Xanthan Stability at Elevated Temperatures

Abstract

Summary Xanthan stability is examined to define more clearly the polymers temperature limitations as a mobility-control agent. Experiments were performed to probe the relative importance of hydrolysis, oxidation, and helix-coil transition in xanthan degradation. In the absence of oxidizing agents (i.e., dissolved oxygen), results indicate that free-radical, oxidation/reduction reactions are not the dominant mechanism for xanthan degradation. Depending on the pH, acid-catalyzed hydrolysis and base-catalyzed fragmentation reactions may play important roles. With Arrhenius calculations, it was estimated that under ideal conditions (no dissolved oxygen, pH 7 to 8, and moderate to high salinities), a xanthan solution could maintain at least half of its original viscosity for a period of 5 years if the temperature does not exceed 75 to 80°C [167 to 176°F]. New polymers will be needed for chemical floods where xanthan does not have sufficient stability. Introduction About 50% of the oil in the U.S. that could be recovered by chemical flooding exists in reservoirs that have temperatures above 60°C [140°F].1 Stable, water-soluble polymers will generally be required during these floods to provide mobility control. Much progress has been made toward understanding the temperature limitations of acrylamide-based polymers - particularly in brines that contain divalent cations.2–6 However, there is still considerable uncertainty about the stability of xanthan solutions at elevated temperatures. The goal of this work is to define more clearly the limits of stability for xanthan at elevated temperatures in the absence of dissolved oxygen. This paper first summarizes the known composition and structure of xanthan. Second, the literature is examined to review those types of mechanisms that could contribute to chemical cleavage of the xanthan-polymer backbone during long-term exposure to reservoir conditions. Next, techniques are briefly described for preparing, monitoring, and maintaining solutions with undetectable concentrations of dissolved oxygen (<2 ppb). Results are reported for experiments that probe the relative importance of hydrolysis, oxidation, and helix/coil transitions in xanthan degradation. Finally, temperature limitations associated with the use of xanthan as a mobility-control agent are discussed. Xanthan Composition and Structure Fig. 1 illustrates the chemical composition7 of xanthan. The backbone of the molecule is composed of glucose monomers connected by ß(1–4) gycosidic linkages. A side chain that contains the trisaccharide sequence mannose/glucuronic-acid/mannose is attached to every other glucose residue in the backbone. In each side chain, an O-acetyl group is usually bound to the mannose residue closest to the main chain of the polymer. Some of the terminal mannose units in the side chains may contain a ketal-linked pyruvate group. Depending on the bacterial strain producing the polymer, the fraction of side chains containing pyruvate may be 0%, 100%, or some intermediate value.7–11 Weight-average molecular weights reported for native xanthan samples have ranged from 2 million to 50 million Daltons.12–16 Most of the molecular weight values are in the low end of this range. Polydispersity indexes (Mw/Mn) between 1.4 and 2.8 have been published.13–15 The radius of gyration of native xanthan has been estimated to be between 0.1 and 0.4 µm in saline solutions.16,17 Xanthan has been modeled as a rigid-rod molecule whose length is between 0.6 and 1.5 µm14,16 and whose diameter is about 2nm [20 Å].16,18,19 Because the contour length of xanthan is thought to be in the range of 2 to 10 µm,15,18 the molecule is presumed to have some flexibility rather than being strictly a rigid rod.14,20,21 Consequently, xanthan has also been modeled as a worm-like chain whose persistence length is between 50 and 120 nm [500 and 1,200 Å].20,22,23 The rigidity of native xanthan is attributed to the helical structure of the molecule. Some researchers have argued that the xanthan helix is composed of a single polysaccharide strand,15,19,22 while others regard xanthan as a double-stranded helix.13,17 Still other researchers suggest that xanthan can assume different ordered configurations (including single- and double-stranded helixes) depending on salinity, temperature, and sample history.11,24 Xanthan Reactions and Degradation Mechanisms Helix/Coil Transition. Xanthan is capable of undergoing a helix/coil transition (or perhaps more correctly, an "order/disorder" transition) as temperature is increased.25,26 In aqueous solutions of very low salinity, this transition can occur near room temperature. The transition or "melting" temperature, Tm, increases in direct proportion to the logarithm of the salt concentration. The following formula, based on the data of Holzwarth,25 relates Tm to the molar sodium concentration, [Na+]:Tm=122+ 30 log[Na+]. (1) For comparison, the data of Milas and Rinaudo26,27 can be described by use ofTm=125+ 43 log[Na+]. (2) The melting temperature is much more sensitive to divalent cations than to monovalent cations. Eq. 3, which is based on Holzwarth's data, quantifies the effect of molar calcium concentration, [Ca++], on Tm:Tm=310+ 70 log[Ca++]. (3) The melting temperature can be influenced by the acetate and pyruvate contents of the xanthan.28 Even so, Tm is expected to be quite high for the salinity and hardness levels that are characteristic of most formation brines. For example, in a 3 wt% NaCl solution, Tm is estimated to be 113°C [235°F] with either Eq. 1 or 2. The presence of calcium raises the melting temperature dramatically. In a 0.3 wt% CaCl2 brine, Eq. 3 predicts a value of 200°C [392°F] for Tm. Xanthan stability at elevated temperatures is much greater in saline solutions than in deionized water.27 The helical conformation of xanthan in saline solutions is thought to protect the molecule from chain scission. In low-salinity solutions, however, xanthan will be in the disordered, coil conformation and, therefore, may be much more susceptible to chemical attack. A key question is whether helix/coil transitions are important during xanthan degradation in saline solutions. Helix/Coil Transition. Xanthan is capable of undergoing a helix/coil transition (or perhaps more correctly, an "order/disorder" transition) as temperature is increased.25,26 In aqueous solutions of very low salinity, this transition can occur near room temperature. The transition or "melting" temperature, Tm, increases in direct proportion to the logarithm of the salt concentration. The following formula, based on the data of Holzwarth,25 relates Tm to the molar sodium concentration, [Na+]:Tm=122+ 30 log[Na+]. (1) For comparison, the data of Milas and Rinaudo26,27 can be described by use ofTm=125+ 43 log[Na+]. (2) The melting temperature is much more sensitive to divalent cations than to monovalent cations. Eq. 3, which is based on Holzwarth's data, quantifies the effect of molar calcium concentration, [Ca++], on Tm:Tm=310+ 70 log[Ca++]. (3) The melting temperature can be influenced by the acetate and pyruvate contents of the xanthan.28 Even so, Tm is expected to be quite high for the salinity and hardness levels that are characteristic of most formation brines. For example, in a 3 wt% NaCl solution, Tm is estimated to be 113°C [235°F] with either Eq. 1 or 2. The presence of calcium raises the melting temperature dramatically. In a 0.3 wt% CaCl2 brine, Eq. 3 predicts a value of 200°C [392°F] for Tm. Xanthan stability at elevated temperatures is much greater in saline solutions than in deionized water.27 The helical conformation of xanthan in saline solutions is thought to protect the molecule from chain scission. In low-salinity solutions, however, xanthan will be in the disordered, coil conformation and, therefore, may be much more susceptible to chemical attack. A key question is whether helix/coil transitions are important during xanthan degradation in saline solutions.