Phase Equilibrium of Methane Hydrate in Aqueous Solutions of

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Phase Equilibrium of Methane Hydrate in Aqueous Solutions of Polyacrylamide, Xanthan Gum, and Guar Gum Pawan Gupta, Vishnu Chandrasekharan Nair, and Jitendra S. Sangwai*

J. Chem. Eng. Data Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 04/01/19. For personal use only.

Gas Hydrate and Flow Assurance Laboratory, Petroleum Engineering Program, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai 600036, India ABSTRACT: The use of water-soluble polymers as natural gas hydrate inhibitors has gained interest in recent years. Variety of polymers have been studied for their kinetic performance in methane hydrate inhibition in the past; however, thermodynamic hydrate inhibition by water-soluble polymers is not fully understood and needs further investigation. This study investigates the effect of molecular weights and concentrations of aqueous solutions of various oilfield water soluble polymers on phase equilibrium of methane hydrate. Water-soluble polymers, such as polyacrylamide (PAM), xanthan gum (XG), and guar gum (GG) with two different molecular weights and varying concentrations, have been considered for the investigations. These are as follows: PAM (Mw: 1.1 × 106 g/ mol, PAM-1 and 1.5 × 105 g/mol, PAM-2), XG (Mw: 6.4 × 105 g/mol, XG-1 and 2.4 × 105 g/mol, XG-2), and GG (Mw: 1.7 × 106 g/mol, GG-1 and 6 × 105 g/ mol, GG-2), with varying concentrations of 100, 200, and 500 ppm each. These are referred to as high-molecular-weight polymers (PAM-1, XG-1, and GG-1) and relatively low-molecular-weight polymers (PAM-2, XG-2, and GG-2). The experiments have been conducted in the pressure and temperature range of 8.63−5.50 MPa and 284.6−279.8 K, respectively. The results indicate that the water-soluble polymers have shown thermodynamic hydrate inhibition with an average temperature depression ranging from 0.25 to 1.05 K. The molecular weight and the concentration of polymers have been shown to affect the hydrate inhibition tendency. We have also proposed a hypothesis for hydrate inhibition based on the mobility of the polymer chain in the solution with a desired functional group in relation to nonelectrolyte/ electrolyte thermodynamic hydrate inhibitors. The presented study on methane hydrate phase stability in the presence of various oilfield polymers is vital for their use in the design and development of hydrate inhibitive drilling fluids for offshore wells, hydrate-bearing formations, and studies related to the recovery of methane from hydrate-bearing sediments using polymer injection.

1. INTRODUCTION Gas hydrates, also referred to as clathrates, are naturally occurring solid icelike crystalline, nonstoichiometric compounds composed of gas molecules held in a cagelike structure. A hydrogen-bonded water framework generates a threedimensional structure (unfilled hydrate lattice) with cavities, which becomes thermodynamically stable with the inclusion of suitably sized guest gas molecules.1 The van der Waals interaction between trapped gas molecules and the surrounding water cage tends to stabilize the individual polyhedral structures. Various components of natural gas, such as methane, ethane, and propane, form gas hydrates under lowtemperature and high-pressure conditions. Gas hydrates are known to form three structures, namely, sI hydrate (cubic crystal structures), sII hydrate (cubic crystal structures), and sH hydrate (hexagonal crystal structures). These structures are different from each other in terms of sizes and number of cages. It is the size of guest gas molecules that decides the type of structure to be formed. For example, methane forms sI hydrate and ethane and propane form sII hydrate, whereas the sH structure is formed in the presence of two guest molecules, one is a smaller, whereas the other is a larger guest gas molecule (e.g., cyclopentane in the presence of methane).1 © XXXX American Chemical Society

There is an additional hydrate structure that has been discovered, which is known as sT hydrate and is formed by dimethyl ether.2 Enormous amounts of natural gas hydrates have been discovered with significant resource potential. These massive resources may be exploited for energy recovery in near future.1,3−9 Furthermore, hydrates have potential applications in CO2 sequestration, desalination, gas storage, gas transportation, and refrigeration.2,10−15 In contrast, gas hydrates are a nuisance for offshore oil and gas industries as they create significant problems resulting in severe blockages in flow pipelines due to production from deep offshore regions.16−18 Additionally, hydrate formation and its rapid dissociation during offshore drilling operations may pose a serious threat such as an increase in the gas kick, increased drill bit pressure, plugging of the blowout preventer, and choking of valves with natural gas hydrates.19−22 Gas hydrate phase equilibrium data is frequently required to understand its stability to suitably design oil and gas facilities.23 A wide range of data on the Received: December 12, 2018 Accepted: March 18, 2019

A

DOI: 10.1021/acs.jced.8b01194 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 1. Details of the Polymer Used in the Study number average molecular weight (g/mol)

polymers

abbreviation

structure

polyacrylamide

PAM-1 PAM-2 XG-1 XG-2 GG-1

(C3H5NO)n (C3H5NO)n (C35H49O29)n (C35H49O29)n (C10H14N5Na2O12P3)n

1.1 1.5 6.4 2.4 1.7

GG-2

(C10H14N5Na2O12P3)n

6 × 105

xanthan gum guar gum

× 106 × 105 × 105 ×105 × 106

phase equilibrium of hydrates has already been generated, which has been frequently utilized to ascertain the conditions at which a natural gas mixture forms hydrates. Moreover, these data are also very helpful in computing the amount of inhibitors necessary to prevent gas hydrate formation.1 Also, several thermodynamic phase equilibrium models have been proposed to predict different hydrate-forming conditions based on an existing experimental data.24−30 Methanol, glycols, and electrolytes are widely used as hydrate inhibitors. Hydrate formation conditions for methane and several gases have already been determined in aqueous solutions of alcohols (methanol and ethylene glycol) and electrolytes (NaCl and ionic liquids).27,31,32 As the oil and gas exploration and production are shifting toward deep offshore regions, this consecutively increases the chances of hydrate encounters during drilling operations. Furthermore, water-based drilling fluids are being preferred over the oil-based mud for offshore conditions, which increases the chances of hydrate formation while drilling through hydrate zones. Studies involving the effect of additives of drilling fluids, such as water-soluble polymers (e.g., polyacrylamide, xanthan gum, and guar gum), on the gas hydrate formation and inhibition are rare. Very few literature reports are available on the thermodynamic studies of these watersoluble polymers, as discussed hereafter. Englezos and Ngan23 first investigated the phase equilibrium of water-soluble polymer “polyethylene oxide” at different concentrations ranging from 0 to 25 wt % and reported that this polymer has very weak hydrate inhibition tendency as compared with the effect of alcohols and electrolytes on the hydrate phase equilibrium conditions. Recently, our group33 has investigated the effect of poly(ethylene glycol) (PEG) on the phase equilibrium of methane hydrate. The thermodynamic phase equilibrium of methane hydrate in aqueous solution of PEG was reported at different concentrations. It was observed that PEG has good thermodynamic inhibition effect on gas hydrate formation. It was also found that the lower-molecular-weight polymer PEG 200 has shown more hydrate inhibition as compared to relatively higher-molecular-weight polymer PEG 400. Information on commonly used oilfield polymers for methane hydrate inhibition is not available in the literature. In recent years, there has been a great interest to investigate the effect of water-soluble polymers on hydrate inhibition as these polymers may impart suitable properties to the drilling fluids if used appropriately. Therefore, there is a need to understand and investigate the effect of various oilfield polymers on the thermodynamics and kinetic behavior of the natural gas hydrate system. Water-soluble polymers that are used in the enhanced oil recovery and drilling fluid applications, such as polyacrylamide, xanthan gum, and guar gum, may have a substantial impact on

polydispersity index (PDI) 1.16 1.3 1.06 1.23 1.16 1.09

source Oil and Natural Gas Corporation Sigma-Aldrich Oil and Natural Gas Corporation Halliburton The Energy and Resources Institute Sisco Research Laboratories, Mumbai

gas hydrate phase stability. Polyacrylamide is widely used in drilling fluid design and enhanced oil recovery applications and has good compatibility with other chemicals along with its cost-effectiveness.34,35 Xanthan gum and guar gum are the polysaccharides with high molecular weight and are known to impart remarkable properties to the drilling fluids. They are used as additives to control rheology in aqueous systems and also act as emulsion stabilizers. These molecules interact with other polymer molecules to make a complex network by conforming to a multiple helix (single, double, and triple).35 Their effect on methane hydrate phase equilibrium has not been understood till date and requires further investigation. The objective of this work is to investigate the effect of water-soluble polymers commonly used in the oil industry on methane hydrate phase equilibrium and to supplement additional experimental data to the literature. Water-soluble polymers, such as polyacrylamide (PAM), xanthan gum (XG), and guar gum (GG) with two different molecular weights and varying concentrations, have been considered for the investigations. These are as follows: PAM (Mw: 1.1 × 106 g/mol, PAM-1 and 1.5 × 105 g/mol, PAM-2), XG (Mw: 6.4 × 105 g/mol, XG-1 and 2.4 ×105 g/mol, XG-2), and GG (Mw: 1.7 × 106 g/mol, GG-1 and 6 × 105 g/mol, GG-2), with varying concentrations of 100, 200, and 500 ppm each. These are referred to as high-molecular-weight polymers (PAM-1, XG-1, and GG-1) and relatively low-molecular-weight polymers (PAM-2, XG-2, and GG-2). The experiments have been conducted in the pressure and temperature range of 8.63−5.50 MPa and 284.6−279.8 K, respectively.

2. MATERIALS AND METHODS 2.1. Material. The details of the polymers and terminology used are given in Table 1. A high-precision analytical weighing machine (Radwag AS-220/X) is used for measuring the weight of the polymers. The uncertainty of the measurement is ±0.00004 mass fraction (mf). All of the aqueous solutions are prepared using deionized distilled water obtained from Labostar (SIEMENS, Germany). 2.2. Polymer Analysis. The polymers used in the study were obtained from various sources; therefore, it is necessary to characterize them before use. The objective of this study is to confirm the functional group and the molecular weight of the polymers. 2.2.1. Gel Permeation Chromatography. The molecular weights of different polymers were determined using gel permeation chromatography (GPC) (model Prominence, Shimadzu, Japan), with a polysep-GFC-P-4000 column and refractive index detector (RID-10A). NaNO3 (0.1 M) was used as a mobile phase at a flow rate of 1 mL/min. A standard curve was prepared using pullulan standards of known molecular weight, and the molecular weight of the sample was B



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Figure 1. FTIR spectra of the polymers used in the study: (a) polyacrylamide (PAM-1, PAM-2), (b) xanthan gum (XG-1, XG-2), and (c) guar gum (GG-1, GG-2).

cm−1 indicate the presence of C-H stretching. One peak at 1405 cm−1 is attributed to the O-H bending group, and the other at 695 cm−1 is attributed to a benzene derivative. Other distinguishing absorption bands of xanthan gum appeared at 1017 cm−1 due to the C-O stretching. The region of FTIR spectra of guar gum (GG-1) at 3300 cm−1 shows a strong O-H stretching and at 2919 cm−1 indicates the presence of strong C-H stretching (see Figure 1c). The peak observed in the spectra at 1636 cm−1 indicates the medium CC stretching and at 1012 cm−1 represents strong C-O stretching modes, while at 868 cm−1 represents the strong C-H bending polymer backbone. The FTIR spectra obtained for PAM-2, XG-2, and GG-2 show almost similar and slightly smaller peaks to those of the corresponding PAM-1, XG-1, and GG-1. This ensures that the studied polymers that are taken for the study are of similar classes. 2.3. Experimental Setup. The schematic of the experimental setup used in the study is shown in Figure 2. The setup entails a high-pressure stirred reactor made of stainless steel (316) with a volume of 250 mL and an operating pressure of 10 MPa. A platinum resistance temperature detector (Pt-100) and a pressure transducer (model Wika A-

determined. The results of the molecular weight of the polymer obtained from GPC used in this work are reported in Table 1. For clarity, for e.g., in PAM-1 and PAM-2, “1” represents the higher-molecular-weight and “2” represents the relatively lower-molecular-weight polymer. 2.2.2. Fourier-Transform Infrared Spectroscopy (FTIR) Analysis. The FTIR analysis gives the functional group distribution of a polymer, which is also very useful in interpreting the results from the study. Characterization was done by an FTIR spectrometer (Cary 660, Agilent Technologies, at NCCRD IIT Madras). The FTIR spectra of different polymers are shown in Figure 1 along with the snapshot of the polymer samples. The typical FTIR spectrum of polyacrylamide (PAM-1) reveals the characteristic absorption frequencies at 3338 and 2930 cm−1 corresponding to a strong and medium N-H group stretching (see Figure 1a). Other spectra at 1653 cm−1 indicate a strong to medium N-H group stretching. Additional absorption bands of PAM-1 were observed at 1450 cm−1 for the C-H group stretching and at 1118 corresponding to the CO group bending. Xanthan gum (XG-1) broad spectra at 3258 cm−1 indicate hydrogen-bonded O-H groups (see Figure 1b). Peaks at 2915 C



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Figure 2. Experimental setup used in the study. Figure 3. Sample isochor obtained during methane hydrate formation in the presence of 200 ppm PAM-2 (this work): (a−c) gas cooling; (c, d) hydrate formation; (d, e) fast heating hydrate dissociation; (e, f) slow heating hydrate dissociation; (b, f) equilibrium point; (f, g) heating of L−V to initial conditions.

10) are used, which work with an uncertainty of ±0.05 K and ±0.005 MPa, respectively. A magnetic top drive stirrer is employed to expedite the gas and liquid interaction at the interface. The speed of the stirrer is kept at 1000 rpm (revolutions per minute) for all of the experiments. A water bath (model A25, Thermo Haake) circulates the water and ethylene glycol mixture in the water jacket of the reactor so as to maintain the desired experimental temperatures. A syringe pump (D-series, Teledyne ISCO, model 500D) was used to pressurize and supply the methane gas from the cylinder to the reactor. A data acquisition system is used to record the pressure and temperature at the interval of 30 s.

formation is indicated by a temperature spike due to the exothermic formation process and a rapid pressure decrease at point c. The hydrate formation is continued up to the point d (as shown in Figure 3) to confirm sufficient hydrate formation in the reactor. Afterward, the temperature of the reactor system is slowly increased at a rate of 1.5 K/h (points d−e), and later, a temperature ramping is used to heat the system at a rate of 0.2 K/h (points e−f) to achieve a precise equilibrium point. Once the equilibrium is reached, the system is heated back to its initial temperature up to point g. The equilibrium point is found from the intersection of the two-phase (L−V) gas cooling line and the three-phase (H−L−V) dissociation line, as shown in Figure 3. The method is followed for every experimental run, and the equilibrium pressure−temperature curve for methane hydrate systems in various polymer aqueous solutions has been plotted. The average temperature depression is given by36

3. EXPERIMENTAL PROCEDURE A high-pressure reactor employed in the study is cleaned and dried properly before use. The required quantity of the polymer sample is measured using analytical weighing balance Radwag AS-220/X, and then, 160 mL of aqueous solution is prepared and mixed properly using a magnetic stirrer with 1000 rpm in a closed beaker for at least 1−2 days so as to ensure proper mixing and that no lumps are formed. Different concentrations of PAM, XG, and GG aqueous solutions (PAM-1, PAM-2, XG-1, XG-2, GG-1, and GG-2), as tabulated in Table 1, are filled into the reactor during individual experiments. The system is sealed air-tight once the solution is poured into the reactor. Subsequently, the reactor is purged with methane to remove the air existing inside the reactor system at about 0.1−0.3 MPa. Then, the vessel is pressurized using methane gas up to a preferred initial pressure (say 8 MPa). The solution within the reactor is then stirred at 1000 rpm. The temperature of the reactor is brought down with the help of a water bath to form methane hydrate. The threephase, hydrate, water, and gas (H−L−V), equilibrium conditions are acquired using an isochoric pressure-search method. Several isochors are produced to acquire the phase equilibrium data for methane hydrate in aqueous polymer systems at different initial temperature−pressure conditions. A sample isochor is presented in Figure 3 for the methane hydrate system in 200 ppm PAM-2 aqueous solution. Figure 3 shows that the reactor temperature decreases along with pressure (from point a to point b) because of the gas cooling of the system. If no metastability is involved within the system,1 hydrate should form at point b, but due to molecular disorder in the system, the molecules take some more time to become ordered and then nucleate to form a hydrate crystal. Hydrate

n

ΔTd =

∑i = 1 ΔT n

where n is the number of equilibrium data and ΔT is the difference between the measured hydrate dissociation temperature in pure water and that in the presence of polymers at the same value of pressure.

4. RESULTS AND DISCUSSION In this study, experimental phase (H−L−V) equilibrium conditions on the temperature and pressure of methane hydrate in the presence of different polymer aqueous systems have been reported. The polymers used in the study are the oilfield polymers, such as PAM, XG, and GG, each with two different molecular weights and three different concentrations of 100, 200, and 500 ppm each (see Table 1). The experiments have been conducted in the pressure and temperature range of 8.63−5.50 MPa and 284.6−279.8 K, respectively. The results discussed here present the effect of molecular weights and effect of concentrations of these polymers belonging to the same and different species on the phase equilibrium of the methane hydrate system. We have also proposed a hypothesis D



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to explain the mechanism of thermodynamic hydrate inhibition based on the molecular mobility of the polymer and compared with other electrolyte/nonelectrolyte additives (alcohol and glycol) in the aqueous solution. The hydrate inhibition capabilities of electrolytes and polymers have also been compared and discussed. 4.1. Phase Equilibrium of Methane Hydrate in Aqueous Solutions of Polymers. Tables 2−4 report the

Table 3. Experimental Phase Equilibrium Data on Methane Hydrates in the Presence of Aqueous Solutions of Xanthan Guma polymer

concentration (ppm)

T (K)

P (MPa)

XG-1

100

283.8 283.0 281.8 283.4 283.0 281.5 279.8 283.4 282.4 281.3 283.8 282.9 281.6 283.3 282.8 282.1 280.9 283.0 282.2 281.3 280.8

8.63 7.83 6.78 7.89 7.54 6.39 5.50 7.81 7.10 6.34 8.62 7.81 6.76 7.93 7.45 6.83 6.10 7.81 7.17 6.57 6.18

200

Table 2. Experimental Phase Equilibrium Data on Methane Hydrates in the Presence of Aqueous Solutions of Polyacrylamidea polymer

concentration (ppm)

T (K)

P (MPa)

PAM-1

50

283.0 282.4 281.6 280.5 283.5 282.6 280.1 280.9 282.8 282.0 280.8 284.6 284.0 282.2 281.4 283.4 282.5 281.8 280.4 282.9 281.9 281.3 280.8 283.4 282.7 281.2 280.2

8.07 7.47 6.76 6.16 8.31 7.57 5.75 6.40 7.77 7.05 6.18 8.58 8.10 6.80 6.20 7.94 7.39 6.76 5.90 7.85 7.15 6.62 6.30 8.06 7.54 6.54 5.88

100

200

500

PAM-2

100

200

500

500

XG-2

100

200

500

a

Expanded uncertainties (including combined instruments and experimental measurements): U(P) = ±0.05 MPa, U(T) = ±0.1 K, U(mass fraction) = ±0.00004 (0.95 level of confidence).

Table 4. Experimental Phase Equilibrium Data on Methane Hydrates in the Presence of Aqueous Solutions of Guar Guma polymer

concentration (ppm)

T (K)

P (MPa)

GG-1

100

283.4 282.8 281.4 283.2 281.5 280.9 280.3 283.3 282.5 281.8 280.4 283.3 282.2 281.6 280.0 283.4 281.9 281.4 280.7 282.9 281.8 281.0 280.2

7.93 7.48 6.70 8.05 6.90 6.46 6.01 7.56 7.00 6.49 5.67 8.07 7.10 6.60 5.83 8.13 7.08 6.60 6.15 7.99 7.00 6.34 5.88

200

a


500

experimental methane hydrate phase equilibrium data in aqueous solutions of various concentrations (ppm) of PAM1, PAM-2, XG-1, XG-2, GG-1, and GG-2 as generated in this study. Table 5 reports average temperature depression, ΔTd (inhibition of hydrate), in the presence of these polymers. To validate the experimental procedure and results, first, the phase equilibrium of the methane hydrate system in pure water has been investigated and then was compared with selected experimental data from the published literature.37 The results on the phase equilibrium conditions for methane hydrate in pure water obtained in this study match well with the data in the literature, ensuring the reliability of the experimental setup and method used in this study (see Figure 4 for the pure methane hydrate case). The same setup has been used earlier for various such studies and found to be reliable.30,38−40 Figure 4a,b shows the phase equilibrium condition of methane hydrate in the presence of PAM-1 and PAM-2 aqueous solutions with varying concentrations. PAM-1 has

GG-2

100

200

500

a


displayed lowest hydrate inhibition at 500 ppm with an average temperature depression (ΔTd) of 0.25 K (see Table 5). Figure E



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Table 5. Average Temperature Depression in the Presence of the Polymers polymer type

concentration (ppm)

ΔTd (K)

PAM-1

50 100 200 500 100 200 500 100 200 500 100 200 500 100 200 500 100 200 500

1.05 0.98 0.94 0.25 0.76 1.06 0.91 0.87 0.66 0.51 0.88 0.76 0.72 0.77 0.90 0.37 0.87 0.93 1.04

PAM-2

XG-1

XG-2

GG-1

GG-2

5a,b shows the H−L−V equilibrium curve of methane hydrate in the presence of xanthan gum. Both XG-1 and XG-2 belonging to the same species of xanthan gum have inhibited hydrate formation and shifted the phase boundary toward low temperature and higher pressure. Figure 6 shows the equilibrium condition of methane hydrate in the presence of GG-1 (Figure 6a) and GG-2 (Figure 6b). Similar to PAM and XG, GG has inhibited methane hydrate by shifting the phase boundary toward low temperature and higher pressure. 4.2. Effect of Different Concentrations of Polymer on Hydrate Inhibition. From Figures 4−6, it has been observed that in all of the cases with varying concentrations of all polymers from 500 to 100 ppm, the phase equilibrium curve has been shifted toward higher-pressure and lower-temperature conditions. In most cases (PAM-1, XG-2, XG-1, and GG-1; see Table 5), as the concentration is reduced, the inhibition of hydrate has been increased. Also, from Table 5, it has been observed that the effect of reducing the concentration beyond a certain value of concentration where the hydrate inhibition is higher, ΔTd is found to decrease in some cases (e.g., PAM-2, GG-1, and GG-2) primarily due to further dilution of polymer in the aqueous solution. For example, it can be observed from Figure 5a and Table 5, say for the case of GG-1, the ΔTd at 500 ppm is 0.37 K, which increases to 0.90 K at 200 ppm and then again decreases to 0.77 K at a low concentration of 100 ppm. Similarly for PAM-2, the ΔTd at 500 ppm is 0.91 K, which increases to 1.06 K at 200 ppm and then decreases to 0.76 K. Also, in the case of GG-2, the hydrate inhibition is higher at 500 ppm with ΔTd of 1.04, which decreases to 0.93 K at 200 ppm and then further decreases to 0.87 K at 100 ppm. Also, from Table 5, for other polymeric systems (such as PAM1, XG-1, and XG-2), it can be seen that the increment in ΔTd is not much significant with the reduction in concentration further below 200 ppm. From Table 5, in the case of PAM-2, it can be seen that if we increase the concentration of polymer, say from 100 ppm, the inhibition effect (ΔTd) increases up to a certain concentration where a maximum hydrate inhibition is obtained (in PAM-2, the maximum inhibition is near 200 ppm). After 200 ppm, we

Figure 4. Phase equilibrium of methane (CH4) hydrate in the presence of polyacrylamide (PAM) aqueous solution. Experimental points: pink open star, pure CH4 hydrate (this work); black solid square, pure CH4 hydrate in ref 37; (a) red solid circle, PAM-1, 500 ppm; green, solid, up triangle, PAM-1, 200 ppm; blue, solid, down triangle, PAM-1, 100 ppm; sky blue solid diamond, PAM-1, 50 ppm; (b) red solid circle, PAM-2, 500 ppm; green, solid, up triangle, PAM2, 200 ppm; blue, solid, down triangle, PAM-2, 100 ppm.

observe that with an increase in polymer concentration the inhibiting effect decreases. A similar observation can be made for GG-1. The particular concentration at which the maximum hydrate inhibition is observed is different for different polymers. For PAM-1, the concentration at which maximum inhibition is expected may be near or below 50 ppm. In this case, with an increase in concentration from 50 to 500 ppm, the inhibition effect is found to reduce. If the lower concentration at which the maximum inhibition is obtained is denoted Cinhibition, then at this relatively low Cinhibition, it is expected that more free water is available for hydrate formation. As there is more free water, more pure methane hydrate will form and thus the inhibition effect will decrease. If the concentration of polymer is increased beyond Cinhibition, the hydrate inhibition effect will also get reduced because, in this case, the concentration of polymer will dominate with lesser chain mobility in the solution. In this F



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Figure 6. Phase equilibrium of methane (CH4) hydrate in the presence of guar gum (GG) aqueous solution. Experimental points: black solid square, pure CH4 hydrate;37 (a) red solid circle, GG-1, 500 ppm; green, solid, up triangle, GG-1, 200 ppm; blue, solid, down triangle, GG-1, 100 ppm; (b) red solid circle, GG-2, 500 ppm; green, solid, up triangle, GG-2, 200 ppm; blue, solid, down triangle, GG-2, 100 ppm.

Figure 5. Phase equilibrium of methane (CH4) hydrate in the presence of xanthan gum (XG) aqueous solution. Experimental points: black solid square, pure CH4 hydrate;37 (a) red solid circle, XG-1, 500 ppm; green, solid, up triangle, XG-1, 200 ppm; blue, solid, down triangle, XG-1, 100 ppm (b) red solid circle, XG-2, 500 ppm; green, solid, up triangle, XG-2, 200 ppm; blue, solid, down triangle, XG-2, 100 ppm.

GG-2, 500 ppm, is 1.04 K. This behavior on the effect of molecular weight of polymers is found to be congruent with the published literature where poly(ethylene glycol) (PEG) 200 g/mol inhibits hydrate formation more than PEG 400 g/ mol.33 Bavoh and co-worker have also found a similar effect of molecular weight of amino acids on methane hydrate inhibition where the average temperature depression of hydrate in the presence of valine amino acids (relatively lowmolecular-weight polymer, 117.151 g/mol) was more than that with threonine amino acids (relatively high-molecular-weight polymer, 119.119 g/mol).36 Thus, the low-molecular-weight polymer has found to show higher methane hydrate inhibition than its high-molecular-weight species, i.e., the hydrate inhibition is more pronounced when the molecular weight of the polymer is lower. A similar comparison has been made for different polymers in Figure 8 at 200 ppm concentration. The reason for the above observation could be that the polymer with relatively lower molecular weight has small chain length as compared to that of the high-molecular-weight polymer and thus imparts lesser viscosity to the solution. As a

study, the selected concentrations may lie below or above Cinhibition. Each polymer may have its unique concentration (Cinhibition) at which the maximum inhibition may be obtained. Consequently, the optimum concentrations of the polymers do play role in their effect on methane hydrate inhibition. The detailed discussion has been made in the subsequent section. 4.3. Effect of Different Molecular Weights of Polymer on Hydrate Inhibition. Figure 7 illustrates the effect of highmolecular-weight polymers (PAM-1, XG-1, and GG-1) and relatively lower-molecular-weight polymers (PAM-2, XG-2, and GG-2) on methane hydrate inhibition. The polymers with relatively lower molecular weights inhibited the hydrate formation slightly more than higher-molecular-weight polymers. For instance, Table 5 shows that the ΔTd value in the presence of PAM-1, 500 ppm, is 0.25 K and in the presence of PAM-2, 500 ppm, is 0.91 K. Similarly, the ΔTd value in the presence of XG-1, 500 ppm, is 0.51 K and in the presence of XG-2, 500 ppm, is 0.72 K. In addition, the ΔTd value in the presence of GG-1, 500 ppm, is 0.37 K and in the presence of G



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Figure 7. Thermodynamic methane (CH4) hydrate phase equilibrium in the presence of different molecular weight polymers. Experimental points: solid black square, pure CH4 hydrate;37 (a) green solid circle, PAM-1, 500 ppm; green open circle, PAM-2, 500 ppm; (b) blue solid circle, XG-1, 500 ppm; blue open circle, XG-2, 500 ppm; (c) pink solid circle, GG-1, 500 ppm; pink open circle, GG-2, 500 ppm.

compared with a high-molecular-weight polymer. In this work, we have observed that a polymer with a relatively lower molecular weight (e.g., PAM-2) inhibited hydrate more than the relatively higher-molecular-weight polymer (PAM-1). Mech and Sangwai also reported a similar observation where a lower-molecular-weight polymer PEG 200 has shown more hydrate inhibition as compared with a relatively highermolecular-weight polymer PEG 400.33 If the molecular weight of polymers such as PAM, XG, and GG would have been much lower than that used in the study, we would expect higher methane hydrate inhibition due to increased molecular mobility of the polymer chains because of their shorter chain length. In addition, if the polymeric chains of XG and GG are small, the hydroxyl groups (−OH) in XG and GG have more chance to interact with the water structure to further disturb the structure. However, producing the polymer with very low molecular weight is a costly affair. Indeed, along with this, the dilution of aqueous solution will also affect the hydrate inhibition tendency of the aqueous solution. Figure 9 shows a comparison between different polymers at different concentrations. It can be observed that the polymers with a low molecular weight (among all polymer species) have a higher tendency to inhibit methane hydrate as compared to

result, the polymer with a lower molecular weight may have higher mobility in the solution. Therefore, the existing structure of water may get disturbed from the proper cage network to form a gas hydrate, resulting in hydrate inhibition. The disturbance in water structure by the polymer with a lower molecular weight will be manifested in lowering of the water activity. There are a few studies reported in the literature that help to demonstrate the above mechanism. Maali and Sadeghi have reported the activity of aqueous solutions of poly(ethylene glycol) (PEG) of two different molecular weights (PEG 400 and PEG 6000; 400 and 6000 g/mol, respectively) at different weight fractions. From their result, it has been found that PEG 400 is effective in reducing the activity of water as compared with the higher-molecular-weight polymer PEG 6000.41 È liassi and co-workers also provided information that relates the activity of water with different molecular weights of the polymers.42 The molecular weight of a polymer is directly related to the viscosity in an aqueous solution by the Mark−Houwink equation.43 Therefore, the viscosity of the solution in the presence of polymers is related to the activity of water. A reduction in activity of water is a major cause of hydrate inhibition.29,44 Therefore, generally, the polymer with a lower molecular weight inhibits the hydrate more as H



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Figure 8. Thermodynamic methane (CH4) hydrate phase equilibrium in the presence of different molecular weight polymers. Experimental points: solid black square, pure CH4 hydrate;37 (a) green solid circle, PAM-1, 200 ppm; green open circle, PAM-2, 200 ppm; (b) blue solid circle, XG-1, 200 ppm; blue open circle, XG-2, 200 ppm; (c) pink solid circle, GG-1, 200 ppm; pink open circle open, GG-2, 200 ppm.

which are responsible for slightly higher hydrate inhibition as compared with PAM (with no hydroxyl group, as confirmed from FTIR analysis; see Figure 1a). 4.4. Comparison of Hydrate Phase Stability in Polymeric and Electrolyte Aqueous Systems. A theoretical comparison is made between the phase equilibrium of methane hydrate in the presence of the polymers and that of electrolyte. It has been observed that the hydrate inhibition in the presence of polymers (as studied in this work) is however not much but quite effective even at very low concentrations of 100 and 200 ppm. The ΔTd in the case of the electrolyte is found to be around 1 K at a concentration of 3 wt % for NaCl,45−48 while the ΔTd in the case of the polymers investigated in this work is around 0.25−1.05 K at 500/200/ 100 ppm. It should also be noted here that with an increase in electrolyte (such as salts) concentration the hydrate inhibition also found to increase, but the same has not been observed in the case of high-molecular-weight polymers used in this work. In most cases (PAM-1, XG-2, XG-1, and GG-1), as the concentration of polymers is reduced, the inhibition of hydrate has been increased. However, for other lower-molecular-weight polymers, such as poly(ethylene glycol) (200 and 400 g/mol),

Figure 9. Average temperature depression of methane hydrate in the presence of water-soluble polymers at different concentrations.

relatively high-molecular-weight polymers. PAM-1 due to its higher molecular weight has shown lesser hydrate inhibition. In addition, it can be observed that xanthan gum and guar gum have shown somewhat higher hydrate inhibition as compared with PAM-1 at 500 ppm. This can be attributed to many hydroxyl groups present in XG and GG (see Figure 1b,c), I



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is playing a major role in hydrate inhibition in the presence of a nonelectrolyte, a more detailed investigation (experimental and modeling) on relating the viscosity with the activity of the water in various aqueous solutions (especially nonelectrolytes) is required to validate this hypothesis. This is the first study carried out to know the effect of the different molecular weights and concentrations of oilfield polymers on methane hydrate phase stability, and additional data has been generated to supplement the existing literature. Also, very recently, the studies on hydrate recoveries56−58 are pitching up very fast, in which polymer and inhibitor injection can play a vital role and therefore the studies on oilfields polymers are highly recommended and useful.

hydrate inhibition increases with an increase in concentration.33,36 It is well-known that the hydrate inhibition in the presence of an electrolyte is due to its colligative property. However, such a property has not been observed in the case of higher-molecular-weight polymers (PAM, XG, and GG) studied in this work. In high-molecular-weight polymers, hydrate inhibition is limited by the combined effect of a higher molecular weight, higher concentration, and dilution (as discussed in the previous section). The possible reason may be that the electrolytes and other thermodynamic hydrate inhibitors with very low molecular weights (such as glycols and alcohol) are found to reduce the thermodynamic activity of the liquid water phase with an increase in concentration, whereas the polymer with a high molecular weight (as studied in this work) increases the viscosity of the solution substantially.23 The viscosity of the aqueous solution has a dominant role in affecting hydrate equilibrium (as described earlier). The explanation of hydrate inhibition in terms of viscosity and mobility of the polymer chain in an aqueous solution for thermodynamic hydrate inhibition also seems to be a reliable theory to explain hydrate inhibition in the presence of methanol and probably other nonelectrolyte inhibitors. As discussed, if the solute present in the solution has higher molecular mobility in the solution, it will inhibit the hydrate formation by interacting with the water structure and distorting it. In addition, the structure of water is further deformed if the functional group present in the aqueous solution is a potential hydrogen bond former. Methanol has been recognized as an excellent thermodynamic hydrate inhibitor. The viscosity of the mixture of water and methanol is lower than the pure water (viscosity of methanol is 0.594 mPa.s and that of pure water is 1 mPa.s at 393.15 K).49 The viscosity of the mixture of water and methanol decreases with an increase in the concentration of methanol. The reduction in the viscosity enhances the mobility of methanol monomer in water, which results in a possible disturbance of the water structure. In addition, methanol is equipped with a -OH group, which will strongly interact with water by forming hydrogen bonds.50−52 Also, as the functional group is attached to the smaller side chain of methanol, there is a very high possibility for methanol to form hydrogen bonding with water. Therefore, the mobility of the monomer coupled with the hydroxyl group in the solution will highly disrupt the structure of water so as to inhibit the hydrate formation. If the concentration of methanol is increased further in the aqueous solution, the viscosity of the solution (water + methanol) will get reduced, which further results in an increased mobility of the monomer (methanol) in the solution, which will further disrupt the water structure responsible for hydrate formation. This effect is also manifested in terms of decrease in activity of aqueous solution. For this, further studies to relate molecular mobility with the activity of the solution are needed. This above discussion is well suitable for nonelectrolytes, and the same cannot be used for understanding hydrate inhibition in the presence of electrolytes. The addition of electrolyte will change the viscosity of water very slightly with an increase in concentration; however, it interacts with the existing water structure due to ionization, which results in a decrease of the activity of water and hence there is an increase in hydrate inhibition with an increase in the concentration of the electrolyte. There are numerous models available that can very effectively predict the activity of water in the presence of electrolyte systems.32,39,53−55 Since the viscosity of the solution

5. CONCLUSIONS The effect of water-soluble polymers commonly used in the oil industry, such as polyacrylamide (PAM), xanthan gum (XG), and guar gum (GG), is investigated in this work. The main focus of this paper is to study the effect of molecular weight and concentration of aqueous solutions of various watersoluble polymers, such as polyacrylamide (PAM), xanthan gum (XG), and guar gum (GG), on phase equilibrium of the natural gas hydrate. Two different molecular weights of polymers, i.e., PAM-1, PAM-2, XG-1, XG-2, GG-1, and GG-2, where “1” represents a high molecular weight and “2” represents a relatively low molecular weight at different concentrations of 100, 200, and 500 ppm, have been used to study their effect on phase equilibrium of methane hydrate. The experiments were performed in the pressure and temperature range of 8.63−5.50 MPa and 284.6−279.8 K, respectively. Water-soluble oilfield polymers have shown thermodynamic hydrate inhibition with average temperature depression ranging from 0.25 to 1.05 K. The molecular weight and the concentration of polymers have been shown to affect the hydrate inhibition tendency. The polymers with relatively lower molecular weights have shown a higher shift in the hydrate phase boundary toward lower temperature and higher pressure as compared with highermolecular-weight polymers. We have proposed a hypothesis for hydrate inhibition based on the mobility of the polymer chain in the solution with the desired functional group in relation to nonelectrolyte thermodynamic hydrate inhibitors. Further detailed investigation (both experimental and modeling) is required for relating the viscosity with the activity of various nonelectrolyte aqueous solutions. The presented study on methane hydrate phase stability in the presence of various oilfield polymers is imperative for their use in the design and development of hydrate inhibitive drilling fluids for offshore wells, hydrate-bearing formations, and studies related to the recovery of methane from hydrate-bearing deposits using polymer injection.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +91-44-2257 4825, Fax: +91-44-2257 4802. ORCID

Jitendra S. Sangwai: 0000-0001-8931-0483 Notes

The authors declare no competing financial interest. J



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Article

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Phase Equilibrium of Methane Hydrate in Aqueous Solutions of

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