Energy Fuels 2011, 25, 33–41 Published on Web 12/30/2010
: DOI:10.1021/ef1011116
Experimental Investigation on the Dissociation Behavior of Methane Gas Hydrate in an Unconsolidated Sediment by Microwave Stimulation Song He,†,‡,§ Deqing Liang,*,†,‡ Dongliang Li,†,‡ and Longlong Ma†,‡ † Key Laboratory of Renewable Energy and Gas Hydrate, and ‡Guangzhou Center for Gas Hydrate Research, Guangzhou Institute of Energy Conversion, Chinese Academy of Science, Guangzhou 510640, People’s Republic of China, and § Graduate University of Chinese Academy of Science, Beijing 100049, People’s Republic of China
Received August 19, 2010. Revised Manuscript Received November 16, 2010
The technique of recovery of methane from a hydrate reservoir by microwave stimulation was experimentally verified for a laboratory scale. The stimulation was carried out in the unconsolidated sediment from the South China Sea by applying 2.45 GHz microwave with the average radiation densities ranging from 3 to 19 kW/m2. We observe that the far-field hydrate layer, which exceeds the microwave penetration depth, dissociated at equilibrium. The hydrate saturation (15.5-54.5%), water saturation (40.7 and 70.4%), freezing and combining with depressurization affect the heating and gas production behaviors, as well as the production efficiency. The formation of ice reduces the gas production efficiency for the frozen sediment but boosts the gas production for the unfrozen sediment combining with depressurization.
thermal stimulation techniques have been proposed and studied in the literature for gas production, such as hot water/brine injection,8-12 in situ combustion,13,14 and electromagnetic heating.15 The primary objectives of thermal stimulation are to heat the hydrate reservoirs above the hydrate decomposition temperature and supply sufficient heat of hydrate decomposition. Therefore, reducing the excessive heat loss to the surroundings and improving the effective heat transmitted to the undissociated hydrate zone are important factors affecting the efficiency of gas production. The tremendous heat loss to the surroundings and the low permeability may limit the feasibility of the conventional thermal stimulation methods, such as injecting hot fluids.15 Therefore, thermal recovery methods invoking in situ direct heating are considered to be energyefficient, contributing to the in situ generation of thermal energy from fuel combustion,13,14 electromagnetic heating,15,16 etc. The optimal case proposed by Islam is to place a distributed electromagnetic heat source horizontally in the hydrate layer.15
Introduction Gas hydrates are ice-like clathrates formed from water and gas molecules under the condition of high pressures and low temperatures. The huge mount of natural gas hydrates found in permafrost regions and continental margins are considered to be a potential energy resource for the 21st century.1 Therefore, method development for the production of natural gas from hydrate reservoirs attracts much attention. For gas hydrate exploitation and gas production, several recovery methods have been proposed, such as thermal stimulation,2,3 depressurization,4 chemical injection,5 and CO2 replacement.6 However, in these methods, the thermal stimulation method is regarded as the most promising choice for gas production and hydrate dissociation, and it is thought to be more effective when combining with depressurization methods.7 Several *To whom correspondence should be addressed. Telephone: þ(86)2087057669. Fax: þ(86)20-87057669. E-mail:
[email protected]. (1) Sloan, E. D. Clathrate Hydrates of Natural Gases, 2nd ed.; Marcel Dekker, Inc.: New York, 1998. (2) McGuire, P. L. Methane hydrate gas production by thermal stimulation. Proceedings of the 4th Canadian Permafrost Conference; Calgary, Alberta, Canada, March 2-6, 1982. (3) Holder, G. D.; Angert, P. F.; John, V. T.; Yen, S. A thermodynamic evaluation of thermal recovery of gas from hydrates in the earth. J. Pet. Technol. 1982, 34 (5), 1127–1132. (4) Holder, G. D.; Angert, P. F. Simulation of gas production from a reservoir containing both gas hydrates and free natural gas. Proceedings of the 57th Annual Fall Technical Conference and Exhibition of the Society of Petroleum Engineers of American Institute of Mining, Metallurgical, and Petroleum Engineers (AIME); New Orleans, LA, Sept 26-29, 1982. (5) Li, G.; Li, X. S.; Tang, L. G.; Zhang, Y. Experimental investigation of production behavior of methane hydrate under ethylene glycol injection in unconsolidated sediment. Energy Fuels 2007, 21 (6), 3338– 3393. (6) Ota, M.; Abe, Y.; Watanabe, M.; Richard, L.; Smith, J.; Hiroshi, I. Methane recovery from methane hydrate using pressurized CO2. Fluid Phase Equilib. 2005, 228, 553–559. (7) Moridis, G. J. Numerical studies of gas production from methane hydrates. SPE J. 2003, 8 (4), 359–370. (8) Kamath, V. A.; Godbole, S. P. Evaluation of hot brine stimulation technique for gas production from natural gas hydrates. J. Pet. Technol. 1987, 39 (11), 1379–1388. r 2010 American Chemical Society
(9) Kamata, Y.; Ebinuma, T.; Omura, R.; Minagawa, H.; Narita, H. Decomposition experiment of methane hydrate sediment by thermal recovery method. Proceedings of the 5th International Conference on Gas Hydrate; Trondheim, Norway, June 13-16, 2005. (10) Tang, L. G.; Xiao, R.; Huang, C; Feng, Z. P.; Fan, S. S. Experimental investigation of production behavior of gas hydrate under thermal stimulation in unconsolidated sediment. Energy Fuels 2005, 19 (6), 2402–2407. (11) Li, X. S.; Wan, L. H.; Li, G.; Li, Q. P.; Chen, Z. Y.; Yan, K. F. Experimental investigation into the production behavior of methane hydrate in porous sediment with hot brine stimulation. Ind. Eng. Chem. Res. 2008, 47 (23), 9696–9702. (12) Lee, J. Experimental study on the dissociation behavior and productivity of gas hydrate by brine injection scheme in porous rock. Energy Fuels 2010, 24, 456–463. (13) Castaldi, M. J.; Zhou, Y.; Yegulalp, T. M. Down-hole combustion method for gas production from methane hydrates. J. Pet. Sci. Eng. 2007, 56 (1), 176–185. (14) Cranganu, C. In-situ thermal stimulation of gas hydrates. J. Pet. Sci. Eng. 2009, 65 (1), 76–80. (15) Islam, M. R. A new recovery technique for gas production from Alaskan gas hydrates. J. Pet. Sci. Eng. 1994, 11 (4), 267–281. (16) Saeger, R. B.; Long, J.; Heinemann, R. F.; Huang, D. D. World Intellectual Property Organization (WO) Patent 9,829,369, 1998.
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: DOI:10.1021/ef1011116
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Electrical heating is generally used to produce crude oil and natural gas from wells. A patent indicating that electrical heating could be performed using connate water in a methane hydrate sediment has been registered.17 Electrical heating of the hydrate reservoir has also been introduced by Zhang et al. in their new method of harvesting natural gas from sea floor gas hydrates and estimated to be economical.18 Islam15 studied the feasibility of electromagnetic heating for dissociating gas hydrate by a series of numerical simulation runs based on the fact that this technique has been shown to be effective in recovering heavy oil. However, Minagawa et al.19 experimentally examined the efficiency of electrical heating of the hydrate core for xenon gas production. They confirmed that it was impossible to heat the methane hydrate sediment without additional injection of electrolyte solution and continued decomposition of methane hydrate under depressurization conditions. Microwaves are a form of electromagnetic energy, which travels in high frequency waves and causes dipoles in materials to heat themselves through high-speed rotations. The wavelengths of microwaves are between 1 mm and 1 m, with corresponding frequencies between 300 GHz and 300 MHz. Microwaves at 915 MHz and 2.45 GHz are widely used to heat materials in industry, laboratory, and daily life. Materials with a high dielectric loss coefficient, such as free water, have better capacity of converting microwave energy to thermal energy. As a special form of energy, microwave has been used in many areas of the petroleum industry. The application of microwave in removal and inhibitor of solid hydrate in hydrocarbon pipelines can be found in several references.20-22 Fatykhov and Bagautdinov reported that 2.45 GHz microwave heating had a better performance in propane hydrate or ice melting in pipelines compared to the natural process. Li et al.23 put forward a scheme of gas recovery from in situ hydrate using microwave heating and found a faster methane hydrate decomposition rate under 2.45 GHz microwave irradiation compared to bath heating from their experiments. However, the hydrates used in their work were formed from sodium dodecyl sulfate solutions without sediments. Because the heat is generated through an electromagnetic field with high frequency, no mass injection is involved in the microwave stimulation. Therefore, microwave stimulation may become another option for gas hydrate recovery that can directly and continuously generate and transport acceptable heat to the hydrate reservoir. Experimental and numerical studies on hydrate dissociation behaviors under different stimulation methods in the sediment have been widely reported. Taking advantage of the mechanisms of multiphase heat and mass transfer, ice melting and formation, intrinsic dissociation kinetics, and hydrate reforming have been developed and involved in the published
Figure 1. Schematic diagram of the experimental setup: 1, microwave source; 2, circulator; 3, water load; 4, dual directional coupler; 5, three stub turner; 6, rectangular-circular transducer; 7, hydrate vessel; 8, quartz glass; 9, methane gas cylinder; 10, vacuum pump; 11, circulator bath; 12, pressure sensor; 13, power sensors; 14, PT100 resistance temperature detectors (RTDs).
numerical models to simulate the gas production behavior of hydrate reservoirs.24 However, experimental work, including thermal stimulation, is still the fundamental aspect on the improvement of the thermal recovery technique and verification of the numerical approaches. The present work focuses on the mechanism of microwave heating and reveals some hydrate dissociation behaviors in the sediment under microwave heating. We experimentally investigate the methane hydrate dissociation behaviors in an unconsolidated sediment by 2.45 GHz microwave heating. The influence of hydrate saturation, water saturation, and initial temperature-pressure conditions on the gas production behavior and efficiency are simultaneously considered. Experimental Section Experimental Apparatus. Figure 1 shows a schematic drawing of the experimental setup used in this work, which is composed of the quiescent hydrate vessel, the microwave system, the gas/ solution injection system, and the circulator bath. The hydrate vessel is made of stainless steel and has a length of 14.9 cm and an internal diameter of 8.0 cm, which permits the propagation of 2.45 GHz microwave. Four armored PT100 thermometers named T1, T2, T3, and T4 are located at different points away from the bottom of the cell for 0.5, 2.4, 4.4, and 8.1 cm, respectively, while their radial positions are 2.5 cm away from the middle axis. A 6301-type pressure sensor from 702 Research Institute with an active range from 0 to 10 MPa is used for pressure measurement. The uncertainties of temperature and pressure are (0.1 °C and (0.5%, respectively. By a microwave radiation test in a vacuum condition, all of the sensors are assured to be well-shielded. The temperature of the hydrate cell is controlled by the flow of ethanol from an external circulating temperature bath from -20 to -80 °C, with a fluctuation less than 0.2 °C. In addition, a layer of insulating material is wrapped around the cell to help maintain the constant temperature. The microwave system includes a microwave generator and several BJ-22 series brass waveguides. The frequency generated is fixed at 2.45 GHz. The output microwave power can be adjusted linearly from 0 to 750 W. The end of the waveguides connected to the hydrate vessel is a rectangular-circular transducer with
(17) Katz, M. L. U.S. Patent 3,916,993, 1975. (18) Zhang, H. Q.; Brill, J. P.; Sarica, C. A method of harvesting gas hydrates from marine sediments. Proceedings of the 6th International Conference on Gas Hydrates; Vancouver, British Columbia, Canada, July 6-10, 2008. (19) Minagawa, H.; Nishikawa, Y.; Takahashi, Y.; Narita, H. Electrical heating of hydrate sediment for gas production. Proceedings of the 8th ISOPE Ocean Mining Symposium; Chennai, India, Sept 20-24, 2009. (20) Rojey, A. U.S. Patent 5,625,178, 1997. (21) John, J. World Intellectual Property Organization (WO) Patent 050819. 2001. (22) Fatykhov, M. A.; Bagautdinov, N. Y. Experimental investigations of decomposition of gas hydrate in a pipe under the impact of a microwave electromagnetic field. High Temp. 2005, 43 (4), 614–619. (23) Li, D. L.; Liang, D. Q.; Fan, S. S.; Li, X. S.; Tang, L. G.; Huang, N. S. In situ hydrate decomposition using microwave heating: Preliminary study. Energy Convers. Manage. 2008, 49 (8), 2207–2213.
(24) Gamwo, I. K.; Liu, Y. Mathematical modeling and numerical simulation of methane production in a hydrate reservoir. Ind. Eng. Chem. Res. 2010, 49 (11), 5231–5245.
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: DOI:10.1021/ef1011116
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consequence of the interactions between microwave fields and dielectric material. There are several mechanisms for energy conversion from microwave into heat, such as dipole polarization, displacement polarization, ionic polarization, and interfacial polarization.25 The relative dielectric loss factor (ε00 ), which sums all of the mechanisms above and indicates the degree to which an externally applied electromagnetic field will be converted to heat, is given by25
an internal diameter of 8.0 cm. While the interior of the waveguides and the hydrate vessel is separated by a microwavetransparent quartz glass, which can bear an inner pressure up to 10 MPa, the hydrate formation and decomposition in the hydrate cell are kept at a constant volume of 0.75 L. Moreover, the incident power and reflected power are real-time measured by a dual directional coupler with two GZ3-0.3W power sensors. The raw sands used come from the submarine sediment of the South China Sea, and the purity of methane gas is 99.9%. Experimental Procedure. The raw dry sands were sieved into the size range of 150-280 μm, and 352 g of them was weighed. We used the wetting and packing method proposed by the previous researchers.32 The initial dry sand was naturally deposited at the bottom of the vessel and then saturated by sufficient water. Methane was slowly flowed into the vessel until a predetermined amount of water was pushed out from the outlet at the bottom, so that the sediment was partially saturated by 95 or 55 mL of water. This results in a partially water-saturated (70.4 and 40.7%) unconsolidated sediment with a bulk volume of about 280 cm3 (5.6 cm depth) and a porosity around 48%. The distances from the top surface of the sediment to the temperature sensors T1, T2, and T3 are 5.1, 3.2, and 1.2 cm, respectively, while the sensor T4 is located at the upper free gas zone, as seen in Figure 1. After the preparation of the partially water-saturated sediment, the air in the vessel was removed by injecting and evacuating 1 MPa of methane gas from the upper outlet. After that, the methane gas was injected up to the required pressure that was sufficiently higher than the hydrate equilibrium pressure at the working temperature and the vessel was closed as an isochoric system. After about 3 days of waiting, the vessel was cooled to start the hydrate formation. The temperature was gradually decreased to about 1 °C by changing the circulator bath temperature. The hydrate formation process generally lasted for 1 day until no pressure drop in the vessel. In some experiments, the sediment was frozen for another 10 h to mimic the permafrost environment. All of the thermal stimulations were carried out in the isovolumetric vessel without any extraction of gas, which means that the pressure increased as methane releasing from the melting hydrate to the bulk gas phase. During the microwave heating process, the circulator bath was removed and the microwave with fixed output power was applied to the sediment, propagating through waveguides, the rectangular-circular transducer, the quartz glass, and the upper free gas zone in the vessel orderly. As a result, the temperature of the sediment increased and the hydrate decomposed into water and gas, which was the main process concerned in this work. The gas exploitation can be conducted during or after the thermal stimulation. To investigate the effect of a combined stimulation method involving the reduction of pressure and the impact of microwave, the system was depressurized rapidly to near the atmospheric pressure, during which most of the associated free gas was extracted. The vent was then turned off before the application of microwave. During all of the experimental runs, the temperature and pressure, the microwave incident power, and the reflected power were recorded at 10 s intervals. In this work, a total of 21 experimental runs were carried out, as shown in Table 1. The sediment samples with different hydrate saturations under the same water saturation were obtained by adjusting the methane gas supply (charge pressure). When we obtain the hydrate sediments with close hydrate saturations (e.g., runs 1-5 and 15-17), different radiation powers can be applied to each run. Stimulation by microwave heating alone was conducted in runs 1-17, where different microwave radiation densities, initial water saturations, and hydrate saturations were considered. In addition, the influence of prefreezing and depressurization is investigated in runs 18 and 19 and 20 and 21, respectively. Conversion of Microwave Energy to Heat. The microwave energy itself is not thermal energy. Heating by microwave is a
ε00 ¼ ε0 tan δ
ð1Þ
0
where ε is the relative dielectric constant and tan δ is the loss tangent. Then, the power dissipation (W/m3) is approximated with25 Pd ¼ 2πf ε0 ε00 Eeff 2
ð2Þ
where f is the frequency (Hz), ε0 is the vacuum permittivity, and Eeff is the effective electrical field strength (V/m). It is seen that an improvement in ε00 and Eeff has a dramatic effect on the power density. However, the electrical field strength decreases with the penetration of microwave in materials, resulting in a fast drop of the power density. The microwave penetration depth, DP, which is the distance from the material surface where the power density falls to e-1 of that at the surface, is given by26 DP ¼
C pffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2πf 2ε0 ½ 1 þ tan2 δ - 11=2
ð3Þ
Materials containing polar molecules, such as free water, have a higher loss factor ε00 at appropriate frequencies, such as 2.45 GHz, because of dipole polarization. It is commonly believed that the penetration depth is significantly limited by the increase of water saturation or the addition of salts.26 On the other hand, the dielectric constant and loss factor of gas hydrate is found to be as low as that of ice and sand compared to liquid water at microwave frequencies.22,27-30 Because the water molecules are tightly and regularly bound in the hydrate phase and the encaged gases are generally nonpolar, it is difficult to rotate the hydrate molecules and heat them directly. Because salt is absent in our experiments, dipole rotation of liquid water is considered to be the most important microwave heating mechanism. It is widely found that ε0 and ε00 are functions of the temperature, frequency, sediment properties, and water saturation.31 Thus, an accurate calculation of the penetration depth requires valid dielectric parameters of the specific material. However, the penetration depth in water-abundant materials typically drops to several centimeters as the water content increases, which can also be observed in our experiments. (25) Metaxas, A. C.; Meredith, R. J. Industrial Microwave Heating; Peter Peregrinus: London, U.K., 1983. (26) Meredith, R. J. Engineers’ Handbook of Industrial Microwave Heating; Institution of Electrical Engineers: London, U.K., 1998. (27) Wright, J. F.; Nixon, F. M.; Patterson, D. E.; Dallimore, S. R. Laboratory investigations of permafrost gas hydrates: selection of index samples for cooperative GSC-Japan testing and assessment of TDR technique for gas hydrate studies. Report to the Institute of Applied Energy; Geological Survey of Canada: Ottawa, Ontario, Canada, 2000. (28) Wright, J. F.; Nixon, F. M.; Dallimore, S. R.; Matsubayashi, O. A method for direct measurement of gas hydrate amounts bulk dielectric properties of laboratory test media. Proceedings of the 4th International Conference on Gas Hydrates; Yokohama, Japan, May 19-23, 2002. (29) Kliner, J. R.; Grozic, J. L. H. Determination of synthetic hydrate content in sand specimens using dielectrics. Can. Geotech. J. 2006, 43 (6), 551–562. (30) Jakobsen, T.; Folgero, K. Dielectric measurements of gas hydrate formation in water-in-oil emulsions using open-ended coaxial probes. Meas. Sci. Technol. 1997, 8 (9), 1006–1015. (31) Jaganathan, A. P.; Allouche, E. N. Temperature dependence of dielectric properties of moist soils. Can. Geotech. J. 2008, 45 (6), 888–894. (32) Winters, W. J.; Pecher, I. A.; Waite, W. F.; Mason, D. H. Physical properties and rock physics models of sediment containing natural and laboratory-formed methane gas hydrate. Am. Mineral. 2004, 89 (8), 1221–1227.
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: DOI:10.1021/ef1011116
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Table 1. Experimental Conditions for All of the Runsa item
2
Rd (kW/m )
T1 (°C)
P1 (MPa)
P2 (MPa)
td (min)
Sw,0 (%)
Sw,1 (%)
Sh,1 (%)
1 2 3 4 5
3.9 7.9 11.9 15.4 18.9
1.1 1.1 1.0 1.0 1.0
3.20 3.25 3.17 3.18 3.14
5.62 5.71 5.66 5.72 5.76
60.6 34.5 25.3 20.6 17.4
70.4 70.4 70.4 70.4 70.4
28.0 29.0 28.0 27.7 26.8
53.0 51.8 53.0 53.4 54.5
6 7 8
8.8 14.1 16.4
1.0 0.9 0.9
3.04 3.00 3.02
4.74 4.80 4.82
25.8 18.2 16.4
70.4 70.4 70.4
41.1 40.6 41.0
36.6 37.2 36.8
9 10 11
7.9 10.5 14.4
0.9 1.0 1.0
2.92 3.03 3.04
3.95 4.04 3.99
16.0 11.8 10.0
70.4 70.4 70.4
52.4 54.6 54.6
22.5 19.8 19.8
12 13 14
7.2 9.7 14.7
1.0 1.0 1.0
3.08 3.06 3.07
4.45 4.49 4.46
19.8 16.0 11.7
40.7 40.7 40.7
16.2 16.1 16.1
30.6 30.8 30.7
15 16 17
6.6 8.0 13.8
1.0 1.0 0.9
3.05 3.08 3.00
3.85 3.90 3.94
10.0 8.5 7.0
40.7 40.7 40.7
27.8 28.3 27.1
16.1 15.5 17.0
18 19
9.7 8.4
-6.5 -6.5
3.11 2.67
5.52 4.03
39.5 36.6
70.4 70.4
28.7 49.4
52.1 26.3
20b 21b
5.2 3.0
1.1 -6.5
3.24 2.74
1.78 1.24
21.7 38.0
70.4 70.4
29.8 53.0
50.7 21.7
a Rd, average microwave radiation density during hydrate decomposition; Sw, water saturation; Sh, hydrate saturation; td, time at 90% gas producton; subscript “0”, the state before hydrate formation; subscript “1”, the state before hydrate decomposition; subscript “2”, the state after hydrate decomposition. b Combined with depressurization.
where Pt, Tgt,f, and Zt are the pressure, temperature, and compressibility, respectively, of the upper free gas at time t and P1, Tg1,f, and Z1 are the initial pressure, temperature, and compressibility, respectively, of the upper free gas before the hydrate decomposition. The compressibility factor is calculated using the Soave-Redlich-Kwong equation of state. Because of the significantly uneven distributions of the temperature and pore volume in the sediment during heating, it is hard to determine the amount of interstitial gas in the sediment. Therefore, in the following discussions, Δnt,f instead of the amount of gas production is calculated throughout the decomposition process to describe the progress of the gas production from the sediment. In other words, Δnt,f cannot reflect the entire released gas from hydrate at time t but does reflect the course of gas production in time, owing to the high porosity and permeability of the unconsolidated sediment. The hydrate saturation in Table 1 is calculated by the mole changes of the injected gas before and after hydrate formation based on a hydration number of 6.0.34
Hydrate Dissociation. During the microwave stimulation, the following processes were involved: propagation of microwave, heating of the hydrate sediment, and heat transfer. The increased temperature, which causes the hydrate to dissociate, can be induced by either the direct heating of microwave or the heat transfer because of a temperature gradient. As a result, the partial energy conversion of sensible heat to hydrate decomposition heat continues, during which the water and gas are produced. The volume of interstitial sedimentary water and injected methane gas is equal to the total volume of residual water, gas, and hydrate after hydrate formation Vw0, s þ Vg0, s þ Vg0, f ¼ Vw1, s þ Vg1, s þ Vg1, f þ Vh1, s
ð4Þ
where subscript “s” denotes the sediment zone, subscript “f ” denotes the upper free gas zone, subscript “0” denotes the state before hydrate formation, and subscript “1” denotes the state after hydrate formation or before hydrate decomposition. We assume that the change of the bulk volume or the porosity of the sediment is negligible during the experiments,32,33 because the volume expansion from water to hydrate is less than 15 cm3, which can be accommodated by the residual gas-filled pore space. In addition, this assumption is confirmed through the visible quartz glass. Hence, the volume of the upper free gas is fixed, making Vg0,f equal to Vg1,f. During the stimulation, the desorbed gas first releases to the pore space of the sediment and then spreads to the upper free gas zone. The cumulative moles of upper free gas at time t during the hydrate decomposition process can be calculated with the following equation:
Results and Discussion
ð5Þ
Microwave Heating Behavior. Panels a-d of Figure 2 show the temperature variation in the sediment during microwave heating for several typical experimental runs, where the influence of water saturation and hydrate saturation on the heating behavior is demonstrated. For run 2, as seen in Figure 2a, a uniform volumetric heating in the earlier stage, before about 50% gas production, is observed through the uniform temperature distribution in the microwave radiation direction. During this stage, most of the liquid water adjacent to hydrate particles in the
(33) Clennell, M. B.; Hovland, M.; Booth, J. S.; Henry, P.; Winters, W. J. Formation of natural gas hydrates in marine sediments 1. Conceptual model of gas hydrate growth conditioned by host sediment properties. J. Geophys. Res., [Solid Earth] 1999, 104 (B10), 22985–23003.
(34) Liu, C. L.; Lu, H. L.; Ye, Y. G.; Ripmeester, J. A.; Zhang, X. H. Raman spectroscopic observations on the structural characteristics and dissociation behavior of methane hydrate synthesized in silica sands with various sizes. Energy Fuels 2008, 22 (6), 3986–3988.
Δnt, f ¼ nt, f - n1, f ¼ ðPt Vg1, f Þ=ðZt RTgt, f Þ - ðP1 Vg1, f Þ=ðZ1 RTg1, f Þ
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Energy Fuels 2011, 25, 33–41
: DOI:10.1021/ef1011116
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Figure 2. Temperature distribution during the gas production by microwave stimulation for runs (a) 2, (b) 15, (c) 19, and (d) 20.
pore space is effective microwave absorber and the heat exchange between hydrate decomposition surfaces and surrounding phases is almost supplied by the direct microwave heating. As the content of released water increases, a considerable temperature gradient from the radiation source to the bottom of the sediment takes place, indicating a decreasing microwave penetration depth. Through the preliminary calculation from eq 3, DP is estimated to be 7.9, 4.3, and 3.2 cm under 20, 40, and 62% water saturation, respectively, based on the reported dielectric data of wet sand at 10 °C.28,31 The water saturation before and after hydrate decomposition in run 2 is 29.0 and 70.4%, with the sample depth of 5.6 cm. Therefore, the effect of the penetration depth will be dramatic in a later decomposition stage, as shown in Figure 2a. When the initial water saturation becomes higher, for runs 6-11, a considerable temperature gradient is found to form earlier. Increasing irradiation density results in a higher heating temperature and larger temperature gradient in the later stage, after about 50% gas production, as shown in Figure 3, although the heating time at 90% gas production for run 5 is just about half of that for run 2. The other factor affecting the heating and temperature behavior is the endothermic reaction of hydrate dissociation or ice melting. From Figure 2b, although the water saturation before decomposition for run 15, about 28.0%, is nearly the same as that for run 2, a more obvious temperature gradation from T3 to T1 is found during the earlier heating process because of the lower hydrate saturation. The heating behavior after depressurization for run 20 is extraordinary, as seen in Figure 2d, because of the combined action of hydrate dissociation and ice formation/melting. The temperature at T1, T2, and T3 will not deviate far from the ice point until most of the hydrate is recovered. The influence of depressurization on the heating and gas production behavior will be further discussed later.
Figure 3. Effect of the radiation density on the temperature distribution at 50 and 90% gas production for runs 1-5.
The places where hydrate dissociates faster can accumulate more free water, absorb microwave better, and thus, simultaneously accelerate the hydrate dissociation. The sediment layer with less residual hydrate and more liquid water warms up faster, indicating no leveling effect on hydrate decomposition or ice melting during microwave stimulation. This is also reflected in the experiments with smaller initial water saturation before decomposition. In Figure 2c, the temperature at T2, located around the middle of the sediment sample, is kept several degrees higher than those at T1 and T3 before 50% gas production after it reaches the freezing point. A reason for this observation is that the aggregated liquid water at T2 can be effectively heated by microwave with deep penetration depth in the condition of small water saturation in the earlier decomposition stage, such as in the case of prefreezing (runs 18 and 19). The decomposition experiments of runs 12-14, which have the initial water saturation of just about 16%, show a similar heating behavior. 37
Energy Fuels 2011, 25, 33–41
: DOI:10.1021/ef1011116
He et al.
Figure 4. Pressure-temperature variation during the microwave stimulation process for runs (a) 1, (b) 2 and 20, (c) 18, and (d) 21. The phase equilibrium data of methane hydrate represented by 9 are found in the literature.1
dissociation has not yet been proven,37 the hydrate dissociation kinetics proposed by Kim et al.,38 which has been wildly used, is used here to help analyze the experimental results, where the thermal effect of microwave is supposed to play an essential role in the hydrate dissociation. On the basis of the Kim-Bishnoi model, the gas production behavior is controlled by both the intrinsic kinetics and the heating rate, which will be reflected through the pressure-temperature (P-T) variation. The P-T variation during microwave heating without depressurization for run 1 is plotted in Figure 4a, where T1, T2, and T3 are treated simultaneously. From the results presented in Figure 4a, a good match between the P-T curves and the reported phase equilibrium data is observed under this low radiation density (3.9 kW/m2) and high hydrate saturation (53%). On the one hand, the significant difference does not appear in equilibrium conditions between porous media and the tank reactor, just as mentioned by other researchers.39 On the other hand, the thermal stability of methane hydrate seems not to be altered in the external 2.45 GHz microwave electromagnetic field, because the practical electric intensity (less than 1 kV/cm) is much smaller than the critical electric intensity, which may alter the hydrate stability, of 0.1 and 10 MV/cm proposed by Makogon40 and English and MacElroy,37 respectively.
It is concluded that the change in the penetration depth causes a gradual transition from microwave directly heating to macroscopic heat transfer through a growing temperature gradient. According to the previous work, the hydrate reservoir is separated into two distinct zones by a moving decomposition front, despite which kind of heating method is applied. The dissociated zone continues to extend as the decomposition front propagates away from the heating source, leaving the rest of the hydrate reservoir as an undecomposed hydrate zone.10,11,13,15,35 As shown in Figure 2, the dramatic rise in T1 at about 90% gas production demonstrates that the dissociated zone or the decomposition front has extended to the bottom of the sediment sample. According to our experimental and calculation results, it is confirmed that a large temperature gradient exists from the microwave heating source to the undecomposed hydrate zone under actual scale, just like that in the down-hole combustion stimulation.13 Gas Production Behavior. Both the kinetic and pure equilibrium models for simulating hydrate dissociation have been used in the numerical studies.24 Generally, the kinetic model reveals a lower dissociation rate than the equilibrium model because of the extra heat needed to make the hydrate temperature deviate from the equilibrium temperature. The deviation between the kinetic and equilibrium models will be larger under a higher heating rate or lower surface area of hydrate particles for a given hydrate.36 Because the non-thermal effect of the practical microwave on the methane hydrate
(37) English, N. J.; MacElroy, M. D. Theoretical studies of the kinetics of methane hydrate crystallization in external electromagnetic fields. J. Chem. Phys. 2004, 120 (21), 10247–10256. (38) Kim, H. C.; Bishnoi, P. R.; Heidemann, R. A.; Rizvi, S. S. H. Kinetics of methane hydrate decomposition. Chem. Eng. Sci. 1987, 42 (7), 1645–1653. (39) Sung, W. M.; Lee, H.; Kim, S.; Kang, H. Experimental investigation of production behaviors of methane hydrate saturated in porous rock. Energy Sources 2003, 25 (8), 845–856. (40) Makogon, Y. F. Hydrates of Hydrocarbons; PennWell Publishing Company: Tulsa, OK, 1997.
(35) Selim, M. S.; Sloan, E. D. Heat and mass transfer during the dissociation of hydrates in porous media. AIChE J. 1989, 35 (6), 1049– 1052. (36) Jamaluddin, A. K. M.; Kalogerakis, N.; Bishnoi, P. R. Modeling of decomposition of a synthetic core of methane gas hydrate by coupling intrinsic kinetics with heat-transfer rates. Can. J. Chem. Eng. 1989, 67 (6), 948–954.
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Figure 5. Gas production behavior during the microwave stimulation process for runs (a) 1, 2, 9, 18, and 19 and (b) 20 and 21.
The gas production processes for several experimental runs are shown in Figure 5. Enhanced microwave radiation is beneficial to the gas production seen from the cumulative upper free gas curves for runs 1 and 2. For the similar hydrate sample in run 2, the temperatures of T2 and T3 plotted in Figure 4b under relatively higher radiation density (7.9 kW/m2) are somewhat higher than the equilibrium temperature. According to the Kim-Bishnoi model, a larger temperature driving force corresponds to a larger decomposition rate and a larger heat consumption rate for a given hydrate sample. Therefore, the temperature deviation at hydrate layers that are effectively heated by microwave is found to be larger as expected under higher heating density, as mentioned above. It has been reported that the kinetic limitations of hydrate dissociation can be important for short-term processes at smaller scales, while for reservoir-scale simulations, both the kinetic and equilibrium models exhibit comparable results.41,42 The far-field hydrate layer mainly heated through heat transfer is found to be at equilibrium even under higher heating density, seen from the bottom temperature at T1 plotted in Figures 3 and 4b. This assumption of local equilibrium is still considered to be reasonable for the practical length and time scales in many numerical studies.42 Besides the stimulation experiments of runs 18 and 19 with prefreezing, the overall P-T variation and gas production behavior during microwave stimulation without depressurization is similar to that shown by Pang et al.43 after their buffering stage during hot water stimulation. At the initial state, as shown in panels a-c of Figure 4, the driving force of hydrate dissociation is zero or so small that a large amount of heat is needed to make the hydrate temperature deviate from the equilibrium temperature, which also increases with the increasing pressure in the isochoric process, because no pressure reduction is applied simultaneously. The warm up time could be longer if the initial temperature is far below the equilibrium temperature, especially in the case of prefreezing (runs 18 and 19), as shown in Figure 5a. The need of the temperature driving force and decomposition heat makes the heating density become the main factor controlling the rate
Figure 6. Average gas production rate with radiation density for all runs.
of gas production for a given hydrate reservoir, as shown in Figure 6. The control effect of the heating density is also reflected in the experimental runs with less initial hydrate saturation, such as run 9, shown in Figure 5a. Although less hydrate saturation means a smaller area of the reaction surface, a larger temperature driving force can be raised to regain the gas production rate and heat consumption rate for a given heating density. However, much more microwave energy will be consumed to heat the co-existing water and sediment, especially in the last decomposition period, taking the limited penetration depth into account. One should note that, as seen in Figure 5a, there is a buffered dissociation stage during which most of the absorbed heat is consumed to melt the co-existing ice, as proposed by Pang et al.,43 at 9-10 and 9-11 min in runs 18 and 19, respectively. There seems no need to melt the co-existing ice completely before the temperature deviates from the ice point to form a sufficient driving force for hydrate decomposition (Figure 4c), especially under lower pressure in run 19. It can be seen from Figure 5a that, when the initial ratio of hydrate to ice decreases, from run 18 to 19, the gas production rate is significantly reduced, indicating that much more energy is consumed for ice melting during the hydrate decomposition process. The close decomposition time at 90% gas production implies that the co-existing ice still has the advantage of melting and heat consumption compared to the hydrate under this P-T condition. For this same reason, the comparison of the ice melting rate with the hydrate decomposition rate should affect the gas production behavior. However, it will be another case when depressurization is applied where the hydrate decomposition kinetics is enhanced.
(41) Kowalsky, M. B.; Moridis, G. J. Comparison of kinetic and equilibrium reaction models in simulating gas hydrate behavior in porous media. Energy Convers. Manage. 2007, 48 (6), 1850–1863. (42) 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), 165–175. (43) Pang, X. Y.; Xu, W. Y.; Sun, C. Y.; Zhang, C. L.; Chen, G. J. Methane hydrate dissociation experiment in a middle-sized quiescent reactor using thermal method. Fuel 2009, 88 (3), 497–503.
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Combining with other recovery schemes could also be beneficial. For example, it may be possible to produce gas from hydrates by depressurization followed by thermal stimulation. Application of depressurization is supposed to improve the gas production mentioned above through enhancing thermodynamic non-equilibrium, while careful treatment of the co-existing ice should be taken into account.44 The gas production behaviors under microwave stimulation after depressurization for runs 20 and 21 are represented in Figure 5b (from point B to point C). The corresponding P-T curves after depressurization are plotted in panels b and d of Figure 4, respectively. The drop of the temperature can be observed during depressurization because of the cooling effect of gas expansion and hydrate decomposition. The line of dashes from point A to point B in Figure 5b means 0.175 mol of methane hydrate has dissociated during the depressurization in run 20 without prefreezing, while none or a tiny amount of hydrate is dissociated during the depressurization in run 21 with prefreezing. Thus, the drop of the temperature during depressurization in run 21 is basically caused by gas expansion. The experimental data also show that the drop of the temperature within the sediment (T1, T2, and T3) is less than 2 °C, while the temperature drop at T4 is about 10 °C, which is not shown in the figure. The observation of an anomalous gas production rate below about -2 °C in run 21 is considered to be the selfpreservation phenomenon reported by many researchers.45,46 The overall gas production behavior that experiences a transition regime around the ice point is consistent with that reported by Circone et al.,47 where the gradually warmed bath passing ice point was employed. Although the driving force of hydrate decomposition is large after self-preservation, as seen in Figure 4d, the gradually increasing gas production rate reflected in Figure 5b is mainly controlled by the gradually increasing heating rate, which is found to be weak before the releasing of adequate free water from hydrate decomposition or ice melting below -2 °C. In contrast, although the temperature of the sediment has dropped below the ice point from about 1 °C after depressurization in run 20, no self-preservation phenomenon is detected. The hydrate continues to dissociate during and after depressurization in run 20, as seen in Figure 5b. One possible reason for this continuous decomposition is that the temperature has not dropped below -2 °C, which is the ceiling temperature of self-preservation found in run 21. Through an estimate of energy conservation during depressurization, it is able to further analyze the decomposition behavior observed in run 20. For run 20, besides the cooling effect of gas expansion, the decomposition of 0.175 mol of methane hydrate should absorb about 9.5 kJ of heat, which is mainly supplied by the sediment itself. A temperature drop of 15 °C within the sediment can be estimated, taking no account of the formation of ice. However, the temperature drop is less than 3 °C, as seen in Figure 4b, which means a
certain amount of ice has formed or the hydrate has decomposed to ice and gas rather than water and gas during the depressurization. Although the formation of ice implies the possibility of hydrate self-preservation, it is hard to cool the sediment itself into the self-preservation region through hydrate decomposition because, once the sediment temperature drops into the self-preservation region, the endothermic reaction of hydrate decomposition will be inhibited and the temperature will re-rise. In other words, the cooling in the self-preservation region through hydrate decomposition is not supported in this situation. The heat supplied by the ice formation from initial co-existing water is helpful to the initial gas production rate in run 20 compared to that after self-preservation in run 21 with prefreezing, as shown in Figure 5b, which is one probable reason for the different influence of ice formation on methane hydrate dissociation reported by Halouane et al.44 The other reason for the higher initial gas production rate is that more initial liquid water with better microwave absorption is found in run 20. It is noted that all of the temperatures are just heated to near the ice point when most of the hydrate has dissociated in both runs 20 and 21, as seen in panels b and d of Figure 4. This phenomenon is also shown in other works by bath heating.47 It is indicated that the ice co-existed throughout the thermal stimulation process after depressurization, before the temperature re-rose above the ice point, just as proposed by Pang et al.43 The longer ice melting time compared to hydrate decomposition after depressurization is also indicated by Kelkar et al.49 and Circone et al.47 With regard to the practical gas production, the formation of solid ice may reduce the permeability of the hydrate reservoir and improve the gas production because of lower hydrate decomposition heat, as proposed by Zhou et al.50 and Tspykin.51 However, ice melting or formation is still considered as occurring at chemical equilibrium in the kinetic model.24 Our results suggest that it is necessary to consider the role of ice according to different thermodynamic conditions. Gas Production Rate and Efficiency. The average gas production rate for a given radiation density is associated with the overall efficiency. The change in the average gas production rates with radiation density for all of the experimental runs is illustrated in Figure 6. The corresponding efficiencies (η) for runs 1-17 are shown in Figure 7. All of the gas production rates and radiation densities plotted in Figures 6 and 7 are calculated on the basis of 0-90% gas production within the hydrate decomposition periods, which exclude the depressurization period for runs 20 and 21 but include the self-preservation period for run 21. The efficiencies are evaluated on the basis of the whole microwave stimulation periods and defined as13
(44) Halouane, A.; Sinquin, A.; Jussaume, L. Influence of ice formation on methane hydrate dissociation. Proceedings of the 4th International Conference on Gas Hydrates; Yokohama, Japan, May 19-23, 2002. (45) Stern, L. A.; Circone, S.; Kirby, S. H.; Durham, W. B. Anomalous preservation of pure methane hydrate at 1 atm. J. Phys. Chem. B 2001, 105 (9), 1756–1762. (46) Ershov, E. D.; Yakushev, V. S. Experimental research on gas hydrate decomposition in frozen rocks. Cold Reg. Sci. Technol. 1992, 20 (2), 147–156. (47) Circone, S.; Stern, L. A.; Kirby, S. H. The role of water in gas hydrate dissociation. J. Phys. Chem. B 2004, 108 (18), 5747–5755.
(48) Dallimore, S. R.; Collett, T. S.; Uchida, T. Overview of science program, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well. Bull.;Geol. Surv. Can. 1999, 554, 11–17. (49) Kelkar, S. K.; Selim, M. S.; Sloan, E. D. Hydrate dissociation rates in pipelines. Fluid Phase Equilib. 1998, 150, 371–382. (50) Zhou, Y.; Castaldi, M. J.; Yegulalp, T. M. Experimental investigation of methane gas production from methane hydrate. Ind. Eng. Chem. Res. 2009, 48 (6), 3142–3149. (51) Tsypkin, G. G. Effect of decomposition of a natural gas hydrate on gas recovery from a reservoir containing hydrate and gas in the free state. Fluid Dyn. 2005, 40 (1), 117–125.
η ¼ ðqout - qin Þ=qout
ð6Þ
where qin is the total input heat and qout is the higher heating value of the released methane gas.
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combing of depressurization before microwave stimulation is found to be efficient, except the experience of the selfpreservation period. The gas production efficiency for run 20 is as high as 0.92, taking the gas produced from the hydrate during depressurization into account. Although the efficiency for run 21 with prefreezing is just 0.80, it is much higher than that of 0.64 for run 19, which is not conducted with depressurization. We believe that the combing recovery method that conducts microwave stimulation followed by depressurization could also be efficient. The thermal stimulation and depressurization method involved in the combing recovery method can be carried out alternately or simultaneously as required.15 Figure 7. Gas production efficiency for runs 1-17.
Conclusions
The overall efficiency, which is in the range of 0.73-0.85, under microwave stimulation here is consistent with the simulated results under down-hole combustion stimulation.13 For run 15, which has a hydrate saturation of 16.1% and exhibits the highest efficiency of 0.85, a daily gas production of 1.0 105 m3 is estimated under the radiation of 6.6 kW/m2 1000 m2. In general, the gas production rate increases with the increasing radiation density, as expected. However, it is not directly proportional to the radiation density because of the decreasing efficiency. It is seen from Figure 6 that a higher radiation density is more helpful to the gas production rate when the initial hydrate proportion is larger in the sediment (more hydrate and less water). As a result, as seen in Figure 7, the efficiency decreases much faster for runs 9-11 and 15-17, which have the smallest proportion of gas hydrate. From Figures 6 and 7, we conclude that lower water saturation is beneficial to the gas production efficiency, while higher hydrate saturation results in better efficiency under higher radiation density. For the experimental runs with prefreezing, one can see that the production rate for run 18 is scarcely affected compared to that for runs 1-5, while the production rate for run 19 is much reduced by the co-existing ice. The reasons for this difference have been discussed above. However, the efficiencies for runs 18 and 19 are just 0.76 and 0.64, respectively, because more energy is consumed to warm the frozen hydrate reservoir and to melt the co-existing ice. The
In this study, the experimental apparatus was set up to investigate the decomposition behavior of methane hydrate in an unconsolidated sediment by 2.45 GHz microwave heating. The limitation in microwave penetration depth causes a gradual transition from microwave directly heating to macroscopic heat transfer through a growing temperature gradient, making the far-field hydrate layer dissociate at equilibrium. The gas production efficiency under microwave stimulation decreases because of the co-existing ice for the frozen sediment, while it increases by combining with depressurization even if there is a self-preservation period below -2 °C. We conclude that, when the sediment itself can supply sufficient heat for hydrate decomposition, which may be sensible heat or latent heat of freezing from initial water, the hydrate decomposition rate is considered to be controlled by the intrinsic kinetics. However, when the current heat is not sufficient, so that the sediment is cooled in the self-preservation region (metastable state) or thermodynamic stable region, additional thermal stimulation is confirmed to play a key role in the hydrate decomposition rate. Acknowledgment. The work was supported by the National Natural Science Foundation (NSFC) of China (50676097), National Basic Research Program of China (2009CB219504), NSFC-Guangdong Union Foundation (U0733003), and Chinese Academy of Science (CAS) (KGCX2-YW-805)
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