Behaviour of Polysaccharides Under Harsh Conditions

Abstract

Abstract The implementation of EOR polymers for high salinity and high temperature reservoirs requires a detailed knowledge of their solution behaviour. This paper describes the results of comparativestudies of the most commonly used polysaccharides, i.e., xanthan and scleroglucan. Improved laboratory techniques, such as equipment for stability tests under anaerobic conditions and high-temperature low-shear viscometer, have recently been designed to characterize their behavior under field conditions. It has been shown that:The stability and good filterability of improved quality polymers arc maintained for several months at 90 ºC.The viscosity of polysaccharides is decreased only slightly by an increase in temperature to 90 ºC,From flow experiments and batch tests, adsorption levels on reservoir rocks decrease with temperature. Introduction The use of polymers in harsh reservoir conditions for EOR applications requires understanding their behaviour in high salinity brine at high temperature. The favourable candidates already known are polysaccharides such as xanthan and scleroglucan. Due to their. Structure in multistranded chains, which gives them molecular rigidity, their viscosities in concentrated brine are higher and less sensitive to multivalent ions than other EOR polymers such as polyacrylamide Moreover, their most interesting characteristic is their good performance at high temperature. The laboratory techniques described in this paper have been improved to analyze polymer solution behaviour under high temperature conditions (≥90 ºC). The stability and the filterability of polysaccharide solutions were studied in a controlled oxygen atmosphere. The rheological characteristics were determined by using a newly devised capillary viscometer. Adsorption on rocks was derived from both batch tests and core flow experiments. The overall results provide criteria for evaluating the respective advantages of both polysaccharides under given field conditions. Experimental Polymers The polymers used, i.e., xanthan and scleroglucan, are polysaccharides both obtained by fermentation process and provided as broths. Their characteristics are given in Table 1. Xanthan (XCPS) is a strong polyelectrolyte containing D-glucuronic acid units and pyruvic acid ketals(1). Scleroglucan (SGPS) is a nonionic polymer made up of linear glucose units branched at every' third residue(2). Polymer solutions were prepared by simple dilution of the broths in salt water. Then, they were clarified by filtration to remove any cells. Low-molecular-weight impurities were eliminated by ultrafiltration. Some solutions, called "high quality" in this paper, were extensively purified by a thermal treatment to remove impurities, especially proteins bound to the polymer molecules(3). This thermal treatment consisted in heating the ultrafiltered solution during 48 hours at 90 ºC and then if necessary, reultrafiltering the solution to eliminate the last impurities. The brine used are a sodium chloride solution or a reconstituted sea water containing 27.8g.l−1 NaCl, 9.54.g.l−1 MgCl22,6H2O, 1.62g.l−1 CaCl2,2H2O, 0.03g.l−1 SrCl22,6H2O, 7.73gl−1 Na2SO4, 10H2O. Procedure Polymer concentrations were determined with a Dohrmann DC80 carbon analyzer, which gives the total carbon content of solutions, and/or with the Technicon Auto Analyser System, which gives the glucose content