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Experimental Investigation of Production Behavior of Gas Hydrate under Thermal Stimulation in Unconsolidated Sediment Liang G. Tang, Rui Xiao, Chong Huang, Z. P. Feng,* and Shuan S. Fan Guangzhou Research Center of Gas Hydrate, Guangzhou Institute of Energy Conversion, The Chinese Academy of Science, Guangzhou, P. R. China, 510640 Received July 21, 2005. Revised Manuscript Received August 31, 2005
This article investigates the temperature distribution and flowing characteristics of the dissociated gas and water from hydrate in porous sediment by utilizing a one-dimensional experimental model setup. With the developed apparatus, the experiments have been run for the thermal stimulation method by injecting hot water with different temperatures and rates. The experimental result suggested that the gas production rate increases with time until it reaches a maximum, and then it begins to decrease. However, the water production rate keeps nearly constant during the whole production process. The injection water temperature and rate, as well as the hydrate content in the sediment, all influence the energy ratio of thermal stimulation production.
Introduction Natural gas hydrates are considered to be a potential energy resource for the 21st century because a huge amount of methane is trapped in hydrate deposits which are found in permafrost regions and continental margins.1 Therefore, developing methods for commercial production of natural gas from hydrates attracts considerable attention. A variety of methods have been proposed, such as (1) thermal stimulation, to increase the deposit temperature above the temperature of hydrate formation at a specified pressure; (2) depressurization, to decrease the deposit pressure below the pressure of hydrate formation at a specified temperature; (3) chemical injection, to inject inhibitors such as methanol to shift the pressure-temperature equilibrium; (4) CO2 replacement, to inject liquid CO2 into offshore reservoirs to displace methane trapped in hydrates by forming CO2 hydrates;2 and (5) gas lift, to lift the hydrate particle as a solid from the sea bottom, which is a new method suggested by Japanese researchers.3 However, among these methods, the thermal stimulation combined with depressurization is regarded as the most promising method for gas hydrate production.4 Extensive mathematical models with simplification of the physical process and self-similarity solutions have * Corresponding author. Tel/Fax: 86-20-87057795. E-mail: fengzp@ ms.giec.ac.cn. (1) Sloan, E. D. Clathrate Hydrates of Natural Gas, 2nd ed.; M. Dekker: New York, 1998. (2) Hirohama, S.; Shimoyama, Y.; Wakabayashi, A. J. Chem. Eng. Jpn. 1996, 29, 1014-1020. (3) Hamaguchi, R.; Nishimura, Y.; Matsukuma, Y.; Minemoto, M. A Fluid Dynamic Study on Recovery System of Methane Hydrate. In The 5th International Conference on Gas Hydrate, 2005, Trondheim, Norway. (4) Makogon, Y. F. Hydrates of Natural Gas; Penn Well Publishing Company: Tulsa, OK, 1997.
been developed in simulating both depressurization and thermal schemes,5-7 and they are proved to be successful in modeling hydrate-bearing sediments with a simple geometry. Three-dimensional reservoir models with numeric solutions, which involve the mechanisms of Darcy flow, heat and mass transfer, intrinsic dissociation kinetics, and so on, have also been developed to predict the gas production behavior of complicated hydrate reservoirs.8,9 Laboratory works in simulating hydrate formation and dissociation behaviors in sediments have also been carried out; even published attempts illustrate the difficulties of simulation of occurrences in an actual hydrate reservoir from an experimental effort.10 Most studies are focused on the depressurization method. For example, Yousif et al.11,12 studied the dissociation of hydrate in Berea sandstone by using a depressurization scheme, and it was suggested that a moving boundary model can be used to describe the dissociation process. Kono13 synthesized methane gas hydrate in porous sediments and dissociated the hydrate by depressurization. On the basis of these experiments, he derived the (5) Ji, C.; Ahmadi, G.; Smith, D. H. Energy Convers. Manage. 2003, 44, 2403-2423. (6) Kamath, V. A.; Mutallk, P. N.; Sira, J. H.; Patll, S. L. SPE Formation Evaluation 1991, 477-484. (7) Selim, M. S.; Sloan, E. D. SPE Reservoir Engineering 1990, 35 (6), 245-251. (8) Masuda, Y.; Fujinaga, Y.; Naganawa, S.; Fujita, K.; Sato, K.; Hayashi, Y. Modeling and experimental studies on dissociation of methane gas hydrates in Berea sandstone cores. In The 3rd International Conference on Gas Hydrates, 1999, Salt Lake City, UT. (9) Moridis, G. J.; Collett, T. S. J. Pet. Technol. 2004, 175-183. (10) Baker, P. E. Natural gases in marine sediments; Kaplan, I. R., Ed.; Plenum: New York, 1974. (11) Yousif, M. H.; Li, P. M.; Selim, M. S.; Sloan, E. D. J. Inclusion Phenom. Mol. Recognit. Chem. 1990, 8, 71-88. (12) Yousif, M. H.; Abass, H. H.; Selim, M. S.; Sloan, E. D. SPE Reservoir Engineering 1991, 69-76. (13) Kono, H. O.; Narasimhan, S.; Song, F.; Smith, D. H. Powder Technol. 2002, 122, 239-246.
10.1021/ef050223g CCC: $30.25 © 2005 American Chemical Society Published on Web 10/13/2005
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Figure 1. Schematic plot of the experimental facility.
apparent kinetic parameters of hydrate dissociation. Sung14 dissociated hydrates in porous rock by depressurization and methanol injection schemes. In their experiments, the flowing characteristics of the dissociated gas and water have been investigated. Experimental work on the dissociation behavior of hydrates by thermal stimulation have also been carried out. Kawamura et al.15 studied the hydrate dissociation kinetics by dissociating pellet-shaped samples, which mimic a naturally occurring hydrate in ocean sediment, with pure water or a viscous fluid of different temperature. The data obtained agree with a 1-D heat conduction model. Kamata et al.16 have carried out a decomposition experiment of methane hydrate sediment by the thermal recovery method. They found that the temperature and pressure in the sample fluctuated between stability and the decomposition region of methane hydrate when the hot water was at a high temperature. No further data and analysis were given in the article. The dynamic processes for hydrate-bearing sediment after thermal stimulation, such as temperature distribution and gas/water production rate, are important for an overall evaluation of the hydrate thermal recovery technique and verification of the mathematical approaches. However, this kind of data is scarce in the literature. The main object of this paper is to investigate the temperature distribution, gas and water production rate, and the thermal efficiency during the hydrate dissociation process after the hot water injection using a 1-D physical model. Experimental Section Setup of Experimental Apparatus. The schematic diagram of the experimental apparatus used in this work is shown in Figure 1. The pressure vessel as the main unit of the setup (14) Sung, W.; Lee, H.; Kim, S. Energy Source 2003, 25, 845-856. (15) Kawamura, T.; Ohga, K.; Higuchi, K.; et al. Energy Fuels 2003, 17, 614-618. (16) Kamata, Y.; Ebinuma, T.; Omura, R.; Minagawa, H.; Narita, H. Decomposition Experiment of Methane Hydrate Sediment by Thermal Recovery Method. In The 5th International Conference on Gas Hydrate, 2005, Trondheim, Norway.
is immersed in an air bath with the temperature varying from -20 to 80 °C. The cell is made of stainless steel and has as internal diameter of 38 mm and a length of 500 mm; it can be operated up to 25 MPa. Four resistance thermometers and two pressure transducers with three differential pressure transducers were placed in four ports evenly along the vessel, as plotted in Figure 1, to measure the temperature and pressure profile along the vessel. To simulate the hot water or chemical inhibitor injection production model, a middle container with an electrical resister has been used, which can prevent damage to the pump due to high temperature or corrosive chemicals. The data acquisition unit records all the information varying with time, which includes pressure or differential pressure, temperature, gas/water injection rate, and gas/water production rate. In the cases using digital sensors and a data acquisition unit, the relationship between measured values and real values has been established for the calibration. The pressure transducers, thermocouples, and mass flow meters were calibrated using a pressure test gauge with an error of (0.05%, a mercury thermometer with a tolerance of (0.01 °C, and a wet gas meter with an accuracy of (10 mL/min, respectively. Experiments. The raw dry sands were sieved into the size range of 300-450 µm and were pushed tightly into the vessel, resulting in a sediment with a porosity around 30% and permeability of 0.11 D. The outlet valve was closed, and the vessel was saturated with a 2.0 wt % NaCl solution using a metering pump. Methane gas was then injected slowly at a pressure that is sufficiently higher than the equilibrium pressure at the working temperature. The amounts of injected NaCl solution and methane gas were recorded. The vessel was then closed and kept at a steady environmental temperature for a whole night to be certain that there is no leak of the system and gases dissolve in water. After that, the temperature of the air bath was decreased to the working temperature, and the data acquisition unit began to work. Gas hydrates began to form in the vessel, and the formation was thought to be finishing when there was no pressure decrease in the system, which lasted in general three to five days. The formation process was repeated again after the hydrate was dissociated by increasing the environmental temperature to obtain homogeneously distributed hydrates in the system. After the hydrate was formed the second time, the backpressure regulator was set to a pressure that is 0.1 MPa higher than that of the inside vessel, and the outlet valve was open. The hydrate began to dissociate after the injection of steam/
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Table 1. Experimental Conditions for Hot Water Injection experimental runs initial pressure (MPa) final pressure (MPa) final temperature (°C) hydrate content (vol, %) hot water injection temperature (°C) hot water injection rate (mL/min) total hot water injected (mL) total gas produced during hot water injection (mL)
1
2
3
4
5
6
8.400 3.180 1.41 15.71 100 11.0 3219.4 10443
9.100 3.412 1.16 11.16 130 11.0 2666.4 6707
9.090 3.485 1.10 16.77 160 11.0 1715.7 10242
9.270 3.395 1.22 7.18 190 11.0 1599.5 4321
9.400 3.382 1.04 17.1 160 8.0 1284.3 9839
9.700 3.602 1.77 13.70 160 2.5 884.0 8022
hot water from the middle container. Gas and water were continuously released from the vessel through the outlet valve. When there is no significant gas released, the hot water injection was stopped and the back-pressure regulator was set at 0.1M Pa with the remaining gas released. To reduce the heat loss during steam/hot water injection, the vessel and loading lines were covered with a heat-resistant material. During the hot water injection simulation, the temperature and flow rate of hot water were changed from one run to another. The detailed experimental plan is given in Table 1.
Results and Discussions 1. Hydrate Formation. NaCl solution instead of distilled water was used for all experiments because of the higher formation rate of NaCl solution than that of the distilled water in sediments.17 The hydrate content in Table 1 is calculated by the volume of injected gas and gas left after hydrate formation, assuming the hydration number is 5.75. A typical pressure-time curve during the hydrate formation in sediment is given in Figure 2. The inlet and outlet pressures of the vessel decrease simultaneously due to the high porosity and permeability of the sediment. The results suggested that the pressure profile can be divided into three sections. In the first section, the pressure decreases with time due to the gas contraction upon cooling at constant volume. The second section is the hydrate nucleation process, during which small hydrate crystals (nuclei) grow and disperse in an attempt to achieve critical size for continued growth. The third section is the hydrate formation process. In this section, the pressure gradu-
ally decreases due to the gas consumption in forming hydrate. This is the slowest stage, which can last three to five days for the NaCl solution. The slow formation rate supports the conclusion in the literature that the sediment somehow inhibits gas hydrate nucleation and growth when forming hydrate in porous media.12 2. Temperature Distribution in the Vessel during Hot Water Injection. The inlet and outlet pressures, during the hot water injection, change simultaneously, which are similar to the hydrate formation and are not given here. The temperature distribution after the hot water injection for run 2 is shown in Figure 3. The changes of temperature with time for all other runs show a similar trend and are not shown here. The maximum temperature and the onset time for temperature increase of each port for all runs are given in Table 2. The temperature of each port increases sharply with time initially, with that of port 1 increasing immediately after the hot water injection and those of ports 2, 3, and 4 increasing consecutively. After a certain time, the temperature increase rate of all ports slows down or becomes stable, as Figure 3 shows. A temperature gradient can be observed along the vessel, with the heat flux from port 1 to port 4. The heat loss is obvious during the hot water injection stage. From Table 2, it can be revealed that the maximum temperature in port 1 for run 1 is only 55.56 °C with the injected hot water temperature of 100 °C. With the continuous increase of the injected hot water temperature, for example, from 100 °C of run 1 to 160 °C of run 3, the maximum temperature of port 1 increases as well but at a lower rate, only from 55.36 to 81.26 °C. With the further increase of the temperature of hot water, that is, from 160 °C of run 3 to 190 °C of
Figure 2. The inlet and outlet pressure profile during hydrate formation. (17) Tang, L. G.; Li, G.; Hao, Y. M.; Fan, S. S.; Feng, Z. P. Effects of Salt on the Formation of Gas Hydrate in Porous Media. In The 5th International Conference on Gas Hydrate, 2005, Trondheim, Norway.
Figure 3. Temperature distribution after hot water injection for run 2.
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Table 2. Temperature Profiles for All Runs runs maximum temperature of port 1 (°C) onset time for temperature increase of port 2 (min) maximum temperature of port 2 (°C) onset time for temperature increase of port 3 (min) maximum temperature of port 3 (°C) onset time for temperature increase of port 4 (min) maximum temperature of port 4 (°C)
1
2
3
4
5
6
55.56 25.5 42.9 57.3 41.76 78.0 29.55
65.57 20.1 49.64 43.6 47.93 65.0 33.52
81.26 18.6 61.85 42.8 58.31 63.7 38.71
81.32 13.4 61.24 31.8 53.85 56.9 37.91
60.38 28.3 42.62 65.9 36.14 91.4 21.43
10.08 67.6 7.15 193.5 5.31 201.1 4.45
run 4, the maximum temperature in port 1 has no obvious increase due to the heat loss to the environment. The onset time for temperature increase of each port is inversely correlated with the temperature of injected hot water, that is, the higher the injected temperature, the shorter the onset time for temperature increase of each port. For example, with the increase of injected hot water temperature from 100 to 190 °C, the onset time for temperature increase of port 2 decreases from 25.5 to 13.4 min. The onset time for temperature increase of each port depends on the total injected heat and the thermal properties of hydrate-bearing sediment, which strongly affect the production behavior of hydrate during the thermal stimulation. Since the four ports are evenly distributed along the vessel, the onset times for temperature increase of ports 2, 3, and 4 of each run, with the exception of run 6, increase in an approximately linear manner. To some extent, this suggested that there is a constant thermal diffusion velocity along the vessel and that the hydrate is homogeneously formed in the vessel. The thermal diffusion velocities are calculated to be approximately 0.62, 0.76, 0.77, and 0.88 cm/min for runs 1 to 4, respectively, by knowing the length and time elapsed of the onset temperature between ports. The hot water injection rate is 11 mL/min, which is equal to a flow rate of 4.01 cm/min in the sediment with a porosity of 30% and hydrate content of 20%. The thermal diffusion velocity is an order lower than the hot water flow velocity along the vessel, which means that the advection instead of thermal diffusion dominates the heat transport in the production process due to the low thermal conductivity of hydrate-bearing sediment. The effect of hot water injection rate on the temperature distribution along the vessel is obvious. For runs 3, 5, and 6, the injection rates of hot water with same temperature (160 °C) are 11.0, 8.0, and 2.5 mL/min, respectively, and the maximum temperatures of port 1 are 81.26, 60.38, and 10.08 °C. For the lowest injection rate run (2.5 mL/min), a higher energy efficiency could be expected, because the maximum temperature along the vessel is only 10.08 °C, which will be discussed later in this article. 3. Gas and Water Production Rate. The gas production rates for all the runs are shown in Figure 4. At the same water injection rate, the gas production rates by injecting hot water of different temperatures show a similar trend, shown in Figure 4a-d. When the hot water was injected into the vessel, the free gas in the vessel was released immediately, causing a large gas release rate initially and then decreasing. The hydrate begins to dissociate and the gas production rate increases with time until it reaches a maximum. After that, the gas production rate begins to decrease.
The hydrate dissociation rate is a function of temperature difference between the dissociation temperature and the equilibrium temperature at dissociation pressure and the total dissociation surface area, suggested by Kim.18 The total dissociation surface area in the sediments is not easily determined. According to Tohidi’s visual experiments in glass micro-models,19 after dissociation the hydrate section begins to break and becomes mobile within the liquid, causing the dissociation surface area to increase at the early stage. After it reaches a maximum, the dissociation surface area begins to decrease with further hydrate dissociation. The temperature difference increases with time at constant dissociation pressure until the heat balance is maintained. This is why a maximum gas release rate can be observed. Although the general trend for gas release rate profile is similar, the time to reach the maximum gas release rate is different from different runs. From Figure 4, the time is around 75 min for two low-temperature runs, that is, 100 °C and 130 °C, while it decreases to 50 min for two high-temperature runs. The total dissociation times for all four runs are different as well. The total running time decreases with the increase of injected hot water temperature, from 300 to 150 min. Another feature that should be mentioned is that the gas dissociation rate from runs 2 and 4 is obviously lower than that of other runs. This is due to the low hydrate content in the sediment, as given in Table 1. With the decrease of injection rate of 160 °C hot water, the gas production rate with 8 mL/min injection shows a similar profile. However, the gas release rate is very scattered, and no gas release peak can be observed for the 2.5 mL/min injection run, shown in Figure 4f. This could be due to the fact that heat injected into the sediment is not enough to dissociate the hydrate simultaneously. The water production rate profile is much simpler than that of the gas production, and Figure 5 gives a typical profile. In general, the water is released later than the gas due to free gas in the vessel. During the whole production process, the water production rate remains nearly constant. Even though there is a peak for gas production rate, the water produced due to hydrate dissociation is much less than the water injection. For example, the water production rate is 0.46 mL/ min with a gas production rate at 100 mL/min, assuming the hydration number is 5.75. 4. Energy Efficiency Analysis. The main disadvantage of producing gas from hydrate using heat stimulation is its low efficiency. To compare the ef(18) Kim, H. C.; Bishnoi, P. R.; Heidemann, R. A.; Rizvi, S. S. H. Chem. Eng. Sci. 1987, 42 (7), 1645-1653. (19) Tohidi, B.; Anderson, R.; Clennell, M. B.; Burgass, R. W.; Biderkab, A. B. Geology 2001, 29 (9), 867-870.
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Figure 4. Gas production rate for all the experimental runs.
Figure 5. Water production rate profile for run 1.
ficiency of different runs, the following parameters have been defined. (1) Heat loss ratio during water injection stage is defined as the ratio of the enthalpy of water with temperature of port 1 to the enthalpy of injected water.
(2) The thermal efficiency is defined as the ratio of the heats used for hydrate dissociation to the total input heats, which is equal to the heats to heat water from environmental temperature to the desired temperature. (3) The energy ratio is defined as the ratio of the combustion heats of produced gas to the total input heats. To focus on the thermal efficiency and energy ratio during the dissociation stage, the heat loss during the water injection stage is not considered, that is, the total input heat is regarded as the heats to heat water from environmental temperature to the temperature of port 1. During the dissociation process, the heat loss to the steel pressure vessel is excluded from total input heats and is approximated from the heats by heating the vessel from the environmental temperature to the average temperature of four ports. The heat loss to the environment is ignored due to the cover of heat-resistant material along the pressure vessel. Table 3 shows the results for two cases, that is, the process when 50% gas was produced and the whole process during hot water injection. The enthalpy of the water or steam is calculated according to IFC-67. The dissociation heat of hydrate is taken as 54.1 kJ/mol, and the combustion
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Table 3. Energy Analysis for the Hydrate Dissociation by Hot Water Injection runs heat loss during the water injection stage (%) thermal efficiency at 50% gas production (%) energy ratio at 50% gas production thermal efficiency for whole production process (%) energy ratio for whole production process
1
2
3
4
5
6
44.14 17.17 2.67 6.87 1.07
49.43 8.20 1.27 4.08 0.64
49.35 15.76 2.45 7.06 1.10
57.54 6.00 0.93 3.28 0.51
62.24 37.05 5.77 18.73 2.92
93.23 54.76 8.52 31.65 4.93
heat of natural gas is 37.6 MJ/m3. The specific heat for the steel pressure vessel is taken as 17.3 J/(°C-mol). After the hot water injection, the input heats are used for the following three parts without considering the heat loss to the pressure vessel. One part is used to heat the sediments, the other part is to heat the dissociated gas and water, and only the part left is used for the hydrate dissociation. The injection water temperature and rate, as well as the hydrate content in the sediment, all influence the thermal efficiency and hence the energy ratio. The hydrate content in sediment has a great influence on the energy ratio. Runs 2 and 4 have the lowest hydrate content, and the energy ratios are all lower than unity. This can also explain why the energy ratios at 50% gas production are all nearly double those of the whole production process. The experimental energy ratios at 50% gas production vary from unity to 8.52. A theoretical model has given an energy ratio lying between 7 and 10 for hot brine stimulation.20 The experimental data are lower overall than the modeling result. If the waste heat from the outlet water and gas is considered, the experimental data can be increased. The injection temperature does not have a strong influence on the energy ratio, as run 1 and run 3. But considering that run 3 (16.77%) has a slightly higher hydrate content than run 1 (15.71%), a conclusion that with increase of injection temperature, the energy ratio decreases at similar hydrate content can still be drawn. With the decrease of hot water injection rate, the energy ratio increases. The energy ratio of runs 3, 5, and 6 increases from 2.45 to 8.52 when the injection rate decreases from 11 to 2.5 mL/min. A very interesting result is that run 6 has the highest energy ratio among all the runs; even the heat loss in the water injection stage is the greatest. This is because the low injection (20) Kamath, V. A.; Godbole, S. P. J. Pet. Technol. 1987, 1379-1388.
rate decreases the temperature of sediment, which then decreases the heat loss during the dissociation stage. However, the drawback of the low rate and temperature injection is the low gas production rate. For practical hydrate reservoir production, there should be an optimal production solution, depending on the hydrate content and thermal properties of sediment. Conclusions In this study, the experimental apparatus was set up to investigate the producing behavior as hydrate is dissociated from unconsolidated porous sands by thermal stimulation. From the experimental results, the following conclusions were drawn. (1) During hydrate formation, a typical pressure-time curve can be divided into three sections, which correspond to gas cool, hydrate nucleation, and crystal growth. (2) After the hot water injection, the hydrate begins to dissociate, and the gas production rate increases with time until it reaches a maximum. After that, the gas production rate begins to decrease. However, the water production rate remains nearly constant during the whole production process. (3) The injection water temperature and rate, as well as the hydrate content in the sediment, all affect the energy ratio. The results suggested that under the experimental condition, a higher hydrate content and lower injection temperature and rate give a higher energy ratio. Acknowledgment. The author thanks Dr. Jun. S. Zhang for helpful comments and suggestions. The financial support by Knowledge Innovation Foundation of the Chinese Academy of Sciences (No. KGZX2-SW309) is also acknowledged. EF050223G
