Analyzing the Process of Gas Production from Methane Hydrate via

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Analyzing the process of gas production from methane hydrate via nitrogen injection Lunxiang Zhang, Yangming Kuang, Xiaotong Zhang, Yongchen Song, Yu Liu, and Jiafei Zhao Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 12 Jun 2017 Downloaded from http://pubs.acs.org on June 13, 2017

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Industrial & Engineering Chemistry Research

Analyzing the process of gas production from methane hydrate via nitrogen injection

Lunxiang Zhang1, Yangming Kuang1, Xiaotong Zhang2,3, Yongchen Song1, Yu Liu1,*, Jiafei Zhao1,* 1

School of Energy and Power Engineering, Dalian University of Technology, Dalian

116024, P.R. China; 2Interdisciplinary Institute of Neuroscience and Technology, Zhejiang University, Hangzhou, 310029, P.R. China;

3

College of Biomedical

Engineering & Instrument Science, Zhejiang University, Hangzhou, 310029, P.R. China

*Corresponding author. Tel: +86 411 84706722; Fax: +86 411 84706722. E-mail addresses: [email protected]; [email protected].

Abstract The application of air or nitrogen injection to extract natural gas from hydrates has attracted considerable attention because both are abundant in space and time; however, few studies have considered the relatively new method. This study employed magnetic resonance imaging to investigate the characteristics of hydrate dissociation via N2 injection. The results show that methane hydrate dissociation using N2 injection can be divided into three main stages: free gas and water liberation, hydrate dissociation driven by chemical potential differences, and restriction of hydrate dissociation due to increased resistance to N2 diffusion and decreased contact area

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between N2 and hydrate. Various factors associated with the N2 injection method were considered, including injection rate and hydrate saturation. Furthermore, a comparison of recovery-injection ratio and methane cumulate production ratio to the results of hydrate dissociation via depressurization suggest that in low hydrate saturation exploitation, N2 injection has advantages over the depressurization method. Keywords: Hydrates; Nitrogen injection; Methane recovery; Water production; Dissociation Process

1. Introduction Natural gas hydrates (NGH) are crystalline solid compounds that contain large volumes of natural gas, which are generally found in permafrost layers and submarine sediments 1. It has been suggested that NGH represent the largest source of hydrocarbons on Earth 2. Therefore, how to economically, safely, and efficiently exploit natural gas from NGH for energy production is attracting considerable attention 3. Current methods for recovering natural gas are based on breaking thermodynamic equilibrium between temperature shifts, pressure changes, and chemical environment alters 4; however, the development of these methods is not yet sufficient for industrial production owing to unresolved complications associated with geological hazards

5

and seabed ecosystem destruction 6. The thermal stimulation method (i.e., increasing the reservoir temperature above that required for NGH formation 7) suffers from poor energy efficiency

8, 9

. Depressurization technology (i.e., decreasing system pressure


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below that required for NGH formation

10

) is inhibited by insufficient heat transfer,

which leads to hydrate reformation, ice generation, and a significant reduction in the gas recovery rate

11-13

. Recent studies have proposed combined treatment methods

involving both thermal stimulation and depressurization is worthy of study as this could potentially solve the problems of poor energy efficiency and low gas production 14

; however, this would represent a complex exploitation process and the applicable

geological conditions of hydrate reservoirs require further investigation

15

. The

inhibitor injection approach, which depends on the unique chemical and physical properties of gas hydrate

16

, is limited by high economic costs and environmental

impacts 17. The CH4/CO2 replacement method represents a promising new approach to NGH exploitation as it can simultaneously achieve CO2 storage for global warming mitigation and CH4 recovery for energy production experimental investigations

19-21

18

; however, extensive

have shown that while CH4/CO2 replacement occurs

easily at the surface of NGH during the initial stage, the replacement rate subsequently slows significantly owing to the diffusion-limited transport of CO2. More recently, NGH exploitation technology has harnessed the chemical potential difference of methane in the hydrate and gas phases 22, 23. Haneda et al. [19] observed hydrate dissociation by sweeping air through hydrate samples. Masuda et al. [20] swept nitrogen through hydrate filled limestone cores and observed the resulting hydrate dissociation. Based on these studies, a novel and promising method was proposed for the remediation of gas hydrate blockages in oil and gas pipelines via nitrogen purging. Hydrate plug dissociation using nitrogen purging was shown to



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cause channel development, which was significantly different from the radial dissociation associated with the depressurization method

24

. Kang et al. [22]

developed a nondestructive method for NGH recovery driven by air and CO2 and demonstrated that the initiation of NGH dissociation leads to replacement of methane by injected air or CO2/air. This method was shown to be more efficient, nondestructive, and more commercially-viable than other methods. Despite the significant body of work into hydrate dissociation, relatively few studies have considered NGH production methods based on exploiting chemical potential through air or N2 injection, both of which are abundant in space and time. NGH dissociation using gas injection is a complex process involving heat and mass transfer. Wang et al. [23] performed a series of experiments on CH4 recovery from hydrate-bearing sediments via gas sweep, and investigated the influences of injection mode (continuous and batch mode), hydrate saturation (between 17.62% and 27.64%), and N2 injection rate (16.7 ml/min and 50 ml/min) on CH4 recovery rate. Their results suggested that the gas sweep method has advantages for exploiting low saturation hydrate reservoirs 25; however, to date few other studies have considered the complex processes and characteristics of hydrate decomposition via gas sweep, the understanding of which is important for the development and promotion of the gas injection method. In this study, we employed for the first time magnetic resonance imaging (MRI) to investigate the characteristics of hydrate dissociation via N2 injection. The reasons for changes in water and CH4 during methane hydrate (MH) dissociation were analyzed


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and the optimal conditions for N2 injection were considered, including injection rate and hydrate saturation. In addition, the recovery-injection ratio and methane cumulate production ratio in each experiment was evaluated separately and compared to the results of hydrate dissociation via depressurization method. These reported experimental results may give support for further investigation of this novel and promising NGH exploitation method.

2. Materials and Methods 2.1 Experimental Materials and Apparatus The experimental apparatus consisted of an MRI system (Varian, Inc., Palo Alto, CA. US), a high-pressure vessel (12 MPa), high-pressure pumps (260D, Teledyne ISCO Inc., USA), refrigerated circulators (F25-ME, JULABO Inc., Germany), a vacuum pump (SHB-111 A, SJSK Exp. Co., Ltd, China), a recycling cylinder, and a data acquisition system (Figure 1). The MRI system (operating at 400 MHz and 9.4 T) was used to measure the 1H contained in liquid water, but did not image the 1H contained in CH4 gas and solids (i.e., MH or crystal ice) because of their much shorter transverse relaxation times. A spin echo multi-slice pulse sequence (SEMS) was used to image the formation and dissociation of MH under the following conditions: time of repetition (TR) = 1000 ms, time of echo (TE) = 4.39 ms, field of view (FOV) = 30 mm × 30 mm, and thickness = 4.0 mm. The MRI image data matrix was 128 × 128 and the acquisition time of the SEMS was 2 min 8 s. The high-pressure vessel, which was made from



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non-magnetic material (polymide), had an inside volume of 35.34 ml, an inner diameter of 15 mm, and a height of 200 mm. Quartz glass beads (ASONE, Co., Ltd., Japan) BZ-01 (0.010–0.120 mm, average 0.1 mm) were used to imitate porous media in the high-pressure vessel. Coolant (3M Fluorinert FC-40, St. Paul, MN, USA) within a jacket surrounding the high-pressure vessel was used to control temperature during experiments. A temperature transducer (Yamari Industries, Japan; range = 245– 473 K at a precision of ± 0.1 K) and pressure transducers (Nagano Co., Ltd., Japan; range = 0–27.6 MPa at a precision of ± 0.1 MPa) were connected to the high-pressure vessel. Additional details concerning experimental apparatus are summarized in a previous study 26.

2.2 Experimental Procedures Two sets of experiments were performed (Table 1). The first set was conducted using varying N2 injection rates (0.1, 0.3, 0.5, 1.0, 2.0 and 5.0 mL/min). Each experiment was repeated several times and imaged by MRI, because the formation of MH is random and the FOV is smaller than the effective size of the MRI vessel. The second set of experiments considered different MH saturation levels, all with an injection rate of 1 mL/min. The experimental process has two parts: (1) the formation of MH in the porous media, and (2) the dissociation of MH via N2 injection. 2.2.1 Hydrate Formation Methane hydrates were formed by CH4 gas (99.99% purity, Dalian Special Gas Co.,


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Ltd., China) and deionized water supplied by a water purification system (Aquapro2S, Aquapro International Company LLC., USA). Controlled by high-pressure pumps and refrigerated circulators filled with the glycol/water mixture, CH4, and deionized water were injected into the high-pressure vessel. During the process of MH formation, the temperature of the experimental system was maintained at 275.15 ± 0.1 K. Previous papers 27 provided the details of the current hydrate formation procedure.

2.2.2 Hydrate Dissociation During the entire process of hydrate dissociation, the coolant-filled refrigerated circulator maintained the same temperature as the MRI vessel (275.15 ± 0.1 K). The equilibrium pressure of the MH was ~3.3 MPa

28

; however, accurate equilibrium

pressure for each experiment was difficult to measure. To avoid the pressure-driven formation and dissociation of MH during decomposition, equilibrium pressure was first set to 3.1 MPa below the equilibrium pressure that existed after the formation of MH, and then few MHs were dissociated. Accurate equilibrium pressure was confirmed when the MRI signal intensity and the pressure of the system had been stable for an hour. After that, the experiment began and the system pressure was maintained. N2 (99.99% purity, Dalian Special Gas Co., Ltd., China) was injected continually from the bottom of the MRI vessel using a high-pressure pump and a temperature of 275.15 ± 0.1 K, which was controlled by a refrigerated circulator filled with the glycol/water mixture. The water and gas produced were collected using a measuring cylinder. Hydrate decomposition was considered complete when no



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further changes in the signal intensity and system pressure were detected. Finally, the inlet and outlet of the MRI vessel were closed and the temperature of the vessel was increased to 293.15 K to decompose any residual MH. Once the temperature and pressure stabilized, the temperature of the vessel was reduced to 273.15 K and the pressure and MRI signal intensity differences between the end of the dissociate experiment and the current conditions were used to determine whether MH had decomposed completely 24.

2.3 Calculations In this study, the MRI system for measuring 1H in liquid was used to directly observe the MH formation and dissociation process. The sagittal plane was selected to display the experimental process in this study. It should be emphasized that all of the mean intensity (MI) data for the images were for a FOV of 30 × 30 mm in this investigation. The MH saturation, which was defined as the volume fraction of the space occupied by MH, was quantified using MI data. It has been reported that 1.25 m3 of MH can be formed from 1 m3 of fresh water at standard temperature and pressure (STP)

29

;

therefore, MH saturation can be calculated using Equation (1), while residual water saturation (RWS) at “i” minute can be described using Equation (2): = 1.25 × RWS =

×

×

× 100%

× 100%

(1) (2)

where is initial water saturation, I0 and Ii are the MI of water at the initial time and “i” minute, respectively. Both saturation and MI are dimensionless.


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The average hydrate dissociation rate (AHDR) and hydrate dissociation rate (HDR) can be expressed using Equation (3) and (4), respectively: AHDR = HDR =

(3)

∆

(4)

∆

where is the initial MH saturation before hydrate dissociation; t is the time of MH dissociation; and △ are MH saturation at the “i” and “i+△ t” minutes, respectively; and △ t is 2 min 8 s, which was the acquisition time of the sequence. The actual average methane gas recovery rate (AMRR) and methane gas recovery rate (MMR) were calculated from Equation (5) and (6), respectively. The relationship between AHDR and AMRR, and between HDR and MMR were described by Equation (7) and (8), respectively. AMMR = MMR =

!"#$

(5)

∆ !"#$ !"#$

(6)

∆

AMMR = AHDR × MMR = HDR ×

%& × '(# )(#

%& × '(# )(#

(7) (8)

where *+,- is the number of moles of recovered methane from completely △ dissociated MH; *+,and *+,are the cumulate number of moles of recovered

methane at t = “i” and “i+△ t” minutes, respectively; ./ is the volume of pores, which is equal to 12.30 mL; 0), and 1), are the molarity and density of MH, which are equal to 124.14 g/mol and 0.92 g/mL, respectively.

3. Result and Discussion



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3.1 Dissociation behavior at different nitrogen injection rates For the first set of experiments, different N2 injection rates of 0.1, 0.3, 0.5, 1.0, 2.0 and 5.0 mL/min was experimented to investigate the dissociation behavior of MH via N2 injection. Figure 2 presents the MRI images of water distribution during MH dissociation in Case 1. The brighter areas in the images represent liquid water, and the blacker areas represent other parts in the vessel. For the first set of experiments, MRI images show the process of N2 injection beginning at 0 min (Figure 2). The N2 injected into the FOV of the vessel was diffused and penetrated the surface of MH, where a boundary between the solid hydrate and gas phase formed. MH was then gradually decomposed as CH4 partial pressure decreased at the boundary with continuous N2 injection. Therefore, the MRI images increasingly became brighten. The process of MH dissociation continued until the chemical potentials of CH4 in the solid hydrate and gas were equal. For Case 1-1, at 125 minutes no noticeable variations were observed in MRI images. At 132 min, the volume of water had increased significantly near the vessel wall. This phenomenon can be explained by the “wall effect” (i.e., porosity near the wall is larger than in the internal space 30), which caused N2 to preferentially penetrate channels packed by porous media near the vessel wall. From 132 to 230 min, MH decomposed continuously and liquid water channels extended from the bottom to the top of the FOV parallel to the direction of N2 injection. This observation differs from characteristic radial dissociation observed during by traditional exploitation methods, such as depressurization and thermal stimulation 26, 27.


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MH axial decomposition was not clearly observed because the flow of N2 in the vessel was a complicated process. From 230 to 300 min, the area surrounded by water decomposed slowly as MH was hard to contact with N2. For Cases 1-2 to 1-6, the characteristics of MH dissociation were similar to Case 1-1, except for significant liquid water flow out of the vessel from 60 to 85 min in Case 1-6. That’s because N2 has lower solubility in water and can discharge liquid water when the velocity of N2 injection is large enough (maximum injection rate in Case 1-6 = 5.0 mL/min). Meanwhile, the phenomenon of “wall effect” can also be detected at the beginning of MH dissociation in Cases 1-2 to 1-6. With an increase in N2 injection rate from 0.1 to 0.5 mL/min (from Cases 1-1 to 1-3), the onset time of MH dissociation was shortened, which can be explained by that the injection rate affected the time of N2 injected to the FOV. However, with a further increase from 0.5 to 5.0 mL (from Cases 1-3 to 1-6), there was no obvious decrease in the onset time. This was because the reduction in the time of N2 injected into the FOV fell as N2 injection rate increased, and this decrease was difficult to monitor using MRI sequences with a 2 min 8s acquisition time. Figure 3 shows the variation in RWS during MH dissociation of Case 1. The water saturation in initial condition, dissociation condition and final condition represented the water saturation before hydrate formation, before hydrate decomposition and after hydrate dissociation, respectively. As shown in the figure, the initial water saturations of Cases 1-1 to 1-6 were 36.98%, 36.86%, 33.10%, 36.30%, 38.59% and 34.94%, respectively, whereas the final water saturations were 34.02%, 34.55%, 28.22%, 27.24%, 23.80%, 11.20%. This observation reflects the fact that some liquid water



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was displaced by N2 through the outlet of the vessel, especially at higher N2 injection rates (i.e., Cases 1-3 to 1-6, where water saturations decreased by 4.88%, 9.06%, 14.69%, and 23.74%, respectively). In Cases 1-3 to 1-6, the maximum RWS was less than the initial water saturation, reflecting the early period where CH4 recovery was significant but water production was low. For Cases 1-3, 1-5, and 1-6, we observed a clear decrease in RWS during the later period of MH decomposition, reflecting a shift to abundant water production and little or no CH4 recovery. Figure 4 shows HDR and AHDR versus time in Case 1. For Cases 1-1 and 1-2, the HDR during MH dissociation can be split into an initial rapid dissociation stage and then a decreased decomposition rate stage. There are two possible explanations for this result: (1) increased resistance of mass transfer between N2 in the gas phase and the surface of MH as the volume of liquid water in pores increased; (2) decreased contact area between N2 and MH with hydrate dissociation.

31

. During MH

decomposition, N2 preferentially flows through growing channels where resistance is lowest, decreasing the contact area between N2 and MH 24. For Cases 1-3 to 1-6, the process of MH dissociation still maintained in a rapid dissociation stage. That’s because, in these Cases, liquid water would be discharged to the outlet of the vessel to provide pathways for N2 to contact with the surface of solid hydrates. For Cases 1-2 to 1-6, the AHDR was 0.14, 0.25, 0.65, 0.67, 0.69, and 0.74%/min, respectively (Figure 4). Initially, AHDR was found to increase with increasing N2 injection rate from 0.01 to 5.0 mL/min; however, higher N2 injection rates (between 1 and 5 mL/min) were not associated with clear AHDR growth. This confirms that an


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optimum N2 injection rate would enhance the economic benefits of MH exploitation.

3.2 Dissociation behavior at different hydrate saturation The saturation of NGH-bearing sediments is an important evaluating index for hydrate exploitation

32

. The dissociation characteristics of the second set of

experiments were similar to the first set, including the wall effect (Figure 5). With an increase in hydrate saturation (from 20.87% to 64.05%), the time of dissociation also increased (from 43 to 137 min). Hydrate saturation had no obvious effect on the increase in RWS (Figure 6). These results show that hydrate saturation influences timescales of MH dissociation but has little impact on the rate of MH decomposition. For further analysis of the effect of MH saturation on hydrate dissociation, water saturation in initial, dissociation and final conditions were calculated in Figure 6. The results show that the initial water saturations of Cases 2-1 to 2-4 were 18.42%, 24.18%, 33.10%, and 62.51%, respectively, while the final water saturation levels were 18.32%, 20.79%, 28.22% and 28.47%, respectively (i.e., the decrease in water saturation equaled 0.10%, 3.39%, 4.88%, and 34.04%, respectively; Figure 6). As the validation process mentioned in 2.2.2, the differences of pressure and MRI signal intensity were used to calculate the residual hydrate saturation if any hydrate remained after N2 injection. The results showed that the residual hydrate saturation of Cases 2-1 to 2-4 were 0.00%, 0.00%, 1.58%, and 39.75%; therefore, actual decreases in water saturation due to liquid water outflow were 0.10%, 3.39%, 3.61%, and 2.24%, respectively. The incomplete decomposition can be explained by increased capillary



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pressure and decreased contact area between N2 and solid hydrate. Capillary pressure was calculated using Equation (11) when the pore shape and the contact angle are known. 23 =

4 × 567689 × :;< => ?

(9)

where the pore shape and the contact angle are known, @? is the receding non-wetting and wetting contact angle, and r is the inscribed radius. As MH saturation increased, the inscribed radius (r) decreased, leading an increase in capillary pressure (23 ). When capillary pressure was higher than the difference between the pressure of N2 and water in water-filled pores (23 > 2B4 − 2D ), N2 was unable to infiltrate pores

33

. In other words, at the same N2 injection of 1 mL/min

which means the same entry pressure of N2, water in the pores was more difficult to be discharged by N2 with an increase in MH saturation. For Case 2-4, water increasingly remained in pores as the pressure of N2 fell below the summation of water pressure and increased capillary pressure (2B4 < 2D + 23 ). Under these conditions, N2 preferred to flow through channels with the least resistance. Meanwhile, the surface area for N2 contact with MHs was relatively less as most pores were occupied by exceedingly high saturation of MH and that would reduce and be plugged by gradually thicken water layers once decomposition water produced, which was similar to the result that the MH decomposition surface area reduced with MH dissociation

31

. Finally, the dissociation of MH ceased, leaving residual hydrate

(39.75%) within the sediment and little water production (2.24%). For Cases 2-1 to 2-3, the decreased capillary pressure and increased contact area showed that MH


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decomposition was almost completed. These results suggest that the N2 injection method is more suitable for low hydrate saturation exploitation.

3.3 Gas Production Potential In order to evaluate this NGH exploitation method, we defined the ratio between the cumulate numbers of moles of CH4 recovered and N2 injected at “i” minute as the recovery-injection ratio (RIR), which was calculated using Equation (10). We also assed the method using the methane cumulate production ratio (MCPR) calculated using Equation (11), which was defined as the ratio between the cumulate numbers of moles of CH4 recovered at “i” minute and the total numbers of moles of CH4 recovered. Equations (10) and (11) are expressed as: RIR = MCPR =

!"#$

(10)

!FG

!"#$

!"#$

× 100%

(11)

where *+,and *B4 are the cumulate numbers of moles of recovered CH4 and

injected N2 at t = “i” minutes, respectively; and *+,- is the number of moles of recovered methane from complete MH disassociation. For Cases 1-1 and 1-2, the PIR during MH dissociation using N2 injection As the RIR shown in the figure, the process of MH dissociation using N2 injection can be divided into three main stage (Figure 7). During the first stage (Figure 7, A-B), N2 was injected into the MH reservoir and gradually invaded pores, while free gas and water were liberated. During this stage, MH remained stable. During the second stage (Figure 7, B-C), N2 diffused and penetrated the surface of MH, where a boundary



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between solid hydrate and gas phase formed. Then, with continuous N2 injection, MH gradually dissociated as the partial pressure of CH4 on the surface of hydrate decreased. During this stage, the chemical potential difference between the solid hydrate and gas phase were the main driving force of the reaction and most MH was dissociated. For Cases 1-1 and 1-2, MCPR increased from 0.00% to 74.56% and 66.51%, respectively. During the third stage (Figure 7, C-D), the rate of MH dissociation fell with increased resistance of N2 diffusion and decreased contact area between N2 and MH. During this stage, the conditions of N2 injection and of the MH deposit (hydrate saturation, water saturation, porosity, etc.) impacted on the rate and amount of hydrate dissociation. Finally, the rate of MH dissociation became extremely slow and sometimes ceased as the chemical potentials of CH4 in both the solid hydrate and gas phases became equal. At this point, some hydrate residues were observed in the MH reservoir. For Cases 1-1 and 1-2, MCPR increased from 74.56% and 66.51% to 95.89 and 95.38%, respectively. Note that the process of continues N2 injection after D point in figure 7, the MH saturation remains the same for an hour at least, which states that the stop of MH decomposition via N2 injection. Figure 8 (a) shows the RIR versus time during MH dissociation in Cases 1-3 and 1-6 and Figure 8 (b) shows that in Cases 2-1 and 2-4. For Cases 1-3 to 1-6, increased N2 injection rate led to a shortened first stage, because N2 was able to diffuse faster to the surface of MH, an extended second stage, and a shortened third stage. These observations reflect the fact that higher N2 injection rates discharged liquid water to the outlet of the vessel and provided pathways for better contact between N2 and MH.


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As a result, MH dissociation continued during the rapid decomposition stage. For Cases 2-1 and 2-4, decreased MH saturation resulted in an extended first stage, reflecting the need for more pores to be invaded by N2 before penetration into the decomposition boundary, a second stage with no obvious change (i.e., the variation in MH saturation had little impact on the rate of MH decomposition), and a significant impact on the third stage, where saturation increased to a limit value (Figure 8b). At this point, it was difficult for MH to dissociate because of increased capillary pressure and a decrease in contact area between N2 and solid hydrate. For Experiment 2-4, the third stage lasted from 19.2 to 113.07 min, which was much longer than that in experiments 2-1 to 2-3, while the MCPR only increased from 15.08% to 37.94%. Note that the decrease and fluctuation in the cure of Figure 8 during the second stage are caused by the vibration in free water production when we use the MRI to monitor the hydrate dissociation. Figure 9 shows the comparison of AHDR between N2 injection method and depressurization method in our previous paper

26, 27

and Wang et al. study

34

. As

shown in figure, the dissociation condition of 1 mL/min represented the rate of N2 injection was 1 mL/min by injection method and 0.3, 0.5, 0.7, 1.1 and 3.3 MPa represented the depressurizing range use depressurization method. When MH saturation was around 21%, the AHDR in injection method was higher than that in depressurization method, which means the N2 injection method has advantage in low MH saturation exploitation than depressurization method

25

. However, as shown in

figure, when MH saturation was around 27%, the AHDR in two cases were higher



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than that in injection method. That’s because the depressurization approach in this two cases were improved by multi-stage depressurization to avoid the reduction in gas recovery rate as the hydrate reformation and ice generation during MH dissociation.

4. Conclusions In this study, we used MRI to investigate the characteristics of hydrate dissociation during N2 injection. Based on the results, a number of conclusions were drawn. (1) At the onset of hydrate dissociation, MH decomposes significantly near the MRI wall because of the wall effect, and axial MH decomposition occurs parallel to the direction of N2 injection. This is in contrast to radial MH dissociation associated with traditional exploitation methods (i.e., depressurization and thermal stimulation). (2) With increasing N2 injection rate, the onset time of MH dissociation shortens and AHDR increases. However, at high N2 injection rates (1.0, 2.0, and 5.0 ml/min), water production occurs, while increases in AHDR cease. These results confirm that the application of optimum N2 injection rates would enhance economic benefits during MH exploitation. (3) Hydrate saturation influences the onset time of MH dissociation, but has little impact on the rate of MH decomposition. With increased hydrate saturation there is an increase in the saturation of undecomposed hydrate, which can be explained by an increase in capillary pressure and a decrease in the contact area between N2 and solid hydrate. These results suggested that the N2 injection method is most suitable for low hydrate saturation exploitation. (4) The process of MH dissociation using N2 injection can be divided into three main stages: free gas and


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water liberation, hydrate dissociation driven by a difference in chemical potential, and hydrate dissociation restricted by increased resistance to N2 diffusion and decreased contact area between N2 and hydrate. (5) The RIR and MCPR show that the N2 injection method has advantages over the depressurization method in low hydrate saturation exploitation. The findings of this study clearly demonstrate that N2 injection rate and hydrate saturation affect hydrate decomposition. The results support the need for further investigation of injection methods for NGH exploitation. In addition, understanding the effects of MH deposit conditions (e.g., excess gas and excess water) on hydrate dissociation via N2 injection also requires further study.

Acknowledgments This study has been supported by the Major Program of National Natural Science Foundation of China (51436003).



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Reference (1) Sloan, E. D. Fundamental principles and applications of natural gas hydrates. Nature 2003, 426 (6964), 353-359. (2) Milkov, A. V. Global estimates of hydrate-bound gas in marine sediments: how much is really out there? Earth-Sci. Rev. 2004, 66 (3-4), 183-197. (3) Makogon, Y. F.; Holditch, S. A.; Makogon, T. Y. Natural gas-hydrates — A potential energy source for the 21st Century. J. Pet. Sci. Eng. 2007, 56 (1-3), 14-31. (4) Makogon, Y. F. Natural gas hydrates – A promising source of energy. J. Nat. Gas Sci. Eng. 2010, 2 (1), 49-59. (5) Dillon, W. P.; Nealon, J. W.; Taylor, M. H.; Lee, M. W.; Drury, R. M.; Anton, C. H. Seafloor collapse and methane venting associated with gas hydrate on the blake ridge: causes and implications to seafloor stability and methanerelease. Natural gas hydrates: Occurrence, distribution, and detection 2001, 211-233. (6) Kvenvolden, K. A. Potential effects of gas hydrate on human welfare. Proc. Natl. Acad. Sci. 1999, 96 (7), 3420-3426. (7) Tsimpanogiannis, I. N.; Lichtner, P. C. Parametric study of methane hydrate dissociation in oceanic sediments driven by thermal stimulation. J. Pet. Sci. Eng. 2007, 56 (1-3), 165-175. (8) Wang, Y.; Li, X. S.; Li, G.; Huang, N. S.; Feng, J. C. Experimental study on the hydrate dissociation in porous media by five-spot thermal huff and puff method. Fuel 2014, 117, 688-696. (9) Kurihara, M.; Sato, A.; Ouchi, H.; Narita, H.; Masuda, Y.; Saeki, T.; Fujii, T. In Prediction of gas productivity from eastern Nankai Trough methane hydrate reservoirs. Offshore technology conference 2008. (10) Chuang, J.; Goodarz, A.; Duane, H. S. Natural gas production from hydrate decomposition by depressurization. Chem. Eng. Sci. 2001, 56, 5801-5814. (11) Wang, Y.; Feng, J. C.; Li, X. S.; Zhang, Y.; Li, G. Analytic modeling and large-scale experimental study of mass and heat transfer during hydrate dissociation in sediment with different dissociation methods. Energy 2015, 90, 1931-1948. (12) Oyama, H.; Konno, Y.; Masuda, Y.; Narita, H. Dependence of Depressurization-Induced Dissociation of Methane Hydrate Bearing Laboratory Cores on Heat Transfer. Energy Fuels 2009, 23 (10), 4995-5002. (13) Wang, Y.; Feng, J. C.; Li, X. S.; Zhang, Y.; Li, G. Large scale experimental evaluation to methane hydrate dissociation below quadruple point in sandy sediment. Appl. Energy 2016, 162, 372-381. (14) Demirbas, A. Methane hydrates as potential energy resource: Part 2-Methane production processes from gas hydrates. Energy Convers. Manage. 2010, 51 (7), 1562-1571. (15) Moridis, G.; Collett, T. Strategies for gas production from hydrate accumulations under various geologic conditions. Lawrence Berkeley Natl. Lab. 2003. (16) Kawamura, T.; Ohtake, M.; Yamamoto, Y.; Haneda, H.; Sakamoto, Y.; Komai, T. Dissociation Behavior of Hydrate Core Sample Using Thermodynamic Inhibitor-Part 3. Inhibitor or Steam Injection Combined with Depressurization and High-Concentration Inhibitor Injection. Int. J. Offshore Polar Eng. 2010, 20 (2), 125-131. (17) Wu, C.; Zhao, K.; Sun, C.; Sun, D.; Xu, X.; Chen, X.; Xuan, L. Current research in natural gas hydrate production. Geol. Sci. and Tech. Info. 2008, 27, 47-52. (18) Lee, H.; Seo, Y.; Seo, Y. T.; Moudrakovski, I. L.; Ripmeester, J. A. Recovering methane from


Page 20 of 33

Page 21 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


solid methane hydrate with carbon dioxide. Angew. Chem. Int. Ed. 2003, 42 (41), 5048-5051. (19) Bai, D. S.; Zhang, X. R.; Chen, G. J.; Wang, W. C. Replacement mechanism of methane hydrate with carbon dioxide from microsecond molecular dynamics simulations. Energy Environ. Sci. 2012, 5 (5), 7033-7041. (20) Zhang, L. X.; Yang, L.; Wang, J. Q.; Zhao, J. F.; Dong, H. S.; Yang, M. J.; Liu, Y.; Song, Y. C. Enhanced CH4 recovery and CO2 storage via thermal stimulation in the CH4/CO2 replacement of methane hydrate. Chem. Eng. J. 2017, 308, 40-49. (21) Lee, B. R.; Koh, C. A.; Sum, A. K. Quantitative measurement and mechanisms for CH 4 production from hydrates with the injection of liquid CO2. Phys. Chem. Chem. Phys. 2014, 16 (28), 14922-14927. (22) Haneda, H.; Sakamoto, Y.; Kawamura, T.; Komai, T. In Experimental study on dissociation behavior of methane hydrate by air. Proceedings of the 5th International Conference on Gas Hydrates 2005, pp 13-16. (23) Masuda, Y.; Konno, U.; Hasegawa, T.; Haneda, H.; Ouchi, H.; Kurihara, M. In Prediction of methane hydrate dissociation behavior by nitrogen injection. Proceedings of the 6th International Conference on Gas Hydrates 2008, pp 6-20. (24) Panter, J. L.; Ballard, A. L.; Sum, A. K.; Sloan, E. D.; Koh, C. A. Hydrate Plug Dissociation via Nitrogen Purge: Experiments and Modeling. Energy Fuels 2011, 25 (6), 2572-2578. (25) Wang, X. H.; Sun, C. Y.; Chen, G. J.; He, Y. N.; Sun, Y. F.; Wang, Y. F.; Li, N.; Zhang, X. X.; Liu, B.; Yang, L. Y. Influence of gas sweep on methane recovery from hydrate-bearing sediments. Chem. Eng. Sci. 2015, 134, 727-736. (26) Song, Y. C.; Zhang, L. X.; Lv, Q.; Yang, M. J.; Ling, Z.; Zhao, J. F. Assessment of gas production from natural gas hydrate using depressurization, thermal stimulation and combined methods. RSC Adv. 2016, 6 (53), 47357-47367. (27) Zhang, L. X.; Zhao, J. F.; Dong, H. S.; Zhao, Y. C.; Liu, Y.; Zhang, Y.; Song, Y. C. Magnetic resonance imaging for in-situ observation of the effect of depressurizing range and rate on methane hydrate dissociation. Chem. Eng. Sci. 2016, 144, 135-143. (28) Kamath, V. A. Study of heat transfer characteristics during dissociation of gas hydrates in porous media. 1984. (29) Sloan, E. D.; Koh, C. A. Clathrate hydrates of natural gases. 3rd ed. Boca Raton: CRC Press. 2008. (30) Kitagawa, A.; Denissenko, P.; Murai, Y., Effect of wall surface wettability on collective behavior of hydrogen microbubbles rising along a wall. Exp. Therm. Fluid Sci. 2017, 80, 126-138. (31) Kumar, A.; Maini, B.; Bishnoi, P. R.; Clarke, M. Investigation of the Variation of the Surface Area of Gas Hydrates during Dissociation by Depressurization in Porous Media. Energy Fuels 2013, 27 (10), 5757-5769. (32) Kumar, A.; Maini, B.; Bishnoi, P. R.; Clarke, M.; Zatsepina, O.; Srinivasan, S. Experimental determination of permeability in the presence of hydrates and its effect on the dissociation characteristics of gas hydrates in porous media. J. Pet. Sci. Eng. 2010, 70 (1-2), 109-117. (33) Mahabadi, N.; Dai, S.; Seol, Y.; Yun, T. S.; Jang, J. The water retention curve and relative permeability for gas production from hydrate-bearing sediments: pore-network model simulation. Geochem., Geophys., Geosyst. 2016, 17 (8), 3099-3110. (34) Wang, S. L.; Yang, M. J.; Wang, P. F.; Zhao, Y. C.; Song, Y. C. In Situ Observation of Methane Hydrate Dissociation under Different Backpressures. Energy Fuels 2015, 29 (5), 3251-3256.



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List of table captions Table 1. Initial experimental conditions for MH dissociation via N2 injection List of figure captions Figure 1. Schematic of the experimental apparatus Figure 2. MRI images of water distribution during MH dissociation in Case 1 Figure 3. Variation in residual water saturation during MH dissociation of Case 1 Figure 4. Rate and average rate of hydrate dissociation versus time in Case 1 Figure 5. MRI images of water distribution during MH dissociation in Cases 2 Figure 6. Variation in residual water saturation during MH dissociation of Case 2 Figure 7. Recovery-injection ratio and methane cumulate production ratio versus time during MH dissociation in Cases 1-1 and 1-2 Figure 8. Recovery-injection ratio versus time during MH dissociation a) in Cases 1-3 to 1-6; b) in Cases 2-1 to 2-4 Figure 9. Comparison of average rate of hydrate dissociation between N2 injection method and depressurization method in our previous paper 26, 27 and Wang et al. study 34


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Table 1. Initial experimental conditions for MH dissociation via N2 injection Pressure

Temperature

Hydrate Saturation

N2 Injection Rate

(MPa)

(K)

(%)

(mL/min)

1-1

3.15

275.15

43.03

0.1

1-2

3.25

275.15

42.49

0.3

1-3

3.24

275.15

38.66

0.5

1-4

3.05

275.15

40.11

1.0

1-5

3.14

275.15

44.20

2.0

1-6

3.27

275.15

41.13

5.0

2-1

3.31

275.15

20.87

1.0

2-2

3.27

275.15

27.65

1.0

2-3

3.05

275.15

40.11

1.0

2-4

3.19

275.15

64.05

1.0

Cases



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Figure 1. Schematic of the experimental apparatus


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Case 1-1

0 min

132 min 160 min 181 min 200 min 230 min 260 min 300 min

Case 1-2

0 min

50 min

66 min

81 min

100 min 120 min 140 min 170 min

Case 1-3

0 min

11 min

30 min

41 min

51 min

60 min 70 min

85

0 min

11 min

30 min

41 min

51 min

60 min 70 min

85

0 min

11 min

30 min

41 min

51 min

60 min 70 min

85 min

0 min

11 min

30 min

41 min

51 min

60 min 70 min

85 min

Case 1-4

Case 1-5

Case 1-6

0

1

Figure 2. MRI images of water distribution during MH dissociation in Case 1



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Figure 3. Variation in residual water saturation during MH dissociation of Case 1


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Figure 4. Rate and average rate of hydrate dissociation versus time in Case 1



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Case 2-1

0 min

15 min

19 min

0 min

11 min

0 min

15 min

30 min

0 min

6 min

21 min

21 min

23 min

30 min

36 min

43 min

Case 2-2

21 min

32 min

43 min

53 min

64 min 75

38 min

47 min

55 min 64 min

34 min

43 min

85 min 119 min 137

Case 2-3

85

Case 2-4

0

1

Figure 5. MRI images of water distribution during MH dissociation in Cases 2


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Figure 6. Variation in residual water saturation during MH dissociation of Case 2



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Figure 7. Recovery-injection ratio and methane cumulate production ratio of hydrate dissociation versus time in Cases 1-1 and 1-2


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(a)

(b) Figure 8. Recovery-injection ratio versus time during hydrate dissociation a) in Cases 1-3 to 1-6; b) in Cases 2-1 to 2-4



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Figure 9. Comparison of average rate of hydrate dissociation between N2 injection method and depressurization method in our previous paper 26, 27 and Wang et al. study 34


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TOC Graphic

Highlights 1. Optimum N2 injection rates enhance economic benefit of hydrate dissociation 2. N2 injection is more suitable for low hydrate saturation exploitation 3. Methane hydrate dissociation occurs in three main stages (S1–3) 4. S1: gas and water liberation; S2: hydrate dissociation 5. S3: increased resistance to N2 diffusion/decreased contact area between N2 and hydrate


Analyzing the Process of Gas Production from Methane Hydrate via

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