Polymères hydrosolubles d'origine naturelle et synthétique Relation structure/propriétés en solution Water-Soluble Polymers of Natural and Synthetic Origin. Structure/Property Relations in Solution

Abstract

Les polymères hydrosolubles utilisés dans les opérations pétrolières (forage, cimentation, stimulation, récupération assistée) peuvent être d'origine très variée (polymères naturels, de fermentation, semi-synthétiques et synthétiques). Leur utilisation et leur efficacité sont directement liées à la connaissance de la relation existant entre leur structure chimique (macrostructure et microstructure) et leurs propriétés en solution. Ce rapport fait la synthèse des divers types de polymères hydrosolubles qui ont un intérêt pratique et définit les paramètres structuraux et fonctionnels gouvernant leur efficacité en fonction d'un certain nombre de paramètres extérieurs (pH, salinité, température). The capacity of water-soluble polymers to modify the rheology of aqueous solutions explains their importance for various oil-recovery operations. The choice of the most appropriate polymer depends on its molecular and macromolecular properties in solution, which are closely related to the nature of their primary, secondary and tertiary structures and of their microstructure. This article describes the different types of water-soluble polymers that are of practical interest, and it defines the structural and functional parameters that govern their efficacy as a function of external parameters (pH, salinity and temperature). There are four main types of polymers, depending on their origin. They are :(a) Natural biopolymers (of vegetable origin) and biotechnological biopolymers (produced by microorganisms), i. e. neutral and/or charged polysaccharides. (b) Modified biopolymers having synthetic side chains. (c) Polyvinylsaccharides (synthetic side chains). (d) Synthetic polymers. For all of them, it is indispensable to know the relationship between structure, conformation and functional properties. The solubility in water and the properties in solution of polysaccharides depend on four main factors: (i) the presence of branched chains, (ii) the presence of charged groups, (iii) the nature of the intermotive bonds , and (iv) the nonuniformity of the repetition structure. Natural polysaccharides can be used either directly or after having been modified to optimize their properties (such as derivatives formed from cellulose and guar gum. Polysaccharides from fermentation (xanthan, scleroglucan) have a unique rheological behavior in relation to the existence of rigid ordered conformations in solution, and their stability range depends on external conditions of pH, salinity and temperature. Water-soluble synthetic polymers and copolymers hold a very important position. Their properties in solution depend to a great extent on their anionicity, on the flexibility of the main chain and on the presence of more or less cluttered side chains. Among these latterk sodium acrylamide/acrylate copolymers resulting from the partial posthydrolysis of polyacrylamide or obtained by direct copolymerization are widely used despite their triple instability (low tolerance to divalent ions, limited mechanical strength and chemical stability), which results in their losing their viscosifying properties (and in the appearance of phase separation). Changing the structural parameters (introducing steric constraints) and functional parameters (replacing carboxylic functions by sulfonic functions) governing their properties in solution greatly improves their resistance to salts and temperature.