In Situ Observation of Methane Hydrate Dissociation under Different

Apr 21, 2015 - To obtain the kinetic data of methane hydrate (MH) dissociation under different backpressures, MH dissociation by depressurization in a...
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In Situ Observation of Methane Hydrate Dissociation under Different Backpressures Shenglong Wang, Mingjun Yang, Pengfei Wang, Yuechao Zhao, and Yongchen Song* Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, Dalian University of Technology, Dalian, Liaoning 116024, People’s Republic of China S Supporting Information *

ABSTRACT: Depressurization has been considered an economic and practicable method for natural gas hydrate (NGH) exploitation. To obtain the kinetic data of methane hydrate (MH) dissociation under different backpressures, MH dissociation by depressurization in a porous medium was investigated in situ using magnetic resonance imaging (MRI). MH was dissociated under backpressures that were varied from 2.8 to 2.2 MPa, and the hydrate saturation variation during dissociation was analyzed. One experimental case was carried out with constant backpressure, and four cases of variable backpressure depressurization experiments were carried out. The radial dissociation pattern during depressurization was confirmed. During hydrate dissociation, free water was observed to move toward the outlet of the vessel and decreased the water saturation after the hydrate totally dissociated in the field of view (FOV). The MRI data provided excellent information on the spatial distribution of water in the porous media during hydrate dissociation.

1. INTRODUCTION Natural gas hydrates (NGHs) are clathrate, ice-like compounds that are formed by natural gas and water under high pressure and low temperature. Found ubiquitously in nature, particularly in subsea environments and permafrost, the total amount of methane trapped in NGHs, which is 1 × 1015 m3 according to the most conservative estimation, may exceed all of the conventional gas reserves by an order of magnitude.1 Because methane is the major component of natural gas, the recovery of methane from hydrate sediments has become an active area of research. Moreover, uncontrolled hydrate dissociation could lead to unexpected methane release and geological hazards.1 Thus, the mechanism of methane hydrate (MH) dissociation under conditions similar to NGH reservoirs needs to be clarified. Hydrate dissociation is a complex process consisting of heat and mass transfer, during which the hydrogen bonds between water molecules would break and, in the hydrate lattice, the van der Waals interaction forces between the methane and water molecules vanish. Several methods for methane production, including depressurization, thermal stimulation, and thermodynamic inhibitor injection, have been proposed by researchers.2−6 The depressurization method, in which the pressure is reduced outside the phase equilibrium region of the hydrate at a certain temperature, has been considered the most feasible and cost-effective solution to exploit methane from a reservoir.7 Hydrate dissociation by depressurization and other methods in the bulk phase and porous media has been extensively studied.8−15 During hydrate depressurization, a high thermal conductivity of the sediment was found to initially accelerate hydrate dissociation but potentially later partially inhibit the process.16 Additionally, a higher pressure drop during dissociation at a constant core temperature caused a higher sediment cooling.17 Researchers found that, if the initial effective permeability of the MH deposit was higher than the © 2015 American Chemical Society

threshold value and if the temperature of the MH deposit was as high as possible, the depressurization-induced gas production would be enhanced.18 The dissociation behavior of MH under pressurized and nonpressurized conditions was measured, and the pressurized process was found to be effective for enhancing the self-preservation of MH.19 Some researchers believed that hydrate dissociation kinetics was important for the production process of a laboratory-scale hydrate core but was negligible for a field-scale hydrate reservoir, and the intrinsic hydration dissociation constant was also determined to be on the order of 104 mol m−2 Pa−1 s−1.20,21 Falser et al. conducted line dissociation tests of MH-saturated soil by combined electrical heating and pressure reduction.22 The effect of the pore size of the sediments on the dissociation of MH was studied. It was found that the hydrate equilibrium condition was determined by the effect of the pore size on the water activity and that, during the initial stage of hydrate dissociation, the recovery rate of methane was strongly dependent upon the pore size.23,24 A study on the effect of ice formation on gas production from sandy porous media indicated that MH dissociation by depressurization could be accelerated by ice formation during hydrate dissociation at a pressure below the quadruple point.25 Moreover, gas production during hydrate depressurization was affected by the permeability of the hydrate zone; for instance, the pores were filled by hydrate regeneration.26,27 Hydrate exploitation by depressurization in field-scale and sediment cores has also been widely investigated.4,28 Researchers found that it was difficult to achieve depressurization and deal with water production in hydrate reserves connected to large aquifers according to the simulation results.29,30 Received: December 2, 2014 Revised: April 13, 2015 Published: April 21, 2015 3251

DOI: 10.1021/acs.energyfuels.5b00486 Energy Fuels 2015, 29, 3251−3256

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Energy & Fuels Raman spectroscopic observations of hydrate dissociation indicated that the ratio of large to small cages was generally consistent with that of bulk hydrate but dramatically decreased after a certain time.31 However, the hydrate occupancy ratio was observed to be constant during dissociation, which meant that the unit cell of structure-I (s-I) hydrate dissociated as a whole and there was no preferential decomposition of s-I cages according to 13C nuclear magnetic resonance (NMR) measurements.32 X-ray computed tomography (CT) imaging was also used to visualize the dissociation of MH in a porous medium. Radial dissociation of the hydrate sample, hydrate reformation, and development of preferential flow pathways during the depressurization during hydrate dissociation were observed.33,34 Although researchers have performed much work on the kinetics and thermodynamics of MH dissociation, the in situ visual observation of the dissociation pattern of MH in porous media and the data analysis of the influence of the backpressure variation on MH dissociation are limited. These studies are essential for understanding the kinetics of MH dissociation and further studies of hydrate exploitation. In this study, we obtained magnetic resonance imaging (MRI) images of MH dissociation in a porous medium by depressurization with different backpressures to provide some fundamental data for this topic. The mean intensity (MI) variation of the images and hydrate saturation during hydrate dissociation were also analyzed. The objective of this research was to acquire and analyze the kinetic data of MH dissociation in a porous medium by depressurization with different backpressures to provide a reference for further research on MH dissociation.

Figure 1. Schematic diagram of the apparatus. 2.2. Hydrate Formation. Glass beads (0.177−0.250 mm, As-One Co., Ltd., Osaka, Japan) were first packed into the vessel. Then, the vessel was connected to the experimental system and placed in the center of the MRI magnet. The porosity of the glass beads was 0.354, and the pore volume of the vessel was 12.51 mL. The deionized water was pressurized to 6.00 MPa to saturate the porous medium after the vessel was vacuumed, and it was kept steady for 1 h before the water in the pores was displaced by high-pressure nitrogen. The volume of the displaced water was recorded to calculate the initial water saturation. Then, the vessel was vacuumed again and pressurized to 6 MPa with CH4 gas (99.99%, Dalian Special Gas Co., Ltd., Dalian, China). The thermostat bath was set to 274.45 ± 0.1 K to cool the vessel. Images were continuously obtained after the vessel temperature and pressure stabilized. 2.3. Hydrate Dissociation. When the hydrate formation process finished, the backpressure control valve was set to a value slightly above the equilibrium pressure. The equilibrium pressure at 274.45 K was 2.98 MPa, as calculated using CSMHyd.1 Thus, for our experiments, the backpressure before dissociation was set to 3.1 MPa. At this pressure, the hydrate was still thermodynamically stable. Subsequently, the pressure of the vessel was reduced to the initial dissociation backpressure. In this study, we chose four different initial backpressures, i.e., 2.8, 2.6, 2.4, and 2.2 MPa, to investigate the influence of the dissociation pressure on the MH dissociation. For case 1, the backpressure was kept constant during hydrate dissociation. For cases 2−5, the backpressure was decreased 0.2 MPa every 10 min from the initial backpressure until the MH fully dissociated. The entire hydrate dissociation process was monitored by MRI, and the imaging time interval was 1 min. The dissociation conditions for five cases are shown in Table 1.

2. EXPERIMENTAL SECTION 2.1. Apparatus and Materials. The experimental system primarily consisted primarily of a polyimide vessel, the MRI system, the data acquisition system that records the pressure and the temperature variation of the porous medium, high-pressure pumps that inject gas and liquid, a backpressure control valve, and the cooling system. The high-pressure vessel was Φ 38 × 314 mm long, and the effective size for packed glass beads was Φ 15 × 200 mm, which was made by a nonmagnetic material (polyimide) to minimize the influence on the radio-frequency (RF) field generated by MRI. The coolant constantly circulated around the vessel in a jacket throughout the experiment. A Varian NMR system (Varian, Inc., Palo Alto, CA) operating at 400 MHz for 1H was used in this work to image the MH dissociation process. The magnetic field intensity of the magnet was 9.4 T, and the maximum gradient strength of the gradient coils was 50 G/cm. The distribution of the liquid in the porous medium could be visualized and quantified by MRI. A spin echo sequence was applied in this study: the repetition time (TR) was 500 ms, and the echo time (TE) was 13.94 ms. The image data matrix is 128 × 128 pixels, and the field of view (FOV) was 30 × 30 mm, with a 4.0 mm thickness. The acquisition time of the sequence was 1 min. A high-precision syringe pump (260D, Teledyne ISCO, Inc., Lincoln, NE) was used to inject both the gas and liquid. A backpressure control valve (BP-2080-M, JASCO, Tokyo, Japan) was used to control the hydrate dissociation backpressure. A pressure transducer (Nagano Co., Ltd., Nagano, Japan) with an estimated error of ±0.1 MPa was connected to the vessel, and a temperature transducer (Yamari Industries, Nagasaki, Japan) with an estimated error of ±0.1 K was connected to the vessel jacket. The temperature and pressure signals were collected by an A/D module (Advantech Co., Ltd., Taiwan, China). The temperature of the vessel was accurately controlled to ±0.1 °C by a thermostat bath (FL300, JULABO Labortechnik GmbH, Seelbach, Germany) filled with Fluorinert (FC-40, 3M Company, Saint Paul, MN). A schematic of the experimental system is shown in Figure 1.

3. RESULT AND DISCUSSION There are five experimental cases in this investigation. In case 1, the hydrate was dissociated under a constant backpressure of 2.8 MPa, and in cases 2−5, the dissociation backpressures were decreased 0.2 MPa every 10 min after the dissociation started. 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 hydrate saturation, which is defined as the volume fraction of hydrate to the pore space, was quantified using the MI data. When brought to the surface of the earth, 1 m3 of MH releases 0.8 m3 of fresh water. Thus, the following equation was applied to estimate the hydrate saturation:35,36 S h = 1.25 3252

(I0 − Ii)Sw0 × 100% I0

(1) DOI: 10.1021/acs.energyfuels.5b00486 Energy Fuels 2015, 29, 3251−3256

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Energy & Fuels Table 1. Experimental Parameters and Results of the MH Dissociation Process experiment number

initial backpressure (MPa)

pressure drop interval (MPa)

final backpressure (MPa)

average dissociation rate (%/min)

initial hydrate saturation (volume fraction) (%)

1 2 3 4 5

2.8 2.8 2.6 2.4 2.2

0.2 0.2 0.2 0.2

2.8 0.1 0.1 0.1 0.1

0.40 1.66 1.89 1.25 1.31

27.0 35.0 37.8 31.2 27.5

where Sw0 is the initial water saturation before hydrate formation, I0 is the MI of the liquid solution at the initial time of hydrate formation, and Ii is the MI at “i” minutes during hydrate dissociation. The MI data of hydrate formation have been provided as Supporting Information. Both the saturation and MI are dimensionless. When the MI during dissociation remains constant, it indicates that the amount of water in the pore does not change any more and MH dissociation has finished. 3.1. MH Dissociation with Constant Backpressure. MH dissociated with a constant backpressure of 2.8 MPa in case 1. The MI and hydrate saturation curves are shown in Figure 2. As

Figure 3. Water distribution variation during hydrate dissociation for case 1.

dissociation pattern was radial dissociation rather than axial dissociation. The heat transfer during dissociation was along the radial direction for the coolant that was circulated around the vessel in a jacket, as mentioned in section 2.1. A similar phenomenon was found by other researchers in the investigations of gas-hydrate-bearing sandstone samples. Hydrate dissociation occurred from the outside into the core sample during depressurization because of heat transfer from the temperature-controlling bath surrounding the vessel.37 Moreover, some researchers showed that the dissociation process of hydrates was determined by the radial heat flow from the environment into the dissociating zone in saturated sediments.38,39 3.2. MH Dissociation with Variable Backpressures. For cases 2−5, the backpressures were continuously decreased by an interval of 0.2 MPa every 10 min. The initial backpressures for these four cases varied from 2.8 to 2.2 MPa to investigate the effect of the initial backpressure on the dissociation of MH by continuous depressurization. Case 3 with an initial backpressure of 2.6 MPa and Case 5 with an initial backpressure of 2.2 MPa were taken as examples to be discussed in this section. In Figure 4, the MI and hydrate saturation variation of case 3 are shown. The hydrate dissociates immediately after the backpressure was decreased to 2.6 MPa, which was different from case 1. This result was likely because the pressure differential between the equilibrium pressure and the dissociation backpressure was relatively high, which allowed the hydrate in the pore to easily dissociate. At 30 min, the process of dissociation ended because the MI reached the maximum value. After 70 min, some fluctuations of the MI could be observed, which were caused by the movement of free water in the pores. Figure 5 shows the dissociation front moving toward the vessel center with time for case 3. The radial dissociation could

Figure 2. MI and hydrate saturation variation during dissociation for case 1.

observed in the figure, MH dissociation began approximately 10 min later after the backpressure was decreased to 2.8 MPa, which indicated that hydrate formed as blockages in the porous medium and that the pressure drop could not be propagated to the inside of the vessel. After several minutes of dissociation, the blockages disappeared and the permeability of the porous medium increased, which could be inferred from the sudden rise of the MI curve in the figure. The MH continuously dissociated after 20 min and ended at approximately 70 min. During dissociation, the volume of free water increased; thus, the MI reached a maximum at 70 min. However, the MI slightly decreased for tens of minutes and then remained constant after 70 min because, after complete dissociation, free water was driven by the gas flow to the outlet, which decreased the signal intensity in the FOV. The MRI images of the water distribution variation during hydrate dissociation are shown in Figure 3. The bright signal in the images indicates the liquid water in the vessel. Because 1H in the solid hydrates could not be imaged using the sequence applied in this experiment, the hydrate is presented as black parts in the images. As shown in the figure, the hydrate 3253

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Energy & Fuels

Figure 6. MI and hydrate saturation variation during dissociation for case 5.

Figure 4. MI and hydrate saturation variation during dissociation for case 3.

moved radially inward and toward the center of the porous medium.

Figure 5. Water distribution variation during hydrate dissociation for case 3. Figure 7. Water distribution variation during hydrate dissociation for case 5.

still be clearly observed. However, unlike case 1, the hydrate in the lower part of the vessel dissociated faster than that in the upper part from 30 to 40 min. Because the lower part of the vessel was close to the outlet where water tended to accumulate, the thermal conductivity could be increased. After the dissociation process ended, the water saturation decreased, as observed in Figure 5 (40−110 min), which indicated that the free water generated by hydrate dissociation in the pore was driven by gas flow toward the outlet. In case 5, the initial backpressure was set to 2.2 MPa, which was the lowest among 5 cases. Figure 6 shows the MI and hydrate saturation curves of case 5. The hydrate saturation significantly decreased after the backpressure was set to 2.2 MPa, as shown in the figure. The dissociation progress ended at 30 min, which was similar to case 3. In addition, a fluctuation of the MI was observed in case 5 after the hydrate totally dissociated. Moreover, a sudden decrease in the MI occurs at 80 min when the backpressure was decreased from 0.6 to 0.1 MPa. Both of the above results were due to the increased permeability of the porous media after hydrate dissociation, which allowed water in the pore to easily migrate toward the outlet, as shown in Figure 7 (30−100 min). The hydrate near the wall dissociated first, and the hydrate dissociation front

The hydrate dissociation patterns of cases 2−5 were primarily raidial, which was not quite different from case 1. This result indicated that the dissociation pattern was determined by the heat-transfer conditions rather than the variation of backpressures because the temperature of the coolant circulating around the vessel during hydrate dissociation was the same for all five experimental cases. However, the hydrate dissociation rate of case 1 was significantly lower than that of the other four cases because the continuous depressurization of the last four cases accelerated the mobility of the gas and free water in the pores. This result was different from the phenomenon described by Lee et al.26 In their investigation, the highest gas production rate occurred when the backpressure was closest to the equilibrium pressure because when the pressure difference between the backpressure and core pressure was high, hydrate regeneration filled the pores and the pressure propagation ceased. In contrast, no hydrate regeneration was observed in the last four cases of our experiments when the outlet pressure was directly decreased to the target backpressure because the porous medium used in our 3254

DOI: 10.1021/acs.energyfuels.5b00486 Energy Fuels 2015, 29, 3251−3256

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Energy & Fuels investigation has a larger average pore space than the core sample. 3.3. Hydrate Saturation and Dissociation Rate Comparisons among the Five Cases. The hydrate saturation curves of all five cases were calculated using the MI of the MRI images according to eq 1, which are shown in Figure 8. A hydrate saturation decrease occurred immediately

Figure 9. Water saturation before and after hydrate dissociation.

whereas the initial hydrate saturations were between 26 and 35%, as shown in Table 1. This result was due to the fact that some of the free water remained in the pore because water in some regions of the porous medium could not contact the methane gas during hydrate formation, which was also supported by the fact that the residual water saturations were all greater than zero. The final water saturation indicates how much water remains in the pore after hydrate fully dissociates. The final water saturations of the five cases were lower than their initial water saturations correspondingly, which was because, during hydrate dissociation, free water was driven to the outlet by the pressure differential. Thus, the water distribution of the entire vessel became inhomogeneous, and the signal intensity of the FOV decreased.

Figure 8. Hydrate saturation curves for cases 1−5.

after 10 min for all of the cases, except cases 1 and 2, because the initial backpressures of these two cases were close to the equilibrium pressure. Thus, the hydrates near the outlet of the vessel dissociated relatively slow and the pressure drop propagating to the inner region was delayed. For all of the experimental cases, after a short period of rapid dissociation, the dissociation progress became continuous and the dissociation rate remained uniform. The average dissociation rates of the five cases are shown in Table 1. The dissociation rate of case 1 was significantly lower than those of the other four cases, as shown in both Figure 8 and Table 1, because the dissociation backpressure of case 1 was very close to the hydrate equilibrium pressure (2.98 MPa). For case 2, the initial dissociation backpressure was also 2.8 MPa; however, the average dissociation rate of case 2 was still significantly higher than that of case 1, which indicated that continuous depressurization would increase the hydrate dissociation rate when the initial backpressure was kept the same. For cases 3−5, the initial backpressure was much lower than the equilibrium pressure; thus, the hydrate immediately dissociates after 10 min. The average dissociation rates for cases 2−5 were similar considering the slight difference between the initial hydrate saturations, as shown in Table 1, which indicated that the influence of the initial backpressures on the hydrate dissociation rate was limited when the depressurization process was continuous. This phenomenon could be explained by the fact that hydrate dissociation was primarily constrained by the heat transfer with the surroundings, as concluded by some research groups.40,41 The initial water saturations (water saturation before hydrate formation), residual water saturations (water saturation after hydrate formation), and final water saturations (water saturation after hydrate dissociation) are shown in Figure 9. The water saturation decreased after hydrate formation and then increased after hydrate dissociation. As shown in the figure, the initial water saturations were between 39 and 42%,

4. CONCLUSION MH dissociation by depressurization in a porous medium was investigated in situ using MRI. Five experimental cases were examined in this study, and the following conclusions were confirmed. MH dissociated in the radial direction rather than the axial direction during depressurization because the hydrate dissociation was constrained by heat transfer from the coolant surrounding the vessel. The dissociation rate of the hydrate that was dissociated by continuous depressurization was higher than that dissociated by a constant backpressure. The impact of the initial backpressure difference on the hydrate dissociation rate was almost negligible for continuous depressurization. The water distribution after dissociation in the pores was different from the initial state because the free water migrated toward the outlet during depressurization. This research supports further investigations of the kinetics of MH dissociation and the development of depressurization methods for hydrate exploitation.

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ASSOCIATED CONTENT

S Supporting Information *

MI, pressure, and hydrate saturation data for the hydrate formation process of all five experimental cases (PDF). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b00486.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 3255

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Energy & Fuels Notes

(29) Reagan, M. T.; Moridis, G. J.; Johnson, J. N.; Pan, L.; Freeman, C. M.; Pan, L.; Boyle, K. L.; Keen, N. D.; Husebo, J. Transp. Porous Media 2014, 1−19. (30) Li, B.; Li, X.-S.; Li, G.; Feng, J.-C.; Wang, Y. Appl. Energy 2014, 129, 274−286. (31) Liu, C.; Lu, H.; Ye, Y.; Ripmeester, J. A.; Zhang, X. Energy Fuels 2008, 22, 3986−3988. (32) Gupta, A.; Dec, S. F.; Koh, C. A.; Sloan, E. D. J. Phys. Chem. C 2007, 111, 2341−2346. (33) Gupta, A.; Moridis, G. J.; Kneafsey, T. J.; Sloan, E. D. Energy Fuels 2009, 23, 5958−5965. (34) Seol, Y.; Myshakin, E. Energy Fuels 2011, 25, 1099−1110. (35) Yang, M.; Song, Y.; Jiang, L.; Wang, X.; Liu, W.; Zhao, Y.; Liu, Y.; Wang, S. J. Ind. Eng. Chem. 2014, 20, 322−330. (36) Yang, M.; Song, Y.; Liu, W.; Zhao, J.; Ruan, X.; Jiang, L.; Li, Q. Chem. Eng. Sci. 2013, 90, 69−76. (37) Kneafsey, T. J.; Moridis, G. J. Mar. Pet. Geol. 2014, 58, 526−539. (38) Oyama, H.; Konno, Y.; Suzuki, K.; Nagao, J. Chem. Eng. Sci. 2012, 68, 595−605. (39) Falser, S.; Uchida, S.; Palmer, A. C.; Soga, K.; Tan, T. S. Energy Fuels 2012, 26, 6259−6267. (40) Davies, S. R.; Selim, M. S.; Sloan, E. D.; Bollavaram, P.; Peters, D. J. AlChE J. 2006, 52, 4016−4027. (41) Hong, H.; Pooladi-Darvish, M.; Bishnoi, P. J. Can. Pet. Technol. 2003, 42, 45−56.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was financially supported by the National Natural Science Foundation of China (51436003, 51106018, and 51227005), the Major National Science and Technology Programs of China (2011ZX05026-004), the High-Tech Research and Development Program of China (2013AA09250302), the Major State Basic Research Development Program of China (2011CB707304), and the Fundamental Research Funds for the Central Universities of China (DUT13LAB19).

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DOI: 10.1021/acs.energyfuels.5b00486 Energy Fuels 2015, 29, 3251−3256