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Numerical Studies of Methane Gas Production from Hydrate Decomposition by Depressurization in Porous Media
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 105 (2017) 250 – 255
The 8th International Conference on Applied Energy – ICAE2016
Numerical Studies of Methane Gas Production from Hydrate Decomposition by Depressurization in porous media Minghao Yua, Weizhong Lia, Mingjun Yanga, Lanlan Jiangb, *, Yongchen Songa,* a
Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, Dalian University of Technology, Dalian 116023, China; b Research Institute of Innovative Technology for the Earth, Kizugawa City, Kyoto 619-0292, Japan
Abstract As a kind of potential new sources of energy, the dissociation processes of gas hydrates using the depressurization method has been investigated by experimental observations and numerical simulations. In this study, on the basis of summarizing the existing model, a one-dimensional mathematical model containing four phase (water phase, gas phase, hydrate phase, ice phase) and three constituents (water, gas, hydrate) using the finite difference method (FDM) was established for methane hydrates decomposition by depressurization in porous media. This paper focuses on the ice generation and distribution characteristics through changing the parameters of the relevant settings, and analyzes the effect of ice generation on the pressure, temperature, permeability, cumulative gas production and other parameters. The results show that, generation of ice increases gradually in the hydrates decomposition process, and occurred early near the side area because of the large pressure gradient. The absolute permeability and instantaneous gas generation rate at the early stage decline with ice generation, and the local pressure rise. © Published by Elsevier Ltd. This © 2017 2016The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy.
Key Words:Methane hydrate; decomposition; ice generating; Mathematic model; Numerical simulation Nomenclature ߩ௪ Ƚߩ
the density of water and methane gas
ݒ௪ ሬሬሬሬറ ሬሬሬሬറȽݒ
the velocity of water and methane gas
݉௪ Ƚ݉ Ƚ݉ the mass of water and gas per unit volume per and time, consumption mass of hydrate ܵ δ݆ ൌ ݓǡ ݃ǡ ݄ǡ ݅ε saturation for each phase
* Corresponding author. Tel.: 86-0411-84708015; fax: 86-0411-84708015. E-mail address: [email protected] and [email protected].
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.310
Minghao Yu et al. / Energy Procedia 105 (2017) 250 – 255
߶
porosity
݇ ሺ݆ ൌ ݎǡ ݓǡ ݃ǡ ݄ǡ ݅ሻ thermal conductivity ܿ ሺ݆ ൌ ݎǡ ݓǡ ݃ǡ ݄ǡ ݅ሻ specific heat at constant pressure ȟܪ
the heat absorption capacity per unit mass hydrate decomposition
ȟܪ
the latent heat of unit mass of water freeze
1. Introduction Gas hydrate is a kind of compounds, in which gas molecules is wrapped by hydrogen bonds in water molecules into the cage structure. Developing methods for production of natural gas from hydrate is attracting considerable attention. Nature gas hydrate in 20 Pa ~ 2000 MPa pressure and temperature range 70 K to 350 K can be stable. And the formation and decomposition of it depends on the pressure, temperature, gas composition, the salinity of the water, the characteristics of porous media and other factors. By increasing the system temperature above the temperature of hydrate formation and decreasing the system pressure, the decomposition of hydrate occurred[2]. Compared with the experimental research the advantage of numerical simulation is the lower cost and easy to adjust the space-time scale. Holder et al considered the variation of temperature and simulated the hydrate decomposition[3]. By using the heat transfer equation, the temperature distribution in the hydrate layer was obtained. And the gas flow and the movement velocity of the surface of decomposition were determined. It assumed that the hydrate decomposition only occurs at the interface between two adjacent reservoirs. But it did not consider hydrate decomposition of the flow of water; gas was only considered single phase flow. Based on this model, three-dimensional model was established on the top of the hydrate considering the hydrate decomposition in the process of the formation of liquid water and the impact on the gas production[4]. However, this work were simple because of the limited understanding on gas hydrate behaviour at that time.To combined with dynamic model and improve the mathematical model, one-dimensional three-phase, finite difference simulator to simulate Berea core decompression decomposition process of hydrate. In particle, the hydrate formation and decomposition coupled the interface movement and physical properties change, i.e., porosity and permeability change[5,6]. The simple models are limited to describe the processes for gas hydrate formation and decomposition. By using adopted elastic and viscoelastic model to simulate the gas hydrates decomposition, the effect on the deformation of solid skeleton was investigated. Although after efforts came from many researchers, the research in numerical simulation of natural gas hydrate decomposition characteristics had significant progress. One of which is that the ice phase was overlooked by most of the model. But ice phase is generated in the process of natural gas hydrate exploitation is real. And its effect is also need to analyze and attention. Therefore, this study consider ice phase into the mathematical model, and analyzed the influence in the hydrate decomposition by depressurization. This paper focuses on the ice generation and distribution characteristics through changing the parameters of the relevant settings, and analyzes the ice with the pressure, temperature, permeability, cumulative gas production. 2. Mathematical model 2.1. Physical model The schematic of the gas hydrate decomposition by depressurization is shown in Fig.1. Initial parameters of the reservoir respectively are pressure ܲ , temperature ܶ , absolute permeability ܭ , each
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Minghao Yu et al. / Energy Procedia 105 (2017) 250 – 255
phase saturation ܵ௪ , ܵ , ܵ , and porosity of the reservoir ߶. The lower mining well pressure ܲଵ leads to the reservoir pressure lower than the equilibrium pressure under the temperature ܶ . The hydrate in the reservoir is decomposed into water and methane gas. Under the action of pressure difference, there are into the production wells, natural gas production.
Fig.1 A schematic of the gas hydrate decomposition by depressurization According to the above the physical model, starting from the research emphasis and summarizing the existing model, a one-dimensional mathematical model for hydrate decomposition by depressurization considering ice phase is set up. The basic assumptions are as following: (1) Model considering four phase (water phase, gas phase and hydrate phase, ice phase) and three components (water, gas and hydrate); (2) Hypothesis hydrate, ice and solid skeleton are stillness, and gas and liquid two phase flow conforms to Darcy law; (3) Regardless of the hydrate second generation process; (4) Regardless of the gas, liquid seepage leakage through the cover layer, ignore the gases dissolved in the water and slippage effect; (5) Gas hydrate for methane hydrates, assumption only water and methane gas hydrate decomposition generated, not consider other hydrocarbons generated; (6) No external heat source and quality source; (7) Assume that each parameter in the reservoir isotropic; (8) Ignore the gravity; 2.2. Mathematical model ሬሬሬሬሬറ˅ డ˄ఘೢ ௩ ೢ
െቂ
డ௫
డ˄ఘ ሬሬሬሬሬറ˅ ௩
డሺథௌೢ ఘೢ ାథௌ ఘ ାథௌ ఘ ାథௌ ఘ ሻ
డ௫
డ௧
ቃ ݉௪ ݉ െ ݉ ൌ
(1)
where ߩ௪ Ƚߩ are the density of water and methane gas, ሬሬሬሬറȽݒ ݒ௪ ሬሬሬሬറ are velocity seepage of water and methane respectively, ݉௪ Ƚ݉ Ƚ݉ are the mass of water and gas by the hydrate decomposition per unit volume per unit time and hydrate consumption quality, ܵ δ݆ ൌ ݓǡ ݃ǡ ݄ǡ ݅εis each phase saturation, and ߶ is porosity. ߲ ߲ܶ ൜ൣሺͳ െ ߶ሻ݇ ߶ܵ௪ ݇௪ ߶ܵ ݇ ߶ܵ ݇ ߶ܵ ݇ ൧ ൠ ߲ݔ ߲ݔ డ െ ൣ൫ߩ௪ ሬሬሬሬറܿ ݒ௪ ௪ ߩ ሬሬሬሬറܿ ݒ ൯ܶ൧ െ ݉ ȟܪ ݉ ȟܪ డ௫ డ
ൌ ൛ൣሺͳ െ ߶ሻߩ ܿ ߶ܵ௪ ߩ௪ ܿ௪ ߶ܵ ߩ ܿ ߶ܵ ߩ ܿ ߶ܵ ߩ ܿ ൧ܶൟ(2) డ௧ where ݇ ሺ݆ ൌ ݎǡ ݓǡ ݃ǡ ݄ǡ ݅ሻ is thermal conductivity, ܿ ሺ݆ ൌ ݎǡ ݓǡ ݃ǡ ݄ǡ ݅ሻ is specific heat at constant pressure, and the subscripts ሺݎǡ ݓǡ ݃ǡ ݄ǡ ݅ሻ are respectively solid matrix, water, methane gas, hydrate and
Minghao Yu et al. / Energy Procedia 105 (2017) 250 – 255
ice. ȟܪ is the heat absorption capacity per unit mass hydrate decomposition, and ȟܪ is the latent heat of unit mass of water freeze. This study uses the finite difference method (FDM) to discrete the mass conservation equation (1). This study is aimed at natural gas hydrate production by depressurization, so the boundary temperatures and boundary pressure of mining well are set to the fixed value. For another boundary pressure settings there are two ways, which are respectively full development boundary and impermeable boundary. The calculation results under different boundary conditions are different obviously. Full development boundary is closer to nature and impermeable boundary is more suitable for laboratory conditions. The results of this study need to compare with the experimental results, so impermeable boundary is adopted. 3. Results and Discussion The ice saturation as a function of time was calculated, as shown in Fig. 2. The ice generated from the mining well and gradually extended to the far wellblock. Also the saturation of ice increased with time. Saturation increased into 0.6% during 10 min reflecting the fast decomposition around the well. The maximum saturation during decomposition was 1.27%. However, the saturation gradually increased at the location after 5 m. The reason was the greater pressure gradient around the mining well. After 60 min, the ice saturation increased along the length. With the faster decomposition, temperature in the surrounding area drop. So icing conditions in the region achieved faster.
Fig. 2. Comparison of the saturation of ice in different time
Fig. 3. The absolute permeability change with the length under included and excluded ice condition The permeability during the decomposition deceased along the length (Fig.3). The effect of ice on the permeability was investigated. Considering of the ice phase, permeability at the same location was lower that under exclude ice phase condition. As the solid phase, the ice phase formed the skeleton and the pore space reduced. It suppressed fluid flow so the permeability with ice generation got low. The permeability difference depended on the ice saturation. The permeability at the location of 5 m was 16.2 mD under include ice condition and 17.8 mD under exclude ice condition, respectively. Around the mining well, the
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Minghao Yu et al. / Energy Procedia 105 (2017) 250 – 255
difference was 2.5 mD because of the large ice saturation; however, it gradually decreased to zero even with ice existing, which mean the permeability did not change with small ice saturation. The pressure distribution during the decomposition at 3000 minutes was calculated. The ice formation had less influence on the pressure (Fig. 4). The figure shows that the pressure under the ice formation condition increased at the same location. Although the pressure increased during the decomposition process, the pressure difference that caused by the ice was little and turn to zero after about 12 mm. The maximize difference was only about 0.1 MPa so that the influence can be ignored.
Fig. 4. The pressure distribution under included and excluded ice condition at 3000 min
Fig. 5.The temperature distribution in the reservoir at different time˄300min, 3000min, 30000min˅ The distribution of temperature within the reservoir at 300 minutes, 3000 minutes, 30000 minutes were obtained. The results are shown in Fig.5. As seen in figure in reaction to 300 minutes, the temperature in the 10 m away from mining well in the reservoir kept the same to the initial reservoir temperature. Along with the mining process, temperature decreased as a whole. Before the 3000 minutes, the temperature curve decreases gradually, especially in area near mining well. It was because that, at the moment in the region, the larger pressure gradient leaded to hydrate decomposition drastically, much heat is absorbed, so the temperature declines significantly. The phenomenon was defined as pressure gradient preponderance. Along with the mining, pressure gradient decreased gradually, and the influence from hydrate dissociation on temperature weaken. Because of boundary temperature, the temperature in the reservoir near the mining well decreased at first and then increased, as the temperature curve of 30000 minutes. The phenomenon was defined as boundary temperature preponderance. 4. Conclusion This study set up mathematical model and numerical model about methane hydrate dissociation by depressurization in porous media considering the ice formation. In the process of hydrate exploration, the amount of the ice generated increases gradually over time. And the saturation of ice declined from the area near mining well to the internal reservoir. The information of ice influenced relevant parameters
Minghao Yu et al. / Energy Procedia 105 (2017) 250 – 255
during process of hydrates mining by depressurization. The absolute permeability of the reservoir decreased. But the influence on the change of pressure can be ignored. Temperature changed as the mining going to a moment. At beginning the temperature went up from the area near mining well to the internal reservoir. In the later period, the temperature in the reservoir near the mining well decreased at first and then increased. Acknowledgments This study has been supported by the National Natural Science Foundation of China (Grant No. 51506024, 51576032) References [1] Sloan ED. Fundamental principles and applications of natural gas hydrates. Nature 2003;426:353–63. [2] Makogon YF, Holditch SA, Makogon TY. Natural gas-hydrates—A potential energy source for the 21st Century. J Pet Sci Eng 2007;56:14–31. [3] Holder GD, Angert PF. Simulation of gas production from a reservoir containing both gas hydrates and free natural gas. SPE Annu. Tech. Conf. Exhib., Society of Petroleum Engineers; 1982. [4] Burshears M, O’brien TJ, Malone RD. A multi-phase, multi-dimensional, variable composition simulation of gas production from a conventional gas reservoir in contact with hydrates. SPE Unconv. Gas Technol. Symp., Society of Petroleum Engineers; 1986. [5] Yousif MH, Dorshow RB, Young DB. Testing of hydrate kinetic inhibitors using laser light scattering technique. Ann N Y Acad Sci 1994;715:330–40. [6] Masuda Y, Fujinaga Y, Naganawa S, Fujita K, Sato K, Hayashi Y. Modeling and experimental studies on dissociation of methane gas hydrates in Berea sandstone cores. 3rd Int. Conf. Gas Hydrates, Salt Lake City, Utah, vol. 7, 1999, p. 18–22.
Biography Minghao Yu is a PHD student in Dalian University of Technology. His research interest is experiment and simulation study on multi-phase dynamics during gas hydrate formation and decomposition.
255
ScienceDirect Energy Procedia 105 (2017) 250 – 255
The 8th International Conference on Applied Energy – ICAE2016
Numerical Studies of Methane Gas Production from Hydrate Decomposition by Depressurization in porous media Minghao Yua, Weizhong Lia, Mingjun Yanga, Lanlan Jiangb, *, Yongchen Songa,* a
Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, Dalian University of Technology, Dalian 116023, China; b Research Institute of Innovative Technology for the Earth, Kizugawa City, Kyoto 619-0292, Japan
Abstract As a kind of potential new sources of energy, the dissociation processes of gas hydrates using the depressurization method has been investigated by experimental observations and numerical simulations. In this study, on the basis of summarizing the existing model, a one-dimensional mathematical model containing four phase (water phase, gas phase, hydrate phase, ice phase) and three constituents (water, gas, hydrate) using the finite difference method (FDM) was established for methane hydrates decomposition by depressurization in porous media. This paper focuses on the ice generation and distribution characteristics through changing the parameters of the relevant settings, and analyzes the effect of ice generation on the pressure, temperature, permeability, cumulative gas production and other parameters. The results show that, generation of ice increases gradually in the hydrates decomposition process, and occurred early near the side area because of the large pressure gradient. The absolute permeability and instantaneous gas generation rate at the early stage decline with ice generation, and the local pressure rise. © Published by Elsevier Ltd. This © 2017 2016The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy.
Key Words:Methane hydrate; decomposition; ice generating; Mathematic model; Numerical simulation Nomenclature ߩ௪ Ƚߩ
the density of water and methane gas
ݒ௪ ሬሬሬሬറ ሬሬሬሬറȽݒ
the velocity of water and methane gas
݉௪ Ƚ݉ Ƚ݉ the mass of water and gas per unit volume per and time, consumption mass of hydrate ܵ δ݆ ൌ ݓǡ ݃ǡ ݄ǡ ݅ε saturation for each phase
* Corresponding author. Tel.: 86-0411-84708015; fax: 86-0411-84708015. E-mail address: [email protected] and [email protected].
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.310
Minghao Yu et al. / Energy Procedia 105 (2017) 250 – 255
߶
porosity
݇ ሺ݆ ൌ ݎǡ ݓǡ ݃ǡ ݄ǡ ݅ሻ thermal conductivity ܿ ሺ݆ ൌ ݎǡ ݓǡ ݃ǡ ݄ǡ ݅ሻ specific heat at constant pressure ȟܪ
the heat absorption capacity per unit mass hydrate decomposition
ȟܪ
the latent heat of unit mass of water freeze
1. Introduction Gas hydrate is a kind of compounds, in which gas molecules is wrapped by hydrogen bonds in water molecules into the cage structure. Developing methods for production of natural gas from hydrate is attracting considerable attention. Nature gas hydrate in 20 Pa ~ 2000 MPa pressure and temperature range 70 K to 350 K can be stable. And the formation and decomposition of it depends on the pressure, temperature, gas composition, the salinity of the water, the characteristics of porous media and other factors. By increasing the system temperature above the temperature of hydrate formation and decreasing the system pressure, the decomposition of hydrate occurred[2]. Compared with the experimental research the advantage of numerical simulation is the lower cost and easy to adjust the space-time scale. Holder et al considered the variation of temperature and simulated the hydrate decomposition[3]. By using the heat transfer equation, the temperature distribution in the hydrate layer was obtained. And the gas flow and the movement velocity of the surface of decomposition were determined. It assumed that the hydrate decomposition only occurs at the interface between two adjacent reservoirs. But it did not consider hydrate decomposition of the flow of water; gas was only considered single phase flow. Based on this model, three-dimensional model was established on the top of the hydrate considering the hydrate decomposition in the process of the formation of liquid water and the impact on the gas production[4]. However, this work were simple because of the limited understanding on gas hydrate behaviour at that time.To combined with dynamic model and improve the mathematical model, one-dimensional three-phase, finite difference simulator to simulate Berea core decompression decomposition process of hydrate. In particle, the hydrate formation and decomposition coupled the interface movement and physical properties change, i.e., porosity and permeability change[5,6]. The simple models are limited to describe the processes for gas hydrate formation and decomposition. By using adopted elastic and viscoelastic model to simulate the gas hydrates decomposition, the effect on the deformation of solid skeleton was investigated. Although after efforts came from many researchers, the research in numerical simulation of natural gas hydrate decomposition characteristics had significant progress. One of which is that the ice phase was overlooked by most of the model. But ice phase is generated in the process of natural gas hydrate exploitation is real. And its effect is also need to analyze and attention. Therefore, this study consider ice phase into the mathematical model, and analyzed the influence in the hydrate decomposition by depressurization. This paper focuses on the ice generation and distribution characteristics through changing the parameters of the relevant settings, and analyzes the ice with the pressure, temperature, permeability, cumulative gas production. 2. Mathematical model 2.1. Physical model The schematic of the gas hydrate decomposition by depressurization is shown in Fig.1. Initial parameters of the reservoir respectively are pressure ܲ , temperature ܶ , absolute permeability ܭ , each
251
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Minghao Yu et al. / Energy Procedia 105 (2017) 250 – 255
phase saturation ܵ௪ , ܵ , ܵ , and porosity of the reservoir ߶. The lower mining well pressure ܲଵ leads to the reservoir pressure lower than the equilibrium pressure under the temperature ܶ . The hydrate in the reservoir is decomposed into water and methane gas. Under the action of pressure difference, there are into the production wells, natural gas production.
Fig.1 A schematic of the gas hydrate decomposition by depressurization According to the above the physical model, starting from the research emphasis and summarizing the existing model, a one-dimensional mathematical model for hydrate decomposition by depressurization considering ice phase is set up. The basic assumptions are as following: (1) Model considering four phase (water phase, gas phase and hydrate phase, ice phase) and three components (water, gas and hydrate); (2) Hypothesis hydrate, ice and solid skeleton are stillness, and gas and liquid two phase flow conforms to Darcy law; (3) Regardless of the hydrate second generation process; (4) Regardless of the gas, liquid seepage leakage through the cover layer, ignore the gases dissolved in the water and slippage effect; (5) Gas hydrate for methane hydrates, assumption only water and methane gas hydrate decomposition generated, not consider other hydrocarbons generated; (6) No external heat source and quality source; (7) Assume that each parameter in the reservoir isotropic; (8) Ignore the gravity; 2.2. Mathematical model ሬሬሬሬሬറ˅ డ˄ఘೢ ௩ ೢ
െቂ
డ௫
డ˄ఘ ሬሬሬሬሬറ˅ ௩
డሺథௌೢ ఘೢ ାథௌ ఘ ାథௌ ఘ ାథௌ ఘ ሻ
డ௫
డ௧
ቃ ݉௪ ݉ െ ݉ ൌ
(1)
where ߩ௪ Ƚߩ are the density of water and methane gas, ሬሬሬሬറȽݒ ݒ௪ ሬሬሬሬറ are velocity seepage of water and methane respectively, ݉௪ Ƚ݉ Ƚ݉ are the mass of water and gas by the hydrate decomposition per unit volume per unit time and hydrate consumption quality, ܵ δ݆ ൌ ݓǡ ݃ǡ ݄ǡ ݅εis each phase saturation, and ߶ is porosity. ߲ ߲ܶ ൜ൣሺͳ െ ߶ሻ݇ ߶ܵ௪ ݇௪ ߶ܵ ݇ ߶ܵ ݇ ߶ܵ ݇ ൧ ൠ ߲ݔ ߲ݔ డ െ ൣ൫ߩ௪ ሬሬሬሬറܿ ݒ௪ ௪ ߩ ሬሬሬሬറܿ ݒ ൯ܶ൧ െ ݉ ȟܪ ݉ ȟܪ డ௫ డ
ൌ ൛ൣሺͳ െ ߶ሻߩ ܿ ߶ܵ௪ ߩ௪ ܿ௪ ߶ܵ ߩ ܿ ߶ܵ ߩ ܿ ߶ܵ ߩ ܿ ൧ܶൟ(2) డ௧ where ݇ ሺ݆ ൌ ݎǡ ݓǡ ݃ǡ ݄ǡ ݅ሻ is thermal conductivity, ܿ ሺ݆ ൌ ݎǡ ݓǡ ݃ǡ ݄ǡ ݅ሻ is specific heat at constant pressure, and the subscripts ሺݎǡ ݓǡ ݃ǡ ݄ǡ ݅ሻ are respectively solid matrix, water, methane gas, hydrate and
Minghao Yu et al. / Energy Procedia 105 (2017) 250 – 255
ice. ȟܪ is the heat absorption capacity per unit mass hydrate decomposition, and ȟܪ is the latent heat of unit mass of water freeze. This study uses the finite difference method (FDM) to discrete the mass conservation equation (1). This study is aimed at natural gas hydrate production by depressurization, so the boundary temperatures and boundary pressure of mining well are set to the fixed value. For another boundary pressure settings there are two ways, which are respectively full development boundary and impermeable boundary. The calculation results under different boundary conditions are different obviously. Full development boundary is closer to nature and impermeable boundary is more suitable for laboratory conditions. The results of this study need to compare with the experimental results, so impermeable boundary is adopted. 3. Results and Discussion The ice saturation as a function of time was calculated, as shown in Fig. 2. The ice generated from the mining well and gradually extended to the far wellblock. Also the saturation of ice increased with time. Saturation increased into 0.6% during 10 min reflecting the fast decomposition around the well. The maximum saturation during decomposition was 1.27%. However, the saturation gradually increased at the location after 5 m. The reason was the greater pressure gradient around the mining well. After 60 min, the ice saturation increased along the length. With the faster decomposition, temperature in the surrounding area drop. So icing conditions in the region achieved faster.
Fig. 2. Comparison of the saturation of ice in different time
Fig. 3. The absolute permeability change with the length under included and excluded ice condition The permeability during the decomposition deceased along the length (Fig.3). The effect of ice on the permeability was investigated. Considering of the ice phase, permeability at the same location was lower that under exclude ice phase condition. As the solid phase, the ice phase formed the skeleton and the pore space reduced. It suppressed fluid flow so the permeability with ice generation got low. The permeability difference depended on the ice saturation. The permeability at the location of 5 m was 16.2 mD under include ice condition and 17.8 mD under exclude ice condition, respectively. Around the mining well, the
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Minghao Yu et al. / Energy Procedia 105 (2017) 250 – 255
difference was 2.5 mD because of the large ice saturation; however, it gradually decreased to zero even with ice existing, which mean the permeability did not change with small ice saturation. The pressure distribution during the decomposition at 3000 minutes was calculated. The ice formation had less influence on the pressure (Fig. 4). The figure shows that the pressure under the ice formation condition increased at the same location. Although the pressure increased during the decomposition process, the pressure difference that caused by the ice was little and turn to zero after about 12 mm. The maximize difference was only about 0.1 MPa so that the influence can be ignored.
Fig. 4. The pressure distribution under included and excluded ice condition at 3000 min
Fig. 5.The temperature distribution in the reservoir at different time˄300min, 3000min, 30000min˅ The distribution of temperature within the reservoir at 300 minutes, 3000 minutes, 30000 minutes were obtained. The results are shown in Fig.5. As seen in figure in reaction to 300 minutes, the temperature in the 10 m away from mining well in the reservoir kept the same to the initial reservoir temperature. Along with the mining process, temperature decreased as a whole. Before the 3000 minutes, the temperature curve decreases gradually, especially in area near mining well. It was because that, at the moment in the region, the larger pressure gradient leaded to hydrate decomposition drastically, much heat is absorbed, so the temperature declines significantly. The phenomenon was defined as pressure gradient preponderance. Along with the mining, pressure gradient decreased gradually, and the influence from hydrate dissociation on temperature weaken. Because of boundary temperature, the temperature in the reservoir near the mining well decreased at first and then increased, as the temperature curve of 30000 minutes. The phenomenon was defined as boundary temperature preponderance. 4. Conclusion This study set up mathematical model and numerical model about methane hydrate dissociation by depressurization in porous media considering the ice formation. In the process of hydrate exploration, the amount of the ice generated increases gradually over time. And the saturation of ice declined from the area near mining well to the internal reservoir. The information of ice influenced relevant parameters
Minghao Yu et al. / Energy Procedia 105 (2017) 250 – 255
during process of hydrates mining by depressurization. The absolute permeability of the reservoir decreased. But the influence on the change of pressure can be ignored. Temperature changed as the mining going to a moment. At beginning the temperature went up from the area near mining well to the internal reservoir. In the later period, the temperature in the reservoir near the mining well decreased at first and then increased. Acknowledgments This study has been supported by the National Natural Science Foundation of China (Grant No. 51506024, 51576032) References [1] Sloan ED. Fundamental principles and applications of natural gas hydrates. Nature 2003;426:353–63. [2] Makogon YF, Holditch SA, Makogon TY. Natural gas-hydrates—A potential energy source for the 21st Century. J Pet Sci Eng 2007;56:14–31. [3] Holder GD, Angert PF. Simulation of gas production from a reservoir containing both gas hydrates and free natural gas. SPE Annu. Tech. Conf. Exhib., Society of Petroleum Engineers; 1982. [4] Burshears M, O’brien TJ, Malone RD. A multi-phase, multi-dimensional, variable composition simulation of gas production from a conventional gas reservoir in contact with hydrates. SPE Unconv. Gas Technol. Symp., Society of Petroleum Engineers; 1986. [5] Yousif MH, Dorshow RB, Young DB. Testing of hydrate kinetic inhibitors using laser light scattering technique. Ann N Y Acad Sci 1994;715:330–40. [6] Masuda Y, Fujinaga Y, Naganawa S, Fujita K, Sato K, Hayashi Y. Modeling and experimental studies on dissociation of methane gas hydrates in Berea sandstone cores. 3rd Int. Conf. Gas Hydrates, Salt Lake City, Utah, vol. 7, 1999, p. 18–22.
Biography Minghao Yu is a PHD student in Dalian University of Technology. His research interest is experiment and simulation study on multi-phase dynamics during gas hydrate formation and decomposition.
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