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Modelling on Gas Hydrate Kinetics in Presence of Saline Water in Porous Media
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 21682–21689
www.materialstoday.com/proceedings
The 3rd International Conference on Green Chemical Engineering Technology (3rd GCET_2017): Materials Science
Modelling on Gas Hydrate Kinetics in Presence of Saline Water in Porous Media Mazlin Idress*, Mazuin Jasamai, Muhammad Syimir Afandi Petroleum Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610 Perak, Malaysia
Abstract The effect of salinity towards methane hydrate formation and induction time was modelled using CMG STARS. Salinity greatly influenced gas hydrate formation and dissociation due to its ability to act as an inhibitor by delaying the nucleation of methane hydrate. However, most of the developed kinetics model did not take into consideration of salts in porous media during the simulation. In this research, the objectives are to simulate and investigate the induction time of methane hydrate formation using Clarke and Bishnoi kinetic model in the presence of saline water. The simulation is conducted using different salinity concentration of 0 ppm, 5000 ppm, 15000 ppm, 25000 ppm,35000 ppm, and 40000 ppm. The presence of salts in porous media increases the induction time from 464 minutes to 515 minutes. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The 3rd International Conference on Green Chemical Engineering and Technology (3rd GCET): Materials Science, 07-08 November 2017. Keywords: salinity; methane hydrate;induction time;CMG STAR
1. Introduction Clathrate hydrates or in other term known as natural gas are mainly composed of cages bonded by the host (water) molecules and guest (gas) molecules by the Van der Waals London forces. A stable clathrate hydrate will only be formed when the size of the guest molecules and the cage space match each other. This natural gas hydrate is found
* Corresponding author. Tel.: +605-3688000 ; fax: +605-3654088 E-mail address: [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The 3rd International Conference on Green Chemical Engineering and Technology (3rd GCET): Materials Science, 07-08 November 2017.
Idress et al. / Materials Today: Proceedings 5 (2018) 21682–21689
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out mostly in marine sediments, discovered in the oil and gas pipeline as well as permafrost region. [1]. Methane (CH4) hydrates exist in large amount beneath the ocean. Methane hydrates is an ice-like substance that trap the guest molecules within. Basically, hexagonal structure (sH), cubic structure I (sI) and structure II (sII) are the three common structures of the gas hydrates whereby typically methane hydrate forms sI hydrate, forming two types of cages; small and large to occlude the gas molecules. At low temperature and high pressure condition, methane hydrate will be stable [1][2]. Generally, CH4 can be produced from natural gas hydrate through depressurization, inhibitor injection, thermal stimulation or combination of techniques. The local pressure and temperature conditions are changing through depressurization and thermal stimulation process where the rising of the temperature and declining of the pressure change the stability of CH4 hydrates. Inhibitor injection introduces the injection of substances to reduce the stability of the hydrate under natural conditions so that the gas will be produced out of the gas hydrate. Majority of the literature had been found to be in bulk with the presence of inhibitor for both formation and dissociation of gas hydrates, while studies on the effect of inhibitor in porous media are very limited. Salt acts as a thermodynamic inhibitor of the formation and dissociation of gas hydrate. Based on the experiment conducted at constant pressure and temperature condition, the rate of formation in seawater experiments was quite low from the initial induction point until the end of the experiment [3]. The kinetics of the CH4 hydrate formation was affected by the existence of salts in porous media resulting in the reduction of CH4 hydrate formation and also changing the gas uptake curve where the hydrates were dissociated at a slow rate in seawater than that of pure water. Research on the effect of salinity in the porous media towards CH4 hydrate formation is still in the experimental stage whereby the effects of different salts present in the seawater towards the formation of gas hydrates are still not extensively studied upon. In fact, most of the developed kinetic models do not consider the salinity effect in porous media when running the simulation that has the possibility to alter the rate of formation of gas hydrates. Thus, research on hydrate production with saline water should be carried out to determine the impacts and consequences during formation of hydrate. The focus of this paper is to evaluate the effect of the salinity towards the CH4 hydrate formation in the porous media. The period when the first nucleation occurs or the induction time of CH4 hydrate formation is observed for different salinity concentrations. The CH4 hydrate formation curve is generated in order to identify the reaction of saline water towards the pressure and temperature of the hydrates. 2. Material and Methods 2.1. Kinetic model of hydrate formation In porous media, the hydrate formation in equilibrium condition can be expressed as ∆μ RTO
+
MT LO
P ∆ P RT
dp −
MT LO
T ∆ TO RT
dp − ∑ v ln 1 + ∑ c φ y p
=
RT
cos θ.
(1)
where the wetting angle, average pore size and surface energy are required for the determination of pressure in vapour phase. Reference [4] discovered that he kinetic reaction of methane hydrate formation could be described as r
= kA f − f
(2)
The reaction surface area, As is classified as the most complicated parameter to be determined among others due to its difficulty to observe the formation of methane hydrates inside the pores. Therefore, As will be determined based on several approaches and assumptions that caused the model to be unique [4-6]. The calculation of the reaction surface area, As can be performed in many ways depending upon the nucleation process of hydrates within the pore space including formation at the pore centres or coating the pore surface [7]. In CMG STARS, As is defined as follow:
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Idress et al. / Materials Today: Proceedings 5 (2018) 21682–21689
A = Φ A S S
(3)
Based on the Arrhenius-type equation [4][8], the kinetic constant, k is expressed as = k exp
∆E
(4)
RT
By substituting the Eq.(3) and Eq.(4) into Eq.(2), the reaction formation equation of hydrate is developed and result in following equation: r
= k exp
∆E RT
Φ A S S
f − f
(5)
The coefficient of fugacity terms, both fg and feq is equal to 1 and being converted to the equivalent pressures. On the other hand, such conversion will result in some errors during the simulation process since it is just an estimation [7]. r
= k exp
r
= k exp
∆E RT ∆E RT
Φ A S S
P − P
Φ A S S
P
1−
(6) P
(7)
P
Peq and Pg are defined as the equilibrium pressure and the gas pressure respectively where the partial equilibrium K-value is the inverse ratio of Peq/Pg that can be written as r
= k exp
∆E RT
Φ A S S
P
1−
(8)
K
From the laboratory data, K-value will be acquired from the three phase equilibrium of Peq vs T or obtained from the following correlation built in CMG STARS K=
KV P
+ KV2 P + KV3 exp
KV
.
T KV
(9)
In CMG STARS, values of KV1, KV2, KV3, KV4, and KV5 are defined within a wide range of pressure and temperature for both methane and water. The vapour pressure of pure component is equal to the partial vapour pressure of liquid of an ideal mixture as stated by the Raoult’s Law multiplied by the mole fraction of the mixture. P
=
P
(10)
Eq.(10) is developed by denoting the partial pressure as yiPg in the gas phase in which xi is assumed as 1 in the three phase system consisting of liquid water, vapour and hydrate. r
=
H
=
K A
ΦS ρ
ΦS ρ
yP
1−
K
exp −
∆E RT
(11)
In this project, the kinetic reaction model to be used for the simulation process is written as (11) by substituting the Eq.(10) into Eq.(8). 2.2. Simulation of hydrate using CMG STARS The system used in this simulation is a homogenous system (Fig. 1) where the bulk volume of the Tubular Hydrate Cell has been upscaled to the bulk volume of the simulation model from 2014.45 m3 to 375000 m3
Idress et al. / Materials Today: Proceedings 5 (2018) 21682–21689
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respectively. All parameters used to describe the methane hydrate and porous media in this simulation are tabulated in Table 1 and Table 2. In order to ensure that the results obtained from this simulation model are relevant when being compared to the lab data, these parameters are acquired from the experiment of the methane hydrate formation that is conducted in lab and therefore the accuracy of the simulation process can be verified.
Fig. 1. Simulation model developed for methane hydrate formation in porous media.
Eq. (11) is being discretized according to grid blocks when the simulation is running and the outputs especially the temperature and pressure distribution are determined over a period of time. The grid blocks of 50 x 20 x 10 are used for X, Y and Z direction respectively, describing the simulated porous media by using the Cartesian coordinate system where each block has the dimension of 2.5m x 5m x 3m. The experiment about the formation of gas hydrate using the Tubular Hydrate Cell is being simulated by placing a single producing well at the grid block with coordinate of (1, 10, 8). Table 1. Properties of porous media and methane hydrate used for simulation using CMG STARS. Parameter
Value
Initial Pressure (kPa)
8000
Initial Temperature (°C) 3
3
Bulk Volume, Vbulk (m )
375 000
Horizontal Permeability, Kh (mD)
100
Vertical Permeability, Kv (mD)
100
Water Saturation, Sw
0.35
Gas Saturation, Sg
0.65
Porosity, Ø
0.40
Hydrate Phase
Solid
Density of hydrate, ρh (kg/ml)
0.919x103
Activation energy of hydrate, EAH (J/gmole)
89 660.025
Reaction Enthalpy (J/mol)
51 857.936
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Idress et al. / Materials Today: Proceedings 5 (2018) 21682–21689
Table 2. Salinity of water used for simulation using CMG STARS. Salinity (ppm)
Type of water
NaCl (wt.%)
0
Fresh water
0.0
5000
Brackish water
0.5
15 000
Moderate Brackish water
1.5
25 000
Heavy Brackish water
2.5
35 000
Typical seawater
3.5
40 000
Mediterranean sea
4.0
3. Results and Discussion In this study, P-T diagram versus Time was generated to observe the behaviour of the methane hydrates during the formation process with and without saline water. The base model is used when running the simulation and then being manipulated with different salinity of water of 0 ppm, 5000 ppm, 15 000 ppm, 25 000 ppm, 35 000 ppm, and 40 000 ppm. 3.1. Pressure and Temperature Profile Initially, the pressure of 8000 kPa is distributed in the porous media throughout the simulation process. From the figures above, it shows that the pressure is declining with same pattern for 0 ppm, 5000 ppm, 15 000 ppm, 25 000 ppm, 35 000 ppm and 40 000 ppm, which indicate the formation reaction is reaching its stabilize condition. From this simulation, it is observed that the pressure drop obtained is not that high when the salinity of water is increased as mention in Fig. 2.
Fig. 2. Pressure - Temperature profile for different salinity.
Based on the P-T profile, the pressure of the system is declining simultaneously with the increase in temperature in the beginning of the simulation with respect to time and stop until it reach the stabilization condition. At the
Idress et al. / Materials Today: Proceedings 5 (2018) 21682–21689
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temperature profile, the sudden spikes or the peaks indicates the onset of hydrate formation or the starting of the CH4 hydrates nucleation in the porous media due to the exothermic process. When water reacts with CH4 gas, heat will be released during the growth of hydrates throughout the process resulting in increased in temperature of the system. The formation of CH4 hydrates is completed when the pressure drop is negligible. In the simulation, the formation pressure and temperature were observed to be constant for simulation with different salinity (Fig. 3). CH4 hydrate was formed at pressure of 6 MPa and temperature of 275.78 K. Therefore, the salinity effect does not show significant impact towards the formation pressure and temperature of CH4 hydrate. In saline water, the pressure is declining before the formation of hydrate takes place resulted from the interaction between the CH4 molecule and the dissolved ions, hence reduces the solubility of CH4.
Fig. 3 CH4 hydrates formation curve
According to the CH4 hydrates formation curve above, increased in concentration of salt in the water will not shift the equilibrium conditions of CH4 hydrates to the lower temperature and higher pressure region. The simulation shows that the CH4 hydrates are formed at formation pressure of 6 MPa and at formation temperature of 275.78 K for different salinity of water. 3.2 Induction Time For the induction time, even though the formation pressure and temperature are not affected by the salinity (Table 3), it shows that as the salinity increases, the induction time will be longer (Fig. 4). Table 3. Induction time at different salinity. Salinity (ppm)
Induction Time (min)
0
464
5000
467
15 000
470
25 000
477
35 000
484
40 000
515
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Idress et al. / Materials Today: Proceedings 5 (2018) 21682–21689
Fig. 4 Induction time versus salinity.
When saline water is presence in the system, the dipoles of water molecules will interact with the ions from the electrolytes bonded by the Columbic forces, which stronger than the Van der Waals forces and the hydrogen bond and therefore water molecules are strongly attracted to the electrolytes ions rather than the hydrates structure. Increasing the salinity concentration actually will make the Columbic forces to become stronger. The presence of NaCl in the porous media kinetically inhibits the formation of hydrates resulting in the induction time to be longer. This phenomenon is the consequence of the existence of salt that increased the barrier during the process of CH4 hydrates nucleation and alters the lattice structure of the gas hydrates. 4. Conclusion Based on the simulation conducted using the kinetic reaction of CH4 hydrate formation for Clarke and Bishnoi model, the presence of saline water significantly affects the kinetic of CH4 hydrates formation at constant formation pressure and temperature described by the induction time. Increasing salinity delayed the formation of methane hydrate from 464 minutes at 0 ppm to 515 minutes at 40 000 ppm. References [1] Z. Duan, D. Li, Y. Chen, and R. Sun, “The influence of temperature, pressure, salinity and capillary force on the formation of methane hydrate,” Geoscience Frontiers, vol. 2, no. 2, pp. 125-135, 2011. [2] A. Demirbas, “Methane hydrates as potential energy resource: Part 1 – Importance, resource and recovery facilities,” Energy Conversion and Management, vol. 51, no. 7, pp. 1547-1561, 7//, 2010. [3] P. Mekala, P. Babu, J. S. Sangwai, and P. Linga, “Formation and Dissociation Kinetics of Methane Hydrates in Seawater and Silica Sand,” Energy & Fuels, vol. 28, no. 4, pp. 2708-2716, 2014. [4] M. Clarke, and P. R. Bishnoi, “Determination of the intrinsic rate of ethane gas hydrate decomposition,” Chemical Engineering Science vol. 55, no. 21, pp. 4869 - 4883, 2000. [5] X. Sun, and K. K. Mohanty, “Kinetic simulation of methane hydrate formation and dissociation in porous media,” Chemical Engineering Science, vol. 61, no. 11, pp. 3476-3495, 2006. [6] B. Li, X.-S. Li, and G. Li, “Kinetic studies of methane hydrate formation in porous media based on experiments in a pilot-scale hydrate simulator and a new model,” Chemical Engineering Science, vol. 105, pp. 220-230, 2014. [7] CMG, “STARS User Manual, Version 2007.10. Calgary, Alberta: Computer Modelling Group,” 2007. [8] H. C. Kim, P. R. Bishnoi, R. A. Heidemann, and S. S. H. Rizvi, “Kinetics of methane hydrate decomposition,” vol. 42 no. 7, pp. 1645-1653, 1987. [9] J. Carroll, Natural Gas Hydrates: A Guide for Engineers, 3, revised ed., p.^pp. 340: Gulf Professional Publishing, 2014. [10] Z. R. Chong, A. H. M. Chan, P. Babu, M. Yang, and P. Linga, “Effect of NaCl on methane hydrate formation and dissociation in porous media,” Journal of Natural Gas Science and Engineering, vol. 27, pp. 178-189, 2015. [11] Z. R. Chong, G. A. Pujar, M. Yang, and P. Linga, “Methane hydrate formation in excess water simulating marine locations and the impact of thermal stimulation on energy recovery,” Applied Energy, vol. 177, pp. 409-421, 2016.
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[12] X. Sun, and K. K. Mohanty, “Kinetic simulation of methane hydrate formation and dissociation in porous media,” Chemical Engineering Science, vol. 61, no. 11, pp. 3476-3495, 2006. [13] Y. Ye, and C. Liu, Natural Gas Hydrates: Experimental Techniques and Their Applications, illustrated ed.: Springer Berlin Heidelberg, 2012. [14] Y. Mingjun, S. Yongchen, L. Yu, C. Yongjun, and L. Qingping, “Influence of Pore Size, Salinity and Gas Composition upon the Hydrate Formation Conditions,” vol. 18, no. 2, pp. 292-296, 2010. [15] B. Li, X.-S. Li, and G. Li, “Kinetic studies of methane hydrate formation in porous media based on experiments in a pilot-scale hydrate simulator and a new model,” Chemical Engineering Science, vol. 105, pp. 220-230, 2014.
ScienceDirect Materials Today: Proceedings 5 (2018) 21682–21689
www.materialstoday.com/proceedings
The 3rd International Conference on Green Chemical Engineering Technology (3rd GCET_2017): Materials Science
Modelling on Gas Hydrate Kinetics in Presence of Saline Water in Porous Media Mazlin Idress*, Mazuin Jasamai, Muhammad Syimir Afandi Petroleum Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610 Perak, Malaysia
Abstract The effect of salinity towards methane hydrate formation and induction time was modelled using CMG STARS. Salinity greatly influenced gas hydrate formation and dissociation due to its ability to act as an inhibitor by delaying the nucleation of methane hydrate. However, most of the developed kinetics model did not take into consideration of salts in porous media during the simulation. In this research, the objectives are to simulate and investigate the induction time of methane hydrate formation using Clarke and Bishnoi kinetic model in the presence of saline water. The simulation is conducted using different salinity concentration of 0 ppm, 5000 ppm, 15000 ppm, 25000 ppm,35000 ppm, and 40000 ppm. The presence of salts in porous media increases the induction time from 464 minutes to 515 minutes. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The 3rd International Conference on Green Chemical Engineering and Technology (3rd GCET): Materials Science, 07-08 November 2017. Keywords: salinity; methane hydrate;induction time;CMG STAR
1. Introduction Clathrate hydrates or in other term known as natural gas are mainly composed of cages bonded by the host (water) molecules and guest (gas) molecules by the Van der Waals London forces. A stable clathrate hydrate will only be formed when the size of the guest molecules and the cage space match each other. This natural gas hydrate is found
* Corresponding author. Tel.: +605-3688000 ; fax: +605-3654088 E-mail address: [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The 3rd International Conference on Green Chemical Engineering and Technology (3rd GCET): Materials Science, 07-08 November 2017.
Idress et al. / Materials Today: Proceedings 5 (2018) 21682–21689
21683
out mostly in marine sediments, discovered in the oil and gas pipeline as well as permafrost region. [1]. Methane (CH4) hydrates exist in large amount beneath the ocean. Methane hydrates is an ice-like substance that trap the guest molecules within. Basically, hexagonal structure (sH), cubic structure I (sI) and structure II (sII) are the three common structures of the gas hydrates whereby typically methane hydrate forms sI hydrate, forming two types of cages; small and large to occlude the gas molecules. At low temperature and high pressure condition, methane hydrate will be stable [1][2]. Generally, CH4 can be produced from natural gas hydrate through depressurization, inhibitor injection, thermal stimulation or combination of techniques. The local pressure and temperature conditions are changing through depressurization and thermal stimulation process where the rising of the temperature and declining of the pressure change the stability of CH4 hydrates. Inhibitor injection introduces the injection of substances to reduce the stability of the hydrate under natural conditions so that the gas will be produced out of the gas hydrate. Majority of the literature had been found to be in bulk with the presence of inhibitor for both formation and dissociation of gas hydrates, while studies on the effect of inhibitor in porous media are very limited. Salt acts as a thermodynamic inhibitor of the formation and dissociation of gas hydrate. Based on the experiment conducted at constant pressure and temperature condition, the rate of formation in seawater experiments was quite low from the initial induction point until the end of the experiment [3]. The kinetics of the CH4 hydrate formation was affected by the existence of salts in porous media resulting in the reduction of CH4 hydrate formation and also changing the gas uptake curve where the hydrates were dissociated at a slow rate in seawater than that of pure water. Research on the effect of salinity in the porous media towards CH4 hydrate formation is still in the experimental stage whereby the effects of different salts present in the seawater towards the formation of gas hydrates are still not extensively studied upon. In fact, most of the developed kinetic models do not consider the salinity effect in porous media when running the simulation that has the possibility to alter the rate of formation of gas hydrates. Thus, research on hydrate production with saline water should be carried out to determine the impacts and consequences during formation of hydrate. The focus of this paper is to evaluate the effect of the salinity towards the CH4 hydrate formation in the porous media. The period when the first nucleation occurs or the induction time of CH4 hydrate formation is observed for different salinity concentrations. The CH4 hydrate formation curve is generated in order to identify the reaction of saline water towards the pressure and temperature of the hydrates. 2. Material and Methods 2.1. Kinetic model of hydrate formation In porous media, the hydrate formation in equilibrium condition can be expressed as ∆μ RTO
+
MT LO
P ∆ P RT
dp −
MT LO
T ∆ TO RT
dp − ∑ v ln 1 + ∑ c φ y p
=
RT
cos θ.
(1)
where the wetting angle, average pore size and surface energy are required for the determination of pressure in vapour phase. Reference [4] discovered that he kinetic reaction of methane hydrate formation could be described as r
= kA f − f
(2)
The reaction surface area, As is classified as the most complicated parameter to be determined among others due to its difficulty to observe the formation of methane hydrates inside the pores. Therefore, As will be determined based on several approaches and assumptions that caused the model to be unique [4-6]. The calculation of the reaction surface area, As can be performed in many ways depending upon the nucleation process of hydrates within the pore space including formation at the pore centres or coating the pore surface [7]. In CMG STARS, As is defined as follow:
21684
Idress et al. / Materials Today: Proceedings 5 (2018) 21682–21689
A = Φ A S S
(3)
Based on the Arrhenius-type equation [4][8], the kinetic constant, k is expressed as = k exp
∆E
(4)
RT
By substituting the Eq.(3) and Eq.(4) into Eq.(2), the reaction formation equation of hydrate is developed and result in following equation: r
= k exp
∆E RT
Φ A S S
f − f
(5)
The coefficient of fugacity terms, both fg and feq is equal to 1 and being converted to the equivalent pressures. On the other hand, such conversion will result in some errors during the simulation process since it is just an estimation [7]. r
= k exp
r
= k exp
∆E RT ∆E RT
Φ A S S
P − P
Φ A S S
P
1−
(6) P
(7)
P
Peq and Pg are defined as the equilibrium pressure and the gas pressure respectively where the partial equilibrium K-value is the inverse ratio of Peq/Pg that can be written as r
= k exp
∆E RT
Φ A S S
P
1−
(8)
K
From the laboratory data, K-value will be acquired from the three phase equilibrium of Peq vs T or obtained from the following correlation built in CMG STARS K=
KV P
+ KV2 P + KV3 exp
KV
.
T KV
(9)
In CMG STARS, values of KV1, KV2, KV3, KV4, and KV5 are defined within a wide range of pressure and temperature for both methane and water. The vapour pressure of pure component is equal to the partial vapour pressure of liquid of an ideal mixture as stated by the Raoult’s Law multiplied by the mole fraction of the mixture. P
=
P
(10)
Eq.(10) is developed by denoting the partial pressure as yiPg in the gas phase in which xi is assumed as 1 in the three phase system consisting of liquid water, vapour and hydrate. r
=
H
=
K A
ΦS ρ
ΦS ρ
yP
1−
K
exp −
∆E RT
(11)
In this project, the kinetic reaction model to be used for the simulation process is written as (11) by substituting the Eq.(10) into Eq.(8). 2.2. Simulation of hydrate using CMG STARS The system used in this simulation is a homogenous system (Fig. 1) where the bulk volume of the Tubular Hydrate Cell has been upscaled to the bulk volume of the simulation model from 2014.45 m3 to 375000 m3
Idress et al. / Materials Today: Proceedings 5 (2018) 21682–21689
21685
respectively. All parameters used to describe the methane hydrate and porous media in this simulation are tabulated in Table 1 and Table 2. In order to ensure that the results obtained from this simulation model are relevant when being compared to the lab data, these parameters are acquired from the experiment of the methane hydrate formation that is conducted in lab and therefore the accuracy of the simulation process can be verified.
Fig. 1. Simulation model developed for methane hydrate formation in porous media.
Eq. (11) is being discretized according to grid blocks when the simulation is running and the outputs especially the temperature and pressure distribution are determined over a period of time. The grid blocks of 50 x 20 x 10 are used for X, Y and Z direction respectively, describing the simulated porous media by using the Cartesian coordinate system where each block has the dimension of 2.5m x 5m x 3m. The experiment about the formation of gas hydrate using the Tubular Hydrate Cell is being simulated by placing a single producing well at the grid block with coordinate of (1, 10, 8). Table 1. Properties of porous media and methane hydrate used for simulation using CMG STARS. Parameter
Value
Initial Pressure (kPa)
8000
Initial Temperature (°C) 3
3
Bulk Volume, Vbulk (m )
375 000
Horizontal Permeability, Kh (mD)
100
Vertical Permeability, Kv (mD)
100
Water Saturation, Sw
0.35
Gas Saturation, Sg
0.65
Porosity, Ø
0.40
Hydrate Phase
Solid
Density of hydrate, ρh (kg/ml)
0.919x103
Activation energy of hydrate, EAH (J/gmole)
89 660.025
Reaction Enthalpy (J/mol)
51 857.936
21686
Idress et al. / Materials Today: Proceedings 5 (2018) 21682–21689
Table 2. Salinity of water used for simulation using CMG STARS. Salinity (ppm)
Type of water
NaCl (wt.%)
0
Fresh water
0.0
5000
Brackish water
0.5
15 000
Moderate Brackish water
1.5
25 000
Heavy Brackish water
2.5
35 000
Typical seawater
3.5
40 000
Mediterranean sea
4.0
3. Results and Discussion In this study, P-T diagram versus Time was generated to observe the behaviour of the methane hydrates during the formation process with and without saline water. The base model is used when running the simulation and then being manipulated with different salinity of water of 0 ppm, 5000 ppm, 15 000 ppm, 25 000 ppm, 35 000 ppm, and 40 000 ppm. 3.1. Pressure and Temperature Profile Initially, the pressure of 8000 kPa is distributed in the porous media throughout the simulation process. From the figures above, it shows that the pressure is declining with same pattern for 0 ppm, 5000 ppm, 15 000 ppm, 25 000 ppm, 35 000 ppm and 40 000 ppm, which indicate the formation reaction is reaching its stabilize condition. From this simulation, it is observed that the pressure drop obtained is not that high when the salinity of water is increased as mention in Fig. 2.
Fig. 2. Pressure - Temperature profile for different salinity.
Based on the P-T profile, the pressure of the system is declining simultaneously with the increase in temperature in the beginning of the simulation with respect to time and stop until it reach the stabilization condition. At the
Idress et al. / Materials Today: Proceedings 5 (2018) 21682–21689
21687
temperature profile, the sudden spikes or the peaks indicates the onset of hydrate formation or the starting of the CH4 hydrates nucleation in the porous media due to the exothermic process. When water reacts with CH4 gas, heat will be released during the growth of hydrates throughout the process resulting in increased in temperature of the system. The formation of CH4 hydrates is completed when the pressure drop is negligible. In the simulation, the formation pressure and temperature were observed to be constant for simulation with different salinity (Fig. 3). CH4 hydrate was formed at pressure of 6 MPa and temperature of 275.78 K. Therefore, the salinity effect does not show significant impact towards the formation pressure and temperature of CH4 hydrate. In saline water, the pressure is declining before the formation of hydrate takes place resulted from the interaction between the CH4 molecule and the dissolved ions, hence reduces the solubility of CH4.
Fig. 3 CH4 hydrates formation curve
According to the CH4 hydrates formation curve above, increased in concentration of salt in the water will not shift the equilibrium conditions of CH4 hydrates to the lower temperature and higher pressure region. The simulation shows that the CH4 hydrates are formed at formation pressure of 6 MPa and at formation temperature of 275.78 K for different salinity of water. 3.2 Induction Time For the induction time, even though the formation pressure and temperature are not affected by the salinity (Table 3), it shows that as the salinity increases, the induction time will be longer (Fig. 4). Table 3. Induction time at different salinity. Salinity (ppm)
Induction Time (min)
0
464
5000
467
15 000
470
25 000
477
35 000
484
40 000
515
21688
Idress et al. / Materials Today: Proceedings 5 (2018) 21682–21689
Fig. 4 Induction time versus salinity.
When saline water is presence in the system, the dipoles of water molecules will interact with the ions from the electrolytes bonded by the Columbic forces, which stronger than the Van der Waals forces and the hydrogen bond and therefore water molecules are strongly attracted to the electrolytes ions rather than the hydrates structure. Increasing the salinity concentration actually will make the Columbic forces to become stronger. The presence of NaCl in the porous media kinetically inhibits the formation of hydrates resulting in the induction time to be longer. This phenomenon is the consequence of the existence of salt that increased the barrier during the process of CH4 hydrates nucleation and alters the lattice structure of the gas hydrates. 4. Conclusion Based on the simulation conducted using the kinetic reaction of CH4 hydrate formation for Clarke and Bishnoi model, the presence of saline water significantly affects the kinetic of CH4 hydrates formation at constant formation pressure and temperature described by the induction time. Increasing salinity delayed the formation of methane hydrate from 464 minutes at 0 ppm to 515 minutes at 40 000 ppm. References [1] Z. Duan, D. Li, Y. Chen, and R. Sun, “The influence of temperature, pressure, salinity and capillary force on the formation of methane hydrate,” Geoscience Frontiers, vol. 2, no. 2, pp. 125-135, 2011. [2] A. Demirbas, “Methane hydrates as potential energy resource: Part 1 – Importance, resource and recovery facilities,” Energy Conversion and Management, vol. 51, no. 7, pp. 1547-1561, 7//, 2010. [3] P. Mekala, P. Babu, J. S. Sangwai, and P. Linga, “Formation and Dissociation Kinetics of Methane Hydrates in Seawater and Silica Sand,” Energy & Fuels, vol. 28, no. 4, pp. 2708-2716, 2014. [4] M. Clarke, and P. R. Bishnoi, “Determination of the intrinsic rate of ethane gas hydrate decomposition,” Chemical Engineering Science vol. 55, no. 21, pp. 4869 - 4883, 2000. [5] X. Sun, and K. K. Mohanty, “Kinetic simulation of methane hydrate formation and dissociation in porous media,” Chemical Engineering Science, vol. 61, no. 11, pp. 3476-3495, 2006. [6] B. Li, X.-S. Li, and G. Li, “Kinetic studies of methane hydrate formation in porous media based on experiments in a pilot-scale hydrate simulator and a new model,” Chemical Engineering Science, vol. 105, pp. 220-230, 2014. [7] CMG, “STARS User Manual, Version 2007.10. Calgary, Alberta: Computer Modelling Group,” 2007. [8] H. C. Kim, P. R. Bishnoi, R. A. Heidemann, and S. S. H. Rizvi, “Kinetics of methane hydrate decomposition,” vol. 42 no. 7, pp. 1645-1653, 1987. [9] J. Carroll, Natural Gas Hydrates: A Guide for Engineers, 3, revised ed., p.^pp. 340: Gulf Professional Publishing, 2014. [10] Z. R. Chong, A. H. M. Chan, P. Babu, M. Yang, and P. Linga, “Effect of NaCl on methane hydrate formation and dissociation in porous media,” Journal of Natural Gas Science and Engineering, vol. 27, pp. 178-189, 2015. [11] Z. R. Chong, G. A. Pujar, M. Yang, and P. Linga, “Methane hydrate formation in excess water simulating marine locations and the impact of thermal stimulation on energy recovery,” Applied Energy, vol. 177, pp. 409-421, 2016.
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