International Conference on Gas Hydrates (ICGH) (6th : 2008)

KINETICS OF HYDRATE FORMATION AND DECOMPOSITION OF METHANE IN SILICA SAND. Nam, Sung Chan; Linga, Praveen; Haligva, Cef; Ripmeester, John A.; Englezos, Peter 2008-07-31

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KINETICS OF HYDRATE FORMATION AND DECOMPOSITION OF METHANE IN SILICA SAND   Sung Chan Nam1, Praveen Linga2, Cef Haligva2, John A. Ripmeester3 and Peter Englezos2*  1. Energy Conversion Research Department,  Korea Institute of Energy Research, Yuseong-gu, Daejeon, 305-343, Korea   2. Department of Chemical and Biological Engineering,  University of British Columbia,  Vancouver, BC, V6T 1Z3, Canada   3. Steacie Institute for Molecular Sciences,  National Research Council Canada, Ottawa, ON, K1A 0R6, Canada   ABSTRACT Kinetics of hydrate formation and decomposition of methane hydrate formed in silica sand particles were studied in detail at three temperatures of 7.0, 4.0 and 1.0? C, respectively. A new apparatus was setup to study the decomposition behavior of the methane hydrate formed in the bed of silica sand particles. Six thermocouples are placed in different locations to study the temperature profiles during hydrate formation and decomposition experiments. Gas uptake measurement curves for the formation experiments and the gas release measurement curves for the decomposition experiment were determined from the experimental data. Percent conversion of water to hydrates was significantly higher for the experiments conducted at 4.0 and 1.0? C compared to 7.0? C. Recovery of methane occurred in two stages during the decomposition experiments carried out with a thermal stimulation approach at constant pressure. Methane recovery in the range of 95 to 98% was achieved.  Keywords: Methane, gas hydrates, silica sand, decomposition, formation   INTRODUCTION: Natural gas hydrates are non-stoichiometic, crystalline, inclusion compounds [1, 2]. They are known to plug hydrocarbon pipelines [3] and to occur naturally in the earth [4]. The first indication of natural gas hydrates outside of the Soviet Union was the report by Stoll et al. [5] observing that the formation of natural gas hydrates can result in significant increases in the acoustic wave velocity in ocean sediments.. Based on wireline logs from two exploratory wells in the Mackenzie Delta, Canada, Bily and Dick [6] concluded that gas hydrates were contained in shallow sand reservoirs. As soon as the significance of the natural gas potential of gas hydrates stored in the earth was realized, efforts to develop extraction methods started [7-12].  Potential reserves of natural gas hydrates are over 1.5 x 1016 m3 and are distributed all over the earth on the land and offshore [13-16].  In order to reliably predict the feasibility of producing natural gas from hydrates several key information are needed namely [17]: (1) Abundance of hydrates in the selected reservoir, (2) Lithology and geologic structure of the reservoir, (3) Presence or absence of a free zone, (4) Arrangement of hydrate within the porous medium, (5) Permeability, relative permeability-saturation relationships, capillary pressure-saturation relationships, and thermal conductivity of the hydrate-bearing and hydrate-free medium, (6) Energy required to dissociate the hydrate, (7) Kinetics of dissociation.  Several of these needs are reservoir specific but others such as dissociation kinetics have components of general applicability that can be Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.  evaluated in the laboratory [17]. Two reports from the Los Alamos National laboratory presented the first state of the art and pointed out the difficulties associated with these gas recovery techniques [10, 18, 19]. Holder et al. [20] concluded that reservoir porosity and the thermal properties of the hydrates and the reservoir are determining factors that would enable a gas hydrate reservoir to produce gas in an energy efficient manner. It was also realized during these early studies that knowledge of the rate of hydrate decomposition is required although the term rate of decomposition is not precisely defined and more importantly it does not distinguish between intrinsic kinetic effects and those attributed to transport factors. Selim and Sloan [21] used thermal stimulation to decompose hydrates formed in porous media in the laboratory. The dissociation rate was found to depend on the thermal properties and the porosity of the reservoir. Further work reported from Sloan?s laboratory found that the endothermic hydrate decomposition process could cause such a temperature drop that would allow hydrates to re-form or ice to freeze [22, 23].    Although significant progress has been made regarding modeling and numerical simulation of natural gas hydrate reservoirs [24-30],  very few papers are available in the literature on experimental data on hydrate kinetics investigated in porous medium [17, 31-33]. Handa and Stupin [31] reported thermodynamic properties and dissociation characteristics of methane and propane in silica gel pores. Stern et al [32] observed the peculiarities of methane clathrate hydrate formation and solid state deformation. However, the conditions at which the observations [31, 32] were made are < 105 K and 10-3 Pa, whereas the natural environment for the samples is moderate pressures (approximately 3 to 10 MPa) and above-freezing temperatures [17].   Kneafsey et al. [17] studied hydrate formation of methane-water system in the presence of silica sand (Foundry 110, U. S. Silica, Berkeley Springs WV) with particle size primarily between 100 to 200?m. They studied methane hydrate formation/dissociation in a large X-ray transparent pressure vessel monitoring pressure and temperature. In addition, they monitored the local density changes using X-ray computed topography during the experiment. They observed that the rate of hydrate formation is not always proportional to the driving force in the porous medium. They also conclude the need for multiple means of measurement as critical for understanding hydrate behaviour during hydrate formation and decomposition. Tang et al. [33] studied the production behaviour of gas hydrate under thermal stimulation in unconsolidated segment. They reported that the gas production rate increased with time until it reaches a maximum, then it begins to decrease. However, hydrate content for all the experiments that were reported was less than 18.0 volume%.    The objective of this work is to study the kinetics of hydrate formation and decomposition of methane hydrates in the presence of silica sand. Key information like the gas uptake and gas recovery curves along with the percent conversion of water to hydrates and methane recovery rates are presented.   EXPERIMENTAL SECTION: Methane used for the present study was supplied by Praxair (UHP grade). The silica sand was supplied by Sigma Aldrich. Water used for the experiments was distilled and deionised. The silica sand has an average diameter of 329 ?m (diameter ranges from 150 to 630 ?m). BET surface area analysis of the sand showed that the silica sand used in this work is micro porous with a pore volume of 0.000152 cm3/g and a pore diameter of 0.90 nm.     Figure 1. Schematic of the apparatus Temperature controlled water bath  CR   R CR     ? Crystallizer       ER ? External Refrigerator R       ? Reservoir       CV ? Control Valve  DAQ  ? Data Acquisition  P1 & P2 ? Pressure transmitters T        ? Thermocouple CV  DAQ & PC G A S  S U P P L Y P1 P2    E R  T to vent A schematic of the apparatus is given in Figure 1. It consists of a crystallizer (CR) with a volume of 1236.05 cm3. The experimental set up also consists of a reservoir (R), which is used to collect the decomposed gas during hydrate decomposition. Both CR and R are immersed in a water bath. The temperature of the water bath is controlled by an external refrigerator. The apparatus is instrumented with two pressure transmitters and several thermocouples connected to a data acquisition system to record the pressure and temperature data. The apparatus also consists of a control valve coupled with a PID controller in order to decompose the hydrates at constant pressure. Six thermocouples are located inside the silica sand bed.   Hydrate formation procedure: The amount of silica sand placed in the crystallizer for each experiment is 914.1g (Height of silica bed = 7 cm). 198.5 ml of water is added into the sand. Once the crystallizer bed is setup the thermocouples are positioned and then the crystallizer is closed. The pressure in the crystallizer is then set to the desired experimental pressure (8.0 MPa) and the temperature is allowed to reach the experimental temperature. This is time zero for the formation experiment. Data is then logged in the computer for every 20 seconds. All hydrate formation experiments are carried out with a fixed amount of water and gas (closed system). The temperature in the crystallizer is maintained constant by an external refrigerator. When hydrate formation occurs, gas will be consumed and hence the pressure in the closed system drops. The experiment is allowed to continue until there is no significant change in the crystallizer pressure.  Pressure and temperature data are used to calculate the moles of methane consumed in the crystallizer (gas uptake) for hydrate formation. At any given time, the total number of moles (nT,t) in the system (crystallizer and the connecting tubing) remains constant and equal to that at time zero (nT,0). The total number of moles at any given time is the sum of the number of moles (nG) in gas phase (G) of the crystallizer and the number of moles (nH) in the hydrate phase. Thus, the number of moles of the gas that has been consumed for hydrate formation at time t = t is given by the following equation. H,t H,0 G,0 G,tn-n =n-n   (1) or H,t H,0G,0 G,tPV PV? n=n-n= -zRT zRT?????????????(2) where  H, ? n ?  is the number of moles consumed at any given time for hydrate formation, z is the compressibility factor calculated by Pitzer?s correlation [34] and G denotes the gas phase of the crystallizer.  Hydrate decomposition procedure: After the completion of hydrate formation experiment, the hydrates are decomposed at a constant pressure as follows. The pressure in the crystallizer is decreased to the desired pressure (20% above the equilibrium hydrate formation pressure). The temperature in the crystallizer is then allowed to become stable. This takes less then about 10 min. ? he temperature of the crystallizer is then increased to the desired value by heating the water bath with an external refrigerator/heater. This is time zero for the decomposition experiment. During the experiment, when the temperature of the crystallizer crosses the equilibrium phase boundary, hydrate starts to decompose and since the pressure in the crystallizer is maintained constant by the PID controller, the excess gas is released from the crystallizer and collected in the reservoir (R). The experiment proceeds until there is no further release of methane gas from the silica sand.  At any given time, the total number of moles (nT,t) in the system remains constant and equal to that at time zero (nT,0). The system in this case includes the crystallizer (CR), the reservoir (R) and the connecting tubing. The total number of moles at any given time is the sum of the number of moles (nG) in gas phase (G) of the crystallizer, the number of moles (nR) collected in the reservoir and the number of moles (nH) in the hydrate phase.  The number of moles of gas released from the hydrate at any time during hydrate decomposition can then be calculated as follows, H,0 H,t G,t G,0 R,t R,0n-n=n-n+n-n             (3) or H, G,t G,0 R,t R,0PV PV PV PV? n= - + -zRT zRT zRT zRT?????????????????????????(4)  The percent methane recovery is calculated for the decomposition experiment based on information obtained from its formation experiment and is calculated by the following equation.  H, H, (? n)% methane recovery 100(? n)??=?       (5) Where  H, ? is the number of moles of methane consumed for hydrate formation at the end of a typical formation experiment and  H, ? Results and discussion: Formation experiments were carried out at three different temperatures of 7.0, 4.0 and 1.0? C, respectively. The initial experimental pressure for all the experiments was 8.0 MPa. The experiment was allowed to continue until there was no further gas consumption for hydrate formation. This is indicated by a near-zero rate of pressure drop as seen in Figure 2 for a typical formation experiment. Time (hr)0 102030405060Rate of Pressure drop (kPa/hr)020406080Formation experiment (7.0 C, Pstart = 8.0 MPa)  Figure 2. Rate of pressure drop due to hydrate growth during hydrate formation  The number of moles consumed for hydrate formation is determined using equation 2 for the formation experiments. Figure 3 shows the gas uptake measurement curve for a formation experiment carried out at 7.0? C along with the temperature profiles of the thermocouples located inside the bed. Hydrate formation is an exothermic crystallization process. Hence during hydrate formation, heat is released which can be seen in the figure at 207 min. This point during hydrate formation is called the nucleation point or turbidity point. The temperature of the system is gradually restored to its set point due to the constant temperature bath controlled by an external refrigerator. The nucleation point is shown as an expanded graph in Figure 3.  Time (min)0 50 100 150 200 250 300Temperature (C)56789Moles consumed0.000.020.040.060.08T1T2T3T4T5T60 1000 2000 3000 4000 5000 6000Temperature (C)56789Moles consumed0.000.050.100.150.200.250.30T1T2T3T4T5T6 Figure 3. Gas uptake measurement curve at 7.0? C.  Figure 4 shows the gas uptake measurement curve for the formation experiment carried out at 1.0? C. It can be seen that the temperature profiles show several nucleation regions occurring during the formation experiment. The first temperature increase at the nucleation point can be seen in section A in the figure. In section B, the temperature increase is not localized and there is a larger consumption of gas for hydrate formation during that region. Overall, as can be seen in figure 4, hydrate formation does not proceed at a constant rate. Similar observation was made by Kneafsey et al.,[17] when hydrates were formed at 6.2 MPa and 1.1? C in the presence of silica sand with a particle size of 100 to 200 ?m. It can also be seen from figure 3 and 4 that the moles consumed for hydrate formation is higher for the experiment carried out at 1.0? C compared to the one conducted at 7.0? C. A similar trend was observed for the experiment carried out at 4.0? C. This is because multiple nucleation points were observed for formation experiments carried out at 4.0 and 1.0? C resulting in a significantly higher methane uptake.  Time (hr)0 102030405060Temperature (C)0123Moles consumed0.00.20.40.60.81.01.21.41.6T1T2T3T4T5T6A B Figure 4. Gas uptake measurement curve at 1.0? C  The conversion of water to hydrate during the formation experiment was determined using the gas uptake information. The hydration number for methane/water system was assumed as 6.1 for our calculations [35]. The conversion of water to hydrate was 11.0, 78.5 and 79.8 mol% for the experiments carried out at 7.0, 4.0 and 1.0? C, respectively. Experiments at 4.0? C and 1.0? C achieve the same level of water to hydrate conversion. However, there is a distinct difference in water to hydrate conversion percentages between 7.0? C and 4.0? C (i.e.11.0% vs. 78.5%).   After the end of each formation experiment, the hydrates are decomposed using thermal stimulation at a constant pressure. Decomposition experiments were carried out at constant pressure of 6.2, 4.6 and 3.5 MPa for the formation experiments carried out at 7.0, 4.0 and 1.0? C, respectively. The number of moles of methane released from hydrates is calculated using equation 4. Figure 5 shows the typical gas recovery curve obtained from the decomposition experiment with temperature as the driving force (? T= 4.0? C) at a constant pressure of 3.5 MPa. As it can be seen in the figure, there is no gas release till the temperature crosses the phase boundary. For this experiment, when the temperature crosses 3.0? C, hydrate starts to decompose and since the pressure in the crystallizer is maintained constant, the gas released is collected in the reservoir (R). Time (hr)048112Temperature (C)012345678Moles released0.00.30.60.91.21.5T1T2T3T4T5T6 Figure 5. Typical gas release measurement curve carried out at 3.5 MPa and a temperature driving force (? T) of 4.0? C . Time (hr)0 5 10 15 20Methane released from hydrates (mol)0.00.20.40.60.81.01.21.41.6Stage 1 Stage 2 Rate=0.15mol/hr Rate=0.037mol/hr Figure 6. Methane recovery from hydrate (Pexp = 3.5 MPa, ? T= 4.0? C)  Figure 6 shows the methane gas recovery as a function of time. Time zero in this figure is the time when methane is released from the hydrate during the decomposition experiment. As it can be seen in the figure, methane recovery occurs in two stages (Stage 1 & Stage 2). The recovery rates were calculated for the stages and are given in the figure. As expected the recovery rates are higher in stage 1 compared to stage 2 because the hydrate zone is being depleted of methane gas during the decomposition experiment. It is noted that the amount of methane is fixed (closed system). The percentage recovery of methane from hydrates for the decomposition experiments was determined using equation 5 and found to be 96.0, 97.6 and 98.1 for the experiments carried out at 8.2, 4.6 and 3.5 MPa, respectively.  CONCLUSIONS: A new apparatus was set up to study methane hydrate formation and decomposition behavior in silica sand. Hydrate formation experiments were studied at there different temperatures of 7.0, 4.0 & 1.0? C, respectively. Multiple nucleation points were observed for formation experiments carried out at 4.0 and 1.0? C resulting in a significantly higher methane uptake. This resulted in achieving a higher percent conversion of water to hydrates in the range of 78 to 80%. Decomposition experimental procedure adopting thermal stimulation approach was carried out for recovering methane from hydrates. Methane recovery occurred in two stages during the decomposition experiments. Methane recovery in the range of 95 to 98% was achieved.  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