MRI analysis for methane hydrate dissociation by depressurization and the concomitant ice generation

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1 Available online at ScienceDirect Energy Procedia 105 (2017 ) The 8 th International Conference on Applied Energy ICAE2016 MRI analysis for methane hydrate dissociation by depressurization and the concomitant ice generation Zhen Fan, Chaomin Sun,Yangmin Kuang, Bin Wang, Jiafei Zhao*, Yongchen Song Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, Dalian University of Technology, Dalian , P. R. China Abstract Hydrate reformation and ice generation may occur under fast depressurizing rate and low production pressure processes caused by lacking sufficient heat transfer in the sediment. This work conducted MRI visualization and analysis for methane hydrate (MH) dissociation by controlling the depressurizing rate gradually to designed production pressures. Obvious hydrate reformation and ice generation can be avoided through this method. MH dissociation behavior was analyzed under different back pressures. Furthermore, ice generation during MH dissociation under rapid depressurization and low back pressure condition was studied. Large amount of ice generated spatially in the vessel. Ambient heat transfer drove ice melt and hydrate dissociation from the surrounding wall in. The saturation of the generated ice in the vessel was also estimated Published The Authors. by Elsevier Published Ltd. This by is Elsevier an open access Ltd. article under the CC BY-NC-ND license ( 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. Keywoeds: Gas hydrate; dissociation; MRI; ice generation; heat transfer 1. Introduction Natural gas hydrates (NGH) are solid, crystalline compounds formed by gas and water molecules [1]. As important contributors in the global carbon cycle, they are formed under high pressure and low temperature conditions, and are buried in permafrost regions and in sediments beneath the sea [2]. NGH contain about 1/3 of the world s mobile organic carbon. And the amount of natural gas trapped in NGH may surpass the available and recoverable conventional methane by two orders of magnitude [3]. Thus NGH have been regarded as a global resource for natural gas [3]. So far three main methods, including depressurization, thermal stimulation, and chemical inhibitor injection have been proposed for hydrate dissociation. Given the feasibility of exploitation and the economic issues, depressurization is considered * Corresponding author. Tel.: ; fax: address: jfzhao@dlut.edu.cn. Corresponding author. Tel.: ; fax: address: songyc@dlut.edu.cn Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi: /j.egypro

2 4764 Zhen Fan et al. / Energy Procedia 105 ( 2017 ) to be the most effective method [4]. The process of methane hydrate dissociation integrates heat transfer and fluid flow with hydrate dissociation kinetics. Numerous experimental and numerical studies have been undertaken to investigate hydrate dissociation induced by depressurization [5-10]. As shown in Fig. 1, researchers have divided the process of MH dissociation into three main stages: free gas liberation, dissociation sustained by reservoir sensible heat, and dissociation under ambient heat transfer [8]. Many of the experimental studies conducted fast depressurization process and the first 2 stages were quite rapid. Phase change such as hydrate reformation and ice generation may occur under fast depressurization and low back pressure processes which lacks sufficient heat transfer in the sediment [8]. Investigations reported that hydrate reformation and ice generation may block the pores and throats of the porous media and thus influence gas production. Several researchers have shown interest in hydrate reformation and ice generation during hydrate dissociation by depressurization [11, 12], but in-situ visualization investigation in porous media has been rarely reported. X-ray computed tomography (CT) and magnetic resonance imaging (MRI) have been used to visualize MH dissociation in porous media recently. Some researchers have studied in-situ formation and dissociation of MH [13]. Several investigations on MH dissociation under different back pressure, different depressurization rate, and different segmented depressurization methods have been conducted [13-15]. A few studies into hydrate reformation and ice generation have been reported. However, visualization of MH dissociation behaviour by depressurization, details related to hydrate reformation and ice generation, and some inherent mechanisms still need to be clarified. In this study, we conduct the dissociation of MH by controlling the depressurization rate gradually to designed back pressures to minimize the effect of lacking sufficient heat transfer which will cause hydrate reformation and ice generation. MRI data and images are obtained to analyze hydrate dissociation behavior under different back pressures. We also focus and study ice generation under rapid depressurization and low back pressure condition. 2. Experimental section Fig. 1. Three main stages of MH dissociation by Fig. 2. Schematic of experimental apparatus. depressurization. As shown in Fig. 2, the experimental apparatus consisted of an MRI system, a high-pressure vessel, 3 high-pressure pumps (A for gas injection, B for water injection, C for back pressure controlling for MH dissociation), a thermostatic bath, and a data acquisition system. The MRI system was operated at 400 MHz to visualize MH formation and dissociation, since MRI can detect the 1 H in liquids and produce liquid (water in our experiment) distribution images. The field of view was 30*30mm, and the MRI image matrix was 128*128 pixels. The size of the cylindrical vessel was Φ38*314mm. BZ-02 glass beads with porosity of 35.4% were packed into the vessel. Details about the experimental apparatus are reported in precious studies [13-15]. Experimental procedure is as follows: The glass beads were tightly packed into the vessel and placed into the MRI system. Then the vessel was evacuated. Deionized water was slowly injected into the vessel until the pressure reached 6 MPa to

3 Zhen Fan et al. / Energy Procedia 105 ( 2017 ) make the porous media water-saturated. High-pressure N2 gas was injected to displace water, and the amount of displaced water was recorded by MRI system. After evacuating the vessel, CH4 gas was injected to the vessel until the pressure reached 6 MPa. The thermostatic bath was set as K for hydrate formation. When pressure and MRI mean intensity became stable, pump C was operated under designed program for MH dissociation. Details of the experimental parameters and initial conditions are shown in Table 1. Table 1. Experimental parameters and initial conditions case Production Depressurizing Rate S wi (%) S hi (%) S gi (%) P i (MPa) S w0 (%) Pressure (MPa) (MPa/min) Constant Pressure Result and Discussion Four different depressurization programs were conducted for MH dissociation. Sagittal plane was chosen to study the experimental process. According to previous studies [13-15], MH saturation and water saturation can be calculated by the following equations: (1) (2) Here, is the initial water saturation, and are MRI mean intensities of liquid water at t=0 and t=i min. 3.1 MH dissociation behaviour under different back pressure Fig. 3 shows the core pressure evolution for cases 1-3. The pressure was depressurized by the rate of MPa/min to 0.6, 1.6, and 2.6 MPa, respectively. The depressurization processes were controlled by pump C steadily to ensure MH dissociated by gradual depressurization without rapid fluctuation. Fig. 4 shows the variation of water distribution for cases 1-3. It s seen that MH dissociated spatially at the first min for all 3 cases. It can be inferred that during this stage sensible heat of the sediment provided most of the heat for hydrate dissociation. Then MH dissociated from the heat transfer wall into the axis of the vessel. Ambient heat transfer played an important factor driving MH dissociation. As can be seen in Fig. 4, a lower production pressure leads to a more rapid hydrate dissociation process. Fig. 4 also indicated that obvious hydrate reformation and ice generation can be minimized by controlling the depressurization rate gradually to designed production pressures. Fig. 3. Evolution of core pressure for cases 1-3. Fig. 5. Increment of MRI mean intensity for cases 1-3.

4 4766 Zhen Fan et al. / Energy Procedia 105 ( 2017 ) Fig. 4. Variation of water distribution for cases 1-3. Fig. 5 shows the increment of MRI mean intensity during MH dissociation for cases 1-3. The MRI mean intensity reflected the amount of water in the sediment and therefore indicated the dissociation of MH. Very small amount of MH dissociated at the first 10 minutes, indicated that MH hydrate didn t dissociate until the pressure was below the equilibrium curve (seen as the 1st stage in Fig.1). Then MI increased continuously for all 3 cases. The equilibrium temperature for case 1 was lower than the other 2 cases, which means the sediment could provide more sensible heat. Thus the dissociation rate of case 1 was more rapid than other 2 cases. There were fluctuations for case 1 during min, for case 2 during min, and for case 3 during min. And the fluctuation range of case 1 was higher than the other 2 cases. Since no obvious hydrate reformation or ice generation but water migration near the vessel wall was observed for case 1 in Fig. 4, so the fluctuation was regarded to be caused by water migration near the vessel wall. The other reason may be the effect of temperature change on MRI mean intensity measurement. 3.2 Ice generation during MH dissociation The pressure of pump C was controlled to constant pressure 1.6MPa in case 4. As shown in Fig. 6, the core pressure decreased rapidly to production pressure in the first 6.5 min. Notable, the pressure increased during min, which indicated there existed hydrate reformation or ice generation in the sediment. Since the equilibrium temperature at 1.6 MPa for MH is -7.5 [15], which is lower than 0, thus we consider ice generation occurred in this case. Fig. 7 shows the variation of water distribution for case 4. MH dissociated spatially in the first 4.27 min, because the core pressure decreased to production pressure rapidly. Due to insufficient heat transfer from the surroundings, the sediment temperature decreased quickly and large amount of ice generated spatially distributed in the vessel. Then the ambient heat transfer drove ice melt and hydrate dissociation from the surrounding wall in. Since the melt of ice absorbs heat, it will hinder hydrate dissociation in the sediment. After about min the distribution of water remained stable in the vessel.

5 Zhen Fan et al. / Energy Procedia 105 ( 2017 ) Fig. 6. Evolution of core pressure for case 4. Fig. 8. Increment of MRI mean intensity for case 4. Fig. 7. Variation of water distribution for case 4. Fig. 8 shows the increment of MRI mean intensity during MH dissociation for case 4. MI increase at the beginning, which agreed with the spatial dissociation in Fig. 7. A rapid reduction of MI occurred between 4.27 and 6.40 min, which indicated ice generation. Then the MI increased continuously caused by ice melt and hydrate dissociation driven by the ambient heat transfer. In this study, the saturation of generated ice at 6.40 min was evaluated by the following equations: (3) (4) (5) Here, is the vessel volume. is heat released by ice generation. is the heat supplied by ambient heat transfer. is the heat of MH dissociation. and are the bulk density and bulk specific heat of the formation composed of sediment, hydrate, and fluids [8]. is the temperature change of the formation. However, the temperature change of the formation was not measured since there was no thermal resistance in the vessel. The input heat from the thermostatic bath was hard to be evaluated without the temperature data of the formation. By assuming that it was the amount of decreased water saturation calculated by Eq. 3 that transformed into ice, the minimum saturation of the generated ice was estimated:. 4. Conclusion This work investigated MRI visualization and analysis for MH dissociation by controlling the depressurization rate gradually to 3 designed production pressures. Obvious hydrate reformation and ice generation can be minimized by controlling the depressurization rate gradually to designed production pressures. MH dissociated spatially at the beginning of depressurization. A low production pressure can supply more sensible heat and MH dissociated rapidly under a low production pressure. MH dissociated from the heat transfer wall into the axis of the vessel later. Ambient heat transfer played an important factor driving MH dissociation. Ice generation during MH dissociation under rapid depressurization and low back pressure condition was further studied. Due to insufficient heat transfer from the surrounding thermostatic bath, large amount of ice generated spatially in the vessel. Then ambient heat transfer drove ice melt and hydrate dissociation from the surrounding wall in. The minimum saturation of the generated ice in the vessel was estimated as 8.44%.

6 4768 Zhen Fan et al. / Energy Procedia 105 ( 2017 ) Acknowledgements This study has been supported by the Major Program of National Natural Science Foundation of China ( ), the National High Technology Research and Development Program of China 863 Program (Grant No.2013AA ), and the Natural Science Foundation of China (Grant Nos , , and ). References [1] Collett TS. Energy resource potential of natural gas hydrates. AAPG bulletin 2002; 86: [2] Collett TS. Natural gas hydrate as a potential energy resource. In: Max MD, editors. Natural Gas Hydrate, Springer Netherlands; 2000, p [3] Beaudoin YC, Boswell R, Dallimore SR, Waite W. Frozen heat: A UNEP global outlook on methane gas hydrates, United Nations Environment Programme, GRID-Arendal; [4] Min LS. A global survey of gas hydrate development and reserves: Specifically in the marine field. Renew Sust Energ Rev 2015; 41: [5] Chong ZR, Yang SHB, Babu P, Linga P, Li XS. Review of natural gas hydrates as an energy resource: prospects and challenges. Appl Energ, 2016; 162: [6] Zhao JF, Liu D, Yang MJ, Song YC. Analysis of heat transfer effects on gas production from methane hydrate by depressurization. Int J Heat Mass Tran 2014; 77: [7] Song YC, Cheng CX, Zhao JF, Zhu ZH, Liu WG, Yang MJ, Xue KH. Evaluation of gas production from methane hydrates using depressurization, thermal stimulation and combined methods. Appl Energ 2015; 145: [8] Zhao JF, Zhu ZH, Song YC, Liu WG, Zhang Y, Wang DY. Analyzing the process of gas production for natural gas hydrate using depressurization. Appl Energ 2015; 142: [9] Tang LG, Li XS, Feng ZP, Li G, Fan SS. Control mechanisms for gas hydrate production by depressurization in different scale hydrate reservoirs. Energ Fuel 2007; 21: [10] Darvish MP, Hong H. Effect of conductive and convective heat flow on gas production from natural hydrates by depressurization. In: Advances in the Study of Gas Hydrates, Springer US; 2004, p [11] Konno Y, Uchiumi T, Oyama H, Jin Y, Nagao J, Masuda Y, Ouchi H. Dissociation behavior of methane hydrate in sandy porous media below the quadruple point. Energ Fuel 2012; 26: [12] Oyama H, Konno Y, Suzuki K, Nagao J. Depressurized dissociation of methane-hydrate-bearing natural cores with low permeability. Chem Eng Sci 2012; 68: [13] Song YC, Wang SL, Yang MJ, Liu WG, Zhao JF, Wang SR. MRI measurements of CO2 CH4 hydrate formation and dissociation in porous media. Fuel 2015;140: [14] Zhang LX, Zhao JF, Dong HS, Zhao YC, Liu Y, Zhang Y, SongYC. Magnetic resonance imaging for in-situ observation of the effect of depressurizing range and rate on methane hydrate dissociation. Chem Eng Sci 2016; 144: [15] Yang MJ, Fu Z, Zhao YC, Jiang LL, Zhao JF, Song YC. Effect of depressurization pressure on methane recovery from hydrate gas water bearing sediments. Fuel 2016; 166: Biography Yongchen Song, PhD, Professor, Executive Vice-President of Institute of Science and Technology of Dalian University of Technology of China. Prof. Song has been long engaged in the researches on the basic properties of gas hydrates, safe and efficient exploitation of gas hydrates, and resource utilization of carbon dioxide.