A number of polymeric scale inhibitors are currently used for downhole application in oilfield "squeeze" treatments. These materials must perform the dual role of inhibiting scale formation at low concentration levels whilst giving acceptably long return curves at the wellbore. Both of these design aspects of polymeric scale inhibitors relate to their adsorption characteristics (either on the growing scale crystal or onto the rock substrate) which, in turn, are functions of the molecular weight of the species. In this paper, we examine the effects of inhibitor molecular weight on its adsorption characteristics onto highly quartzitic substrates and we discuss the importance of this factor in governing the dynamics of the inhibitor return curve. The effects of molecular weight on the inhibition efficiency, during both early nucleation and later crystal growth, are also examined.

The adsorption/desorption characteristics of three polymeric scale inhibitors, each having a range of molecular weights, are studied in this work: viz. polyacrylate (PAA) and phosphino- polycarboxylate (PPCA) scale inhibitors of weight average molecular weight, Mw < 10,000 g/mol, and polyvinyl sulphonic acid (PVS) inhibitors of Mw < 20,000g/mol. Results using several commercial polymeric materials show that preferential adsorption of higher molecular weight components is observed. In order to examine this effect further, a range of polyelectrolyte inhibitors were synthesised and fractionated in order to give pure, low polydispersity polymers for investigation. Using these polymers, we show that the preferential adsorption of the higher molecular weight components occurs. This is probably because of the increased strength of the binding between the inhibitor and the surface due to increased multiplicity of segment/surface binding sites. It is also demonstrated that the adsorption of these higher molecular weight species may result in the displacement of previously adsorbed low molecular weight species. Corresponding bulk "precipitation" (or phase separation) experiments on the PPCA also show preferential loss from solution of the higher molecular weight species.

Results on the scale inhibition efficiency of barium sulphate, obtained for the same range of polymeric inhibitors, are also presented as functions of molecular weight. The factors required to ensure a long return curve are not necessarily the same as those for efficient inhibition under certain solution conditions (e.g. solution pH). This illustrates the importance of reaching a compromise in terms of molecular weight between inhibition efficiency and squeeze lifetime. The significance of these findings for field squeeze treatments using polymeric inhibitors is discussed.

Subject
Water, Inhibitor performance, Precipitation (solids), Adsorption, Scale inhibitors, Molecular weight, Adsorption isotherms, Desorption, Polymers, Chemical inhibition, Inhibition efficiency, Corrosion inhibitors, Scale
Keywords:
Scale inhibitors, molecular weight, polyelectrolytes, polyacrylates, phosphino-polycatboxylates, polyvinylsulphonates, inhibitor adsorption, displacement and precipitation
© 1994 Association for Materials Protection and Performance (AMPP). All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of AMPP. Positions and opinions advanced in this work are those of the author(s) and not necessarily those of AMPP. Responsibility for the content of the work lies solely with the author(s).
1994
Association for Materials Protection and Performance (AMPP)
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