does not provide a mechanism for acceptance or rejection, since it is the ability of the near well bore formation to effectively retain squeezed chemicals and then subsequently deliver small but adequate quantities of chemical in produced waters, that is vital for field squeeze economics. The aqueous phase behaviour of inhibiting agents under conditions equivalent to those that may be encountered during chemical squeeze applications is essential to understand and optimise placement strategies. Some general phase behaviour work has been addressed in the literature but this is largely associated with injectivity demands rather than placement optimisation. The absence of detailed phase maps can lead to coreflood investigations which have ill-defined depositional processes that become meaningless to model. This study identifies phase boundary regimes where coreflood experiments can be performed that address adsorption or precipitation phenomena directly.

There has been a tendency to drift from blended inhibitor formulations to single components because of the fear of chromatographic separation, preferential uptake and monitoring demands. Synergistic effects of dual component formulations have been reported in simple batch tests but again little concern has been given to the fundamental rock/fluid interactions and their demands.

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