Chemical Stability of Biopolymer Solutions

Abstract

Abstract Biopolymers such as xanthan are being used increasingly in oilfield operations where the stability of the biopolymer is a prime consideration. The stability of xanthan depends on the brine salinity. Good viscosity retention over many months is observed in sea water at 90C. The successful application of xanthan at temperatures higher than 90C requires an understanding of the factors that limit stability. These factors include the nature of the polymer and the mechanisms of degradation. The stability features of a novel biopolymer are also described. Introduction Biopolymers produced by microbes (e.g. xanthan, scleroglucan) are a class of water-soluble polymers whose properties are being exploited increasingly in oilfield operations. Their high viscosifying power, coupled with excellent stability to salts and shear degradation makes them suitable for thickening applications in, for example, mobility control, profile control, drilling, workover and completion profile control, drilling, workover and completion fluids. Biopolymers may be used in place of or in combination with other water soluble polymers, e.g. HEC, guars. Dilute solutions of biopolymers are highly pseudo-plastic, show negligible visco-elasticity and yield stress and give low adsorption, leading to good injectivity and flow in porous media. More concentrated solutions have a high apparent yield that may be enhanced by controlled crosslinking, giving fluids with good solids carrying capacity or that are suitable for profile control. The versatility of biopolymers together with a potential for declining costs arising from economies potential for declining costs arising from economies of scale and advances in biotechnology suggest that the use of this class of water-soluble polymers in oilfield operations will continue to grow. In oilfield applications the stability of the viscosity of the fluid is frequently an important consideration. The time scales are various. For enhanced oil recovery (EOR) the viscosity of the polymer solution has to be stable for years, while in drilling operations good stability over hours or days only is essential. In both these applications the trend is towards higher temperature so that considerations of thermal stability become dominant in the selection of the polymer and of its formulation and in the design of the field handling procedures. In this study the stability of xanthan solutions has been measured for the purpose of selecting appropriate polymers and additives for two higher temperature EOR projects. Following these specific studies the factors that limit the stability of xanthan are being further examined in a programme that is providing a sound understanding for the applications of xanthan at higher temperatures. Finally the stability characteristics of a novel biopolymer type are presented. This polymer shows promise as a thickener not only for EOR at moderate temperatures but also for workover and completion fluids. EXPERIMENTAL Samples Xanthan samples were selected from commercial and development polymers available as concentrates (active matter 5-15%), powders (active matter greater than 85%) or broth (active matter 1-3%). Concentrate A is a sample of ENORFLO-X TM and the novel heteropoly-saccharide is a developmental polymer, ENORFLO-S TM, both supplied by Shell International Chemical Company. Viscosity Measurement Viscosities were measured on the following instruments, Brookfield micro cone and plate (with 1.565 cone), Brookfield LVT with UL adapter or Haake CV100. Temperatures were controlled to better than 0.1C using a specially constructed jacket for the UL adapter.