Effect of silica sand size on the formation kinetics of CO2 hydrate in porous media in the presence of pure water and seawater relevant for CO2 sequestration

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Effect of silica sand size on the formation kinetics of CO2 Hydrate in porous media in the presence of pure water and seawater relevant for CO2 sequestration Prathyusha Mekala, Marc Busch, Deepjyoti Mech, Rachit S. Patel, Jitendra S. Sangwai

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S0920-4105(14)00266-6 http://dx.doi.org/10.1016/j.petrol.2014.08.017 PETROL2771

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Journal of Petroleum Science and Engineering

Received date: 28 May 2014 Accepted date: 13 August 2014 Cite this article as: Prathyusha Mekala, Marc Busch, Deepjyoti Mech, Rachit S. Patel, Jitendra S. Sangwai, Effect of silica sand size on the formation kinetics of CO2 Hydrate in porous media in the presence of pure water and seawater relevant for CO2 sequestration, Journal of Petroleum Science and Engineering, http: //dx.doi.org/10.1016/j.petrol.2014.08.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Research Article Effect of Silica Sand Size on the Formation Kinetics of CO2 Hydrate in Porous Media in the Presence of Pure Water and Seawater Relevant for CO2 Sequestration Prathyusha Mekala, Marc Busch2, Deepjyoti Mech, Rachit S. Patel, Jitendra S. Sangwai* Gas Hydrate and Flow Assurance Laboratory, Petroleum Engineering Program, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai – 600 036, India 2

Exchange Student, Department of Mechanical Engineering, RWTH Aachen – 52072, Germany

Corresponding Author: JitendraSangwai: [email protected] Phone: +91-44-2257-4825 (Office) Fax: +91-44-2257-4802 Abstract Understanding the kinetics of carbon dioxide (CO2) hydrate formation in pure water, seawater and porous media aids in developing technologies for CO2 gas storage, carbon capture and sequestration (CCS) and potentially for methane production from methane hydrates. The present work is focused on understanding the kinetics of CO2 hydrate formation in pure water and seawater at an initial formation pressure of 6 MPa (providing a driving force of about 4.0 MPa) and a formation temperature of 276.15 K with 75% water saturation in three silica sand particle sizes (0.16 mm, 0.46 mm, 0.92 mm). The seawater (3.3 wt% salinity) used in the present study is obtained from sea coast of Chennai (India). It is observed that the gas consumption of

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CO2 in hydrate is more for smaller silica sand particle and decreases as the size of the sand increases. The total gas consumed at the end of the seawater experiment is found to be less than the gas consumed at the end of the pure water experiment. This is due to the fact that salts in seawater acts as a thermodynamic inhibitor resulting in lower gas consumption of CO2 in hydrate. The average rate of hydrate formation observed is optimum in 0.46 mm particles and is observed to be higher as compared to 0.16 and 0.92 mm particles over 10 h experimental time. This indicates that 0.46 mm silica sand provides an optimum environment for efficient hydrate formation. The study can be useful to understand the suitability of potential sandstone reservoir for CO2 sequestration in the form of hydrate in the presence of saline formation water. Keywords: CO2 hydrate; Carbon sequestration; Formation kinetics; Seawater; Silica sand. 1. Introduction Earth is facing problems due to global warming of the atmosphere. Some of the major effects of global warming includes, encroaching sea line on coastal areas, reduction in agricultural output, health effects on human beings, and melting of glaciers. The increased emission of gases, such as carbon dioxide (CO2), are responsible for global warming and are due to the increased burning of fossil fuels and the use of renewable energies. This needs solutions to capture and safe disposal of carbon dioxide so as to reduce implications of global warming. A grand challenge for the next generation petroleum engineer is to develop efficient methods for capturing CO2 and disposing it safely. Carbon dioxide capture and sequestration (CCS) is considered to be the prime remedy for global warming. Typically, the use of chemicals along with absorption column to capture and store CO2 have shown benefits. Though the capturing of CO2 has been studied widely, the knowledge related to sequestration of CO2 needs to be developed. 2

In case of upstream oil and gas industrial applications, CO2 injection is used for enhancing oil recovery from mature oil fields. It has dual advantages, firstly, it helps in increasing the oil and gas production from mature oilfields by dislodging the oil trapped in the pores of underground rocks and secondly, to store the CO2 in the reservoirs. The abandoned oil and gas fields shows potential reservoir for storing CO2. In addition, the sedimentary rocks containing brine which are observed well below the source of drinking water shows potential reserves for the storage of CO2. However, there are indications that the CO2 may leak even in such cases (Hawkes et al., 2005). The faults may occur due to subsurface activities, including earthquakes. The storage of CO2 in the form of hydrates avoids lot of risks which is, mainly, its release to the atmosphere. CO2 storage in the form of hydrate in subsea reservoirs avoid release of CO2 gas back through ocean into the earth’s atmosphere. CO2 storage in the form of hydrates is an efficient method and has gained interest from the scientific community to develop CO2 storage technologies. Husebo et al. (2009) through their experiments, observed that the salinity of the formation water affect the hydrate formation in geological environments. For the storage of CO2 in ocean sediments other parameters like ocean environmental conditions, sediment depth, and physical properties of rock and fluid plays a major role for CO2 hydrate stability. Qanbari et al. (2012) studied the effects of ocean and sediment depth on the storage of CO2 at dynamic temperature and pressure conditions of the ocean. They observed that the gravitational stability of CO2 in geothermal gradient is suitable for hydrate formation in the areas of ocean which can restrict upward flow of CO2 toward the seabed. CO2 sequestration in gas hydrate reservoir located at subsurface and seafloor has dual advantage, which are mainly, the storage of CO2 and production of methane from methane hydrate (Goel, 2006). This technology is considered to be an economical one which could

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address the need of the hour of finding an energy solution. In such situation, the formation of CO2 gas hydrates are expected to happen in the porous surroundings which may show sensitive kinetics and phase behavior with respect to various characteristics of porous media and formation water. Hence, it is essential to understand the phase stability and formation kinetics of CO2 hydrate in porous media using pure water and seawater for developing efficient CO2 sequestration procedures. The phase stability of CO2 hydrate in porous media is sufficiently studied in an open literature either for bulk phase or for porous media but seldom reported for silica sand. Several studies are being reported for prediction of phase stability of hydrates for various system of gases at bulk phase conditions (Sloan, 2007; Sami et al., 2013; Mekala and Sangwai, 2014; Sangwai and Oellrich, 2014; Kang and Lee, 2012; Mohammadi and Richon, 2012; etc.). The studies on phase stability of hydrates using porous media were primarily reported on sample porous system such as silica gel (Handa and Stupin, 1992; Seo et al., 2002; Seo and Lee, 2003; Barmavath et al., 2014 Adeyemo et al., 2010), porous glass beds (Uchida et al., 2002; Anderson et al., 2003; Ilani-Kashkoulia et al., 2013; etc.) and Lane Mountain sand (Buffet and Zetsepina, 2000; Zetsepina and Buffet, 2001). Few researchers explored the formation kinetics of CO2 hydrates in distilled water at different experimental conditions. Malegaonkar et al. (1997) conducted experiments on the solubility of CO2 gas hydrate formation in distilled water. They stated that the gas consumption during hydrate formation is proportional to the formation driving force. Buffet and Zetsepina (2000) studied the formation of gas hydrate in natural porous media, such as Lane mountain sand with a size of 0.4 to 0.6 mm. They observed that the CO2 hydrate formation is viable from the dissolved CO2 in natural porous media by the nucleation of hydrate crystal in the absence of free gas. Zetsepina and Buffet (2001) investigated the phase stability of CO2 gas hydrate in natural

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porous medium and observed that the solubility of CO2 hydrate decreases as the formation temperature falls inside the hydrate stability field. Adeyemo et al. (2010) conducted gas hydrate formation experiments from flue gas (CO2/N2) and fuel gas (CO2/H2) mixtures in a crystallizer using a column of silica gel particles. They concluded that the larger pores and particle size of silica gel increase the CO2 gas consumption and water to hydrate conversion. Sun and Englezos (2014) investigated the CO2 hydrate formation in a bed of silica sand partially saturated with water at near-pressure and temperature conditions to that of a typical depleted gas reservoir located in a Northern Alberta using two injection methods. They observed 39-47% and 51-57 % of water to hydrate conversion in gas cap mode and spiral tube mode injection experiments, respectively. Lee and Kang (2011) reported phase equilibria behavioral and kinetic studies of CO2 hydrate formation in NaCl (3.5 wt%) solution. They observed that the phase stability of hydrate inhibited with proportional to the inhibitor concentration. Zhang and Lee (2008) succeeded CO2 hydrate formation in dilute cyclo-pentane bulk solution using a non-stirred batch reactor under static conditions. They observed faster growth rate for hydrate formation which depend on water volume and pressure of the system. Clarke and Bishnoi (2005) measured intrinsic rate constant for CO2 hydrate formation kinetic data obtained by conducting experiments in a semi-batch stirred tank reactor. Kang and Lee (2010) studied the effect of driving force on the formation kinetics of CO2 hydrate in silica gel using surfactant. They observed that the hydrate conversion and formation rate of hydrate are higher for initial high pressure or low temperature for hydrate formation is used and that the formation is higher for driving force of more than 1 MPa. Linga et al. (2012) studied the formation kinetics of various natural gas mixtures in silica sand using stirred vessel and fixed bed reactor of silica sand. They observed that the hydrate formation rate in fixed bed of silica sand are significantly greater than

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that of stirred vessel resulting in higher percentage of water to hydrate conversion. Kumar et al. (2013) studied the effect of different surfactant, viz., cationic dodecyl trimethyl ammonium chloride (DTACl) and anionic sodium dodecyl sulphate (SDS) on the formation kinetics of CO2 hydrate in porous silica gel. They observed that SDS is more effective in reducing the induction time and also in enhancing the rate of hydrate formation. Sun et al. (2014) carried out phase stability studies of methane hydrate formation in silica sand particles. They observed that the phase equilibrium of methane hydrate in coarse grained silica sand are in line with the results for bulk water, thus neglecting the effect of coarse size silica sand on the phase stability. Babu et al. (2013) carried out studies on CO2 hydrate formation kinetics in silica sand and silica gel. They observed that water to hydrate conversion was about 36% with silica sand compared to about 13% conversion in the silica gel bed. This showed that the silica sand provides effective medium for CO2 hydrate formation than silica gel. In our recent study (Mekala et al. 2014) the formation and dissociation kinetics of methane hydrate in sample silica sand is studied using pure water and seawater. We observed 6 time reduction in water to hydrate conversion in case of seawater than pure water in the presence of porous media. However, the effect of size of silica sand on kinetics and water to methane hydrate conversion in presence of seawater and pure water was not investigated (Mekala et al. 2014). Kang et al. (2013) studied the pre-combustion capture of CO2 in silica gel pores from a simulated flue gas. They observed that, due to the presence of pore in silica gel matrix, a higher rate of hydrate formation CO2 hydrates is observed which is suitable for CO2 capture. Mohammadi et al. (2014) carried out kinetic study of CO2 hydrate formation in the presence of surfactant and silver nanoparticles. They observed that the presence of surfactant and silver nanoparticles increases the storage capacity for CO2 hydrate formation but does not decrease the induction time for hydrate formation. Lee et al. (2014) studied the dissociation

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enthalpies of CO2+N2 mixture for application related to CO2 capture and storage. Their study provided background for the estimation of heat absorbed or dissociated during hydrate formation which may help for predictions of suitable operational conditions for CO2 capture and storage. In practical situations, storage of CO2 hydrates is to be carried out in reservoirs under the ocean seafloors, and hence it is essential to understand the CO2 hydrate formation in seawater and porous media. Direct seawater consists of many dissolved salts (NaCl, MgCl2, Na2SO4, and MgSO4), microbes, etc., contributing to the overall salinity of the seawater. However, the information on the formation kinetics of CO2 hydrate formation in porous media using pure water and seawater are seldom known in an open literature. In the present study, an attempt is made to understand the kinetics of CO2 hydrate formation in pure water and seawater in the presence of different sizes of silica sand as a porous media in an experimental set-up representing a marine environment. CO2 hydrate formation kinetics was studied at 6 MPa and 276.15 K experimental conditions using pure water and seawater in three different silica sand particles (0.16, 0.46 and 0.92 mm) with a 75% water saturation. The silica sand particle sizes are chosen so as to be in geometric series with common ratio of approximately 2, so as to provide better understanding on the formation kinetics. In reality, the lower water saturation may be funicular and may have isolated gas bubbles. However, simulating such complex geological condition is not possible with the current set-up, neither the current focus of the study. In the study of Mekala et al. (2014), we studied the hydrate formation of methane for two water saturations, viz., 100 % and 75 %. We observed that the water saturation with 75% provide better gas uptake in the hydrate formation and the same is being followed in this study.

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2. Materials The seawater (3.3 wt% salinity and pH of 8) used in the present study is obtained from sea coast of Chennai (India) and used as it. Three silica sand samples of different particle sizes (0.16; 0.46; and 0.92 mm) are used and are shown in the Figure 1. The silica samples with small, medium and large are checked for the size range by sieve analysis. Samples were sorted by mechanical sieves followed by washing several times with distilled water. Small size of 0.16 mm (in the range of 0.15-0.18 mm); medium size of 0.46 mm (in the range of 0.43-0.5 mm); and large size of 0.92 mm (in the range of 0.85-1.00 mm). The mean diameter is computed after conducting sieve analysis of small, medium and large sand samples as 0.16 mm, 0.46 mm, 0.93 mm, respectively. Images of the silica sand (as shown in Figure 1) visualizes the silica sand texture according to particle sizes. Figure 2 shows the Scanning Electron Microscope (SEM) images of silica sand particles used in this study and provides information confirming on the average particle size. The SEM device also works as an energy dispersive X-ray spectroscope and is able to analyse the chemical composition of the samples. It is observed from SEM study that the silica sand contains silica (33±5 wt%), oxygen (41.76 ±5 wt%), aluminum (11.45 ±5 wt%), and potassium (13.79 ±5 wt%). The presence of oxide (as regards to oxygen) are also be found in the sands. In addition, de-ionized water is used as a pure water for hydrate formation in silica sand. CO2 gas used is 99.5% pure and is supplied by Bhuruka Gas Agency, Chennai, India. The details of the particle size and pore volume for each silica sand estimated are listed in Table 1. Pore volume increase as the particle size increased. It is known that as the particle size increases, surface area decreases.

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3. Experimental Setup The schematic diagram of the experimental setup used for gas hydrate kinetic studies is shown in Figure 3. Different components of the experimental setup are discussed below in detail. The principal part of the experimental setup is a high pressure jacketed batch reactor, which is made of stainless steel vessel (SS-316) designed for pressures up to 10 MPa. The schematic of the reactor is shown in Figure 4. The reactor capacity is of 1.4 L in volume. The reactor consists of three ports, two at the top of reactor flange for gas inlet and outlet, respectively. The third port at the bottom is used for liquid outlet. The ports are fitted with two way needle valves. Julabo® water bath (Model FP 50) is used to keep steady experimental temperature throughout the bed in the reactor. The external part of the reactor is jacketed for circulation of water-glycol mixture in order to facilitate temperature control for either heating or cooling. The temperature of the circulating fluid is digitally controlled by a PID (proportional integral derivative) programmable temperature controller. To sense temperature and pressure in the sand bed and the reactor, a platinum resistance thermometer (Pt-100) and a pressure transducer are fixed in the reactor. The Pt-100 rests in the middle of the bed. The Pt-100 used is a class-A type as per DIN 43760 and calibrated using American Society for Testing and Materials (ASTM) 1137 procedure prior use. A piezo-resistive pressure transducer (Model HD20V4T) is used for measuring the internal pressure of the reactor. The temperatures and pressures are measured with an expanded uncertainty of ± 0.11 K and ± 0.02 MPa, respectively. The signals given out by Pt-100 and pressure transducer are fed to a data acquisition unit and are stored in the computer online.

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4. Experimental Procedure Experimental procedure can be well elucidated with the assistance of Figures 3 and 4. The kinetic study of hydrate formation are studied using porous bed of three different sizes of silica sand particles separately. The batch reactor is charged with desired amount of silica sand which makes the bed height of 7 cm. The bed height is chosen such that the Pt-100 dips wells into the bed till center and measures the temperature correctly without fluctuations. Details of the bed height, the amount of sand used and other details are listed in Table 1. Having pore volume of 15%, the bed requires 88.09 ml of seawater or pure water to acquire 75% water saturation. The bed is made by uniform layering of sand and seawater alternatively in 5 steps to avoid any air pockets (Mekala et al., 2014). The reactor with bed is properly closed from top to align and position Pt-100 and pressure transducer perfectly. The outlet and inlet ports of water bath is connected to the inlet and outlet ports of reactor jacket by means of hoses. Temperature of the water bath is set for constant experimental temperature of 276.15 K for all experimental studies. An anti-freeze heater is provided at the outlet of CO2 cylinder to avoid ice formation. Before injecting CO2 gas, reactor with silica sand bed is flushed thrice with CO2 gas at 1 MPa to remove inside trapped atmospheric air. At this point, the reactor is checked for any leaks to fix. As soon as the temperature in the bed becomes steady at 276.15K (±1K), reactor is pressurized by injecting CO2 gas from the cylinder. Temperature and pressure in the sand bed and the reactor can be visualized on the control panel connected to the reactor. Subsequently, the data logger is started to record temperature and pressure at every 20s interval for about 10 h. While filling the gas in the reactor to 6 MPa, with pressure, temperature will also rise, but due to the simultaneous cooling at desired condition slowly the temperature falls back to the steady experimental temperature. Until steady

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experimental temperature is reached, gas injection is continued. Since CO2 is highly compressible, this took a while to attain steady conditions in the reactor. After experimental temperature and pressure is attained, the gas injection is stopped. During the injection of gas, hydrate nucleation is observed by monitoring the increase in temperature and gradual decrease in the reactor pressure. Since porous media acts as an inhibition to the hydrate formation mechanism, the equilibrium pressure required to form hydrates in porous media of ‘silica sand’ is marginally higher compared to the equilibrium pressure required to form hydrates in the bulk phase however it is observed that the coarse silica sand does not affect the phase stability (Sun et al., 2014) and, hence, is not considered for determining the driving force for hydrate formation for the present study. The initial driving force for hydrate formation experiments for pure water and seawater experiment at formation pressure and temperature condition of 6 MPa at 276.15 K is observed to be about 4 MPa. It is to be noted here that the kinetic experiments were carried out in batch mode and that the driving force represent the initial driving force suitable for hydrate formation.

5. Results and Discussion CO2 hydrate formation kinetics experiments in porous media are conducted in pure water and seawater using three types of silica sand with sizes 0.16 mm, 0.46 mm, and 0.92 mm to represent porous media. Table 2 provides details of experiments carried out in this study. For repeatability memory experiments (M), viz., S2, S4 and S6 (see Table 2) are conducted after conducting the fresh experiments (F) and leaving the bed overnight (12 h). The effect of particle size of the silica sand sample on the CO2 hydrate formation kinetics is studied in detail in pure water and seawater and presented here relating their significance for CO2 sequestration.

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The amount of gas consumed during the experiment is calculated from the Eq. 1.



PR   P  − R     zRT t =0  zRT t  (1)

( ∆nH )t = VR 

( ∆nH )t is represented as the number of moles of gas consumed at time t. It is the difference in initial number of moles of gas at time t=0 and number of moles of gas at given time t. z is the compressibility factor for given temperature, T and pressure, PR, which is the reactor pressure at given time t. z is computed by Pitzer’s correlations which is being used widely by many researcher for the said use (Smith et al., 2001; Linga et al., 2012; Kumar et al., 2013; Babu et al., 2013; Mekala et al., 2014). VR is the crystallizer volume. R is the universal gas constant. Figures 5 and 6 shows the comparison of pressure and temperature profiles during the hydrate formation experiment in pure water and seawater in porous media in the reactor. The experiments are performed for 10 h so as to ensure that there is no further pressure drops. The temperature of the bed during the onset of hydrate formation increases sharply, which is a clear indication of hydrate formation process. This is due to the fact that hydrate formation is an exothermic process. The rise in temperatures is typically referred to as nucleation phenomenon. The nucleation phenomenon in case of bulk process (without porous media) is indicated with high rise in the temperature of the system which is due to the formation (nucleation) of several hydrate crystal around the same time. However, in case of porous media, the nucleation phenomenon is quite distributed across the bed, and also due to the presence of small interstitial water and gas available around the pore volume, the rise of temperature may not be that high as the case with bulk phase without porous media (Figures 5-6). During the 10 h experimental duration, a one sharp nucleation point is observed at the start of the experimental run. The induction point which is an indication of the initial nucleation occurrence where the hydrate starts formation can be identified easily by means of increase in temperature and decrease in pressure at the onset. As the CO2 forms hydrates readily as compared to other hydrate formation

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gases such as methane, it is important to capture the pressure temperature behavior at the earlier stage of the hydrate formation reaction. Inset in Figures 5 and 6 shows temperature and pressure profiles during first one hour of the experiment. Thereafter, the nucleation points were not clearly visible, both in case of pure water and seawater, probably due to the presence of porous media and presence of small interstitial water and gas available at in-situ conditions. Also that the required hydrate formation conditions for CO2 hydrates are much lower than that of other gases like methane, the nucleation phenomenon was not that severe in case of CO2 hydrates formation in porous media. This observations are in line with literatures studies (Kang and Lee, 2010; Kumar et al., 2013; Linga et al., 2012). The time at which the first nucleation event is observed is referred to as induction time. As the hydrate formation starts in the porous media, the gas from the gas phase gets consumed in the form of hydrate in porous media and results in decrease of the reactor pressure. The number of CO2 gas moles consumed in hydrate per water molecule are plotted over for 10 h experimental duration and the results for the same are shown in Figure 7. It is observed that CO2 gas consumption in the form of hydrate is higher in pure water than in seawater for all three silica sand particles, viz., 0.16 mm, 0.46 mm, and 0.92 mm, as shown in Figure 7. The details on the gas moles consumed per mole of water at the end of experimental run for various studies are shown in Table 2.The total gas consumed at the end of the seawater experiment found to be less than the gas consumed at the end of the pure water experiment. The reason behind this can be because, salts in seawater acts as thermodynamic inhibitor resulting in less consumption of CO2 gas molecules. It is also observed that the salt also acts as kinetic growth inhibitors as in case of methane hydrate formation in porous media resulting in lower gas consumption and kinetically slowing the process of hydrate formation (Mekala et al., 2014). The details on the induction time

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observed for various experimental studies using pure water and seawater in silica sand are reported in Table 2. It is well-known fact that the hydrate formation process is more stochastic in nature. However, the trends on the induction time shows a peculiar observation on the induction time as observed from Table 2. The induction time for hydrate formation in case of seawater is observed to be lower than that of pure water. This indeed is in contrary with known behavior of the effect of salt on the inhibition of hydrate. However, recent studies by Farhang et al. (2014) indicates that the salts at various concentration does affect the kinetic behavior and induction time for CO2 hydrate formation. They observed that that, even though the salts are thermodynamic inhibitors, they can act as kinetic promoters at lower concentrations enhancing the rate of hydrate formation and reducing the induction time for hydrate formation. They stated that the halide ions affects the bulk and surface water structure thus significantly influencing the rate of CO2 hydrate formation. Qi et al. (2012) investigated the effect of the Na+ and Cl− ions on the phase stability and the structure of pure gas hydrates using molecular dynamics simulations. They observed that the ions of salt interacts with the water molecules of pure hydrate in a stronger way than the van der Waals forces alone. These studies, in general, indicate a complex behavior of slats on the kinetics and phase stability of hydrate structures. Our observation are on similar lines on the induction time for the case of CO2 hydrate formation kinetics for seawater and pure water in porous media. However, the focus of this study was not to investigate in details the effect of slats on the induction time, this study indeed forms a precursor for further investigations on these lines. Comparisons of CO2 gas consumptions in pure water and three sizes of silica sand particle (0.16mm; 0.46mm and 0.92mm) are shown in Figure 8. Form the gas consumption profiles for the gas mole consumed per water mole over 10 h experimental duration for 0.16 mm, it is

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observed that the gas consumption is higher for 0.16 mm silica sand particle than in 0.46 and 0.92 mm silica sand particles. In fact, the trends shows that the gas consumption of CO2 in hydrate is more with smaller silica sand particle and decreases as the size of the sand increased. This is because, even the pore volume is higher in case of 0.92 and 0.46 mm silica sand particles, surface area of contact is expected to be higher in case of 0.16 mm silica sand particle resulting in better gas-liquid contact suitable for hydrate formation. Figure 9 shows the CO2 gas consumptions in hydrate per water molecule using seawater and three sizes of silica sand particle 0.16 mm, 0.46 mm and 0.92 mm. From the profiles, it is observed that the CO2 gas consumed in hydrate per water molecule over 10 h for 0.16 mm is found to be higher compared to the 0.46 mm and 0.92 mm particle size silica sand. The order of gas consumption decreased as particle size increased. Water to hydrate conversion percentage is computed from Eq.2 for various experimental studies. Water to hydrateconversion ( % ) =

∆nH × Hydration number × 100 nH 2 o (2)

Hydration number used for the present study is 7.23 (Sloan, 2007; Kang et al., 2001). nH 2O is the number of moles of water present in the reactor. Table 2 lists percentage of water to hydrate conversion at the end of the kinetic experiment for various experimental studies conducted in pure water and seawater. In general, the maximum hydrate conversion is observed in the case of pure water than in seawater. As the size of silica sand increases, the hydrate conversion decreased. The maximum hydrate conversion in case of pure water is observed to be 58.76 % for the case of 0.16 mm silica sand. Maximum seawater to hydrate conversion percentage is achieved is 45.14 % with 0.16 mm silica sand particle. Experiments conducted in 0.46 mm particle and pure water also showed nearly 34.20 % 15

conversion. Also, it is observed that only in 0.46 mm particle, gas consumption is only 1% less in seawater compared to experiment conducted in pure water. For the experiments conducted in 0.92 mm particle, hydrate conversion percentage in pure water and seawater are found to be 25.33 and 22.97 %, respectively, showing more conversion in pure water than in sea water similar to other systems. Surface area provided by smaller particle size is expected to be more than the larger particle size which provides large area of contact between gas and liquid necessary to form hydrates. It is to be noted that, the percentage of hydrate formed is much more in case of CO2 hydrates as compared to the kinetics of hydrate formation for methane gas hydrate in seawater and pure water (Mekala et al., 2014). The average rate of hydrate formation is computed using Eq. 3.  d ∆nH  ( ∆nH )t +∆t − ( ∆nH )t   = ∆t  dt t (3)

The average rate is computed for every half an hour throughout the experiment as shown in Figure 10a. In general practice, the sampling time for data acquisition of pressure and temperature used is small enough (20 s) to capture the insights into the hydrate formation process. However, for the calculation of the rate of hydrate formation, a time step of 30 min is suitable which indicate a hydrate formation rate for that time step. This is done since the data is too large, and the average rate for every half an hour help proper visualization of the rate change over 10 h. Inset shows the rate profile during the first one and a half hour of the experiment. The average rate of hydrate formation in 0.46 mm particle is found to be higher as compared to the average rates of hydrate formation in 0.16 and 0.92 mm particles. The rate of hydrate formation, moles of gas per mole of water per hour in 0.46 mm particle is high from the induction time and gradually decreased towards the end of 10 h of the experiment. This is because of the 16

accessibility of both pore volume and surface area in 0.46 mm particles. As 0.16 mm particles has higher surface area but less pore volume and 0.92 has higher pore volume but low surface area, the rates of hydrate formation is low. Therefore, pore volume and the surface area provided by 0.46 mm particle size silica sand can provide better environment for optimum conversion of water to hydrate. Average rates of hydrate formation in pure water in all the three silica sand particles are shown in the Figure 10b. It can be inferred that the rate of hydrate formation in smaller (0.16 mm) is higher than medium and large particle sizes. However, it is to be noted that (see Table 1) more amount of 0.16 mm silica sand is needed to make the same bed height. So, it is clear that the smaller particle size provides larger surface area required for the gas to have contact with the liquid phase necessary to initiate hydrate formation. As mentioned earlier, that the sand bed is made by uniform layering of sand and seawater alternatively in 5 steps to make a bed of 75 % water saturation to ensure uniform water saturation, the hydrate formation reaction will happen at the water-gas interface within the sand bed in addition to the entire porous media. The surface area here refers to the gas-liquid interface area plus the area provided by the porous media. The hydrate formation process begins both at the water-gas interface and also in the entire porous media due to the presence of dissolved CO2. It is expected that the hydrate formation occurs both by means of soluble gas and the gas available at the water-gas interface in the sand bed. As it is known, the hydrate nucleation can be increased with vigorous stirring of gas-water phase in bulk reactor (without porous media) to increase the contact between two phases. In case of porous medium, the hydrate nucleation kinetics in the sand is affected because of the increased surface area available for natural gas to get in contact with water. The average rates of hydrate formation in pure water and seawater in 0.46 mm particles are compared in Figure 11. Inset shows the rate profile during the first one and

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a half hour of the experiment. It is observed that the rate of hydrate formation is marginally higher in pure water than in seawater for the case of CO2 hydrate formation. The repeatability on the tracking of gas consumption profile during the course of hydrate formation experiments is, though, difficult owing to the stochastic hydrate formation process in porous media, however, repeatability of the gas consumption and final water-to-hydrate conversion can be achieved within the range of ±15 % of the final value. The results in general throws light on the effect of sizes of the silica sand in the hydrate storage reservoir for formation of CO2 hydrate in the presence of pure water and seawater. The study can be useful to understand the suitability of potential sandstone reservoir for CO2 sequestration the form of hydrate in the presence of saline formation water and also for possible methane production from hydrate reservoirs.

6. Conclusion CO2 hydrate formation kinetics is studied at 6 MPa in pure water and seawater at 276.15 K experimental conditions using three different silica sand particles (0.16, 0.46 and 0.92 mm) with a 75% water saturation. It is observed that the gas consumption of CO2 in hydrate is more with smaller silica sand particle and decreases as the size of the sand increased. The total gas consumed at the end of the seawater experiment is found to be less than the gas consumed at the end of the pure water experiment. The average rate of hydrate formation observed is higher in 0.46 mm particles as compared to 0.16 and 0.92 mm silica sand particles providing an optimum environment for efficient hydrate formation. The induction time for hydrate formation in case of seawater is observed to be lower than that of pure water, which is in-line with the selected results on the effect of salt on the hydrate formation mechanism.

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Acknowledgement: Authors would like to acknowledge the office of the Industrial Consultancy and Sponsored Research (ICSR), Indian Institute of Technology Madras, Chennai for financial support through project number OEC/10-11/530/NFSC/JITE.

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Table 1: Properties silica sand bed for CO2 kinetic experiments in pure water and seawater Sand nomenclature

Dry weight (g)

Pore volume (%)

SS1

Sand particle size (mm) 0.16

997.80

15

Water required for 75% saturation (ml) 88.09

SS2

0.46

829.75

25

155.5

SS3

0.92

774.8

31

185.5

23

Height of sand bed (cm)

7

Table 2: Water to hydrate conversion in pure water and seawater observed in this work Silica

Experiment

State

Induction

Moles of gas

Hydrate

sand

nomenclaturea

of

time (h)

consumed/mole

conversion

of water

(%)

grain

water

b

size

(mo1/mol)

(mm)

SS1

SS2

SS3 a

P1

F

0.13

0.0816

58.76

S1

F

0.10

0.0627

45.14

S2

M

0.11

0.0554

39.85

P2

F

0.90

0.0475

34.20

S3

F

0.15

0.0464

33.39

S4

M

0.27

0.0374

27.91

P3

F

0.71

0.0352

25.33

S5

F

0.27

0.0319

22.97

S6

M

0.08

0.0313

22.55

P: Pure water; S: Seawater;

b

F: Fresh, M: Memory

Figure Captions Figure 1:

Images of silica sand samples (SS1, SS2, and SS3) of different particle size.

Figure 2:

SEM images of the three silica sand samples with different particle size.

Figure 3:

Schematic of the experimental setup for studies on CO2 hydrate formation kinetic experiments.

Figure 4:

Schematic of the jacketed reactor used for CO2 hydrate formation kinetic experiments.

Figure 5:

Pressure vs. temperature profiles for CO2 hydrate formation kinetics in seawater and 0.46 mm silica sand (SS2)

Figure 6:

Pressure vs. temperature profiles for CO2 hydrate formation kinetics in pure water and 0.92 mm silica sand (SS3) 24

Figure 7:

Comparison of gas consumption in pure water and seawater in three sizes of silica sand particles. S: seawater; P: pure water (see Table 2 for experiment nomenclature).

Figure 8:

Comparison of CO2 gas consumption in pure water and three sizes of silica sand particle

Figure 9:

Comparison of CO2 gas consumption in seawater and three sizes of silica sand particle

Figure 10: Comparison of average rate of hydrate formation in three sizes of silica sand. (a) S: seawater; (b) P: pure water Figure 11: Comparison of average rate of hydrate formation in pure water and seawater in0.46 mm size silica sand. S: seawater; P: pure water (refer Table 2 for nomenclature)

Figure 1: Images of silica sand samples (SS1, SS2, and SS3) of different particle size.

25

(a) 0.16 mm silica sand, SS1

(b) 0.46 mm silica sand, SS2

(c) 0.92 mm silica sand, SS3 Figure 2: SEM images of the three silica sand samples with different particle size.

26

Figure 3: Schematic of the experimental setup for studies on CO2 hydrate formation kinetic experiments.

Figure 4: Schematic of the jacketed reactor used for CO2 hydrate formation kinetic experiments.

27

290

6.5 7

300

6

P T

295

288

5

6.0

290

286

4 285

280

284

2

5.5

275

1

0

282

270 0.0

0.1

0.2

0.3

0.4

0.5

5.0

280

Tem perature (K)

Pressure (MPa)

3

278 4.5 276 4.0 0

1

2

3

4

5

6

7

8

9

10

Time (h)

Figure 5: Pressure vs. temperature profiles for CO2 hydrate formation kinetics in seawater and 0.46 mm silica sand (SS2)

28

6.5

290 284

P T

4.4

6.0

282

288

4.2 280

286 4.0 278

3.8

284

276

5.0 3.6

274

282 0.0

4.5

0.2

0.4

0.6

0.8

1.0

280

Tem perature (K)

Pressure (M Pa)

5.5

4.0 278 3.5

276

3.0 0

1

2

3

4

5

6

7

8

9

10

Time (h)

Figure 6: Pressure vs. temperature profiles for CO2 hydrate formation kinetics in pure water and 0.92 mm silica sand (SS3)

29

0.04

0.08

m oles of gas/ m ole of w ater

m oles of gas/ m ole of water

0.10

0.06

0.04

0.02

0.03

0.02

0.01 P2 S3

S1 P1 0.00

0.00

0

1

2

3

4

5

6

7

8

9

10

0

1

2

3

4

Time (h)

5

6

7

8

9

10

Time (h)

0.16 mm silica sand (SS1)

0.46 mm silica sand (SS2)

moles of gas/ mole of water

0.04

0.03

0.02

0.01 S5 P3 0.00 0

1

2

3

4

5

6

7

8

9

10

Time (h)

0.92 mm silica sand (SS3) Figure 7: Comparison of gas consumption in pure water and seawater in three sizes of silica sand particles. S: seawater; P: pure water (see Table 2 for experiment nomenclature).

30

0.09

moles of gas/ mole of water

0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.16 mm 0.46 mm 0.92 mm

0.01 0.00 0

1

2

3

4

5

6

7

8

9

10

Time (h)

Figure 8: Comparison of CO2 gas consumption in pure water and three sizes of silica sand particles

0.07

moles of gas/ mole of water

0.06

0.05

0.04

0.03

0.02 0.16 mm 0.46 mm 0.92 mm

0.01

0.00 0

1

2

3

4

5

6

7

8

9

10

Time (h)

Figure 9: Comparison of CO2 gas consumption in seawater and three sizes of silica sand particles 31

Average rate of hydrate formation (moles of gas/ mole of water/h)

0.005

0.005

0.16 S1-S2 0.46 S3-S4 0.92 S5-S6

0.004

0.004

0.003

0.002

0.003 0.001

0.000

0.002

0.0

0.5

1.0

1.5

0.001

0.000 0

2

4

6

8

10

Time (h)

(a)

Average rate of hydrate formation (moles of gas/ mole of water/h)

0.006 0.16 P1 0.46 P2 0.92 P3

0.005

0.004

0.003

0.002

0.001

0.000 0

2

4

6

8

10

Time (h)

(b) Figure 10: Comparison of average rate of hydrate formation in three sizes of silica sand. (a) S: seawater; (b) P: pure water

32

Average rate of hydrate formation (moles of gas/ mole of water/h)

0.005 0.46 S3-S4 P2

0.005 0.46 S3-S4 P2 0.004

0.004 0.003

0.002

0.003 0.001

0.000 0.0

0.002

0.5

1.0

1.5

0.001

0.000 0

2

4

6

8

10

Time (h)

Figure 11: Comparison of average rate of hydrate formation in pure water and seawater in 0.46 mm size silica sand. S: seawater; P: pure water

•

Formation of CO2 hydrate in depleted reservoir offers solution for CO2 storage.

•

CO2 hydrate formation kinetics is investigated for seawater and porous media.

•

Effect of silica sand size on the CO2 hydrate formation kinetics is presented.

•

Gas consumption of CO2 in hydrate decreases as the size of the sand increases.

•

Gas consumed in seawater is found to be less than the pure water experiment.

33