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Experimental Investigation on Deformation of Natural Gas Hydrate in the Process of Decompressing
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Energy Procedia 00 (2018) 000–000 Energy Procedia 158 Energy Procedia 00(2019) (2017)5510–5516 000–000
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10th International Conference on Applied Energy (ICAE2018), 22-25 August 2018, Hong Kong, China 10th International Conference on Applied Energy (ICAE2018), 22-25 August 2018, Hong Kong, Experimental on Deformation of Natural Gas Hydrate The Investigation 15th International Symposium and Cooling China on District Heating
in the Process of Decompressing AssessingInvestigation the feasibility using the heat demand-outdoor Experimental onof Deformation of Natural Gas Hydrate a,b,c,d,e a b,c,d,e b,c,d,e Zhen-He Liin the,Jian-Xing Yu Wang ,Xiao-Sen Li * forecast temperature function for a long-term district heat demand Process of,Yi Decompressing a
State Key Laboratory of Hydraulic Engineering Simulation and Safety,Tianjin University,Tianjin 300072,China
a,b,c a b c b,c,d,e Guangzhou of Energy Chinese Academy of Sciences, Guangzhou 510640, China I. Andrić *,Institute A. Pina , P.Conversion, FerrãoaYu , J.a,Yi Fournier B. ,Xiao-Sen Lacarrière , O. Le* Correc Zhen-He Lia,b,c,d,e ,Jian-Xing Wang.,b,c,d,e Li b
a
c CAS Key Laboratory of Gas Hydrate, Guangzhou 510640, China d Guangdong Provincial Key Laboratory New and Renewable Energy Research and Development, Guangzhou 510640, China IN+ Center Technology and of Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1,300072,China 1049-001 Lisbon, Portugal a for Innovation, State Key Laboratory of Hydraulic Engineering Simulation and Safety,Tianjin University,Tianjin b e Guangzhou Center forof Gas Hydrate Research, Chinese Academy ofDaniel, Sciences, Guangzhou 510640,China China Veolia Recherche & Innovation, 291 Avenue Dreyfous 78520 Limay, 510640, France b Guangzhou Institute Energy Conversion, Chinese Academy of Sciences, Guangzhou c Département Systèmes Énergétiques et Environnement - IMT Atlantique, rue Alfred Kastler, 44300 Nantes, France c CAS Key Laboratory of Gas Hydrate, Guangzhou4 510640, China d
Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China e Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou 510640, China
Abstract Abstract The study of the mechanical properties of natural gas hydrate sediments is an important part in the research of gas hydrate District heating networks are commonly in researches the literature as one the most effectiveexperiments, solutions forand decreasing the extraction technology. However, in the past,addressed most of the focused on of triaxial compression few people Abstract greenhouse gas emissions the building sector. These systems require hightoinvestments are returned the the heat studied the deformation in from the process of depressurization mining. In order study this which problem, we havethrough simulated sales. Dueofbetween tothethemechanical changed conditions andgas building policies, heat demand inthethe future could relationship the axial climate deformation natural gas hydraterenovation sediments and in theinprocess of depressurization. In The study properties ofofnatural hydrate is anaerogenesis important part research of gas decrease, hydrate prolonging the investment returninperiod. this experiment, the in-situ formation decompression decomposition of on hydrate arecompression simulated, and the overlying of extraction technology. However, theand past, most of the researches focused triaxial experiments, and pressure few people The main of and this the paper todeformation assessof thedepressurization feasibility using theisheat outdoor temperature for heat demand 13.5MPa isscope loaded, of hydrateofsediment recorded. conclusion is that, infunction thehave process of hydrate studied the deformation in axial theis process mining. In demand order The to– study this problem, we simulated the forecast. The district Alvalade, located in Lisbon used a the caserelationship study. district is consisted of gas 665 decompression decomposing, the strain increased withgas the(Portugal), amountsediments ofwas gas produced, between value and relationship between theofaxial deformation of natural hydrate andas aerogenesis in theThe process of strain depressurization. In buildings that vary inisboth construction periodand andthe typology. Threecoefficient weather scenarios (low,the medium, and three district production percentage a proportional function, proportional increases with increase of hydrate saturation. this experiment, the in-situ formation and decompression decomposition of hydrate are simulated, and thehigh) overlying pressure of renovation scenarios developed (shallow, intermediate, deep). To estimate error, obtained heat were 13.5MPa is loaded, andwere the axial deformation of hydrate sediment is recorded. The the conclusion is that, in thedemand process values of hydrate ©compared 2019 The Authors. Published by Elsevier with results from athe dynamic heatLtd. demand previously by the authors. Keywords: Sediment Deformation; Hydrate Experiment decompression decomposing, strainDecomposition; increased withmodel, the amount of gasdeveloped produced,and thevalidated relationship between strain value and gas This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) The results showed that only weather change is considered, thecoefficient margin ofincreases error could acceptable forhydrate some applications production is awhen proportional function, and the proportional withbethe increase of saturation. Peer-reviewpercentage under responsibility of the scientific committee of ICAE2018 – The 10th International Conference on Applied Energy. (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation scenarios, the error value increased to 59.5% (depending Keywords: Sediment Deformation; Hydrateup Decomposition; Experimenton the weather and renovation scenarios combination considered). 1. Introduction The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the decrease thehydrate number is of aheating 22-139h duringbythe heating season (depending the combination weather and Naturalingas kind ofhours cageofcrystal formed water molecules under highonpressure and low of temperature, renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending the 1. Introduction which envelop natural gas molecules (such as methane, etc.) formed by cage crystal. Hydrate sediment isonthe coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and sediment that includes sand, clay, mixed soil, etc of hydrate. Hydrate crystals play the role of supporting or improve accuracy of is heat demand estimations. Naturalthegas hydrate a kind of cage crystal formed by water molecules under high pressure and low temperature,
cementing the structure in sediments. It is estimated that the global carbon reserves in natural gas hydrate are 1.8 × 12 which envelopis natural gas twice molecules (suchamount as methane, etc.) found formedinby crystal. Hydrate sediment is the t, which morePublished than the total of carbon thecage world's conventional fossil fuels.[1] 10 © 2017 The Authors. by Elsevier Ltd. sediment that includes sand, clay, mixed soil, etc of hydrate. Hydrate crystals play the role of supporting or Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and cementing the structure in sediments. It is estimated that the global carbon reserves in natural gas hydrate are 1.8 × Cooling. +86-20-87057037; fax: +86-20-87034664. t, which is author. more Tel: than twice the total amount of carbon found in the world's conventional fossil fuels.[1] 10*12Corresponding E-mail address: [email protected] Keywords: Heat demand; Forecast; Climate change
* Corresponding author. Tel: +86-20-87057037; fax: +86-20-87034664. E-mail address: 1876-6102 [email protected] © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 10th International Conference on Applied Energy (ICAE2018). 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. 1876-6102 Copyright © 2018 Elsevier All rights reserved.of The 15th International Symposium on District Heating and Cooling. Peer-review under responsibility of theLtd. Scientific Committee 1876-6102 © 2019 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of the scientific committee of the 10th International Conference on Applied Energy (ICAE2018). 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 ICAE2018 – The 10th International Conference on Applied Energy. 10.1016/j.egypro.2019.01.594
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However, Ning[2] found that there are still many problems in hydrate extraction. For example, the chemical stability of hydrate is relatively poor, so the rising temperature and the decreasing pressure and other external factors will make it decompose. This unexpected decomposition will destroy the cementation and support of the hydrate to the settled layer. Free water and free gas generated by decomposition will increase pore pressure, and reduce the effective stress of sediments. The result will lead to instability of seabed reservoirs and easily induce submarine earthquakes, landslides, or even tsunamis. If methane gas enters seawater, It will result in overmethanation of the sea water, and trigger ecological problems. In addition, methane is a greenhouse gas, and methane entering the atmosphere will cause greenhouse effect, which will cause more serious effects. The schematic diagram of the associated disaster is shown in Fig. 1.
Fig. 1 Schematic representation of a possible mechansim for initiating submarine landslides by methane hydrate dissociation
Over the past two decades, researchers have conducted a number of experimental studies on the mechanical properties and mechanical behaviors of gas hydrate sediments, including the following typical figures: Hyodo[3], Yoneda[4], Kajiyama[5] and other researchers have been working on triaxial compression experiments on gas hydrate in recent years.(a) He concludes that the strength of gas hydrate increases with the increase of confining pressure and the pore pressure, and the decline of temperature. (b)The initial strength and stiffness of the sediments containing methane hydrate are higher than those of pure sand, which has a strong dilatancy effect.(c)The shear zones of sediments containing methane hydrate are thinner than those without hydrate. He also studied the triaxial compression experiment of hydrate sediments with the pure glass beads as the skeleton of the sediments. (d)They draw a conclusion that the triaxial compression experiment of pure glass beads shows stick-slip phenomenon, which is more obvious with the increase of confining pressure, but the formation of hydrate in the void will reduce this effect. (e) the more smooth the glass beads particles grind, methane hydrates appear to be more resistant to pressure. In recent years, Miyazaki[6] has carried out a series of triaxial compression experiments on toyoura sand containing methane hydrate. (a) The strength and rigidity of methane hydrate sediments increase with the increase of hydrate saturation.(b)The increase of effective confining pressure can increase the strength and rigidity of the hydrate, while the Poisson ratio decrease. (c) The strength of the sediment is independent of the sediment skeleton, but the stiffness is related to the sediment skeleton. The finer the sand is, the weaker the stiffness of the sediment is. He also carried out the related numerical simulation work and proposed a nonlinear elastic model which can be applied to methane hydrate sediments. This model is based on Duncan-Chang model, and fits his previous research on mechanical properties of methane hydrate. A series of triaxial compression experiments on gas hydrate deposits were carried out by Li Yanghui[10] and
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others. (a)The results show that the temperature has a certain effect on the mechanical properties of methane hydrate sediments. And the higher the temperature is, the lower sediment strength is.(b) The confining pressure has a great effect on the deformation behavior. When the confining pressure is lower than 10MPa, the failure intensity increases significantly. (c) The higher the methane hydrate saturation, the higher the strength and stiffness of the structure. According to the investigation, the author found that the previous researches on the strength of hydrate sediments were mainly based on acoustic wave experiment and triaxial compression experiment, which were classical soil mechanics experiments. However, it can only study the different mechanical properties of hydrate sediments with specific saturation. That means that hydrate cannot be formed and decomposed in the experimental process. But, the hydrate sediment in exploitation cannot be static. The saturation of water, gas and hydrate in hydrate sediments is always in a dynamic state in the actual mining process, so it is difficult to simulate this complex process by traditional experimental schemes and methods. At the same time, in the process of hydrate exploitation, the overlying strata is a definite value, which is contrary to the design idea of triaxial compression experiment. Based on this idea, we have designed and commissioned an apparatus for measuring the decomposition deformation of hydrate formation. 2. Experimental section 2.1. Experimental facility Fig. 2 is an apparatus for measuring the decomposition deformation of hydrate formation which is developed by Guangzhou Institute of Energy Research, Chinese Academy of Sciences. The apparatus combines the hydrate formation decomposition reaction chamber with a vertical pressure system, plus a gas supply system, a water supply system, an exploitation system and a data collection system, and air bath water bath way to maintain low temperature. This device can simulate in-situ formation and decomposition process of hydrate deposits, and provide stable axial pressure for ballast to simulate the overlying strata pressure. Besides, by controlling the extraction pressure through the counterbalance valve, the simulation experiment’s depressurization of hydrate sediment is realized. Main technical parameters as shown in Table 1. Table 1. Main technical parameters Projects Geometric dimensioning in reaction kettle Working pressure range and control accuracy Working temperature range and control accuracy
Parameters ϕ50mm×240mm,Max effective volume 471mL ≦30MPa;±0.1MPa -15℃~90℃；±0.5℃
The system can observe the axial deformation, forming data and images of the sediment structure in the process of depressurization and decomposition of hydrates in real time. and it can use software for post processing and analysis. And it can simulate the different distribution forms of hydrate sediments and obtain the influence of the distribution form on the mechanical properties of the sediments.
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Fig. 2 Schematic diagram of the experimental device
2.2. Experimental steps The steps of the experiment are as follows: 1. The quartz sand needed in the experiment is forecasted and three groups of sand samples are taken. The real density of the sand is determined to be 2.65g/cm 3. 2. A certain quality quartz sand is filled in the reactor kettle and the reaction device is assembled. The reactor kettle is placed under the hydraulic cylinder to load the axial compression to 23MPa, and the quartz sand skeleton is compacted. 3. The total volume of the reactor kettle is calculated and the pore volume is calculated according to the mass and real density of the quartz sand. 4. In order to obtain the initial hydrate sediment sample with target saturation, the quantity of deionized water and methane gas needed to be filled into the reactor was calculated. According to the result of calculation, the scheme of water injection and gas injection is made. 5. After the formation of hydrate with target saturation is completed according to the established scheme, simulated pressure reduction mining is carried out. The axial pressure of the hydraulic cylinder is adjusted to the target formation pressure of 13.5MPa, and the outlet pressure (i.e. pore pressure) is reduced to 4.5MPa at the rate of depressurization pressure of 0.45MPa/min. The experimental data such as axial variables and outlet gas flow are recorded and processed later. Table 2. Experimental data of hydrate production The experimental group
The group 1
The group 2
The length of sediment/cm
18.0
18.0
Porosity
0.45
0.45
Water saturation
0.65
0.45
Hydrate saturation
0.25
0.45
Gas saturation
0.10
0.10
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3. Results and discussions
Fig. 3 The relationship between gas production, strain and time
Fig. 3 shows the relationship between gas production, strain and time in the experiment. It can be seen from Fig. 3 that (1) gas production and strain increase synchronously with the development of exploitation; (2) gas production and maximum strain of the second group of experiments with higher hydrate saturation are higher than those of the first group. The figure shows a high degree of consistency between increases in strain and increases in gas production, therefore, we next study the relationship between strain and gas production.
Fig. 4 The relationship between strain and gas production
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Fig. 4 shows the relationship between strain and gas production. It is obvious that the second group of experiments with higher hydrate saturation has greater strain when they produced the same amount of gas, and the two lines can be approximated to synthesize a straight line. So our next step is to dimensionless gas production. The percentage of gas production is obtained by dividing the gas production of the two experiments by total gas production.
Fig. 5 The relationship between strain and the percentage of gas production
Fig.5 shows the relationship between strain and the percentage of gas production. Moreover, it is compared with the result of numerical fitting with the proportional function, and the value of R is very close to 1. It shows that the relationship between strain and percentage of gas production is basically a proportional function. At the same time, the positive proportional coefficients of the second group of experiments with higher hydrate saturation are higher than those of the first group, indicating that sediments are more likely to be compressed when hydrate saturation is higher. The reason for this phenomenon is that hydrate crystals and sediment particles together contribute to the structural strength, and the higher the hydrate saturation, the greater the contribution to the strength. Therefore, in the process of decomposition, the greater the decrease in structural strength of this sediment, the easier it is to be compressed. 4. Conclusion This group of experiments studied the compression deformation of hydrate sediments with different levels of saturation under the same conditions during the downward pressure exploitation, and reached the following conclusions: 1. In the process of depressurization exploitation of natural gas hydrate, the hydrate sediment will undergo compression deformation under the action of stable axial pressure, and the strain and gas production will increase with time. 2. The higher the saturation of hydrate, the higher the total strain, and the higher the strain corresponding to the same amount of gas. 3. The relationship between strain value and gas production percentage is a proportional function, and the proportional coefficient increases with the increase of hydrate saturation. Acknowledgments This work is supported by Key Program of National Natural Science Foundation of China (51736009), National Natural Science Foundation of China (51676190), Pearl River S&T Nova Program of Guangzhou (201610010164), International S&T Cooperation Program of China (2015DFA61790), Science and Technology Apparatus Development Program of the Chinese Academy of Sciences (YZ201619), Frontier Sciences Key Research Program of the Chinese Academy of Sciences (QYZDJ-SSW-JSC033), National Key Research and Development Plan of
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China (2016YFC0304002, 2017YFC0307306), Youth Science and Technology Innovation Talent of Guangdong (2016TQ03Z862), and Natural Science Foundation of Guangdong (2017A030313313), which are gratefully acknowledged. References [1] Boswell R, Collett T S. Current perspectives on gas hydrate resources. Energy & Environmental Science. 2011; 4(4):1206-1215. [2] Ning F, Yu Y, Kjelstrup S,et al. Mechanical properties of clathrate hydrates: status and perspectives. Energy & Environmental Science. 2012; 5(5):6779-6795. [3] Hyodo M, Li YH, Yoneda J, Nakata Y, Yoshimoto N, Nishimura A, Song YC. Mechanical behavior of gas-saturated methane hydratebearing sediments. Journal of Geophysical Research: Solid Earth. 2013; 118(10):5185-5194. [4] Yoneda J, Hyodo M, Yoshimoto N, Nakata Y, Kato A. Development of high-pressure low-temperature plane strain testing apparatus for methane hydrate-bearing sand. Soils & Foundations. 2013; 53(5):774-783. [5] Kajiyama S, Yang W, Hyodo M, Nakata Y, Nakashima K, Yoshimoto N. Experimental investigation on the mechanical properties of methane hydrate-bearing sand formed with rounded particles. Journal of Natural Gas Science & Engineering. 2017; 45:96-107. [6] Miyazaki K, Tenma N, Aoki K, Yamaguchi T. A Nonlinear Elastic Model for Triaxial Compressive Properties of Artificial MethaneHydrate-Bearing Sediment Samples. Energies. 2012; 5(10):4057-4075. [7] Miyazaki K, Tenma N, Aoki K, Sakamoto Y, Yamaguchi T. effects of confining pressure on mechanical properties of artificial methanehydrate-bearing sediment in traxial compression test. International Journal of Offshore & Polar Engineering. 2011; 21(2). [8] Miyazaki K, Tenma N, Aoki K, Sakamoto Y, Yamaguchi T. Loading-Rate Dependence of Triaxial Compressive Strength of Artificial Methane-Hydrate-Bearing Sediment Containing Fine Fraction. Twenty. 2012. [9] Miyazaki K, Masui A, Aoki K, Sakamoto Y, Yamaguchi T, et al. Strain-Rate Dependence of Triaxial Compressive Strength of Artificial Methane-Hydrate-Bearing Sediment. International Journal of Offshore & Polar Engineering. 2010; 20(4):256-264. [10] Li YH, Song YC, Liu WG, Yu F, Wang R, Nie XF. Analysis of Mechanical Properties and Strength Criteria of Methane Hydrate-Bearing Sediments.International Journal of Offshore & Polar Engineering. 2012; 22(4):290-296. [11] Li Y, Zhao H, Yu F, Song Y, Liu W, et al. Investigation of the Stress–Strain and Strength Behaviors of Ice Containing Methane Hydrate. Journal of Cold Regions Engineering. 2012; 26(4):149-159.
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Available online at www.sciencedirect.com Availableonline onlineatatwww.sciencedirect.com www.sciencedirect.com Available Energy Procedia 00 (2018) 000–000
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10th International Conference on Applied Energy (ICAE2018), 22-25 August 2018, Hong Kong, China 10th International Conference on Applied Energy (ICAE2018), 22-25 August 2018, Hong Kong, Experimental on Deformation of Natural Gas Hydrate The Investigation 15th International Symposium and Cooling China on District Heating
in the Process of Decompressing AssessingInvestigation the feasibility using the heat demand-outdoor Experimental onof Deformation of Natural Gas Hydrate a,b,c,d,e a b,c,d,e b,c,d,e Zhen-He Liin the,Jian-Xing Yu Wang ,Xiao-Sen Li * forecast temperature function for a long-term district heat demand Process of,Yi Decompressing a
State Key Laboratory of Hydraulic Engineering Simulation and Safety,Tianjin University,Tianjin 300072,China
a,b,c a b c b,c,d,e Guangzhou of Energy Chinese Academy of Sciences, Guangzhou 510640, China I. Andrić *,Institute A. Pina , P.Conversion, FerrãoaYu , J.a,Yi Fournier B. ,Xiao-Sen Lacarrière , O. Le* Correc Zhen-He Lia,b,c,d,e ,Jian-Xing Wang.,b,c,d,e Li b
a
c CAS Key Laboratory of Gas Hydrate, Guangzhou 510640, China d Guangdong Provincial Key Laboratory New and Renewable Energy Research and Development, Guangzhou 510640, China IN+ Center Technology and of Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1,300072,China 1049-001 Lisbon, Portugal a for Innovation, State Key Laboratory of Hydraulic Engineering Simulation and Safety,Tianjin University,Tianjin b e Guangzhou Center forof Gas Hydrate Research, Chinese Academy ofDaniel, Sciences, Guangzhou 510640,China China Veolia Recherche & Innovation, 291 Avenue Dreyfous 78520 Limay, 510640, France b Guangzhou Institute Energy Conversion, Chinese Academy of Sciences, Guangzhou c Département Systèmes Énergétiques et Environnement - IMT Atlantique, rue Alfred Kastler, 44300 Nantes, France c CAS Key Laboratory of Gas Hydrate, Guangzhou4 510640, China d
Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China e Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou 510640, China
Abstract Abstract The study of the mechanical properties of natural gas hydrate sediments is an important part in the research of gas hydrate District heating networks are commonly in researches the literature as one the most effectiveexperiments, solutions forand decreasing the extraction technology. However, in the past,addressed most of the focused on of triaxial compression few people Abstract greenhouse gas emissions the building sector. These systems require hightoinvestments are returned the the heat studied the deformation in from the process of depressurization mining. In order study this which problem, we havethrough simulated sales. Dueofbetween tothethemechanical changed conditions andgas building policies, heat demand inthethe future could relationship the axial climate deformation natural gas hydraterenovation sediments and in theinprocess of depressurization. In The study properties ofofnatural hydrate is anaerogenesis important part research of gas decrease, hydrate prolonging the investment returninperiod. this experiment, the in-situ formation decompression decomposition of on hydrate arecompression simulated, and the overlying of extraction technology. However, theand past, most of the researches focused triaxial experiments, and pressure few people The main of and this the paper todeformation assessof thedepressurization feasibility using theisheat outdoor temperature for heat demand 13.5MPa isscope loaded, of hydrateofsediment recorded. conclusion is that, infunction thehave process of hydrate studied the deformation in axial theis process mining. In demand order The to– study this problem, we simulated the forecast. The district Alvalade, located in Lisbon used a the caserelationship study. district is consisted of gas 665 decompression decomposing, the strain increased withgas the(Portugal), amountsediments ofwas gas produced, between value and relationship between theofaxial deformation of natural hydrate andas aerogenesis in theThe process of strain depressurization. In buildings that vary inisboth construction periodand andthe typology. Threecoefficient weather scenarios (low,the medium, and three district production percentage a proportional function, proportional increases with increase of hydrate saturation. this experiment, the in-situ formation and decompression decomposition of hydrate are simulated, and thehigh) overlying pressure of renovation scenarios developed (shallow, intermediate, deep). To estimate error, obtained heat were 13.5MPa is loaded, andwere the axial deformation of hydrate sediment is recorded. The the conclusion is that, in thedemand process values of hydrate ©compared 2019 The Authors. Published by Elsevier with results from athe dynamic heatLtd. demand previously by the authors. Keywords: Sediment Deformation; Hydrate Experiment decompression decomposing, strainDecomposition; increased withmodel, the amount of gasdeveloped produced,and thevalidated relationship between strain value and gas This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) The results showed that only weather change is considered, thecoefficient margin ofincreases error could acceptable forhydrate some applications production is awhen proportional function, and the proportional withbethe increase of saturation. Peer-reviewpercentage under responsibility of the scientific committee of ICAE2018 – The 10th International Conference on Applied Energy. (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation scenarios, the error value increased to 59.5% (depending Keywords: Sediment Deformation; Hydrateup Decomposition; Experimenton the weather and renovation scenarios combination considered). 1. Introduction The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the decrease thehydrate number is of aheating 22-139h duringbythe heating season (depending the combination weather and Naturalingas kind ofhours cageofcrystal formed water molecules under highonpressure and low of temperature, renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending the 1. Introduction which envelop natural gas molecules (such as methane, etc.) formed by cage crystal. Hydrate sediment isonthe coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and sediment that includes sand, clay, mixed soil, etc of hydrate. Hydrate crystals play the role of supporting or improve accuracy of is heat demand estimations. Naturalthegas hydrate a kind of cage crystal formed by water molecules under high pressure and low temperature,
cementing the structure in sediments. It is estimated that the global carbon reserves in natural gas hydrate are 1.8 × 12 which envelopis natural gas twice molecules (suchamount as methane, etc.) found formedinby crystal. Hydrate sediment is the t, which morePublished than the total of carbon thecage world's conventional fossil fuels.[1] 10 © 2017 The Authors. by Elsevier Ltd. sediment that includes sand, clay, mixed soil, etc of hydrate. Hydrate crystals play the role of supporting or Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and cementing the structure in sediments. It is estimated that the global carbon reserves in natural gas hydrate are 1.8 × Cooling. +86-20-87057037; fax: +86-20-87034664. t, which is author. more Tel: than twice the total amount of carbon found in the world's conventional fossil fuels.[1] 10*12Corresponding E-mail address: [email protected] Keywords: Heat demand; Forecast; Climate change
* Corresponding author. Tel: +86-20-87057037; fax: +86-20-87034664. E-mail address: 1876-6102 [email protected] © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 10th International Conference on Applied Energy (ICAE2018). 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. 1876-6102 Copyright © 2018 Elsevier All rights reserved.of The 15th International Symposium on District Heating and Cooling. Peer-review under responsibility of theLtd. Scientific Committee 1876-6102 © 2019 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of the scientific committee of the 10th International Conference on Applied Energy (ICAE2018). 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 ICAE2018 – The 10th International Conference on Applied Energy. 10.1016/j.egypro.2019.01.594
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Zhen-He Li et al. / Energy Procedia 158 (2019) 5510–5516 ZhenheLi/ Energy Procedia 00 (2018) 000–000
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However, Ning[2] found that there are still many problems in hydrate extraction. For example, the chemical stability of hydrate is relatively poor, so the rising temperature and the decreasing pressure and other external factors will make it decompose. This unexpected decomposition will destroy the cementation and support of the hydrate to the settled layer. Free water and free gas generated by decomposition will increase pore pressure, and reduce the effective stress of sediments. The result will lead to instability of seabed reservoirs and easily induce submarine earthquakes, landslides, or even tsunamis. If methane gas enters seawater, It will result in overmethanation of the sea water, and trigger ecological problems. In addition, methane is a greenhouse gas, and methane entering the atmosphere will cause greenhouse effect, which will cause more serious effects. The schematic diagram of the associated disaster is shown in Fig. 1.
Fig. 1 Schematic representation of a possible mechansim for initiating submarine landslides by methane hydrate dissociation
Over the past two decades, researchers have conducted a number of experimental studies on the mechanical properties and mechanical behaviors of gas hydrate sediments, including the following typical figures: Hyodo[3], Yoneda[4], Kajiyama[5] and other researchers have been working on triaxial compression experiments on gas hydrate in recent years.(a) He concludes that the strength of gas hydrate increases with the increase of confining pressure and the pore pressure, and the decline of temperature. (b)The initial strength and stiffness of the sediments containing methane hydrate are higher than those of pure sand, which has a strong dilatancy effect.(c)The shear zones of sediments containing methane hydrate are thinner than those without hydrate. He also studied the triaxial compression experiment of hydrate sediments with the pure glass beads as the skeleton of the sediments. (d)They draw a conclusion that the triaxial compression experiment of pure glass beads shows stick-slip phenomenon, which is more obvious with the increase of confining pressure, but the formation of hydrate in the void will reduce this effect. (e) the more smooth the glass beads particles grind, methane hydrates appear to be more resistant to pressure. In recent years, Miyazaki[6] has carried out a series of triaxial compression experiments on toyoura sand containing methane hydrate. (a) The strength and rigidity of methane hydrate sediments increase with the increase of hydrate saturation.(b)The increase of effective confining pressure can increase the strength and rigidity of the hydrate, while the Poisson ratio decrease. (c) The strength of the sediment is independent of the sediment skeleton, but the stiffness is related to the sediment skeleton. The finer the sand is, the weaker the stiffness of the sediment is. He also carried out the related numerical simulation work and proposed a nonlinear elastic model which can be applied to methane hydrate sediments. This model is based on Duncan-Chang model, and fits his previous research on mechanical properties of methane hydrate. A series of triaxial compression experiments on gas hydrate deposits were carried out by Li Yanghui[10] and
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others. (a)The results show that the temperature has a certain effect on the mechanical properties of methane hydrate sediments. And the higher the temperature is, the lower sediment strength is.(b) The confining pressure has a great effect on the deformation behavior. When the confining pressure is lower than 10MPa, the failure intensity increases significantly. (c) The higher the methane hydrate saturation, the higher the strength and stiffness of the structure. According to the investigation, the author found that the previous researches on the strength of hydrate sediments were mainly based on acoustic wave experiment and triaxial compression experiment, which were classical soil mechanics experiments. However, it can only study the different mechanical properties of hydrate sediments with specific saturation. That means that hydrate cannot be formed and decomposed in the experimental process. But, the hydrate sediment in exploitation cannot be static. The saturation of water, gas and hydrate in hydrate sediments is always in a dynamic state in the actual mining process, so it is difficult to simulate this complex process by traditional experimental schemes and methods. At the same time, in the process of hydrate exploitation, the overlying strata is a definite value, which is contrary to the design idea of triaxial compression experiment. Based on this idea, we have designed and commissioned an apparatus for measuring the decomposition deformation of hydrate formation. 2. Experimental section 2.1. Experimental facility Fig. 2 is an apparatus for measuring the decomposition deformation of hydrate formation which is developed by Guangzhou Institute of Energy Research, Chinese Academy of Sciences. The apparatus combines the hydrate formation decomposition reaction chamber with a vertical pressure system, plus a gas supply system, a water supply system, an exploitation system and a data collection system, and air bath water bath way to maintain low temperature. This device can simulate in-situ formation and decomposition process of hydrate deposits, and provide stable axial pressure for ballast to simulate the overlying strata pressure. Besides, by controlling the extraction pressure through the counterbalance valve, the simulation experiment’s depressurization of hydrate sediment is realized. Main technical parameters as shown in Table 1. Table 1. Main technical parameters Projects Geometric dimensioning in reaction kettle Working pressure range and control accuracy Working temperature range and control accuracy
Parameters ϕ50mm×240mm,Max effective volume 471mL ≦30MPa;±0.1MPa -15℃~90℃；±0.5℃
The system can observe the axial deformation, forming data and images of the sediment structure in the process of depressurization and decomposition of hydrates in real time. and it can use software for post processing and analysis. And it can simulate the different distribution forms of hydrate sediments and obtain the influence of the distribution form on the mechanical properties of the sediments.
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Fig. 2 Schematic diagram of the experimental device
2.2. Experimental steps The steps of the experiment are as follows: 1. The quartz sand needed in the experiment is forecasted and three groups of sand samples are taken. The real density of the sand is determined to be 2.65g/cm 3. 2. A certain quality quartz sand is filled in the reactor kettle and the reaction device is assembled. The reactor kettle is placed under the hydraulic cylinder to load the axial compression to 23MPa, and the quartz sand skeleton is compacted. 3. The total volume of the reactor kettle is calculated and the pore volume is calculated according to the mass and real density of the quartz sand. 4. In order to obtain the initial hydrate sediment sample with target saturation, the quantity of deionized water and methane gas needed to be filled into the reactor was calculated. According to the result of calculation, the scheme of water injection and gas injection is made. 5. After the formation of hydrate with target saturation is completed according to the established scheme, simulated pressure reduction mining is carried out. The axial pressure of the hydraulic cylinder is adjusted to the target formation pressure of 13.5MPa, and the outlet pressure (i.e. pore pressure) is reduced to 4.5MPa at the rate of depressurization pressure of 0.45MPa/min. The experimental data such as axial variables and outlet gas flow are recorded and processed later. Table 2. Experimental data of hydrate production The experimental group
The group 1
The group 2
The length of sediment/cm
18.0
18.0
Porosity
0.45
0.45
Water saturation
0.65
0.45
Hydrate saturation
0.25
0.45
Gas saturation
0.10
0.10
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3. Results and discussions
Fig. 3 The relationship between gas production, strain and time
Fig. 3 shows the relationship between gas production, strain and time in the experiment. It can be seen from Fig. 3 that (1) gas production and strain increase synchronously with the development of exploitation; (2) gas production and maximum strain of the second group of experiments with higher hydrate saturation are higher than those of the first group. The figure shows a high degree of consistency between increases in strain and increases in gas production, therefore, we next study the relationship between strain and gas production.
Fig. 4 The relationship between strain and gas production
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Fig. 4 shows the relationship between strain and gas production. It is obvious that the second group of experiments with higher hydrate saturation has greater strain when they produced the same amount of gas, and the two lines can be approximated to synthesize a straight line. So our next step is to dimensionless gas production. The percentage of gas production is obtained by dividing the gas production of the two experiments by total gas production.
Fig. 5 The relationship between strain and the percentage of gas production
Fig.5 shows the relationship between strain and the percentage of gas production. Moreover, it is compared with the result of numerical fitting with the proportional function, and the value of R is very close to 1. It shows that the relationship between strain and percentage of gas production is basically a proportional function. At the same time, the positive proportional coefficients of the second group of experiments with higher hydrate saturation are higher than those of the first group, indicating that sediments are more likely to be compressed when hydrate saturation is higher. The reason for this phenomenon is that hydrate crystals and sediment particles together contribute to the structural strength, and the higher the hydrate saturation, the greater the contribution to the strength. Therefore, in the process of decomposition, the greater the decrease in structural strength of this sediment, the easier it is to be compressed. 4. Conclusion This group of experiments studied the compression deformation of hydrate sediments with different levels of saturation under the same conditions during the downward pressure exploitation, and reached the following conclusions: 1. In the process of depressurization exploitation of natural gas hydrate, the hydrate sediment will undergo compression deformation under the action of stable axial pressure, and the strain and gas production will increase with time. 2. The higher the saturation of hydrate, the higher the total strain, and the higher the strain corresponding to the same amount of gas. 3. The relationship between strain value and gas production percentage is a proportional function, and the proportional coefficient increases with the increase of hydrate saturation. Acknowledgments This work is supported by Key Program of National Natural Science Foundation of China (51736009), National Natural Science Foundation of China (51676190), Pearl River S&T Nova Program of Guangzhou (201610010164), International S&T Cooperation Program of China (2015DFA61790), Science and Technology Apparatus Development Program of the Chinese Academy of Sciences (YZ201619), Frontier Sciences Key Research Program of the Chinese Academy of Sciences (QYZDJ-SSW-JSC033), National Key Research and Development Plan of
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China (2016YFC0304002, 2017YFC0307306), Youth Science and Technology Innovation Talent of Guangdong (2016TQ03Z862), and Natural Science Foundation of Guangdong (2017A030313313), which are gratefully acknowledged. References [1] Boswell R, Collett T S. Current perspectives on gas hydrate resources. Energy & Environmental Science. 2011; 4(4):1206-1215. [2] Ning F, Yu Y, Kjelstrup S,et al. Mechanical properties of clathrate hydrates: status and perspectives. Energy & Environmental Science. 2012; 5(5):6779-6795. [3] Hyodo M, Li YH, Yoneda J, Nakata Y, Yoshimoto N, Nishimura A, Song YC. Mechanical behavior of gas-saturated methane hydratebearing sediments. Journal of Geophysical Research: Solid Earth. 2013; 118(10):5185-5194. [4] Yoneda J, Hyodo M, Yoshimoto N, Nakata Y, Kato A. Development of high-pressure low-temperature plane strain testing apparatus for methane hydrate-bearing sand. Soils & Foundations. 2013; 53(5):774-783. [5] Kajiyama S, Yang W, Hyodo M, Nakata Y, Nakashima K, Yoshimoto N. Experimental investigation on the mechanical properties of methane hydrate-bearing sand formed with rounded particles. Journal of Natural Gas Science & Engineering. 2017; 45:96-107. [6] Miyazaki K, Tenma N, Aoki K, Yamaguchi T. A Nonlinear Elastic Model for Triaxial Compressive Properties of Artificial MethaneHydrate-Bearing Sediment Samples. Energies. 2012; 5(10):4057-4075. [7] Miyazaki K, Tenma N, Aoki K, Sakamoto Y, Yamaguchi T. effects of confining pressure on mechanical properties of artificial methanehydrate-bearing sediment in traxial compression test. International Journal of Offshore & Polar Engineering. 2011; 21(2). [8] Miyazaki K, Tenma N, Aoki K, Sakamoto Y, Yamaguchi T. Loading-Rate Dependence of Triaxial Compressive Strength of Artificial Methane-Hydrate-Bearing Sediment Containing Fine Fraction. Twenty. 2012. [9] Miyazaki K, Masui A, Aoki K, Sakamoto Y, Yamaguchi T, et al. Strain-Rate Dependence of Triaxial Compressive Strength of Artificial Methane-Hydrate-Bearing Sediment. International Journal of Offshore & Polar Engineering. 2010; 20(4):256-264. [10] Li YH, Song YC, Liu WG, Yu F, Wang R, Nie XF. Analysis of Mechanical Properties and Strength Criteria of Methane Hydrate-Bearing Sediments.International Journal of Offshore & Polar Engineering. 2012; 22(4):290-296. [11] Li Y, Zhao H, Yu F, Song Y, Liu W, et al. Investigation of the Stress–Strain and Strength Behaviors of Ice Containing Methane Hydrate. Journal of Cold Regions Engineering. 2012; 26(4):149-159.