Concentration, Brine Salinity and Temperature effects on Xanthan Gum Solutions Rheology

Xanthan gum is a biopolymer used in several different industries for a variety of applications. In the Petroleum Industry, xanthan gum has been applied in Enhanced Oil Recovery (EOR) methods for mobility control due to its Non-Newtonian rheological behavior, relative insensitivity to salinity and temperature compared to other conventional synthetic polymers, as well as its environmentally-friendly characteristics. As challenging reservoir conditions arise, candidate polymers should meet the screening factors for high salinity, high temperatures and heterogeneous reservoirs. This paper aims to evaluate the effects of temperature and monovalent salts on the rheological behavior of xanthan gum for Enhanced Oil Recovery purposes. We tested polymer solutions with brine salinities of 20,000/110,000/220,000 ppm of Sodium Chloride in a rheometer at temperatures of 23, 50, and 77∘C. The results acquired showed that temperature plays a key role in viscosity and salinity protected the solution viscosity against negative thermal effects, unusually a turning point is observed where the increase in the monovalent salt content enhanced the polymeric solution viscosity. Such investigations coupledwith a detailed discussion presented in the paper contribute to understand critical aspects of xanthan gum and its capability to provide basic requirements that fit desired screening factors for EOR.


Introduction
In the petroleum industry, xanthan gum is widely employed in EOR and drilling applications [46]. Regarding the EOR methods, polymer flooding stands out as a chemical method for mobility control, where soluble polymers are added to the injection water to enhance oil displacement and sweep efficiencies of the reservoir [22]. Polymer flooding also delays the water breakthrough due to the lower volume of injection water needed to reach the residual oil saturation and because of the reduction in water relative permeability caused by polymer adsorption [25].
As more challenging reservoir conditions arise, more efficient and lower cost polymers are needed that meet the screening criteria. In conditions of high temperature, high salinity and very heterogeneous carbonate reservoirs, candidate polymers, especially biopolymers [3,6,7,20,32,37] must provide the required rheology at minimum solution concentration. The polymer solutions also need to be thermally and mechanically stable and result in manageable adsorption on the formation rock.
The first step is a rheological assessment to design the target polymer solution to meet the screening factors for an EOR polymer flood project. Extracting data and information about the rheological behavior of the polymer solutions are essential to progressing to more detailed and complex evaluations and reservoir simulation.
Polymer solutions are classified as pseudoplastic fluids under most conditions. These types of fluids show a reduction of viscosity as shear rate increases. They are known as shear thinning fluids [44] and are represented by a curve with three distinct regions. The first region is a plateau characterized by a constant viscosity at very low shear rates or stress (η 0 ). The second region describes the shear thinning behavior, known as a pseudoplastic region. The third region is also a plateau and indicates the end of the pseudoplastic region and the beginning of the constant behavior of viscosity for high shear rates (η ∞ ).
There are several empirical models to describe the shear rate-dependence on viscosity [45], but the most commonly used is the power law relationship, also called Ostwald-de Waele model [2,34], which describes the pseu-doplastic region.
where η is the apparent viscosity (cP), is the shear rate (1/s), nis the flow behavior index (dimensionless), and K is the consistency index (Pa·s n ). For pseudaplastic fluids, n < 1. The equation accurately describes only the pseudoplastic regime. By plotting the polymer concentration against viscosity, it is possible to observe the transition between the dilute and the semi-diluted regime, which is called the critical concentration. These regimes express the degree of interaction between the macromolecules. In the semidiluted regime, the macromolecules are entangled, where the friction forces are imposed on each other, increasing the viscosity. In the diluted region, the macromolecules lie separated within the solution showing an independent behavior [1,45].
The higher the polymer concentration in the solution, the higher the interaction between the polymer molecules and the solvent. As the polymer concentration decreases, the fluid solution loses its pseudoplastic characteristic resembling a Newtonian fluid, where the viscosity of dilute solutions presents negligible changes with shear rate increase [44]. Additionally, polymer concentrations for offshore field applications need to take into account the storage and processing capacity of the facilities. In such circumstances, the feasible range of concentrations lies between 500 ppm to 1200 ppm [10,18,50].
Xanthan gum is a polysaccharide, which is produced through fermentation of glucose or fructose by different bacteria [24]. Figure 1 shows the chemical structure of xanthan gum. The backbone structure of xanthan gum is similar to cellulose. The side chains of the polymer contain charged structures composed of acetate and pyruvate groups leading to a polyelectrolyte polymer.  [49] Xanthan gum has been modeled as a rigid-rod molecule [14,47], comprising a helical structure where the side groups fold down along the helix creating a stiff rodlike macromolecule [30]. Holzwarth & Prestidge [17] and Holzwarth [15] say that the xanthan gum helix is composed of a single polysaccharide strand, while [41] indicates a double-stranded helix. Some other researchers assume different ordered configurations depending on salinity, temperature, and production origin [23,29].
According to [13,27,41,43], under high temperature, xanthan gum solution is subjected to a helix/coil transition or, also called, an order/disorder transition profile. This transition corresponds to the disruption of its structure, thus setting the molecule into a completely random coil conformation. The authors also showed in their studies that this relationship is also salinity dependent. Adding more salt to the solution yields a disordered conformation to an ordered and more rigid structure of the xanthan gum, meaning that low salinity polymer solutions exhibit a low transitional temperature and vice versa. Some xanthan gum in distilled aqueous solutions can reach this transition point even at room temperature [43]. In brines composed of divalent cations, this relationship tends to be more pronounced. Regarding molecular structure, the threshold temperature or so-called "melting temperature" is influenced by the acetate and pyruvate content [16].
Once the order-disorder transition point is achieved, the xanthan gum starts to behave as a Newtonian fluid, losing its ability to build viscosity entirely. At the region below the transition point, the xanthan gum in solution adopts a partial self-associated conformation due to polymer entanglements and hydrogen bonds that shows high viscosity at low shear-rates [38,39]. This weaker structure partially breaks and the polymer chains are oriented under shear in a preferential direction towards the flow, showing a shearthinning behavior as the shear rate increases.
Temperature also plays a key role as regards viscosity, as it favors a progressive transition from partially ordered helix conformation to a random coil conformation. As a result, the xanthan gum becomes less viscous and less pseudoplastic until it reaches the Newtonian plateau. The molecules are abruptly rearranged into a fully random coil conformation, which dominates the rheological behavior [31,35]. Several authors [8,26,52] documented that the relationship between the apparent viscosity of polymeric solution and temperature satisfies the Arrhenius equation: where η is the apparent viscosity of the polymeric solution (Pa·s), A is a constant characteristic of polymeric solution (Pa·s). T is the absolute temperature (K), ∆Eη the viscous activation energy or the activation energy for flow (kJ/mol), and R is the universal gas constant (kJ·K −1 ·mol −1 ). The viscous activation energy is related to the dependence of the viscosity on temperature of the polymeric solution, higher viscous activation energy indicates greater influence of the temperature on the viscosity [42].
In summary, temperature combined with the brine salinity are triggers for the conformational transition and play a key role in the ability of the polymer solution to build viscosity, as illustrated in Figure 2. However, the xanthan gum response to ionic variation is unlike that of most other polyelectrolytes. Polyelectrolytes usually exhibit an expanded structure under conditions of low ionic strength, as the result of neighboring group electrostatic charge repulsion. The addition of salt results in a screening of the neighboring group repulsive forces and leads the molecule to collapse into a compact coil, reducing the intermolecular interaction, and thereby the viscosity.
Studies of the effects of salts on xanthan gum viscosity have produced contradictory results [51]. Some authors show that the thickness of dilute xanthan gum slightly increased in the presence of Sodium Chloride [11,19,33,48,51]. Dário et al. [5] suggest that increasing the concentration of soluble salts, decreases viscosity. However, a turning point is reached when very high concentrations of salt are incorporated into the solution, allowing the recovery of the ordered state conformance. Other authors show a moderate decrease in viscosity, less than other conventional polyelectrolyte structures [9,12,36,40].

Methodology
The polymer used in this study is Xanthan gum obtained from Xanthomonas Campestris by Sigma-Aldrich TM . According to [21], the average molecular weight is 2.65 Milion Daltons, the Pyruvate content is 0.9%, and the Acetate Content is 3.51%. A more detailed description of these measurements can be found in his paper.
The polymeric solution was prepared based on API-RP-63. Initially a stock solution of 5,000 ppm was prepared and the desired concentrations were obtained by diluting the stock solution. To make the Stock, dried polymer powder was uniformly sprinkled into the brine for 3 minutes. The polymeric solution was agitated for 9 -12 hours and the magnetic stirrer was set to a medium degree of agitation to avoid powder clumping to ensure homogeneous solubilization. Finally, the solution was left overnight to hydrate.
Three different brine salinities composed of Sodium Chloride (NaCl) at three different temperatures were evaluated, as shown in Table 1.
The rheological tests were performed in a HAAKE MARS III rheometer, which is a high precision instrument. The sensor used was the concentric cylindrical (DG41). The temperature control used was the THERMO HAAKE C25P refrigerated bath with a Phoenix II Controller. A new sample of solution was applied for each test.   The flow curves were recorded at shear rates between (10 −1 and 10 3 ) s −1 with 20 measurement points. The acquisition mode was controlled by the shear rate.    Table 1. The Power Law model fitted the flow curves and Tables 3-11 list the parameters. For each condition, a clear pseudoplastic behavior is observed at polymer concentrations ranging from 5,000 -2,000 ppm, comprising the semi-diluted region, and then a noticeable gap is observed between 2,000 ppm and 1,000 ppm. Concentrations below 1,000 ppm exhibit a slight pseudoplastic behavior and the initial viscosity plateau at low shear rates can be seen in the majority of the data. Figure 12 clearly shows that most curves lost the Non-Newtonian rheological behavior at concentrations around 500 ppm. The critical concentration was found at the intersection between a linear fitted curve within the 2,000 -5,000 ppm concentration and a linear fitted curve within the 1,000       Table 2). The critical concentration increases sharply from the 23 ∘ C/ 20,000 ppm data and all the flow curves fall within the diluted region below the 2,000 ppm. The temperature showed a significant effect on the solution viscosity. All of the concentrations for each analyzed condition presented more than 60% reduction of its original thickness at room temperature (23 ∘ C). As seen in Figure 13, the increase in temperature linearly decreases the log η. Figure 14 shows a plot of the measured points at 23 ∘ C and 77 ∘ C for each concentration, where the percentage of viscosity reduction caused by the increase in temperature was calculated (based on both temperatures, that is, how much of the viscosifying capacity of the xanthan gum at room temperature conditions decreased by the highest temperature measured). In the semi-diluted region, the viscosity reduction by temperature gradually decreased in higher salinities and in higher xanthan gum concentration. In solutions with higher salinities, the salt content protected the viscosity in the semi-diluted region; this is due to the salt content causing the molecules to form more ordered and rigid structures, avoiding thermal breakages      [4,5,43] however, in the diluted region, the influence of temperature on viscosity reduction remained constant, independently of the salt content and xanthan gum concentration. This observation suggests that the xanthan gum in such conditions is not capable of forming complex entanglements by interacting macromolecules on its conformance configuration, which are responsible for the pronounced Non-Newtonian behavior, thus their thermal disruption would lead to more severe viscosity reductions. Analyzing the salinity effect (Figure 12), the increase in monovalent salt content from 20,000 ppm to 110,000 ppm showed a moderate to low reduction in the viscosity for a given temperature. However, the opposite behavior was observed when the salt content increased from 110,000 to 220,000 ppm, where the viscosity actually increased for a fixed temperature. The latter behavior has also been seen by previous works [11,19,33,48,51], even though it is not clear what mechanism drives this unusual effect (increase in viscosity with increasing salinity) within the range from 110,000 ppm to 220,000 ppm. The results obtained leads to a similar behavior described by [5], whereby extremely high salinity allowed the xanthan gum to recover its entangled partially ordered conformance, resulting in higher viscosities.
As regards the first step of addressing the screening criteria for a polymer flood project with respect to target solution, xanthan gum can build higher viscosities (with the same polymer concentration) than other high molecular weight synthetic polymers such as partially hydrolyzed polyacrylamide (HPAM). This also indicates that  high salinity reservoirs mostly comprised of Sodium Chlorine could increase the xanthan gum viscosity over the long term due to the interaction between the injected brine and the reservoir brine. However, xanthan gum also presented some important negative aspects that should be carefully evaluated before moving to further steps. A target viscosity of xanthan gum relies on shear rate, temperature, polymer concentration, and salinity. Literature suggests a feasible range of polymer concentration between 500 ppm to 1,200 ppm for offshore field applications. Concentrations above 1,200 ppm are prone to cause severe injectivity loss especially in tight formation because of its high viscosity. Concentrations below 500 do not show potential viscosities for Enhanced Oil Recovery, moreover a weak pseudoplacticity means an inefficient injectivity, since the shear-thinning effect is mild in the well vicinity. In summary a target solution in a rheometric test relies on shear rate, temperature, polymer concentration, and salinity. Simultaneously, for better injectivity, the solution should present the pseudoplastic behavior in ordered to thin in high shear rate regions close to the well and then recovering its target viscosity while flowing through the vast majority of reservoir under lower shear rates.

Conclusions
Xanthan gum presented a strong pseudoplastic behavior, especially in the semi-diluted region. Its ability to build viscosity in the diluted region (above 2,000 ppm) is significantly reduced when compared to the semi-diluted region (below 2,000 ppm) and a slight pseudoplastic behavior is observed. Most of the flow curves show a Newtonian behavior below 500 ppm of xanthan gum concentration leading to a narrow range of feasible concentrations for field applications in Enhanced Oil Recovery by polymer flood Temperature is a very important parameter for the xanthan gum polymer solution as, for a fixed polymer concentration, viscosity decreases with the increase of temperature. Even though salinity also has an impact, its effect in reducing viscosity is smaller compared to that of temperature. It is important to note that a turning point within 110,000 -220,000 ppm of NaCl range showed an opposite behavior where the increase in salinity increased the solution viscosity. Additionally, the salinity increase protected the xanthan gum in the semi-diluted region against the adverse impact of temperature on viscosity.
As regards the first step of addressing the screening criteria for a polymer flood project with respect to tar-get solution, xanthan gum can build higher viscosities (with the same polymer concentration) than other high molecular weight synthetic polymers such as partially hydrolyzed polyacrylamide (HPAM). This also indicates that high salinity reservoirs could increase the xanthan gum viscosity over the long term due to the interaction between the injected brine and the reservoir brine.
The rheological assessment accomplished in this study reopens possibilities for xanthan gum applications for Enhanced Oil recovery under conditions found in the Brazilian pre-salt.