International Journal of Engineering & Technology IJET-IJENS Vol:16 No:03 1

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1 International Journal of Engineering & Technology IJET-IJENS Vol:6 No:03 Rheological Approach of HPAM Solutions under Harsh Conditions for EOR Applications Bruno M. O. Silveira, Leandro F. Lopes, Rosângela B. Z. L. Moreno - University of Campinas Abstract The hydrolyzed polyacrylamide (HPAM) is the water-soluble polymer most often used in flooding applications. However, the temperature, salinity and hardness of the reservoirs affect its viscosifying properties reducing the sweep efficiency of the polymer solution. This work aims to tailor HPAM (Flopaam 55SH) solutions prepared with synthetic produced water (SPW) or with distilled water only, and evaluate their rheological properties. The flow curves confirm the detrimental effects of salinity and hardness. For the target viscosity ( mpa.s at ~7.8 s - and 23 C) chosen to perform further polymer flooding tests, the required polymer concentration is 250 ppm. At this concentration, the solution presents a thermothinning behavior as the temperature increases. Regarding the viscoelastic properties, the viscous modulus is predominant for fluids prepared with SPW, while the elastic modulus is prevalent for fluids without salts. These results contribute to the design of a polymer solution composition under interest conditions. Index Term Polymers, Viscosity, Rheology, Polymer flooding, Enhanced Oil Recovery (EOR) INTRODUCTION Enhanced oil recovery (EOR) methods encompass advanced techniques such as thermal, chemical, miscible and microbial. They are used when the primary and secondary recoveries had already been implemented, and a considerable portion of the original oil in place (OOIP) remains in the reservoir [-3] The polymer flooding is a chemical method to enhance oil recovery (CEOR). This method consists in changing the rheological properties of the displacing fluid by the addition of water-soluble polymers. The polymer addition increases the viscosity and viscoelasticity of the injected/displacing fluid, leading to a more favorable mobility. This mobility control improves the sweep efficiency and also lowers the total volume of water needed to achieve the residual oil saturation [], [2], [4]. Polyacrylamide-based polymers, such as partially hydrolyzed polyacrylamide (HPAM), are widely used in CEOR, mainly due to its availability and relatively low cost compared to polysaccharides [5]-[8]. However, under high temperatures and high salinity/hardness (HTHS) conditions, the effectiveness of HPAM is severely affected [9], []. At low salinity solutions, the chains of HPAM are stretched due to the charge repulsion of the carboxylic groups, resulting in higher viscosities. When any cation is dissolved in the solution, the negative charges are neutralized or shielded, promoting a compression of the flexible chains, resulting in a molecular shrinkage and thus a decrease in the fluid viscosity [2], [9], []. Moreover, the viscosity reduction is more pronounced in the presence of divalent cations such as calcium (Ca 2+ ) and magnesium (Mg 2+ ), than monovalent ones such as sodium (Na + ) and potassium (K + ), when added in equivalent mass percentage [9], []-[5]. This effect is explained by the ionic strength of the solution (I S = /2Σm i z i ², where m i is the molar concentration of the ion and z i is its charge). For example, the ionic strength of molar solution of CaCl 2 is three times greater than that of NaCl. Due to that, CaCl 2 compresses the electrical double layer of polymer chain more than NaCl [2]. Furthermore, the binding forces of the divalent ions are stronger, owing to their higher charge and polarizability [2], [], [3]. Thus, the polymer chain can wraps in the presence of low levels of Ca 2+, due to its higher effectiveness in shielding the negative charge of the polymer chains than Na + [9]. The detrimental effect on the viscosity of the HPAM solution is also correlated to the temperature. At higher temperatures, there is an increase in thermal motion of molecules, leading to a decrease in the interaction time with the neighboring polymer molecules. At this condition, the intermolecular forces among polymer chains and the viscosity of the polymer solution are decreased [6], [7]. Also, higher temperatures can increase the degree of hydrolysis of the polymer, which converts amide groups (CONH 2 ) to carboxyl groups (COO ). This effect introduces more negatives charges to the polymer backbone, allowing more interaction with divalent ions, which may cause their precipitation, and thus promote a change in the rheological properties of HPAM solutions [5], [],[8], [9]. According to the literature, HPAM copolymerized with the sulfonated monomers, such as a 2-acrylamidotertbutylsulfonic acid (ATBS group), is more resistant to salinity and temperature [20], [2]-[24]. This resistance contributes to minimize the losses on the rheological properties, such as viscosity and viscoelasticity [2]. The molecular structure of HPAM and HPAM with ATBS group are illustrated in Fig. and, respectively IJET-IJENS June 206 IJENS

2 International Journal of Engineering & Technology IJET-IJENS Vol:6 No:03 2 Fig.. Molecular structure of HPAM and HPAM with ATBS group. Adapted [22], [25]. A rheological study allows setting/design the solution composition under interest conditions [2], [24]. Focused on this purpose, this work aims to evaluate some rheological properties, such as viscosity, shear stress, overlap concentration and viscoelastic characteristics of the polymer solutions tailored with Flopaam 55SH for a given synthetic produced water (SPW) composition (high salinity and hardness conditions). Additionally, the procedure was used to determine the fluid composition with a target viscosity for further core flooding through specific conditions. The fluid viscosity was set as ~ mpa.s at 23 C for ~7.8 s - of shear rate, which can be correlated to the shear rate that the polymer solution faces at low flow velocity into the porous medium [26]. EXPERIMENTAL SECTION Materials The polymer fluids were prepared with an SPW composition. On Table I is shown the type of salt, its concentration, and its ionic strength. The polymer used was Flopaam 55SH (4-6 x 6 g/mol of molecular weight (Mw) and 5% of hydrolysis degree) from SNF. According to the supplier, this is an ATBS-based polymer less sensitive to temperature and salinity, recommended for reservoir temperatures up to 95 C. Furthermore, this polymer has 5% of ATBS (2- acrylamido-tertbutylsulfonic acid), and total anionicity of 25% [23]. Fluid Handling The formulation of the polymeric fluids was based on API- RP-63 [27]. According to this standard, a stock solution with 5000 ppm of polymer concentration should be first prepared. After that, the stock solution is diluted to obtain the desired polymer concentration. It is also required that the solution exhibits a homogeneous aspect, i.e., no insoluble particle (fisheye) should be present. To prepare the stock solution, the distilled water or the SPW solution was vigorously stirred in a beaker within 30 seconds. Meanwhile, the polymer in powder form was uniformly sprinkled into the vortex of the solution. Then, the stirring speed was reduced in order to avoid mechanical degradation of the polymer. The homogenization was kept for three hours. After that, the stock solution was stored overnight. The stock solution was diluted with distilled water or SPW until the desired polymer concentration was reached. The solutions were put into a beaker and homogenized by a magnetic stirrer at low speed (20 rpm) for minutes. Salt Molecular weight (Mw) (g/mol) Table I Synthetic produced water (SPW) composition Concentration (g/l) Ionic strength (mol/l) CaCl 2.2 H 2 O MgCl 2.6 H 2 O KCl NaCl Na 2 SO TOTAL.84 0 (%) Rheological apparatus and procedure The rheological tests were performed using a HAAKE MARS III rheometer, which is a high precision instrument. The sensor used was the double gap (DG4) because this cylindrical sensor is preferable for low-viscous fluids. The temperature control used was the THERMO HAAKE C25P IJET-IJENS June 206 IJENS

3 Viscosity (mpa s) Shear Stress (Pa) Viscosity (mpa s) Shear Stress (Pa) International Journal of Engineering & Technology IJET-IJENS Vol:6 No:03 3 refrigerated bath with a Phoenix II Controller. A new sample was applied at each test, and every data analyzed were within the measuring range of the sensor. The analysis of viscosity, shear stress, temperature and overlap concentration was made by flow curve tests, and the last one was performed at the shear rate of interest. The viscoelasticity analysis was performed by frequency sweep tests. These tests require stress values within the linear viscoelastic response (LVR), wherein the total resistance of the material, at a given strain, is independent of the stress imposed [28]-[30]. Thus, amplitude sweep tests were run for each fluid, and the shear stress was chosen to perform the frequency sweep. RESULTS AND DISCUSSIONS Flow curves Viscosity and shear stress versus shear rate of stock and dilute solutions The rheological behavior of the stock and diluted solutions, prepared with distilled water or SPW, are presented in Fig. 2 and Fig. 3 respectively. In Fig. 2 and Fig. 3 it is shown that the apparent viscosity of the fluids increases as the polymer concentration increase. This behavior is related to the rise in the intermolecular entanglement [2], [9], [3]. Also, the shear thinning behavior is observed as the shear rate increases. This effect is due to the uncoiling and the aligning of the polymer chains when they are subjected to shearing [9], [3]. It can also be observed that the salt content negatively affects the viscosity of the polymer solutions, as described in the literature [2], [9], []-[5]. From the shear stress versus shear rate curves (Fig. 2 and Fig. 3 ), the pseudoplastic behavior can be fitted by Ostwald de Waele model (. The rheological parameters of each fluid according to this model, formulated with distilled water or SPW, are summarized in Table II ppm 500 ppm 20 ppm 750 ppm 30 ppm 00 ppm 50 ppm 250 ppm 75 ppm 500 ppm 0 ppm 3000 ppm 300 ppm 5000 ppm ppm 500 ppm 20 ppm 750 ppm 30 ppm 00 ppm 50 ppm 250 ppm 75 ppm 500 ppm 0 ppm 3000 ppm 300 ppm 5000 ppm Shear Rate (s - ) Shear Rate (s - ) Fig. 2. Viscosity and shear stress vs. shear rate of dilutions with distilled water (at 23 C) ppm 00 ppm 20 ppm 250 ppm 30 ppm 500 ppm 50 ppm 2000 ppm 75 ppm 2500 ppm 0 ppm 3000 ppm 300 ppm 3500 ppm 500 ppm 5000 ppm 750 ppm SPW ppm 00 ppm 20 ppm 250 ppm 30 ppm 500 ppm 50 ppm 2000 ppm 75 ppm 2500 ppm 0 ppm 3000 ppm 300 ppm 3500 ppm 500 ppm 5000 ppm 750 ppm SPW Shear Rate (s - ) Shear Rate (s - ) Fig. 3. Viscosity and shear stress vs. shear rate of dilutions with SPW (at 23 C) IJET-IJENS June 206 IJENS

4 International Journal of Engineering & Technology IJET-IJENS Vol:6 No:03 4 It can be observed that the higher the polymer concentration is, the higher the parameter K will be, which will provide a higher resistance for the fluid to flow. Furthermore, it implies that the rheological behavior of the polymer solution will drift away from the Newtonian behavior (n ) [24], [30]. For polymer solutions prepared with SPW, these effects are less pronounced. From Fig. 3, one can see that 250 ppm of polymer concentration provides the target viscosity (~ mpa.s at ~7.8 s - ) for flooding. This value is within the range of polymer concentrations for field applications that usually ranges from 500 to 2500 ppm [32], [33]. An analysis of the influence of each salt on its given concentration (Table I) is presented in Fig. 4. The thermal evaluation of the bulk composition is presented in Fig. 4. Both analyses were performed for solutions containing 250 ppm of polymer concentration. On Fig. 4 is shown that the addition of any salt negatively affects the apparent viscosity when compared with the fluid without salt. Furthermore, it is possible to see the higher detrimental effect caused by divalent ions. The fluids prepared with MgCl 2.6H 2 O or CaCl 2.2H 2 O, even having lower ionic strength values than that prepared with NaCl only, promoted a similar reduction in the viscosity of the polymer solution. In another work, similar results were obtained with Flopaam AN SH (HPAM) from SNF, which is also an ATBS-based polymer [24]. From Fig. 4 the thermothinning behavior is observed with the increase in the temperature [34]. This behavior is caused by the reduction of the intermolecular forces among the polymer chain [6], [7]. Taking the results at 23 C and 7.8 s - as a reference, the increase in the temperature to 30, 40, 50, 60 and 70 C reduced the solution viscosity in 20%, 34%, 43%, 47% and 49% respectively. Overlap concentration (c*) Plotting the viscosity versus polymer concentration for a specific shear rate, two concentration regions can be highlighted, a dilute and a semi-dilute. In the dilute regime, the macromolecules behave independently because they are separated from each other. In the semi-dilute regime, frictions are imposed on the macromolecules due to the polymeric entanglement. The transition between both regimes is called overlap concentration (c*) and its is characterized by a change in the slope of the viscosity versus polymer concentration curve [35]. Polymer concentration Table II Rheological parameters of the polymer solutions K (Flow consistency index) n (Flow behavior exponent) r (Correlation coefficient) ppm Distilled water SPW Distilled water SPW Distilled water SPW IJET-IJENS June 206 IJENS

5 Viscosity (mpa s) Viscosity (mpa s) Viscosity (mpa s) Viscosity (mpa s) International Journal of Engineering & Technology IJET-IJENS Vol:6 No: HPAM - No Salt HPAM - All Salts HPAM + CaCl2 HPAM + MgCl2 HPAM + KCl HPAM + NaCl HPAM + Na2SO C 30 C 40 C 50 C 60 C 70 C 80 C Shear Rate (s - ) Fig. 4. Viscosity vs. shear rate. Influence of each salt on solutions with250 ppm of HPAM at 23 C (g/l CaCl 2.2H 2O; 6.3 g/l MgCl 2.6H 2O; 0.6 g/l KCl; 86.6 g/l NaCl and.3 g/l Na 2SO 4). Temperature effect on solutions formulated with 250 ppm of HPAM and SPW. In Fig. 5 it is possible to see both regions for polymer solutions prepared with distilled water or SPW. Dashed lines represent the dilute region, and continuous lines represent the semi-dilute one. The c* of the polymer solutions prepared with distilled water (Fig. 5) or SPW (Fig. 5 ) were evaluated for fixed shear rates. The c* values were defined in the intersections between the dashed and the continuous lines. The parameters of each trend line and the resulting c* are shown in Table III. The solutions with SPW presented similar values for c* (average value was ppm at 23 C) regardless of the shear rate, i.e., little change was observed within the shear rate range under study Shear Rate (s-) This value of c* (337.7 ppm) is close to values reported by some authors. For a solution prepared with HPAM (27.8% acrylate, 72.2% acrylamide monomers and 8.5x 6 g/mol of Mw, supplied by SNF) and 2% of KCl at 30 C, 295 ppm was obtained [35]. In another work, the solution with HPAM (SNF Flopaam 3230S, 8x 6 g/mol of Mw and 30% degree of hydrolysis, approximately) and 2% NaCl at 25 C, 300 ppm was reported for c* [36]. Concerning the Flopaam 55SH solutions with distilled water, one can see that the higher the shear rate, the higher was the c* value. Similar results were reported for polystyrene solutions, attributing this behavior to the deformations of the chains induced by the flow of the solutions [37] s s Polymer concentration (ppm) Polymer concentration (ppm) Fig. 5. Critical overlap concentration (c*) for fluids prepared with: distilled water and SPW (at 23 C) IJET-IJENS June 206 IJENS

6 G' G'' (Pa) G' G'' (Pa) International Journal of Engineering & Technology IJET-IJENS Vol:6 No:03 6 Shear Rate Trend line * (Dashed lines) Table III Parameters of the dilute and the semi-dilute concentration regions Trend line 2* (Continuous lines) Viscosity η (mpa.s) c* (ppm) a b a b No salts s s s s s SPW s s s s s * adjusted by potential trend line: Viscoelastic Measurements (G and G ) The viscoelastic properties, especially the elastic modulus (G ), plays a significant role in increasing oil recovery owing to its microscopic features [38], [39]. The amplitude sweep tests were performed for both conditions, with and without salts. Among several results, some were chosen to illustrate the polymer concentration and the salinity effects on the LVR. The results are presented in Fig. 6. Comparing the results between the fluids prepared with distilled water (Fig. 6 ) and SPW (Fig. 6 ), it is evident that the salinity affects the LVR. For the solutions with SPW, LVR was not identified for the solutions with polymer concentrations lower than 250 ppm. Considering solutions without salts, the LVR was not observed only for polymer concentrations lower than 50 ppm. On Table IV are presented the shear stress chosen to perform the frequency sweep test for each fluid composition, and Fig. 7 presents the elastic (G ) and viscous (G ) moduli as a function of the angular frequency. For both conditions, the magnitude of the moduli of G and G increased as the polymer concentration increased, such that the viscoelastic characteristics of the fluids can be guided by the type and concentration of the polymer [30]. Also, if the polymer concentration in the solution is higher than c*, the elastic modulus will be dominant over the viscous modulus [40]. This condition was observed for fluids without salts, where the values of G were higher than those of G for polymer concentrations over 0 ppm (see Fig. 7 ). However, for fluids containing salts, the viscous behavior was predominant for all polymer concentrations evaluated. 0 0 G' G" 0 ppm 0 ppm 250 ppm 250 ppm 5000 ppm 5000 ppm G' G" 50 ppm 50 ppm ppm 0 ppm 250 ppm 250 ppm 5000 ppm 5000 ppm Shear stress (Pa) Shear stress (Pa) Fig. 6. Amplitude sweep test for fluids without salts and fluids formulated with SPW, both at 23 C IJET-IJENS June 206 IJENS

7 G' G'' (Pa) G' G'' (Pa) International Journal of Engineering & Technology IJET-IJENS Vol:6 No:03 7 Table IV Shear stress choices to perform frequency sweep tests Shear Stress (Pa) Polymer concentration (ppm) Fluids with distilled water Fluids with SPW No LVR < 50 < / 75 / / 500 / 2000 / 2500 / / 500 / 750 / 00 / 250 / / / G' G" 50 ppm 50 ppm ppm 0 ppm 250 ppm 250 ppm ppm 5000 ppm ω (Rad/s) 0.00 G' G" ppm 250 ppm 2500 ppm 2500 ppm 5000 ppm 5000 ppm ω (Rad/s) Fig. 7. Frequency sweep test for fluids without salts and fluids formulated with SPW, both at 23 C. CONCLUSIONS The solutions prepared by mixing HPAM Flopaam 55SH with either SPW or distilled water presented a shear thinning behavior (pseudoplastic behavior) that can be fitted by the Ostwald de Waele model. It was observed that the salt content in the polymer solutions reduces the polymer viscosity and the shear stress. Furthermore, this detrimental effect was more severe for salts with divalent cations (Ca 2+ and Mg 2+ ) in their respective concentration. It was found that the salt content directly influences the determination of the overlap concentration (c*). The polymer solutions prepared with distilled water indicate that the overlap concentration increases as the shear rate increases, unlike the results observed for fluids containing salts. For these fluids, the overlap concentration was almost independent of the imposed shear rate. From the viscosity versus shear rate data, we determined that the polymer concentration required to obtain the target viscosity was 250 ppm at 23 C (considering mpa.s at 7.8 s - as shear rate). The rheological evaluation also indicates that, at this polymer concentration (250 ppm), thermothinning behavior is identified as the solution temperature increases. From room temperature (23 C) to 70 C at 7.8 s -, the decrease in the viscosity of the polymer solution was around 49%. Regarding the viscoelastic properties of the polymer solutions, it was observed that the viscous behavior (G ) was predominant for the fluids prepared with SPW. For fluids without salt, the elastic behavior (G ) was more pronounced, except for fluids with polymer concentrations lower than 0 ppm. ACKNOWLEDGMENTS The authors wish to thank STATOIL, ANP, CEPETRO and the Division of Petroleum Engineering/DE-FEM-UNICAMP for their support in this work. REFERENCES [] RAMIREZ, W. F. Application of Optimal Control Theory to Enhanced Oil Recovery, Developments in Petroleum Science, v. 2, Elsevier. The Netherlands, 987. [2] SORBIE, K. S. Polymer-Improved Oil Recovery. st Ed., Boca Raton: CRC Press, p , , 99. [3] VEERABHADRAPPA S. K., URBISSINOVA, T., KURU, E. Polymer screening for EOR application - A rheological characterization approach. Paper SPE presented at SPE Western North American Regional Meeting held in Anchorage, Alaska, 7- May, p., 20. [4] CHEN Q, WANG Y, LU Z, FENG Y. Thermoviscosifying polymer used for enhanced oil recovery: rheological behaviors and core flooding test. Polymer Bulletin, v. 70, n. 2, p , 202. [5] LAKE L. W. Enhanced Oil Recovery. New Jersey, USA: Prentice Hall. p. -2, , 989.Osterloh, W. T., Law, E. J. SPE 39694, IJET-IJENS June 206 IJENS

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