INVESTIGATIONS IN ACRYLATE POLYMER COMPOSITION, IN ITS STRUCTURE AND DRAG-REDUCING PROPERTIES UNDER CONDITIONS OF HIGH TEMPERATURES AND HIGHLY MINERALIZED AQUEOUS MEDIA ABSRTACT A.I. Nechaev, V. A. Valtsifer and V. N. Strelnikov Institute of Technical Chemistry of UB RAS, Acad. Koroleva St., 3, Perm, Russia toxambj@gmail.com Terpolymers of acrylamide (AA), acrylonitrile (AN), and 2-acryl-amido-2- methylpropanesulfonic acid (AMPS) were prepared by radical copolymerization in aqueous solutions. Terpolymer containing 71,6 %mol of AA, 10,5 %mol of AN, and 17,9 %mol of AMPS is stable to thermal and hydrothermal destructions. Structure of this terpolymer is AA blocks linked by AN and AMPS units statistically located in the polymer chain. The maximal magnitude of the drag reduction effect, DRmax, for this terpolymer prepared is in the interval 70 80 %. The terpolymer provides the DR performance by not less than 70 % under conditions of high temperatures up to 180 C and highly mineralized aqueous media with CaCl 2 up to 70 g/l. INTRODUCTION Nowadays, drilling of deep/ultradeep prospect and production oil/gas wells under complicated mining and geological conditions is actively progressing. In this connection, a drilling fluid and all its components should retain their performance characteristics when subjected to elevated thermal, salt, and acidic aggressions. Water-soluble polymers with high molecular mass (>10 6 Da) appear to be one of basic components in drilling fluid that provide drag reduction (the Toms effect) during drilling. Currently, high-molecular ionic and nonionic acrylamide copolymers (AA) are most often usable to provide drag reduction of turbulent fluid flows in aqueous media [1]. The given polymer provides drag reduction by 50-60 % as compared with non-treated water [2]. However, polyacrylamide is amenable to such processes as destruction, partial cross-linking, hydrolysis etc. when subjected to thermal, salt (polyvalence metal cations), and acidic aggressions resulting in total loss of functional properties. The AA copolymers with sulfonate substituents in side chains, such as acrylamide copolymers with 2-acrylamido-2-methylpropanesulfonic acid (AMPS) and its sodium salt, are more stable under conditions of thermal and salt aggressions [3]. Availability of units containing -C N groups in side chains of polyacrylamide increases thermal stability of copolymers [3, 4]. To attain sufficient stability of acrylate copolymers at elevated temperatures (thermal hydrolysis), they should be featured by the following: (a) properly opted diapason and ratio of anionic and non-ionic macromolecular fragments, and (b) drag reduction of turbulent water flows by not less than 50-60 %. 91
Earlier [5, 6], the authors had shown the AA-AN-AMPS terpolymers with starting contents of AN monomer (14 % mol) and AMPS monomer (over 20 % mol) to be most stable to thermal and hydrothermal destructions at temperatures up to 180 C, to CaCl 2 solutions at concentrations up to 70 g/l, and to ph values down to 1.65. Therefore, investigations aimed at the study of the structure and hydrodynamic properties of compositionally-optimized terpolymer AA-AN-AMPS containing monomers at the molar ratio of 65:15:20 and characterized by the drag-reducing properties under conditions of thermal and salt aggressions are of current importance. Experimental part The following reagents were used in these experiments: acrylamide (AA, 98+ %, Alfa Aesar), acrylic acid nitrile (AN, 99+ %, Alfa Aesar), 2-acrylamido-2-methylpropanesulfonic acid (AMPS, 98+ %, Alfa Aesar), sodium hydroxide of analytical purity, potassium persulfate ( 99,0 %, Sigma-Aldrich), sodium sulfite of analytical purity. Stabilizing agent contained in acrylic acid nitrile was preliminary removed by distillation. AA, AN, and AMPS were copolymerized in an aqueous medium in the presence of potassium persulfate in accord with the method described in [5]. 2 M solution of NaOH was added to aqueous solution of AA and AMPS taken in calculated proportions at the molar ratio 3,25:1 to maintain ph value of the solution close to 9. This value was requisite to keep the initiation rate constant at variable ionic force of the solution. Next, the calculated proportion of AN (15 % mol of total portion of monomers) was added. Cumulative concentration of monomers in the solution equaled 1,6 mol/l. The co-monomer solution was then bubbled with nitrogen through the finely porous Schott filter for 15 min. Next, potassium persulfate as initiator at concentration of 1 10-3 mol/l and sodium sulfite as oxygen scavenger at concentration of 1 10-3 mol/l were added. The solution was poured into weighing bottles, whereupon the weighing bottles were hermetically plugged up, placed into the thermostat and kept at 60 C. Terpolymer was isolated from the reaction medium by precipitation into acetone. The copolymers were thrice washed with acetone, and dried under vacuum at 50 C to attain a constant mass value. Composition of terpolymer was determined from the findings of elemental analysis and thermogravimetric analysis. Elemental composition was determined on the LECO CHNS-9321P Elemental Analyzer (Netherlands). Weighing portions of specimens equaled 2 mg, with coefficient of variation between 0,05-0,29 %. 1 H and 13 C NMR spectra were registered on the Bruker spectrometer (300 and 75.5 MHz, respectively), with DMSO-d 6 as a solvent. The 13 C NMR spectra were registered with broadband proton suppression and in the JMOD regime. The FTIR spectra were registered on the Bruker IFS 66/S spectrometer. Specimens to be analyzed were prepared as KBr-pressed tablets at the ratio of 1 mg of specimen to 299 mg of KBr. Thermogravimetric analysis of the specimens was performed on the TGA/DSC 1 device (Mettler Toledo, Switzerland) in the air atmosphere at the heating rate 10 C/min and in temperature interval 25-1000 C. Molecular mass of the copolymer was indirectly judged from values of characteristic viscosity [η] on the basis of the Mark-Houwink-Kuhn equation [ηη] = KKMM αα ηη. Measurements were made on the VPZh-1 viscosimeter with d k =1 mm. On the basis of viscosimetric 92
measurements in 0,5 M NaCl solution at 30 C, linear concentration profiles η sp /C p = f(c p ) were plotted, wherein C p stands for concentration of copolymer. With the obtained direct relations extrapolated to zero concentration value, characteristic viscosity of the copolymer [η] was calculated: [η] = lim(η уд /С п ), при С п 0. Average molecular mass MM ww of the copolymer and size distribution of its macromolecular aggregations in highly-diluted aqueous solutions were determined with use of the dynamic light-scattering method on the ZetaPals device (Brookhaven Inst. Co., USA). The findings were processed using the Particle Solutions, ver.3.0 software (Brookhaven Inst. Co., USA). The drag-reducing ability of the copolymer was appraised on the laboratory turbulent capillary-type rheometer. The layout of the rheometer is presented in Fig.1. Measurements were performed under overpressure of 8 MPa and at 25 C, Re=28670, with water as a solvent. Fig.1. Layout of turbulent capillary rheometer: nitrogen cylinder (1), receiver (2), pumping main (3), feed tank (4) and capillary (5) equipped with jacket, thermostat (6), thermo-couple (7), tensometric sensor (8), analog-digital converter (9), personal computer (10), receiving tank (11), discharge tank for tested fluid (12). Hydrodynamic performance of the copolymer specimens was appraised from relative values of drag reduction (DR) [2, 7]. The outflow duration of both pure solvent (water) and variously concentrated polymer solutions at the same pre-set pressure drops between the pipe ends ( P s = P p = const) of the capillary rheometer was measured. The DR value was calculated in accord with the formula: DDDD = 1 tt pp 2 tt ss 2 100 %, wherein t s and t p values are outflow durations of fixed volumes of pure solvent and polymer solution, respectively, through the capillary, in turbulent flow regime. Drag-reducing performance of the copolymers was appraised from the drag reduction value of the copolymer solution when its concentration is decreased by 1 unit of 93
concentration; at this point, the solution is progressively diluted to endlessness [8]. To this end, the concentration dependence C п /DR = f(c п ) was plotted as a direct line at low concentration values of the copolymer. With the obtained direct line extrapolated to zero concentration value, characteristic value [C п /DR] was derived, from which the drag-reducing performance f was calculated: 1/f = [C п /DR] = lim (C п /DR), при С п 0. Results and discussion Composition of terpolymer synthesized at the initial molar ratio of monomers [AA]:[AN]:[AMPS]=65:15:20 was analyzed by the elemental and thermogravimetric methods. The findings evinced the terpolymer to be composed of 71,6 %mol units of AA, 10,5 %mol units of AN, and 17,9 %mol units of AMPS. In FTIR spectra of the copolymer with the ratio [65]:[15]:[20] (71,6 %mol of AA, 10,5 %mol of AN, and 17,9 %mol of AMPS), several bands usable to structurally characterize the copolymer were observed (Fig.2). So, we observe characteristic bands between 3100 cm -1 and 2850 cm -1 corresponding to vibrations of aliphatic methylene groups, and the band at 1670 cm -1 assigned to vibrations of the C=O acrylamide groups. The lowintensive band at 2240 cm -1 is assigned to vibrations of the CN groups in an acrylonitrile unit. The bands at 1213, 1189, and 1043 cm -1 are assigned to vibrations of the SO 3 H group, the band at 628 cm -1 to vibrations of the C S group. Fig.2. FTIR spectrum of the copolymer with the ratio [65]:[15]:[20] The structure of the polymers obtained was explored with use of 1 H and 13 C NMR spectroscopy. Concentrated aqueous solutions of polymers were used as specimens for NMR spectroscopy. Assignment of the group signals in the spectra of terpolymers was carried out using the reference data [9, 10] and while taking into account the influence of the SO 3 group. Fig.3 displays the 1 H NMR spectrum of the copolymer with the ratio [65]:[15]:[20] (71,6 %mol of AA, 10,5 %mol of AN, and 17,9 %mol of AMPS). In the 1 H NMR spectrum 94
of poly(aa-an-amps), chemical shifts of the methylene proton signals in the main chain are observed at δ 1.41-1.6 ppm. The peaks observable at δ 1.1-1.3 ppm correspond to protons of CH 3 group of AMPS. The spectrum contains also signals at δ 2.08 ppm that correspond to protons of CH 2 -CH- group in the polymer chain of AA and AMPS; the peaks observable at δ 2.61-2.9 ppm correspond to protons of CH 2 -CH- group of the AN unit. The peak at δ 3.34 ppm is assigned to methylene protons of C-CH 2 -S unit; the broad peak at δ 3.41 ppm is assigned to proton signals of water. Chemical shifts of proton signals of NH 2 group of AA and NH group of AMPS are observable at δ 6.69-7.15 and 7.49-8.39 ppm, respectively. Fig.3. 1 H NMR spectrum of the copolymer with the ratio [65]:[15]:[20] and DMSO-d 6 13 C NMR spectrum of the terpolymer with the ratio [65]:[15]:[20] is presented in Fig.4. The peaks at 171-177 ppm in the spectra correspond to carbon atoms of carbonyl group; the peak at 123 ppm to the carbon atom of nitrile group. The spectrum contains also signals of quarternary carbon C 10 at 51.70 ppm and signals of methylene group of the AMPS unit bound to SO 3, at 60.11 ppm. The peak at 25.73 ppm in the spectra is assigned to C 11 carbon atoms of two methyl groups. 95
Fig.4. 13 C NMR spectrum of the copolymer with the ratio [65]:[15]:[20] and DMSO-d 6 Analysis of the broadly swept carbonyl and nitrile group signals of polymers permits a conclusion that the compensational dyads AA-AA, AA-AMPS, AA-AN AMPS-AA, AMPS- AN, AN-AA and AN-AMPS are observable in the copolymer with the ratio [65]:[15]:[20]. For the terpolymers, availability of blocks in AA-AA units is typical at the expense of high contents of the latter. The AMPS-AMPS and AN-AN dyads are absent in the spectrum of the terpolymers; consequently, sulfo-acid blocks and polyacrylonitrile blocks are not formed during synthesis of terpolymers. The characteristic viscosity [η] and molecular mass values of the synthesized terpolymer measured by the dynamic light-scattering method showed the average molecular mass M w to be within 1.1-1.5 MDa. Dimensions of molecular aggregates contained in the highly-diluted aqueous solution of the synthesized copolymer are governed by the logarithmically normal distribution and sized 0,05-1µm. The acrylate copolymers [AA]:[AN]:[AMPS] were explored to ascertain physicchemical properties as described in [14,15], and additionally including stability to elevated temperatures up to 180 C, to acidic aggressions of down to ph 1.65, and to salt aggressions in C(CaCl 2 ) medium at concentrations of up to 70 g/l. The findings evinced the opted terpolymer specimen of [65]:[15]:[20] (71,6 %mol of AA, 10,5 %mol of AN, and 17,9 %mol of AMPS) to be fit for further investigations in its drag-reducing (DR) performance under conditions of elevated temperatures and acidic/salt aggressions. The terpolymer specimen of [65]:[15]:[20] provides minimal properties requisite under pre-set conditions. The drag-reducing (DR) performance was investigated on the ELS-PT-230 test bench (Fig.1). The salt aggression test was conducted at 25 C, ph 6.86 with added portions (10, 30, and 70 g/l) of CaCl 2. Concentration profiles of DR performance were compared with the one for the terpolymer [65]:[15]:[20] dissolved in distilled water. 96
70 DR, % 60 50 40 30 20 10 0 % CaCl2 1 % CaCl2 3 % CaCl2 7 % CaCl2 С(poly), % 0 0.05 0.1 0.15 0.2 0.25 0.3 Fig.5. Concentration profiles of DR performance of terpolymer [65]:[15]:[20] vs. CaCl 2 concentration in medium As is apparent from Fig.5, the concentration profiles and DR values remain constant with increasing concentrations of CaCl 2 up to70 g/l. The evinced effect of terpolymer [65]:[15]:[20] is related to introduction of AMPS containing sulfonate anion and being a strong acid that does not form poorly soluble salts with bivalent calcium cations. The findings evince the terpolymer [65]:[15]:[20] to be stable and efficient under conditions of salt aggression in media at CaCl 2 concentrations of up to 70 g/l. Besides, at concentrations above 0,2 %, the terpolymer [65]:[15]:[20] provides drag reduction values not less than by 73 % in saline media, as compared with the same conditions without the additive. The acidic aggression test was conducted with the terpolymer [65]:[15]:[20] dissolved in buffer solutions with ph 4.01, 3.56, and 1.68. Concentration profiles of DR performance were compared with the one for the terpolymer [65]:[15]:[20] dissolved in the buffer solution with ph 6.86 at 25 C (Fig.6). DR, % 70 60 50 40 30 20 10 ph 6,85 ph 4,01 ph 3,56 ph 1,68 С(poly), % 0 0.05 0.1 0.15 0.2 0.25 0.3 97
Fig.6. Concentration profiles of DR performance of terpolymer [65]:[15]:[20] vs. ph values of medium As is apparent from Fig.6, with decreasing ph value of the medium from 6.86 down to 1.65, drag reduction values of the terpolymer [65]:[15]:[20] in a diapason of diluted solutions (0<C<C opt ) negligibly decrease by 3-5 %. The mentioned decrease is related to suppressed dissociation of carboxylic groups in copolymer, with decreasing ph values. As a consequence, this phenomenon results in decreasing dimensions of polymer aggregations. In turn, decreased dimensions of polymer aggregations lead to decreased DR values. At this point, the ph value decreased down to 1.65 did not influence the DR max value at C opt values. Similarly, the concentration profiles retain their shapes. Stability of the terpolymer [65]:[15]:[20] to acidic aggressions is related to a decreased total number of carboxylic groups in copolymer at the expense of an increased number of nitrile- and sulfo-groups. At concentrations above 0,2 % and under conditions of acidic media with ph values of down to 1.68, the terpolymer provides drag reduction not less than by 71 %, as compared with the same conditions without the additive. The temperature aggression test was conducted with the terpolymer [65]:[15]:[20] dissolved in the buffer solution with ph 6.86 at temperatures up to 180 C. Concentration profiles of DR performance were compared with the one for the terpolymer [65]:[15]:[20] dissolved in the buffer solution with ph 6.86 at 25 C (Fig.7). 80 DR, % 70 60 50 40 30 25 C 60 C 100 C 140 C 180 C 20 10 С(poly), % 0 0.05 0.1 0.15 0.2 0.25 0.3 Fig.7. Concentration profiles of DR performance of terpolymer [65]:[15]:[20] vs. temperature of medium As is apparent from the profiles in Fig.7, with temperature increasing up to 100 C, a slight increase (by 3-5 %) in DR performance is observable. At this point, concentration profiles retain their shapes in the entire diapason of concentrations under investigation. With further increasing temperature above 100 C, a steep decrease in the DR value occurs. At 140 C, the concentration profile becomes identical to the initial one measured at 25 C. At 180 C, the DR value attains 70-75 % only at 0,2-0,3 % concentration of terpolymer [65]:[15]:[20]. Such a decreased effect is related to two proceeding processes. Firstly, an increase of temperature leads to a hindered strain and a hindered orientation of copolymer 98
molecules in the flow due to highly intensive thermal motion. Secondly, an increase of temperature causes destruction of polymer molecules, thus resulting in a decreased DR performance. Conclusion The optimally composed terpolymer [65]:[15]:[20] has been ascertained to contain 71,6 %mol of AA, 10,5 %mol of AN, and 17,9 %mol of AMPS, to be characterized by average molecular mass 1,13 MDa, and structured as AA blocks linked by AN and AMPS units statistically located in the polymer chain. Investigations in the DR performance of turbulent aqueous flows have evinced the optimally composed terpolymer [65]:[15]:[20] at concentration above 0,1 % to provide a decrease in friction loss by not less than 70 % as compared with non-treated water. At concentration above 0,15 % and a content of CaCl 2 in medium of not less than 70 g/l, the terpolymer provides the DR performance by not less than 70 % as compared with the same conditions without the additive. Besides, at a concentration above 0,18 % and in a medium with ph value reduced down to 1.65, the terpolymer [65]:[15]:[20] provides the DR performance by not less than 70 % as compared with the same conditions without the additive. The findings obtained evince also stability and efficiency of the terpolymer [65]:[15]:[20] at concentrations above 0,2 % under conditions of temperatures elevated up to 180 C. Acknowledgement The study was financially supported by the Ministry of Education and Science of the Russian Federation, Agreement 14.607.21.0121 (RFMEFI60715X0121). References [1]. Pereira A.S., Andrade R.M., Soares E.J.: 'Drag Reduction Induced by Flexible and Rigid Molecules in a Turbulent Flow into a Rotating Cylindrical Double Gap Device: Comparison between Poly (ethylene oxide), Polyacrylamide, and Xanthan Gum'. Journal of Non- Newtonian Fluid Mechanics 2013 202 72-87. [2]. Kamel A., Shah S.N.: 'Effects of salinity and temperature on drag reduction characteristics of polymers in straight circular pipes'. Journal of Petroleum Science and Engineering 2009 67 23-33. [3]. Malcolm A. Kelland. Production Chemicals for the Oil and Gas Industry, Second Eition. CRC Press, 2014. [4]. Eshrati M., Al-Hashmi A.R., Al-Wahaibi T. et al: 'Drag Reduction using High Molecular Weight Polyacrylamides during Multiphase Flow of Oil and Water: A Parametric Study'. Journal of Petroleum Science and Engineering 2015 135 403-409. [5]. Nechaev A.I., Lebedeva I.I., Val tsifer V.A., Strel nikov V.N.: 'Infl uence of the Composition of Acrylamide Acrylonitrile 2-Acrylamido-2-Methylpropanesulfonic Acid Terpolymer on Its Resistance to High Temperatures and Salts'. Russ. J. Appl. Chem. 2016 89 (8) 1296-1301. [6]. Nechaev A.I., Lebedeva I.I., Vasil eva O.G., Chashchukhin A.S., and Val tsifer V.A.: 'Reduction of the Hydrodynamic Resistance to Turbulent Water Flow with Copolymers of 99
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