Effect of NaCl on methane hydrate formation and dissociation in porous media

Journal of Natural Gas Science and Engineering xxx (2015) 1e12

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Effect of NaCl on methane hydrate formation and dissociation in porous media Zheng Rong Chong a, Adeline Hui Min Chan a, Ponnivalavan Babu a, Mingjun Yang a, b, Praveen Linga a, * a b

Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585, Singapore Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, Dalian University of Technology, Dalian 116024, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 April 2015 Received in revised form 23 August 2015 Accepted 24 August 2015 Available online xxx

In this study, we investigate the effect of NaCl (1.5 wt. % and 3.0 wt. % concentration) on methane hydrate formation and dissociation kinetics in the presence of porous media. It was found that hydrate formation was significantly slower in the presence of NaCl as compared to experiments conducted without the presence of salts despite subjected to a higher kinetic driving force, affirming the significant kinetic inhibition effect of NaCl on methane hydrate formation in porous media. We also observed that methane hydrate formation in water at overpressures of 1.5 MPa and 4.2 MPa resulted in the completion of hydrate formation kinetics within 100 h, which was significantly faster than that of salt containing systems that took more than 150 h to reach a plateau. To study the difference in methane hydrate dissociation, water at a temperature of 297.2 K was used to thermally stimulate the hydrate beds externally. The rate of gas produced was not significantly different between hydrates formed from water and NaCl solution, with NaCl system showing slightly faster dissociation kinetics. © 2015 Elsevier B.V. All rights reserved.

Keywords: Gas hydrates Methane hydrates Energy recovery Thermal stimulation Marine sediments

1. Introduction Gas hydrates are crystalline, ice-like solid composed of gas molecules of small molecular diameter (also known as ‘guest’ molecule) entrapped within cages formed by water molecules (the ‘host’ molecules) under suitable conditions of high pressure and low temperature (Sloan and Koh, 2008; Davidson and Franks, 1973). Three commonly cited structures of gas hydrates are cubic structure I (sI), structure II (sII) and hexagonal structure (sH), whereby water molecules arrange in different combinations and types of small and large cages to occlude the gas molecules (Sloan and Koh, 2008; Davidson and Franks, 1973; Englezos, 1993; Veluswamy et al., 2014; Jeffrey and McMullan, 2007). The structure of gas hydrate for each guest molecule is dependent on the temperature and pressure condition during hydrate formation as well as the size and chemical nature of the guest molecules (Sloan and Koh, 2008). Over the years, methane hydrate, which typically forms sI hydrate, has been identified as an abundant resource of natural gas. The estimate of hydrate resource varies from different research techniques

* Corresponding author. E-mail address: [email protected] (P. Linga).

and models (Sloan and Koh, 2008; Kvenvolden, 1988; Gornitz and Fung, 1994; Macdonald, 1990; Milkov, 2004; Klauda and Sandler, 2005; Johnson, 2011; Archer et al., 2009), but recent literature have reported that natural gas recoverable from hydrate resources is in the order of 103 Trillion cubic meters (TCM) (Milkov, 2004; Johnson, 2011; Archer et al., 2009; Boswell and Collett, 2011; Chong et al., 2015). The abundance of natural gas hydrate has attracted the interest from researchers around the world, and increasing exploration tests around the world have proven the existence of methane hydrate as well as the possibility in producing methane from these resources. To produce natural gas from hydrate resources, three main techniques have been proposed: thermal stimulation, depressurization and inhibitor injection. More recently, a gas-exchange approach which involve the recovery of methane from hydrate reservoir with the exchange of carbon dioxide has been proposed (Ohgaki and Inoue, 1994; Ohgaki et al., 1996; Ebinuma, 1993; Lee et al., 2013, 2015; Shin et al., 2008; Park et al., 2006). The technical feasibility of this approach has been demonstrated in Ignik Sikumi field trial conducted in Alaska, with 40% of injected CO2 recovered during the flow-back period as compared to 70% for N2, implying that a fraction of injected CO2 were sequestered (Schoderbek et al., 2013). Due to the high cost involved in field trials, the difficult locations

http://dx.doi.org/10.1016/j.jngse.2015.08.055 1875-5100/© 2015 Elsevier B.V. All rights reserved.

Please cite this article in press as: Chong, Z.R., et al., Effect of NaCl on methane hydrate formation and dissociation in porous media, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/j.jngse.2015.08.055

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of their presence (remote permafrost and oceanic location) and the environmental concerns, only about 4 field trials have been conducted so far for the production of methane from natural gas hydrates. Hence, there is a need to form synthetic representative hydrate samples in laboratory scale to better elucidate the hydrate formation/dissociation behaviour (Linga et al., 2009; Kneafsey et al., 2007). Synthetic methane hydrates have been prepared in different laboratories using different methods, including excess gas (Linga et al., 2009; Li et al., 2011; Waite et al., 2004; Priest et al., 2005), excess water (Falser et al., 2013; Loh et al., 2015), engrained ice (Deusner et al., 2012; Lee et al., 2014), surfactant addition (Pang et al., 2009; Zhou et al., 2008), and a reduced water saturation to increase the interfacial area between liquid and gaseous phase (Kono et al., 2002; Oyama et al., 2009; Ors and Sinayuc, 2014). The wide variability in methane hydrate formation procedure has inevitably caused a difference in the hydrate sample formed, posing a challenge in comparing the extent of recovery from different methods. As a huge proportion of hydrate exists in oceanic locations where salt is present, the effect of salt on hydrate formation and dissociation has to be studied to establish the knowledge on the naturally occurring hydrates and to facilitate the prediction of their occurrence, distribution and production response. The earlier research focused on the thermodynamic inhibition of salt on hydrate stability conditions (Dickens and Quinby-Hunt, 1994; Englezos and Bishnoi, 1988; De Roo et al., 1983; Dholabhai et al., 1991). The effect of salt on the kinetics of hydrate formation and dissociation has been looked into in more recent studies as field exploration involves transient response. In a pioneering work on the kinetics of methane hydrate formation in electrolyte system, it was shown that the rate of hydrate formation was reduced in the presence of NaCl and KCl as compared to water at the same experimental temperature and pressure (Dholabhai et al., 1993a). In another study, the effect of NaCl at 0.5 wt. % and 1.0 wt. % on the kinetics of CH4/CO2 exchange in stirred tank reactor (STR) was studied (Li et al., 2009). From the analysis of CH4 produced in gas phase and CO2 trapped in hydrate phase, the study reported a slightly lower gas exchange for the higher NaCl concentration. However, as naturally occurring hydrates are dispersed in porous media, the production response from STR may not be representative. In another study investigating the effect of salts in seawater on the formation and dissociation kinetics of CH4 hydrate dispersed in Toyoura sand, significant kinetic inhibition on CH4 hydrate formation was observed in the presence of salt (Mekala et al., 2014). Nonetheless as the dissociation stage in the study started off with vastly different hydrate saturations, the effect of salt on hydrate dissociation was not well elucidated in that study. The production of methane from hydrates formed with water (no salts) and seawater at similar hydrate saturation using a combination of thermal stimulation and depressurization had also been studied employing a dual wellbore scheme (Loh et al., 2015). The study reported a slightly lower recovery factor for the hydrate formed with seawater as compared to hydrates formed in water (without salts) under the production condition of 6 MPa and 288.2 K. Although the cumulative gas production was looked into, the study did not provide insight as to how salt affect the kinetics of hydrate dissociation. In this study, we aim to look into the effect of a typical salt, NaCl on the kinetics of hydrate formation and dissociation in porous media. The kinetics that we observe in this study includes not only the intrinsic kinetics of hydrate formation and dissociation, which have been studied previously in well-stirred systems, but also capturing the transport of gas and water within porous media in the presence of salt. In our previous study which applied seawater from Tekong Island in Singapore (Mekala et al., 2014), a significant

kinetic inhibition in seawater was observed. However, as seawater is typically comprised of a variety of salts, the individual effect of each salt on the kinetics of hydrate formation and dissociation cannot be resolved from the previous study. Therefore in this work, we employed NaCl as it is the major contributor for seawater salinity. 2. Experimental section 2.1. Materials Toyoura sand of size ranging between 100 and 500 mm, 0.217 cm3/g pore volume and 1.56 g/cm3 bulk density was used as the porous media. The same type of sand was used in our previous works (Mekala et al., 2014; Babu et al., 2013a; Yang et al., 2014). As this study looked into the effect of salt on hydrate formation and dissociation, the sand samples were digested and analysed using ICP-OES to verify that they are free of salt content. The analysis revealed a close-to-zero concentration of cations (0.01 ppm Na, 0.004 ppm Mg and 0.12 ppm Ca). Deionized water was used as the water for hydrate formation. Sodium Chloride (NaCl, Merck Millipore, CAS # 7647-14-5) was dissolved in deionized water to make salt water of 1.5 wt. % and 3.0 wt. %. Methane (CH4) gas (99.9% purity) was obtained from Soxal Private Limited, Singapore. 2.2. Apparatus description The schematic diagram presenting the crystallizer (CR) in which methane hydrate was formed in this study is shown in Fig. 1A. The crystallizer is made of stainless steel, with an inner diameter of 10.2 cm and inner height of 26.7 cm, giving a volume of 2182 cm3. A cooling jacket is welded to the crystallizer to control the reactor temperature. For temperature measurement, a multipoint thermocouple (Omega copper-constantan T-type, ±0.1 K) capable of measuring temperatures at different heights within the crystallizer (as can be seen in Fig. 1A) is inserted through a port on the cover of the crystallizer. An additional Omega thermocouple (Tw) is included to monitor the temperature of the coolant. The experimental setup is illustrated in Fig. 1B. Both the crystallizer and a 1000 cm3 gas reservoir are welded with cooling jacket connected with an external refrigerator (ER) to maintain the temperature. Each pressure vessel is connected to a Rosemount smart pressure transmitter (±20 kPa) and a Wika analog gauge. The crystallizer and gas reservoir are connected with a control valve coupled with a PID controller (OMEGA® CN2120 Ramp/Soak controller) to maintain a constant pressure during the decomposition process. All temperature and pressure readings are acquired in a personal computer (PC) using the data acquisition system and LabView software (National Instruments). 3. Experimental procedure 3.1. Methane hydrate formation Prior to the experiment, preliminary calculation had been carried out to ensure the water to gas ratio within the current setup was such that excess gas was supplied for the predetermined amount of water (Available in Page S2eS3 in Supporting information). Preliminary experiments had also been conducted to select water saturation suitable for this study such that the experimental design would enable us to study the kinetic effect of salt within the duration of experiments. The hydrate formation was done similar to the steps outlined in our previous studies (Mekala et al., 2014; Yang et al., 2014). Silica sand bed of 5 cm height was prepared at 75% water saturation.

Please cite this article in press as: Chong, Z.R., et al., Effect of NaCl on methane hydrate formation and dissociation in porous media, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/j.jngse.2015.08.055

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Fig. 1. Experimental setup for the study of hydrate formation and dissociation in porous media. (A) Crystallizer with thermocouple positions and sand bed height; and (B) Schematic diagram of the overall setup.

Accordingly, the crystallizer was filled with 637.36 g of silica sand and dispersed with 103.73 mL of deionized water. For the other experimental runs investigating the effect of salinity on formation kinetics, NaCl solution of 1.5 wt. % and 3.0 wt. % were used in place of water to saturate the sand bed. The sand bed was then prepared by layering uniformly the sand and water in 5 stages alternatively. Once the preparation of sand bed was completed, the crystallizer was closed together with a multipoint thermocouple inserted into positions shown in Fig. 1A. Subsequently, the crystallizer was connected with the gas supply line, vent line and the gas reservoir vessel, and coolant was introduced into the cooling jacket. The chiller temperature was then adjusted such that the crystallizer temperature reaches 277.2 K, 276.6 K and 275.9 K when water, 1.5 wt. % NaCl and 3.0 wt. % NaCl salt solution were used to ensure that the initial kinetic driving force was constant across all experimental runs for a fair comparison between systems of different salinity. The crystallizer was purged with methane gas to 1 MPa for three times in order to flush out the residual atmospheric air within the crystallizer. Once the temperature within the bed stabilized, the crystallizer was pressurized with methane gas to 8 MPa and 5.3 MPa at a rate of approximately ~50 kPa/s for experiments of different driving forces. After pressurization, the crystallizer was

Fig. 2. Temperature and pressure profile during pressurization and stabilization stage illustrating time zero of the experiment and induction period for the first 1300 s of experiment D3.

allowed to stabilize for a few minutes such that the temperature increase after pressurization reaches 90% from the set point (Fig. 2 & Figs. S1 e S3 in ESI). This stabilization step on an average took about 2.2e3.2 min for all the experimental conducted in this work. The experiment time was started when the temperature and pressure conditions met the stabilization criteria, and the data acquisition system was activated to record the temperature and pressure profiles of the hydrate formation runs at an interval of 20 s. Gas hydrate formation can be identified by sharp increase in temperature (due to exothermic nature of hydrate formation), followed by a sustained period of pressure drop, indicating gas uptake during hydrate formation. The period between time zero and the first exothermic peak, which is a detectable signal for hydrate formation, was defined as the induction period, as can be seen in Fig. 2. Hydrate formation was considered complete when no further gas uptake was observed and pressure drop was negligible (~1 kPa/h).

3.2. Methane hydrate dissociation Dissociation of gas hydrates in this study was carried out by thermal stimulation method. For all the 8 MPa systems, the pressure of the crystallizer was first decreased to 4.8 MPa (24% above the equilibrium pressure at the experimental temperature) through slow venting of the gas to ensure all hydrates were stable in the beginning of dissociation process. After the crystallizer pressure stabilized, the hydrate sample was thermally stimulated while the pressure of the crystallizer was maintained at 4.8 MPa throughout dissociation experiment using a control valve coupled with a PID controller. The gas produced during thermal stimulation was collected in the gas reservoir. The thermal stimulation of hydrate was done by two methods in this study: gradual heating and constant temperature heating method. The gradual heating method was adapted in our previous studies (Mekala et al., 2014; Yang et al., 2014) and one of the water experiment in this study. In this method, water of increasing temperature (from experimental temperature during formation stage to the higher final temperature) was introduced to perturb the hydrate bearing sediment. However in a field exploitation, a constant temperature fluid (e.g. 343.2e353.2 K in Mallik 2002 test) is injected continuously to achieve thermal stimulation instead of a fluid with an increasing temperature profile (Hancock et al.,). To mimic a scenario closer to this reality, V8 and V9 were closed at the

Please cite this article in press as: Chong, Z.R., et al., Effect of NaCl on methane hydrate formation and dissociation in porous media, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/j.jngse.2015.08.055

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beginning of dissociation step, and water was heated to the desired experimental temperature (297.2 K) prior to thermally stimulate the artificial hydrate at a rate of 1.6 K/min. The temperature and pressure profiles of the hydrate dissociation were recorded at an interval of 20 s using the online data acquisition system. Once the water temperature reached 297.2 K, V8 and V9 were opened, introducing the heated water into the cooling jacket of the crystallizer. Pressure within the reactor was kept constant at 4.8 MPa using a PID controller of high sensitivity. Additionally, it is important to note that since this study was conducted using excess gas formation method, the expansion of gas overlying the hydrate bed will also be collected in the gas reservoir, which has to be accounted for in the final gas production calculation (Daraboina et al., 2011; Babu et al., 2013b). This was done by conducting control experiments using the same operating procedure as the dissociation process, but in the absence of methane hydrates in the crystallizer. The actual gas released from hydrate dissociation was then adjusted using the results obtained from the control experiments.

   
 P P DnG;[ t;ctrl ¼ VGR GR  VGR GR zRT t zRT t¼0

where DnG;[ is the amount of methane gas released in moles, PGR and T are the pressure (±20 kPa) and temperature (±0.1 K) within the gas reservoir at time t, z is the compressibility factor calculated by Pitzer's correlations, VGR is the gas reservoir volume and R is the universal gas constant. Equation (4) was applied to compute the number of mole of gas released during control experiment which accounts for the thermal expansion of free gas, ðDnG;[ Þctrl . The amount of methane gas produced from hydrate, ðDnH;[ Þt ; is then calculated as the amount of gas released from dissociation experiment excluding the contribution from thermal expansion of overlying gas, which is computed from:




   DnH;[ t ¼ DnG;[ t;dis  DnG;[ t;ctrl




 P DnH;Y t ¼ VCR CR zRT
NGt ¼

DnH;Y nH2 O



  VCR

t¼0

PCR zRT

 (1a) t

 t

ðmol of gas=mol of waterÞ

(1b)

where DnH;Y is the amount of methane gas consumed in moles, PCR and T are the pressure (±20 kPa) and temperature (±0.1 K) of the crystallizer at any time t, z is the compressibility factor calculated by Pitzer's correlations (Smith et al., 2005), VCR is the crystallizer volume, R is the universal gas constant and nH2 O is the number of moles of water dispersed in sand bed during the experiment. The percentage of water consumed is determined from Equation (2):

Conversion of water into hydrate : CWH ð%Þ DnH;Y  NHyd  100 ¼ nH2 O

(2)

where DnH;Y is the amount of methane gas consumed at the end of the formation process, which can be obtained from Eq. (1) and nH2 O is the number of moles of water introduced into the crystallizer. Nhyd refers to the hydration number, which is defined as the number of water molecules required to encapsulate one guest molecule. A hydration number of 6.1 is used in this present study (Uchida et al., 1999; Tulk et al., 2000). 3.3.2. Methane hydrate dissociation data The number of moles of methane gas released during dissociation experiment and control experiment at a given time t can be computed from Equation (3)

   
 P P DnG;[ t; dis ¼ VGR GR  VGR GR zRT t zRT t¼0

(3)

(5)

As dissociation experiments were conducted from samples of different hydrate saturation, the gas production in this study is normalized with the hydrate volume:

3.3. Methods of calculation 3.3.1. Methane hydrate formation data In our study, the number of moles of CH4 consumed during hydrate formation and the normalized gas uptake (NGt) at any given time t was calculated using Pitzer's correlation for compressibility factor estimation, as shown in Equation (1a) and Equation (1b):

(4)

Vg;t


 DnH;[ t  22400 ¼ Vh
 .  DnH;[ t  22400  3 
m STP m3 hydrate ¼ DnH;Y rh

(6)

where rh represents the molar density of hydrate (7345.16 mol/m3), calculated from hydrate density of 925 kg/m3 (Waite et al., 2009) and hydration number of 6.1 (Tulk et al., 2000). Ideal gas molar volume of 22,400 L/mol is computed according to IUPAC definition of standard temperature and pressure (STP, 273.2 K, 100,000 Pa). To ensure the integrity of the experimental data obtained, percentage recovery of methane from hydrates were also computed from this study using Equation (7) which couples results from hydrate formation and dissociation steps:


 DnH;[ t  f  100% Recovery ð%Þ ¼
DnH;Y t

(7)

4. Results and discussion All experimental studies of CH4 hydrate formation were conducted in Toyoura sand with grain size ranging between 100 and 500 mm at an initial pressure of approximately 8 MPa in water (W) and salt solution (1.5 wt. % NaCl, denoted as ‘D’ and 3.0 wt. % NaCl, denoted as ‘C’) systems. Experimental temperature of the sand bed was varied for the different systems to maintain a constant initial overpressure of 4.2 MPa for experiment in water, 1.5 wt. % NaCl and 3.0 wt. % NaCl. Another set of experiment (L) was done at a lower overpressure of 1.5 MPa in water condition to match with the results from our previous work (Yang et al., 2014) and investigate how formation behaviour changes with different driving forces. Table 1 shows a summary of the experimental conditions, as well as the methane hydrate formation results obtained for different systems. 4.1. Analysis on hydrate formation 4.1.1. Methane hydrate formation in water The gas consumption and the temperature profiles during methane hydrate formation for experiment W3 conducted in water is shown in Fig. 3. The onset of hydrate formation is represented by the sudden spikes in the hydrate bed temperature, as represented

Please cite this article in press as: Chong, Z.R., et al., Effect of NaCl on methane hydrate formation and dissociation in porous media, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/j.jngse.2015.08.055

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Table 1 Summary of experimental conditions and results of CH4 Hydrate formation in porous media. Exp. No.

NaCl (wt%)

T (K)

Initial P (MPa)

Final P (MPa)

Induction time, IT (s)

Methane mol. Fraction at IT

tend (hr)

DnH;Y (mol)

CWH (%)

W1 W2 W3 Wc D1a D2 D3 C1 C2 C3 L1 L2 L3c

0 0 0 0 1.5 1.5 1.5 3.0 3.0 3.0 0 0 0

277.2 277.2 277.2 277.2 276.6 276.6 276.6 275.9 275.9 275.9 277.2 277.2 277.2

8.0 7.9 8.0 8.0 7.8 7.9 8.0 8.0 7.8 8.0 5.3 5.3 5.3

7.4 7.1 7.3 7.5 7.5 7.4 7.5 7.6 7.3 7.6 4.6 4.6 5.3

-b 200 20 -b 300 280 680 840 5820 4940 8760 3860 5580

e 0.0054 0.0010 e 0.0088 0.0067 0.0080 0.0085 0.0137 0.0119 0.0022 0.0018 0.0011

87.60 92.92 68.42 75.00 90.48 194.34 160.00 238.33 195.98 197.58 71.95 75.07 92.22

0.7456 0.8401 0.7952 0.5236 0.4234 0.5930 0.6304 0.5338 0.5716 0.5140 0.7376 0.7471 0.6789

78.92 88.92 84.18 55.42 44.92a 62.77 66.99 56.50 60.50 54.41 78.08 79.08 71.87

W: Water; D: Dilute Salt Solution (1.5 wt% NaCl); C: Concentrated Salt Solution (3.0 wt% NaCl); L: Water at 5.3 MPa. a Experiment D1 was stopped before plateau off. b Exothermic peak appeared prior to system stabilization (See Supporting information Page S4eS6 for more details). c Experiment L3 was a semi-batch formation experiment at constant temperature of 5.3 MPa.

Fig. 3. (A) Gas uptake curve and temperature profile of experiment W3 in water at 277.2 K; (B) Close-up profile from t ¼ 0 h to t ¼ 5.0 h; and (C) Close-up profile from t ¼ 5.0 h to t ¼ 20 h.

by T1 e T3 in Fig. 3 due to the exothermic nature of hydrate formation. Following the rise in the bed temperature, there was a simultaneous drop in pressure in the crystallizer vessel, which was translated into an increase in gas uptake in Fig. 3 employing Equation (1a). Subsequently, crystals of hydrate started to grow within the sand bed, translating into an increase in gas uptake as shown in Fig. 3. Fig. 3B shows a close up of the first 5 h of formation experiment and it is evident that hydrate growth has taken place within the sand bed within the first hour. Similarly, several temperature spikes within sand bed have been observed, coupled with

a continuous drop in crystallizer pressure at other experimental times, as reflected in Fig. 3C. Multiple exothermic peaks were also observed in other experiments conducted in water. The observation of multiple peaks of temperature increase implied the occurrence of multiple nucleation events in the presence porous media and is well documented in the literature (Linga et al., 2009; Kneafsey et al., 2007; Mekala et al., 2014; Haligva et al., 2010). 4.1.2. Methane hydrate formation in NaCl solution Fig. 4 shows the gas uptake and temperature profiles for

Please cite this article in press as: Chong, Z.R., et al., Effect of NaCl on methane hydrate formation and dissociation in porous media, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/j.jngse.2015.08.055

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condition (277.2 K, 3.8 MPa) has been conducted (available in Page S10eS11 in ESI). The results show a decrease of 2% and 12% in methane solubility for 1.5 wt% and 3 wt% NaCl respectively from water. The methane solubility is very low in water (e.g. 0.0011 mol fraction from Fig. S8 at 3.8 MPa, 277.2 K) as compared to methane composition in hydrate (for hydration number of 6.1, the corresponding mole fraction of methane is 0.1408). In addition, the extent of decreased solubility is not significant for the presence of NaCl. Therefore, we believe that solubility is not the key issue for the kinetic inhibition we observed for hydrate formation.

Fig. 4. Gas uptake curve and temperature profile of experiment C1 conducted in 3.0 wt % NaCl solution at 275.9 K.

experiment C1 conducted in 3 wt. % NaCl solution at a temperature of 275.9 K. Similar to Fig. 3, large temperature spikes in the sand bed were observed in the beginning of the experiment, as shown in the close up graph in Fig. 4, indicating the occurrence of catastrophic hydrate growth event. However, after the initial temperature spikes, no other temperature peak was observed in the salt containing systems, which is in contrast with the observation in Fig. 3. The absence of multiple temperature spikes is also consistently observed in 1.5 wt % NaCl containing systems. The low occurrence of temperature peak in salt containing systems despite subjected to the same initial kinetic driving force may imply that the presence of salt kinetically inhibits hydrate formation by suppressing the hydrate growth. This observation is in line with our previous study on the effect of salt on CO2 hydrate formation (Yang et al., 2014). The possible explanation for this phenomenon is that the presence of salt alters the activity of water and disrupts the lattice structure of gas hydrates, thereby increasing the barrier for hydrate nucleation. Considering the duration of experiments (i.e. the time taken for hydrate formation to attain a plateau), the hydrate formation in salt solution at 3.0 wt. % and 1.5 wt. % takes a significantly longer period to reach a plateau (approximately 238 h and 194 h for C1 and D2 respectively) as compared to experiments conducted in water (approximately 68 h for W3). Despite carrying out the formation experiments for a longer time period, the amounts of gas consumed for the formation experiments conducted in salt solution are lower than that conducted in water, as shown in Table 1. From both Figs. 3 and 4, it can be seen that the thermocouple location whereby the most prominent exothermic signal were detected was T2 region, which is located at 2.5 cm below the sand bed-gas phase interface. It can be expected that for dissolved methane to reach a depth of 2.5 cm, the induction time reported in Table 1 is insufficient (Haligva, 2009; Spangenberg et al., 2005) However, it must be noted that a 75% water saturation was applied in this study to increase the contact area between gas and water phase. Methane hydrate formation mechanism in partiallysaturated sand/water system is discussed in detail by Kneafsey et al. (Kneafsey et al., 2007). Kneafsey et al. (Kneafsey et al., 2007) postulated that capillary driven flow of water through sand contribute to a moving methane hydrate formation front that drags the water saturation front. On top of that, it is also expected that with an increasing salt concentration, the methane solubility will decrease due to an increased polarity of the aqueous phase. To study the effect of NaCl concentration on methane solubility, dissolution experiments below equilibrium hydrate formation

4.1.3. Comparison of methane hydrate formation results In order to have a clearer comparison between different runs of different formation durations, the normalized gas uptake curves for hydrate formation in water and different concentrations of NaCl solutions are compared in Fig. 5. As evident from the figure, hydrate formation in water was significantly faster than in the presence of NaCl, with each water run reaching a plateau within 60 h. The formation profile for run W1 also displayed a secondary catastrophic hydrate growth phase around the 50th hour, which is consistent with the formation behaviour of methane observed in other studies (Mekala et al., 2014; Haligva et al., 2010; Fitzgerald et al., 2014). On the other hand, it is evident from Fig. 5 that the gas uptake profile in salt solution system is gradual and continuous, which takes a longer period before reaching a plateau. With the exception of experiment D1 which was terminated at 90th hour before a plateau was observed, all the experimental runs in the presence of NaCl took more than 150 h to complete. This observation clearly demonstrates the significant kinetic inhibition effect of NaCl. The normalized gas uptake curves for the experiments conducted at different salt concentrations are also rather similar in trends, with the growth curves of both salt concentrations almost coincide at the first 60 h. Nonetheless, a slightly faster formation rate was observed for formation experiments conducted at lower salt concentration (1.5 wt% NaCl) after the 60th hour.

Fig. 5. Comparison of normalized gas uptake curves between individual formation experiments conducted at 8 MPa in water systems (W1eW3), 1.5 wt. % NaCl (D1eD3) and 3.0 wt. % NaCl (C1eC3).

Please cite this article in press as: Chong, Z.R., et al., Effect of NaCl on methane hydrate formation and dissociation in porous media, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/j.jngse.2015.08.055

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A comparison between the normalized gas uptake profiles in water at different overpressure (4.2 MPa and 1.5 MPa) is shown in Fig. S4 in ESI. As can be seen from Table 1 and Fig. S4, significant hydrate formation (>70%) can be achieved even at the lower overpressure, with L1 and L2 reached a water conversion around 78% within 100 h. Secondary catastrophic hydrate growth was observed in all lower driving force runs at different time point, indicating the stochastic nature of which catastrophic growth can occur. Besides, it was also observed that the induction time for experiments at 1.5 MPa overpressure was an order of magnitude longer than that of 4.2 MPa overpressure, as can be seen from Table 1. Nonetheless, significant water conversion within 100 h at an initial overpressure of 1.5 MPa indicated that the effect of driving force on kinetics of hydrate formation was not significant in our experimental system. Despite we did not experiment on even lower driving force, it can be expected the kinetic effect of driving force will become more pronounced as we further decrease the driving force. As our formation experiments were mostly conducted in a batch system, the decreasing gas pressure during hydrate formation will result in a diminished driving force. To illustrate the diminishing driving force during hydrate formation, we selected the three runs of highest conversion within their categories: W2, C2 and L2, and the driving force profile is illustrated in Fig. 6, expressed in terms of fugacity. In water system, the equilibrium pressure is constant at the experimental temperature of 277.2 K. However, in the presence of thermodynamic inhibitor (for example, NaCl), hydrate formation consumes free water, which in turn affecting the concentration of inhibitor in the aqueous phase and the equilibrium condition. As shown in Fig. 6, for the case of salt water (C2), the equilibrium pressure at the experimental temperature increased from 3.8 MPa to 4.7 MPa at the end of the experiment due to an increasing NaCl concentration (see Supporting information Page S8eS9 for detailed computational approach). Despite both runs started with the same overpressure, the change in gas phase pressure as well as the equilibrium pressure (for NaCl containing systems) resulted in a diminished driving force for both cases, with salt water diminished

more drastically (e.g. C2, final driving force ¼ 1.5 MPa) as compared to water (e.g. W2, final driving force ¼ 2.2 MPa). The lower kinetic driving force may be an argument for the slower formation kinetics in NaCl solution. However, Fig. 6 also demonstrated that for the lower driving force run (experiment L2, diminished to a final kinetic driving force of 0.25 MPa), a faster formation kinetics than salt containing system was still observed despite subjected to a lower driving force. Prior to this study, some previous work had already been done on stirred tank system. The kinetic inhibition observed in this study is in line with a nearly study which applied stirred tank system to investigate the kinetics of methane hydrate formation in the presence of NaCl (Dholabhai et al., 1993b). The study reported a 24.4% and 51.2% decrease in gas uptake for 3 wt% and 8 wt% NaCl solution respectively after 45 min. However, since the temperature and pressure condition for that study was the same across different NaCl concentration, the authors attributed the kinetic inhibition as a result of a reduced driving force. Our study revisits the effect of salt with the consideration of porous media and difference in overpressures for NaCl containing systems. The findings from our study show that the kinetic inhibition of NaCl was significant in the presence of porous media, and it was not solely dependent on the driving force. In future studies which aims to nullify the effect of driving force, semi-batch formation experiment at a controlled pressure can be designed for the case of water. However, to maintain a constant kinetic driving force in the presence of a thermodynamic inhibitor, an increasing pressure profile has to be maintained in response to the changing thermodynamic equilibrium condition, which could be difficult to achieve technically. Nonetheless, backed by results from our lower overpressure runs, our study clearly demonstrated the effect of salt as kinetic inhibitor, decreasing the rate of formation even at a higher kinetic driving force as compared to the lower overpressure water systems. A summary of the water conversion to hydrates is tabulated in Table 1 and Fig. 7. It can be seen that approximately 84.01%

Fig. 6. Schematic of fugacity changes during experiment and variation of equilibrium fugacity demonstrating the diminishing pressure driving force for experiment conducted in the presence of NaCl.

Fig. 7. Summary of water conversion results and average formation time from this study along with finding from Mekala et al. (Mekala et al., 2014) at water saturation of 75%.

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(±5.00%), 76.34% (±3.91%), 64.75% (±2.79%) and 57.14 (±3.09%) water conversion is obtained for water at 8 MPa, water at 5.3 MPa, 1.5 wt% NaCl and 3.0 wt% NaCl solutions respectively. The comparison of the formation results has shown that even at a lower kinetic driving force, the formation extent for water experiment was consistently higher than those in the presence of NaCl, affirming the kinetic inhibition effect of NaCl on hydrate formation. This agrees with the observations made by Mekala et al. (Mekala et al., 2014) whereby similar kinetic inhibition effect has been observed in natural seawater hydrate formation for CH4 in porous media at overpressures of 4.2 MPa and 6.2 MPa. The water conversion to hydrate in seawater at 100% water saturation was only 7.12% (±2.48%) and 8.44% (±4.51%) for driving forces of 4.2 MPa and 6.2 MPa respectively in that study. As the water conversion was very low in that study, a water saturation of 75% was adapted in the last experimental set, and an average final conversion of 26.31% was achieved over an average experimental time of 55 h, as shown in Fig. 7. It can be expected that an even higher conversion can be achieved shall a longer experimental duration was provided in that study. On the other hand, Yang et al. (Yang et al., 2014) observed a weak inhibitory effect on the kinetics of CO2 hydrate formation in the presence of salt in porous media at a driving force of 1.5 MPa using NaCl and the same seawater employed by Mekala et al. (Mekala et al., 2014). This may be an advantage for the CH4eCO2 gas exchange method for the production of CH4 from the natural gas hydrate. Since insignificant kinetic inhibition effect is observed for the formation of CO2 hydrate in the presence of salt, CO2 gas can be captured more easily and quickly within the hydrate lattice structure as compared to methane; whereas more CH4 can be harnessed during the exploration since reformation of CH4 hydrate is kinetically inhibited by the presence of NaCl. Nonetheless, further investigation of CH4eCO2 exchange method in presence of salts is required in order to provide a detailed and accurate conclusion on the impact of salts on the exchange CH4eCO2 method.

Fig. 8. Improved Process Control for dissociation kinetics at constant crystallizer pressure of 4.8 MPa for experiment W1.

4.2. Methane hydrate dissociation During hydrate dissociation stage for the 8 MPa experiments, the pressure within the crystallizer vessel was kept constant at 4.8 MPa to ensure that the hydrates were stable before the thermal stimulation. The hydrate samples in this study were dissociated using 2 different thermal stimulation methods: ‘gradual heating’ and ‘constant temperature heating’ method, as discussed in Section 3.2. While both methods introduce a final coolant temperature of 297.2 K into the crystallizer vessel to thermally stimulate the hydrate bearing sands, the ‘gradual heating’ method involves gradually heating the entire system to a temperature of 297.2 K, which is not the case for real field exploitation. On the other hand, to enable a good depiction of hydrate dissociation kinetics under the fast thermal stimulation, the process control has to be improved. Therefore, we applied a new PID controller (self-tuned) to provide a better and more precise control of the dissociation kinetics. The deviation from set point of the new control system is ±5 kPa (as illustrated in Fig. 8), which is better than the former PID controller that gave a deviation of ±25 kPa typically (Mekala et al., 2014; Yang et al., 2014). 4.2.1. Pressure-temperature profile during dissociation Fig. 9 shows the reservoir pressure and crystallizer temperature profiles for experiment W2 and W1 which were carried out using the gradual heating and constant heating method respectively. The temperature Tw representing the temperature of water within the cooling jacket shows how different thermal stimulation was applied. From Fig. 9A applying gradual heating, it can be seen that the temperature readings in the gaseous region (Tg1) closely mirror

Fig. 9. Pressure profile of reservoir vessel along with temperature profile of sand bed in the crystallizer during hydrate dissociation for (A) experiment W2 via gradual heating method; and (B) experiment W1 using constant temperature heating stimulation method.

the heating profile of the water bath. Equilibrium temperature of hydrate at the predetermined pressure is represented by a period of steady and constant temperature. As observed in Fig. 9A, the temperature profiles of T1 and T2 initially rise and level off for a short period of time at a temperature of around 279.4 Ke279.8 K, which was close to the equilibrium temperature computed by

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CSMGem program and the model proposed by Loh et al. (279.3 K at 4.8 MPa) (Loh et al., 2012). Due to the endothermic nature of gas hydrate dissociation process, the rise in temperature was delayed as the heat supplied by the water bath was used as the latent heat of dissociation of CH4 hydrates at a comparable rate. The delay in the rise in temperature of T1 and T2 during thermal stimulation indicates the presence of hydrate around these regions. A similar trend is observed in the case for constant temperature thermal stimulation, as shown in Fig. 9B. Under this method, the coolant was first heated to 297.2 K, and once it reached the target temperature (297.2 K), it was introduced into the cooling jacket of the crystallizer vessel, thereby thermally stimulating the hydrate bed. The temperature profile within the reactor was similar to that in gradual thermal stimulation, with the exception of T3. At that location, the temperature rise was observed to be impeded between 0.4 and 0.65 h. Possible reason behind this phenomenon is that T3 is located at a height of 4.5 cm, which is a region immersed in the sand bed. For an initial water saturation of 75%, some of the pore space is occupied by gaseous species or hydrates after the formation. Under the force of gravity, if most of the water sediments at the bottom of the bed, the height of bed which is occupied by water would be 75% of the bed height (3.75 cm), which is below the height of which T3 is sensing. As such, when the bed is stimulated by heat, the temperature T3 measured will be a combination of effect from gaseous phase, solid phase (silica sand and hydrate present) and liquid phase. Though this subtle explanation requires more sophisticated heat transfer calculation and modelling to support, it logically explains the intermediary temperature response of T3, which increases significantly faster than the underlying temperature readings yet distinctive from the overlying gaseous phase temperature readings in both thermal stimulation methods in Fig. 9. Another interesting observation is that when we compare T1 and T2 profiles in Fig. 9, T2 experiences a longer period of delay in the temperature rise as compared to T1. This observation holds true for all the other experimental runs conducted, inferring the possibility that the hydrate distribution across fixed bed is not homogeneous and less hydrate formation occurs around T1. Since T1 is located deeper within the sand bed, the transport of methane gas to this region may be limited within the experimental time scale, which could give rise to the heterogeneity in hydrate distribution. 4.2.2. Quantification of thermal expansion As an excess gas condition was applied during hydrate formation step, the overlying free gas region on top of the hydrate bed will expand during thermal stimulation. The thermal expansion of free gas has to be excluded in order to quantify the amount of gas released from hydrate dissociation. Therefore, control experiments without the presence of hydrate were conducted to determine the amount of free gas expansion during thermal stimulation. This procedure is consistent to that reported in the literature to accommodate the excess gas collected due to thermal expansion from heating (Daraboina et al., 2011; Babu et al., 2013b). Table 2 shows a summary of the dissociation experimental conditions and results obtained using both the gradual heating and constant heating thermal stimulation methods at 297.2 K. The close-to-100% recovery for each experiment, with the exception for experiment D1, shows the integrity in the experimental analysis. The lower recovery for experiment D1 is a result of experimental error as some gas was lost during venting step for that particular run. 4.2.3. Dissociation rate comparison Fig. 10 shows a comparison of the hydrate recovery curves for water experiments W1 and W2 using constant temperature heating and gradual heating method respectively. Time zero on the

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figure represents the time when water was introduced into the water bath. The rate of dissociation was computed by linear fitting of the experimental data during the first stage, which is determined to be 10 min and 25 min for constant heating and gradual heating method respectively. The selection of the duration of first stage was under the consideration of the closeness of the dissociation data to linearity and the percentage of hydrate dissociated from their final amount, such that the dissociation rate represents the kinetic when a considerable amount of hydrate dissociates. As evident in Table 2, more than 65% of hydrate has been dissociated within this stage. It is evident from Fig. 10 that the dissociation rate for gradual heating method was significantly slower than that of constant heating method, with its recovery achieved a plateau at about 60 min as compared to 30 min for constant heating method. From Table 2, the 1st stage dissociation rates for W1 and W2 are 11.633 min1 and 5.791 min1 respectively, quantitatively showing that the dissociation rate for constant heating was twice as high as gradual heating method. To evaluate the effect of salt on the rate of dissociation, the gas production curves for experiment W1, D2 and C1 were plotted in Fig. 11. As the dissociation of each experimental set began with different hydrate saturation, the gas production is normalized with the volume of hydrate in Equation (6). The rates of dissociation during the 1st stage were fitted and tabulated in Table 2. From Table 2, it can be seen that during the 1st stage, both 1.5 wt. % and 3.0 wt. % NaCl solution systems have slightly higher rates of dissociation as compared to water. The improved dissociation rate in the presence of salt is in agreement with the findings from a molecular simulation study (Yi and Liang, 2014). According to the simulation results, it was proposed that the presence of NaCl improved the dissociation rate by two mechanisms, Firstly, the salt ion, especially Cl was embedded into water cage, attracting Hþ from the water molecule, thereby disrupting the original hydrogen bonding that forms the water cage. Secondly, from the density profile of methane after dissociation, it was found that methane forms bubbles in the presence of salt due to limited solubility in salt solution, whereas for the case of water, methane dissolves more uniformly in the liquid phase. Therefore, there was a larger methane concentration gradient between hydrate phase and surrounding liquid phase in the case of salt water, which drives a faster dissociation rate. With these arguments in mind, it can be postulated that a higher salt concentration solution will result in a better heat conduction and stronger attraction to the water molecules, yielding a faster dissociation rate. However in our study, the dissociation rates for each systems using fast heating method are very close to each other. Therefore, the effect of salt concentration on methane hydrate dissociation is not well-resolved from the current system. To better resolve the gas production rate, constant heating method at a lower temperature could be employed in the future studies. It was also noted that different initial hydrate saturation may result in different heat transfer mechanism, which cannot be accounted for by normalizing against initial hydrate content. Therefore, we further carried out an additional water experiment (Wc) which was controlled to achieve a lower final water conversion which is closer to the NaCl containing systems (55.42%). The dissociation behaviour of this run was compared with D2 and C3, which have the most comparable water conversions, as shown in Fig. S9 in ESI. It can be seen that the initial dissociation rate from salt containing system was faster than that of water, with D2 achieving a higher rate and quantity dissociated than Wc. On the other hand, C3 which had a lower initial hydrate saturation than Wc also displayed a faster initial rate despite having a lower final dissociated amount. These results are in line with the findings from Fig. 11, further affirming that dissociation in the presence of NaCl is

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Table 2 Summary of experimental conditions and CH4 hydrate dissociation results. Exp. No. NaCl (wt%) Heating method Stage 1

Vg,

100 min

(m3 gas [STPa]/m3 hydrate) Recovery (%)

Rate of CH4 production [R2] (m3 gas [STPa]/m3 hydrate$min) % Dissociated W1 W2 W3 Wc D1 D2 D3 C1 C2 C3 L1 L2 L3

0 0 0 0 1.5 1.5 1.5 3.0 3.0 3.0 0 0 0

Const. Grad. Const. Const. Const. Const. Const. Const. Const. Const. Const. Const. Const.

11.633 [0.9840] 5.791 [0.9923] 11.043 [0.9911] 12.260 [0.9629] 7.641 [0.9635] 12.333 [0.9548] 11.891 [0.9515] 12.974 [0.9719] 12.147 [0.9681] 12.429 [0.9513] 11.186 [0.9740] 11.154 [0.9609] 11.429 [0.9793]

67.32 76.77 67.50 74.90 79.81 76.76 76.99 80.69 77.71 77.69 69.81 75.14 69.50

162.91 164.84 152.59 166.66 93.04 165.98 162.00 163.02 153.67 152.81 162.29 151.70 164.78

97.95 99.54 91.55 98.76 77.28 99.68 97.81 98.61 93.19 93.00 98.17 91.35 98.75

W: Water (8 MPa); D: Dilute Salt Solution (1.5 wt% NaCl); C: Concentrated Salt Solution (3.0 wt% NaCl); L: Water (5.3 MPa). a STP: Standard Temperature and Pressure according to IUPAC definition (273.15 K, 100,000 Pa).

Fig. 10. Comparison of gas production profile from constant heating method (W1) and gradual heating method (W2).

Fig. 11. CH4 release curve from hydrates formed in water (W1), 1.5 wt% NaCl (D2) and 3.0 wt% NaCl solution (C1).

more favourable than that in water system. One more interesting observation was on the temperature

profile for T2 in water and salt water systems, as shown in Figs. S10 and S11 (in ESI). As mentioned in the previous section, the region around T2 was where we observed the longest delay for temperature increase, implying a higher concentration of hydrate found around this region. In Fig. S10 (in ESI), for the case of water experiment, it is observed that a higher water conversion experiment will result in a longer delay in the constant temperature zone, as can be intuitively expected. The distinctive region of constant temperature was observed consistently for all water experiments. However, such trend was not that obvious for salt water systems, most evidently from the concentrated salt conditions, as shown in Fig. S11 in ESI. This observation implies that the presence of salt affects the thermodynamic property of the sediment, which cause it to dissociate at various temperature. When hydrate was formed, salt was excluded from the hydrate bearing region, causing an increase in salinity in the vicinity of hydrate formation, as schematically shown in Fig. S5 in ESI. Therefore, it was possible that different points within the bed may be of different salinity and resulting in a variance in the dissociation temperature. It has been reported in a molecular simulation studies that minute amount of Naþ and Cl ion may be included in water cages within the hydrate structure, and those which do not participate in hydrate structure would concentrate on the surface of hydrate (Qi et al., 2012). However, more detailed studies have to be carried out in order to better elucidate this unusual thermal behaviour. Hot water (ranging between 353.2 K and 373.2 K) is commonly injected into the hydrate samples to dissociate hydrates (Hancock et al.,; Tang et al., 2005; Li et al., 2012). However, the rate of dissociation of hydrates using this temperature range may be too fast in this study to observe the kinetics of dissociation. As such, the temperature perturbation during the course of this study was done at room temperature (297.15 K), which was deemed as a low temperature for thermal stimulation. Yet, judging from Table 2, the dissociation rate was not significantly different between water and NaCl solution systems under constant thermal stimulation method. Therefore, one improvement for the followeup of this study is that a lower temperature water injection can be used for thermal stimulation in order to slow down the dissociation rate, thereby allowing the dissociation kinetics in the presence of salt to be better studied. On the other hand, a fundamental limitation of the current setup in mimicking the real deepwater reservoir is that the hydrate samples in this study was formed using excess gas approach, by which gas is supplied in excess to a known amount of water (Ruffine, 2015; Hyodo et al., 2013). It has been reported that for oceanic hydrate deposit, the majority of the interstitial pores is

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filled with water (Boswell et al., ; Reagan et al., 2015; Thakur and Rajput, 2011). Recent studies have also revealed that excess water approach, by which water is pumped in excess to a known amount of gas, is capable of forming a more representative hydrate sample for the production studies (Best et al., 2013; Boswell et al., 2011; Priest et al., 2009; Seol et al., 2015). However, excess gas approach is still widely implemented due to its ability to form hydrates at a faster rate as compared to excess water method. In this study, an experimental duration of more than 200 h was taken to fully depict the kinetics of methane hydrate formation in 3 wt% NaCl solution, justifying the use of excess gas method to study the difference in methane hydrate formation behaviors under different salinity. 5. Conclusion The kinetics of methane hydrate formation and dissociation formed in sand (100e500 mm) under the presence of NaCl of different concentrations (0 wt. %, 1.5 wt. % and 3.0 wt. %) were studied. Under the same initial kinetic driving force of 4.2 MPa, a significant kinetic inhibition effect on hydrate formation was observed for the experiments conducted with NaCl, which resulted in lower water conversion to hydrates, slower formation rate and a longer duration to reach a plateau for methane hydrate formation. The dissociation behaviour for each synthetic hydrate samples was studied under thermal stimulation. It was found that the presence of salt slightly increased the first stage dissociation rate and resulted in a faster recovery of methane gas than water system. Interesting insights on the thermal behaviour during methane hydrate dissociation has also been observed and discussed in this study. Acknowledgement The financial support from the National University of Singapore (R-279-000-420-750; R-261-508-001-646 and R-261-508-001733) is greatly appreciated. Zheng Rong Chong would like to thank NUS for the President's Graduate Fellowship. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jngse.2015.08.055. References Archer, D., Buffett, B., Brovkin, V., 2009. Ocean methane hydrates as a slow tipping point in the global carbon cycle. Proc. Natl. Acad. Sci. 106, 20596e20601. Babu, P., Yee, D., Linga, P., Palmer, A., Khoo, B.C., Tan, T.S., Rangsunvigit, P., 2013. Morphology of methane hydrate formation in porous media. Energy Fuels 27, 3364e3372. Babu, P., Kumar, R., Linga, P., 2013. Pre-combustion capture of carbon dioxide in a fixed bed reactor using the clathrate hydrate process. Energy 50, 364e373. Best, A.I., Priest, J.A., Clayton, C.R.I., Rees, E.V.L., 2013. The effect of methane hydrate morphology and water saturation on seismic wave attenuation in sand under shallow sub-seafloor conditions. Earth Planet. Sci. Lett. 368, 78e87. Boswell, R., Collett, T.S., 2011. Current perspectives on gas hydrate resources. Energy Environ. Sci. 4, 1206e1215. Boswell, R., Moridis, G., Reagan, M., Collett, T.S., 2011. Gas hydrate accumulation types and their application to numerical simulation. In: 7th International Conference on Gas Hydrates, Edinburgh. R. Boswell, K. Yamamoto, S.-R. Lee, T.S. Collett, P. Kumar, S. Dallimore, Methane hydrates: chapter 8. Chong, Z.R., Yang, S.H.B., Babu, P., Linga, P., Li, X.-S., 2015. Review of natural gas hydrates as an energy resource: prospects and challenges. Appl. Energy. http:// dx.doi.org/10.1016/j.apenergy.2014.12.061. Daraboina, N., Linga, P., Ripmeester, J., Walker, V.K., Englezos, P., 2011. Natural gas hydrate formation and decomposition in the presence of kinetic inhibitors. 2. Stirred reactor experiments. Energy Fuels 25, 4384e4391. Davidson, D.W., 1973. Clathrate hydrates. In: Franks, F. (Ed.), Water in Crystalline

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Global estimates of hydrate-bound gas in marine sediments:

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Please cite this article in press as: Chong, Z.R., et al., Effect of NaCl on methane hydrate formation and dissociation in porous media, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/j.jngse.2015.08.055