Energy Fuels 2010, 24, 5912–5920 Published on Web 10/27/2010
: DOI:10.1021/ef100367a
Experimental Study on Gas Production from Methane Hydrate-Bearing Sand by Hot-Water Cyclic Injection Xin Yang, Chang-Yu Sun,* Qing Yuan, Ping-Chuan Ma, and Guang-Jin Chen* State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China Received March 25, 2010. Revised Manuscript Received September 26, 2010
A three-dimensional middle-size reactor was used to simulate gas production from methane hydratebearing sand by hot-water cyclic injection. The gas production process and energy efficiency in the whole process, which was divided into injecting hot water, closing well, and producing gas (three steps), were investigated using 16 thermocouples distributed in hydrate-bearing sand samples. The experimental results indicates that the overall temperature trend increases with hot-water injection and decreases with gas production. The temperature distribution and fluctuation in the reactor depend upon the location of the injecting/producing well as well as the porosity and permeability of hydrate samples. Heat transfer is controlled by hot-water seepage flow during the injection of hot water. The affecting factors on the energy efficiency, such as hydrate saturation, hydrate sample temperature, hot-water temperature, mass of hot water injected, and well pressure, were examined. It was found that, when other conditions are similar, the energy efficiency ratio increases with the increase of the hydrate-bearing sand saturation and hydrate sample temperature but decreases with the increase of the hot-water temperature and well pressure.
equilibrium temperature at the specified pressure,3-17 depressurization, to depressurize the gas hydrate reservoir below the equilibrium pressure at the specified temperature,18-23 and
1. Introduction Natural gas hydrates are occurring crystalline substances composed of water and gas, mainly methane, which are found most predominantly in permafrost zones and deep marine sediments. The amount of methane in natural gas hydrates in permafrost zones and marine sediments is very vast. Even the most conservative estimate of the total quantity of gas in hydrate is twice the energy contents of the total fossil fuel reserves recoverable by conventional modes.1 Some methods of producing gas from hydrates have been presented,2 such as thermal stimulation, to produce gas from hydrates by increasing the temperature of the gas hydrate reservoir above the
(8) Selim, M. S.; Sloan, E. D. Hydrate dissociation in sediments. SPE Reservoir Eng. 1990, 5, 245–251. (9) Islam, M. R. A new recovery technique for gas production from Alaskan gas hydrates. J. Pet. Sci. Eng. 1994, 11, 267–281. (10) Masuda, Y.; Kurihara, M.; Ohuchi, H.; Sato, T. A field-scale simulation study on gas productivity of formations containing gas hydrates. Proceedings of the 4th International Conference on Gas Hydrates; Yokohama, Japan, 2002; pp 40-46. (11) Kamata, Y.; Ebinuma, T.; Omura, R.; Minagawa, H.; Narita, H.; Masuda, Y.; Konno, Y. Decomposition experiment of methane hydrate sediment by thermal recovery method. Proceedings of the 5th International Conference on Gas Hydrates; Trondheim, Norway, 2005; pp 81-85. (12) 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. (13) Tsimpanogiannis, I.; Lichtner, P. C. Parametric study of methane hydrate dissociation in oceanic sediments driven by thermal stimulation. J. Pet. Sci. Eng. 2007, 56 (1-3), 165–175. (14) 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. (15) Pang, W. X.; 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, 497–503. (16) Linga, P.; Haligva, C.; Nam, S. C.; Ripmeester, J. A.; Englezos, P. Recovery of methane from hydrate formed in a variable volume bed of silica sand particles. Energy Fuels 2009, 23 (11), 5508–5516. (17) Phirani, J.; Mohanty, K. K.; Hirasaki, G. J. Warm water flooding of unconfined gas hydrate reservoirs. Energy Fuels 2009, 23 (9), 4507–4514. (18) Ji, C.; Ahmadi, G.; Smith, D. H. Natural gas production from hydrate decomposition by depressurization. Chem. Eng. Sci. 2001, 56 (20), 5801–5814. (19) Kono, H. O.; Narasimhan, S.; Song, F.; Smith, D. H. Synthesis of methane gas hydrate in porous sediments and its dissociation by depressurizing. Powder Technol. 2002, 122 (2-3), 239–246.
*To whom correspondence should be addressed. Fax: þ86-1089732126. E-mail:
[email protected] (C.-Y. Sun);
[email protected] (G.-J. Chen). (1) Burshears, M.; O’Brien, T. J.; Malone, R. D. A multi-phase, multi-dimensional, variable composition simulation of gas production from a conventional gas reservoir in contact with hydrates. Proceedings of the Society of Petroleum Engineers (SPE) Unconventional Gas Technology Symposium; Louisville, KY, May 18-21, 1986; SPE Paper 15246. (2) Sloan, E. D. Clathrate Hydrates of Natural Gas, 2nd ed.; Marcel Dekker, Inc.: New York, 1998. (3) McGuire, P. L. Methane hydrate gas production by thermal stimulation. Proceedings of the 4th National Research Council of Canada Permafrost Conference; Calgary, Alberta, Canada, 1981; pp 356-362. (4) Holder, G. D.; Angert, P. F.; Godbole, S. P. Simulation of gas production from a reservoir containing both gas hydrates and free natural gas. Proceedings of the Society of Petroleum Engineers (SPE) Annual Technical Conference and Exhibition; New Orleans, LA, Sept 26-29, 1982; SPE Paper 11105. (5) Bayles, G. A.; Sawyer, W. K.; Anada, H. R.; Reddy, S.; Malone, R. D. A steam cycling model for gas production from a hydrate reservoir. Chem. Eng. Commun. 1986, 47, 225–245. (6) Kamath, V. A.; Holder, G. D. Dissociation heat transfer characteristics of methane hydrates. AIChE J. 1987, 33 (2), 347–350. (7) Ullerich, J. W.; Selim, M. S.; Sloan, E. D. Theory and measurement of hydrate dissociation. AIChE J. 1987, 33 (5), 747–752. r 2010 American Chemical Society
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inhibitor injection, to inject inhibitors, such as methanol and brine, into the gas hydrate reservoir to shift the temperaturepressure equilibrium point.24,25 For the thermal stimulation method, Holder et al.4 demonstrated the feasibility of gas production from hydrate through the perspective of thermodynamics. McGuire3 considered that if there was a hydrate reservoir with high permeability or the reservoir was beneath the existence of salt water in the aquifer, then thermal stimulation was the most attractive method of gas recovery from hydrate. Some experimental work has been performed using thermal stimulation to investigate the decomposition rate of hydrate formed in bulk water or ice.6,7,15 For hydrate formed in porous media, Selim and Sloan8 found that the dissociation rate was a strong function of the thermal properties of the system and the porosity of the porous medium. Kamata et al.11 applied the thermal recovery method to dissociate methane hydrate in the hydrate sediment sample by hot-water injection from one side and gas production from another side. It was found that the temperature and pressure in the sample fluctuated between the stability and decomposition regions of the methane hydrate sample when the temperature of the hot water was high. Using a similar recovery method as Kamata et al.,11 Tang et al.12 investigated temperature distribution and flowing characteristics of the dissociated gas and water from the hydrate in porous media. Li et al.14 investigated the influence of salinity and temperature on the gas production behavior from methane hydrate in porous sediment using a one-dimensional apparatus. The results indicated that the dissociation rate was fast with the increase of the salinity within a certain range. For hydrate samples synthesized in different size volume beds, Linga et al.16 found that the rate of methane recovery depended upon the bed size when dissociated by thermal stimulation. Besides, some numerical models were also developed to assess gas production schemes of heating and hot-water injection from hydrates.9,10,13 Islam9 reported electromagnetic heating for dissociating gas hydrates using a numerical simulation method. The effects of the porosity and thickness of the hydrate reservoir on gas recovery and energy efficiency were investigated, and the range of the energy efficiency ratio is from 5 to 40. However, in Islam’s work,9 the influence of salt was not considered. No heating medium existed in hydrate sediments, and the temperature distribution was not investigated. From these experimental investigations, it could be found that hydrate dissociation by thermal stimulation was in general simulated using a one-dimensional, small reactor.
Three-dimensional temperature distribution during hydrate dissociation was hardly obtained, which is very important to understand the process of gas production from hydrate sediment. In addition, the thermal stimulation was usually performed by a single hot-water injection process in the literature. For a low-permeability hydrate reservoir, the injecting pressure will rapidly rise with the hot-water injection, which limits the volume of hot water injected in a single-injection period. In addition, gas and water produced can hardly penetrate the location of the producing well because of low permeability of the sediment if the gas and water produced were not discharged from the injecting well. This problem may be solved by a hot-water cyclic injection to the hydrate sediment from a single injecting/producing well. However, the hot-water cyclic injection for gas production from hydrate-bearing sediments, especially in a three-dimensional apparatus, was hardly reported. In this work, a new method of gas production by thermal stimulation was developed, that is, a cyclic injecting hot-water method. Meanwhile, the experiments of producing gas from hydrate-bearing sediments were conducted in a three-dimensional apparatus by hot-water cyclic injection to hydratebearing sand from a single injecting/producing well, and the hydrate-bearing sand sample formed in this apparatus is closer to the real natural gas hydrate. The gas recovery, gas production rate, energy efficiency ratio, and three-dimensional temperature distribution in the hydrate sediments during the whole gas production process were investigated. These parameters are very important to understand the process of gas production from the hydrate sediment, which are hardly reported in the literature. 2. Experimental Section 2.1. Apparatus. The experimental apparatus used in this work is shown in Figure 1, which included four sections: the reacting system, water injection system, gas production system, and monitor and control generated system (MCGS). The main part of the reacting system is a high-pressure reactor with an inner diameter of 300 mm and an effective height of 100 mm, and the maximum operating pressure is 16 MPa. The reactor is immerged in a water-ethylene glycol aqueous solution, which can be maintained at a constant temperature with a precision of 0.1 K. A total of 16 thermocouples, with an accuracy of (0.1 K, are inserted into the reactor from the reactor lid, and their positions are shown in Figures 1 and 2. The hot water, prepared through the water heater with thermostatic control, is injected into the reactor from a well with a diameter of 3 mm by a metering pump to control the injecting rate of hot water. A balance is used to record the mass variation of the hot water injected. The gas production system is mainly composed of a gas-water separator, a filter, a back-pressure regulator, and two mass flow transducers with different scales. Gas and water produced during hydrate dissociation are separated in the gas-water separator. The produced water is collected and weighed, and the amount of gas produced is measured by the mass flow transducers. The MCGS is used for recording the temperature, pressure, and gas flow rate data during the experiments. 2.2. Procedure. The experimental procedure was divided into two parts. In the first stage, a representative hydrate-bearing sand sample was prepared as follows. First, 10 360 g of quartz sand (20-40 mesh) with a porosity of about 39% and 1400 g of aqueous brine of 3.35 mass % were weighed precisely, then cooled to about 273.5 K, and kept for at least 12 h. Thereafter, the cold sand and brine were mixed and stirred evenly to obtain a mixture with a brine saturation of 51.3 vol % in sand pore space. The mixture of sand and brine was packed into the reactor,
(20) Tsypkin, G. G. Effect of decomposition of a gas hydrate on the gas recovery from a reservoir containing hydrate and gas in the free state. Fluid Dyn. 2005, 40 (1), 117–125. (21) Alp, D.; Parlaktuna, M.; Moridis, G. J. Gas production by depressurization from hypothetical Class 1G and Class 1W hydrate reservoirs. Energy Convers. Manage. 2007, 48 (6), 1864–1879. (22) 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. (23) Liu, Y.; Strumendo, M.; Arastoopour, H. Simulation of methane production from hydrates by depressurization and thermal stimulation. Ind. Eng. Chem. Res. 2009, 48 (5), 2451–2464. (24) Sira, J. H.; Patil, S. L.; Kamath, V. A. Study of hydrate dissociation by methanol and glycol injection. Proceedings of the Society of Petroleum Engineers (SPE) Annual Technical Conference and Exhibition; New Orleans, LA, Sept 23-26, 1990; SPE Paper 20770. (25) Dong, F. H.; Zang, X. Y.; Li, D. L.; Fan, S. S.; Liang, D. Q. Experimental investigation on propane hydrate dissociation by high concentration methanol and ethylene glycol solution injection. Energy Fuels 2009, 23 (3), 1563–1567.
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Figure 1. Schematic graph of the experimental apparatus: (1) gas cylinder, (2) water bath, (3) reactor, (4) water-injecting/gas-producing well, (5) thermocouples, (6-9) pressure transducers, (10-16) valves, (17) gas-water separator, (18) filter, (19) back-pressure regulator, (20 and 21) mass flow transducers, (22) metering pump, (23) balance, (24) water heater with thermostatic control, and (25) computer.
Figure 2. Distribution of the thermocouples and well in the reactor. The 16 thermocouples were divided into four groups. In each group, four thermocouples were placed in different radii of 132, 99, 66, and 33 mm, respectively, and at the same depth. T1, T2, T3, and T4 groups were at the same depth of 82 mm. T5, T6, T7, and T8 groups were at the same depth of 58 mm. T9, T10, T11, and T12 groups were at the same depth of 34 mm. T13, T14, T15, and T16 groups were at the same depth of 10 mm. The well was located in the middle of the reactor at a depth of 65 mm.
as follows. For the first cycle, the water bath temperature was set to about 275 K, at which the gas production from hydrates by hot-water injection was processed. After the hydrate sample temperature was stable, valves 11 and 13 in Figure 1 were opened and hot water with a constant temperature was injected into the reactor at a constant rate of about 2.4 L/h, controlled by a metering pump. During the injection of hot water, the backpressure regulator was set to a pressure value and the outlet valves 15 and 16 were opened. When the mass of hot water injected into the reactor attained a certain value, the metering pump was shutdown and valves 11 and 13 were closed. Meanwhile, the hydrate began to dissociate into gas and water because the hydrate sample has been below the hydrate-stable conditions as a result of the increase of the temperature. In addition, the reactor pressure would also increase with an increasing of system temperature. When the reactor pressure did not rise anymore, valves 11 and 12 were opened and the mixture of methane gas and water began to flow out from the well. During the gas production process, two mass flow transducers with different scales were switched to collect the released gas when at
where the temperature of the water bath was stabilized at 272.7 K, ensuring that water in the sand exists as ice after the mixture was loaded into the reactor. In addition, it will also prevent water migration during the formation of the hydrate. This is important to synthesize a representative hydrate-bearing sand sample in which the hydrate distributes homogeneously. Next, nitrogen gas was injected into the reactor and kept for about 4 h to ensure that there was no leak in the system, while the water bath temperature was kept constant. Afterward, the whole system was washed by injecting methane gas and vacuumed. Methane continued to be introduced into the reactor until the specified pressure value was attained. Thereafter, the whole system was closed, and the hydrate nucleated and formed among the sediment gradually. Hydrate formation was considered to finish if there were no changes of the temperature and pressure in the system for a certain long time. In general, the time for hydrate formation in the reactor was about 27 h, except the time for cooling the sand and brine. After the representative hydrate sample was prepared, the gas production stage, including two cyclic injections, was performed 5914
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Table 1. Experimental Conditions and Results for Each Group of Hot-Water Injection runs
Thydrate (K)
SH (vol %)
Thot water (K)
Mhot water (g)
Pwell (MPa)
Vgas (vol %)
VRG (vol %)
EER
η (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
275.1 276.1 277.0 275.0 274.8 275.0 274.9 274.8 274.9 274.8 274.9 274.8 275.0 275.1 274.9
29.3 26.8 25.2 23.1 35.4 28.9 29.3 30.3 29.3 30.7 29.5 29.3 29.3 29.6 29.9
333.2 333.2 333.2 333.2 333.2 313.2 353.2 333.2 333.2 333.2 333.2 333.2 333.2 333.2 333.2
2000 2000 2000 2000 2000 2000 2000 800 900 1000 1200 1450 2000 2000 2000
3 3 3 3 3 3 3 3 3 3 3 3 3.5 4 1
25.1 33.8 42.3 32.0 26.5 21.8 27.0 16.5 15.5 18.7 24.7 18.5 22.6 12.7 89.6
74.9 66.2 57.7 68.0 73.5 78.2 73.0 83.5 84.5 81.3 75.3 81.5 77.4 87.3 10.4
4.9 6.8 8.3 3.8 5.9 9.2 3.2 8.1 6.6 7.4 7.8 5.1 4.1 2.6 18.2
20.6 28.3 34.5 16.0 24.8 29.2 15.1 33.9 27.5 30.9 32.6 21.5 17.1 10.9 75.8
different gas flow rates. After the gas flow rate was lower than 0.05 L/min, the residual gas production amount was very small, where the back pressure was controlled at the specified well pressure. A large heat loss will arise if gas is continuously collected until the gas flow rate decreases to 0, which will decrease the total energy efficiency in the two-cycle gas production by injecting hot water. Therefore, when the gas flow rate was lower than 0.05 L/min, valves 11 and 12 were closed and the first cycle of gas production was assumed finished. Thereafter, valves 11 and 13 were opened, and the second cycle of water injection started with the same steps as the first cycle. The temperature of hot water injected during the second cycle was also the same as that in the first cycle. After the second cycle of producing gas finished, valves 11 and 12 were closed and the back-pressure regulator was set to 0 MPa. At the same time, the temperature of the water bath was increased to release the remaining gas in the reactor. The residual gas in the reactor was collected, and its volume was recorded by the computer.
3. Results and Discussion
Figure 3. Variation of the pressure at the top of the reactor versus time during the hydrate formation for run 1.
In this work, 15 experimental runs were performed to evaluate the effect of different parameters on gas production from hydrate by injecting hot water. The experimental conditions of hydrate formation and experimental results for each run are given in Table 1, in which the hydrate sample temperature (Thydrate) refers to the average temperature of T1-T16 before the injection of hot water. The mass of hot water injected (Mhot water) and gas production volume percentage (Vgas) for runs 1-7 and 13-15 are the total amount of two cycles. For runs 8-12, Mhot water and Vgas are the amount of a single cycle. Hydrate saturation (SH) is determined by assuming that the hydration number is 5.75.2,12 The well pressure of gas production (Pwell) is the outlet pressure of the well. The volume fractions of produced gas (Vgas) and residual gas (VRG) are obtained from the ratio of the corresponding gas volume/total gas volume. During the gas production, the water bath temperature is always kept constant. 3.1. Hydrate Formation Process. The variation of pressure at the top of the reactor (for short as the top pressure) versus time during the hydrate formation for run 1 was shown in Figure 3. The corresponding fluctuations of temperatures at different locations were shown in Figure 4. From point A to B in Figure 3, the top pressure drops a little because of the decrease of the injecting gas temperature. From point B to C, the top pressure decreases gradually, showing the process of hydrate formation. At the same time, the temperatures at different locations in the reactor first rise to a maximum value and then decrease, as shown in Figure 4. At the initial period, the hydrate formation rate is high and the temperatures
rise because of the exothermic reaction. With the decline of the hydrate formation rate because of the decrease of the pressure driving force and pore space, the rising trend of temperature weakens and the temperatures even decrease gradually because the exothermic effect of hydrate formation cannot make up for the refrigeration effect of the water bath. From point C to D, the pores of the sediment are packed with formed hydrate, which resulting in the fact that gas at the top of the reactor can hardly permeate into the inner sediment. Therefore, little hydrate forms, and pressure and temperature are nearly kept constant in this stage, although the top pressure of about 5.0 MPa is much higher than the equilibrium pressure of about 3.2 MPa, which is determined by the corresponding temperature of about 274 K. From point D to E, hydrate continues to form again, which can be implied from the magnitude of pressure decrease in Figure 3 and the increase in the temperatures in Figure 4. After point E, no hydrate formed and the top pressure and temperature were kept constant gradually. During the whole process of hydrate formation, there are two stages (from B to C and from D to E) of a rapid increase of the temperature caused by the exothermic effect of hydrate formation. From Figure 4, it can be found that all of the temperatures at different locations (T1-T16) for these two stages rise nearly at the same time and the same magnitude, suggesting that hydrate formed in the sand sediment was nearly even and the distribution of hydrate was nearly uniform. The similar results were also obtained for the other experimental runs. 5915
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Figure 4. Temperature distribution during the hydrate formation for run 1 (a) with the same depth of 82 mm, (b) with the same depth of 58 mm, (c) with the same depth of 34 mm, and (d) with the same depth of 10 mm.
3.2. Gas Production Process. In this work, only experimental run 1 is taken, for example, to analyze the gas production process in view of its similarity for all 15 groups of experiments. The experimental parameters for run 1 have been given in Table 1. During the whole gas production process, the water bath temperature is constant and kept at 275.0 K. The pressure-time curves and gas production rate during the first and second cycles are given in panels a and b of Figure 5, respectively. In every cycle, the gas production process can be divided into three steps: injecting hot water, closing the well, and producing gas. Before the gas production process is introduced, some concepts, such as the top pressure, injecting pressure, and well pressure, are defined. The top pressure, as described in section 3.1, refers to the pressure of the free space at the top of the reactor. The injecting pressure refers to the outlet pressure of the pump during the injecting of hot water, which is not only controlled by the meter pump but also related to the hydrate-bearing sand in the reactor. The well pressure refers to the outlet pressure of the well as described before. During the step of injecting hot water, the top pressure of the reactor and the injecting pressure increase gradually with time because of continuous injection of water into the reactor, gas released from hydrate, and thermal expansion. It is noticed that the top pressure of the reactor is always higher than the injecting pressure, as shown in Figure 5. The pressure difference between the two pressures decreases gradually during the injection of hot water. Moreover, the pressure difference in the first cycle is larger than that in the
second cycle, which may be related to the variation of porosity because of hydrate dissociation. During the closing well stage, the injecting pressure decreases gradually because water injection stopped and the water in the pipeline flows back into the pump. However, the top pressure of the reactor rises continuously because of gas released from hydrate. As seen in Figure 5, the time of closing the well in the second cycle is much shorter than that in the first cycle. The main reason is that, after gas production in the first cycle, the hydrate content in sediments reduces greatly in the second cycle. After the closing well stage, gas production starts. The top pressure of the reactor for run 1 drops fast from 4.7 to 3.1 MPa in the first cycle because of 19.9 vol % gas and 39 g of water produced and from 8.8 to 3.1 MPa in the second cycle because of 5.2 vol % gas and 879 g of water produced. In comparison to the first cycle, a smaller quantity of gas is produced in the second cycle. As can be seen from Figure 5b, although the top pressure dropped fast, little gas was produced during the time period from 28 to 33 min. The chief part produced from the well in this stage was water. It is known that, when the well is submerged with enough water injected, gas will accumulate at the top of the reactor before the gas production starts. Once the outlet valve of the well is opened, the water above the well head will rush out first. The top pressure drops rapidly with water and gas produced. When the top pressure of the reactor is close to the well pressure, little gas or water can discharge out because of the limit of the driving force. As a result, only 5.2 vol % gas is 5916
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mainly depends upon the seepage rate of hot water in sediments instead of thermal diffusion, which can be shown from the big temperature difference between T4 and T13. That is to say, the seepage flow controls the heat transport in this process at these locations, which is different from the fact that thermal diffusion is the control step in the electromagnetic heating process proposed by Islam.9 The similar case was also observed by Tang et al.12 If the heat loss is ignored, the maximum seepage flow rate at the radius direction in the hydrate sample is about 11 mm/min at a water injecting rate of 2.4 L/h. In addition, the temperature distribution also implied the injected hot-water distribution in the reactor to a certain extent. The temperatures of sediments near the well, such as T4 and T8, rise more rapidly. In contrast, the temperatures of sediments far away from the well, such as T13 and T14, have little change during the gas production cycle. The maximum temperature difference appears at 25 min in the first cycle, and its value is 24.5 K (between 305 K of T4 and 275.5 K of T13). This indicates that the location of the well has an important effect on the temperature distribution in the hydrate sediment, thereby affecting the gas production. From the variation of the temperature at different positions, as shown in Figures 6 and 7, the temperature gradients in the reactor can also be determined; that is, the temperature gradually increases from the bottom to the top of the reactor and from the edge to the center of the reactor during the whole gas production process, which is also implied from the maximum temperature data in Table 2. When the valves of injecting water are closed, the temperatures at most locations in sediments reach a maximum value, as given in Table 2. The temperatures of sediments drop fast with gas production because partial heat was taken away by the dissociated gas and water from the reactor. The decrease of the temperature, especially for T1, T5, T9-T11, and T13-T16 in the first cycle, is mainly due to the endothermic effect of hydrate dissociation during the gas production. 3.4. Affecting Factors of Gas Production. An important parameter used to evaluate efficiency of gas production from a natural gas hydrate reservoir is the energy efficiency ratio (EER), which is defined as8,9 QΔHg ð1Þ EER ¼ cw mw ðTw - T0 Þ
Figure 5. Variation of pressure and gas production rate with time for run 1 during the (a) first cycle and (b) second cycle.
produced during the second cycle, while a large amount of gas (74.9 vol %) still remains in the reactor, which can be obtained by specifying the well pressure to atmosphere and improving the temperature of the water bath. Therefore, the well depth in the hydrate sediment also has an effect on gas recovery, especially in the second cycle of water injection. 3.3. Temperature Distribution in the Reactor. The temperature distributions in the reactor during gas production for all of the experimental runs are similar. Therefore, only run 1 was chosen to present the typical temperature-time curves during the gas production process. Figures 6 and 7 showed the variation of temperatures with time at different locations for run 1 during the first and second cycles, respectively. As shown in Figures 6 and 7, it can be found that the overall temperature trend increases with hot-water injection and decreases with gas production. At the initial stage of water injection, the temperatures rise slowly because much heat is consumed to warm the flowing channel of hot water. Afterward, the temperatures at different locations rise rapidly. The onset time for the temperature jump is given in Table 2. It is noted that, for the local positions of the hydrate sample, which are away from the injecting well and near the reactor wall, such as T1, T5, T9, and T13, the temperature-jump phenomenon is not observed. It is known that the hot water can hardly spread to those locations in a shorter time. Strong heat exchange with the environment may also occur on those positions near the reactor wall with high conductivity. In contrast, at other locations, in view of the low conductivity of the hydrate sample, the fluctuation of the temperature
where Q (L) is the total volume of methane gas, ΔHg is the combustion heat of methane gas (39.7 kJ/L), cw is the specific heat of water (4.2 J g-1 K-1), mw (g) is the mass of the hot water injected, Tw (K) is the temperature of hot water, and T0 is the environmental temperature (293.2 K), because the injecting water temperature was 293.2 K before it was heated in this work. In fact, the consumption of energy is just the part of heat in which the water temperature increased from 293.2 K to Tw. Another important parameter is thermal efficiency (η), which is defined as12 ΔHd mh 100 ð2Þ η ¼ cw mw ðTw - Thydrate Þ where ΔHd is the dissociation enthalpy of methane hydrate (453.5 J/g),26 mh (g) is the total mass of methane hydrate, and (26) Handa, Y. P. Compositions, enthalpies of dissociation, and heat capacities in the range 85 to 270 K for clathrate hydrates of methane, ethane, and propane, and enthalpy of dissociation of isobutene hydrate, as determined by a heat-flow calorimeter. J. Chem. Thermodyn. 1986, 18 (10), 915–921.
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Figure 6. Variation of the temperature with time at different locations during the first cycle for run 1 (a) with the same depth of 82 mm, (b) with the same depth of 58 mm, (c) with the same depth of 34 mm, and (d) with the same depth of 10 mm.
Thydrate is the temperature of the hydrate sample as defined above. The EER and thermal efficiency values for different runs are listed in Table 1. The range of EER obtained in this work is from 2.6 to 18.2, which is much wider than that (ranging from 6.2 to 11.4) calculated by Selim and Sloan.8 The processes occurring during the experiment affect the range of the EER. For example, for the reservoir with a high saturation of free gas, more gas will be produced with the same injecting hot water. A part of gas production is produced by depressurization but not because of thermal injection, while gas production by depressurization needs little energy input. This will result in the increase of EER. Therefore, the EER range obtained in this work may be different from that of Selim and Sloan.8 The influences of operating conditions on gas production efficiency are discussed as follows. For experimental runs 1, 4, and 5, hydrate samples formed with different saturations of 0.293, 0.231, and 0.354, while other parameters, such as the hydrate sample temperature, hot-water temperature, mass of hot water injected, and well pressure, were similar. The hydrate saturation was determined by controlling the methane gas volume of injection into the reactor for forming hydrate. As seen in Table 1, it is obvious that high hydrate saturation corresponds to high EER and thermal efficiency. Similar results were also reported by Tang et al.12 Moreover, it is observed that the EER value in the first cycle is higher than that in the second cycle for all experimental runs. This is due to the lower hydrate saturation in the second cycle. For experimental runs 1, 2, and 3, as shown in Table 1, the hydrate was synthesized at different temperatures of 275.1,
276.1, and 277.0 K by controlling the water bath temperature, while other parameters, such as hot-water temperature, mass of hot water injected, and well pressure, were similar. It is noticed that the hydrate saturation decreases with the increase of the environmental temperature at the same volume of methane gas injected into the reactor for these experiments. The hydrate sample formed at 277.0 K has the highest EER of 8.3, although it has the lowest saturation of 0.252. That is, the EER increases with the increase of the temperature that hydrate formed. For experimental runs 1, 6, and 7, as shown in Table 1, only the hot-water temperature was different, that is, 333.2, 313.2, and 353.2 K, respectively, while other parameters, such as the hydrate sample temperature, hydrate saturation, mass of hot water injected, and well pressure, were similar. It is obvious that the EER and thermal efficiency decrease sharply with the increase of the hot-water injection temperature. This may attribute to the increase of heat loss at higher hot-water injection temperatures. For experimental runs 8-12 listed in Table 1, hydrate samples formed at a similar temperature to saturation of about 0.30. The hot-water temperature and well pressure were also similar, but the mass of hot water injected was different. For these groups of experiments, the gas production process is only one cycle. If the volume expansion because of partial water converting to hydrate is ignored, the potential maximum mass of water injected to the reactor is 2200 g. The ratio of the mass of hot water injected/potential maximum mass (Mw/Mmax) was shown in Table 3. The variation of EER with Mw/Mmax was shown in Figure 8. It can be found that, when the Mw/Mmax is less than 0.41, the EER 5918
Energy Fuels 2010, 24, 5912–5920
: DOI:10.1021/ef100367a
Yang et al.
Figure 7. Variation of the temperature with time at different locations during the second cycle for run 1 (a) with the same depth of 82 mm, (b) with the same depth of 58 mm, (c) with the same depth of 34 mm, and (d) with the same depth of 10 mm. Table 2. Maximum Temperature and Onset Time for the Temperature Jump for Run 1 maximum temperature (K)
T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14 T15 T16
first cycle
second cycle
277.7 280.1 288.0 305.0 277.9 279.3 282.1 296.6 277.2 277.2 278.5 282.5 275.5 276.3 276.8 277.6
277.0 281.7 292.0 311.2 277.0 281.9 291.6 307.2 276.8 278.1 282.5 290.3 275.5 276.8 278.8 280.4
onset time for the temperature jump (min) first cycle
second cycle
18 8 2
8 3 0
17 4
9 3 0
15
10 3
Figure 8. Variation of EER with the mass of hot water injected for runs 8-12.
dissociated gas from the hydrate. When Mw/Mmax is between 0.41 and 0.51, the EER increases with the increase of the hotwater amount injected because the dissociated gas from the hydrate rather than free gas accounts for most of the gas produced. When Mw/Mmax is about 0.51, the EER attains a maximum value of about 8.0, as shown in Figure 8. After that, the EER decreases with the increase of the hot water injected. It is obvious that there exists an optimal ratio of Mw/Mmax for the highest EER during the gas production experiments. For experimental runs 15, 1, 13, and 14, hydrate samples formed at similar conditions and with the same hot-water temperature and mass of hot water injected, except that the
Table 3. Energy Efficiency for Runs 8-12 at Different Masses of Hot Water Injected experimental runs
Mw/Mmax EER thermal efficiency (%)
8
9
10
11
12
0.36 8.1 33.9
0.41 6.6 27.5
0.45 7.4 30.9
0.55 7.8 32.6
0.66 5.1 21.5
decreases with the increase of the hot-water amount injected because the gas produced is mainly free gas rather than 5919
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: DOI:10.1021/ef100367a
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well pressure of gas production is from 1 to 4 MPa. From Table 1, it can be found that, when the well pressure rises from 1 to 4 MPa, the EER drops sharply from 18.2 to 2.6. During the industrial gas production from hydrates, however, a lower well pressure will induce a faster gas production rate, which may lead to safety problems, such as geological disaster or pipeline packing. As a result, an optimal well pressure for gas production from the hydrate should be designed to ensure a safe and economic gas recovery from the hydrate reservoir.
the location of the well through which hot water is injected. The porosity and permeability of hydrate samples also have an important effect on temperature distribution. For hotwater cyclic injection to produce gas from the hydrate, EER increases with the increase of the hydrate saturation and hydrate-forming temperature but decreases with the increase of the hot-water temperature and well pressure, when other conditions are similar. The highest EER occurs in the first cycle. For single-cycle injection, the optimal ratio of Mw/Mmax is about 0.51. The cyclic injection method is useful for the field with a high saturation hydrate and low permeability, in which the released gas and water are difficult to penetrate the tight sediment zone if using other methods, such as the continuous injection method.
4. Conclusions The gas production process and energy efficiency from methane hydrate-bearing sand via single-well hot-water cyclic injection were investigated using a three-dimensional middle-size reactor. The process of gas production from the hydrate by cyclic injection of hot water was divided into three steps: injecting hot water, closing the well, and producing gas. It was found that the overall temperature trend increases with hot-water injection and decreases with gas production. The temperature distribution in the reactor mainly depends upon
Acknowledgment. The financial support received from the National Natural Science Foundation of China (20925623, U0633003, and 21076225), National 863 Project, NCET-07-0842, Targeted Advanced Item of China University of Petroleum (2010QZ02), and National 973 Project of China (2009CB219504) is gratefully acknowledged.
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