Effect of Hydrate Saturation on the Methane Hydrate Dissociation by

Article pubs.acs.org/IECR

Effect of Hydrate Saturation on the Methane Hydrate Dissociation by Depressurization in Sediments in a Cubic Hydrate Simulator Yu Zhang, Xiao-Sen Li,* Zhao-Yang Chen, Yi Wang, and Xu-Ke Ruan Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, P. R. China Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou 510640, P. R. China S Supporting Information *

ABSTRACT: This study presents the three-dimensional (3D) cubic hydrate simulator (CHS) to analyze the methane dissociation phenomena from hydrate-bearing sediment with different hydrate saturation. The experiments by depressurization have been carried out at the environmental temperature of 281.15 K, the dissociation pressure of 5.0 MPa, and in the hydrate saturation range of 17.0−43.2%. The hydrate dissociation process consists of two periods: the depressurizing period and the steady-pressure period. In the depressurizing period, the hydrate dissociation occurs in the whole reactor. The cumulative gas production is similar, and the gas production rate is affected by the depressurization rate and the water production in this period. The cumulative water production increases with the decrease of the hydrate saturation in the whole depressurization process. In the steady-pressure period, the cumulative gas production increases with the increase of the hydrate saturation. The average gas production rate first increases with the increase of the hydrate saturation and then decreases at hydrate saturation of 43.2%. The water production during the steady-pressure period only occurs in the experiments with the high hydrate saturation. The temperatures in different regions in the reactor change with similar degrees in the depressurizing period and have the similar lowest values for different experiments. In the steady-pressure period, the temperatures increase gradually from the inner-wall region to the center region. On the basis of the calculation of the energy balance, it was found that the ratio of the sensible heat of the reservoir to the latent heat of the hydrate dissociation decreases with the increases of the hydrate saturation and the dissociation pressure.

1. INTRODUCTION Since the early 1980s around the world, gas hydrate is being recognized as an alternative energy resource for fossil energy. It is because of this that gas hydrate is vastly distributed throughout both marine and permafrost areas and represents a potentially significant resource.1−3 In order to recover natural gas from the gas hydrate, several schemes have been proposed, including depressurization, thermal simulation, inhibition injection, and CO2 swapping. Recently, a new method of the combination of depressurization and wellbore heating was improved and tested by Falser et al.4 with experimental and numerical simulation. They found that the gas production from hydrates could be increased significantly with additional heating. The depressurization technique is to dissociate hydrate by reducing the pressure below the equilibrium pressure at the specific temperature of the reservoir. It is one of the earliest methods to be proposed and considered as an economic and promising technique to produce natural gas from hydrate reservoirs.5−7 Moreover, it has been applied commercially to produce natural gas from the Messoyakha hydrate gas reservoir in Russia.8 However, depressurization also demonstrates a low productivity because of the low production rate. Thus, it is still essential to investigate the hydrate dissociation mechanism by depressurization further for safely, efficiently, and commercially viable producing natural gas. Recently, the hydrate dissociation behaviors via the depressurization method have been investigated through a variety of experimental and numerical studies. Babu et al.9 © 2015 American Chemical Society

studied the hydrate formation and dissociation in a specially designed crystallizer, which is used for mophology observation. They reported that pore space and its interconnectivity of the porous media have important effect on the methane hydrate formation. Oyama et al.10 studied the dissociation characteristics during depressurization from methane hydrate in sediments through the experiments and a numerical dissociation model. The experiments were performed under various production pressures. The model can describe the experimental results well, and it is demonstrated that the heat transfer from the surrounding is predominant in the experiments. Lee et al.11 carried the depressurization experiments to investigate into the hydrate dissociation behaviors and the productivity. The experiments were performed under isothermal condition of 273.65 K and different pressure conditions. Their results showed that the degree of depressurization is a significant factor influencing the gas production rate in a hydrate reservoir. Linga et al.12,13 and Haligva et al.14 experimentally studied the formation and dissociation of methane hydrate in porous media with different sample size. They found that the gas production rate could be classified as distinguishable as two stages for depressurization and three stages for thermal stimulation.12,14 In the depressurization experiments, the rate of the gas recovery Received: Revised: Accepted: Published: 2627

October 30, 2014 February 16, 2015 February 23, 2015 February 23, 2015 DOI: 10.1021/ie5042885 Ind. Eng. Chem. Res. 2015, 54, 2627−2637

Article

Industrial & Engineering Chemistry Research

Figure 1. Schematic of the three-dimensional experimental apparatus.

is strongly dependent on the silica sand bed size during the first stage and depends weakly on the silica sand bed size during the second stage. Multiple nucleation events at different times and different locations for methane hydrate formation were observed with the location of multiple thermocouples within the bed.13−15 Zhou et al.16 carried the hydrate formation and dissociation experiments in a water-saturated silica sand matrix by using a cylindrical reactor. The hydrate was dissociated by depressurization and the experiments were conducted at 2.2 °C and 4137 kPa in a 72 L reactor. The pore space volume of the hydrate forming in the reactor for the experiments is 11% based on the mass balance calculation. Kono et al.17 measured the dissociation rate of methane gas hydrate via the depressurizing method in various porous sediments. The hydrate sample was synthesized at experimental conditions of 273.5 K and pressures between 6.8 and 13.6 MPa. They reported that the dissociation rate can be adjusted by changing the sediment properties. Zhao et al.18 bulilt a two-dimensional axisymmetric simulator and modeled methane hydrate dissociation in sediments by depressurization method. The relationship among changes of water saturation, temperature, pressure, and hydrate saturation were analyzed by the simulations. They found that the water phase plays an important role in late stage thermal conduction. In our previous work, we have reported the investigations into the hydrate dissociation behaviors from methane hydrate in sediments by depressurization using different experimental apparatuses. The experiments were carried in a three-dimensional cubic hydrate simulator (CHS) and a three-dimensional pilot hydrate simulator (PHS).19,20 The effective volumes of the CHS and PHS are 5.8 and 117.8 L, respectively. The experimental results in different apparatuses show that the gas production process has three periods: free gas production, mixed gas production, and dissociated gas production. The cumulative gas production, the hydrate dissociation rate and the water production increase as the dissociation pressure decreases. With the comparison of the experimental results in CHS and PHS, it was found that the pressure reduction rate is predominant in first and second periods. In the third period, the heat transferred from the ambient is the main driving force

for the hydrate dissociation. The hydrate reservoir size has a significant effect on the gas production rate and time. It was reported that in the Shenhu Area the sediment porosities are in the range of 33%−48%, the hydrate saturation ranges from 1.0% to 47.3% and has an average value of around 22%.21,22 The hydrate saturation determines the final gas amount could be dissociated from the hydrate and may has significant effects on the physical properties of the hydrate reservoir, such as the permeability and thermal conductivity, which will influence the dissociated gas and the water flow behaviors. Therefore, the hydrate saturation is an important factor that should be considered for the economic efficiency of the industrial exploitation of the natural gas hydrate (NGH). The experimental studies on the impact of the hydrate saturation on the hydrate dissociation behaviors, such as the water and gas production rate, will be helpfull to assess the economic efficient and feasibility of producing natural gas from the natural gas hydrate reservior with different hydrate saturation, and to design the industrial operations in the production process. Xiong et al.23 briefly discussed the effect of the hydrate saturation on the methane hydrate dissociation behaviors in a one-dimensional apparatus. The inside of the reactor is cylindrical, with a diameter of 38 mm and a length of 250 mm. There are four pairs of electrodes and four resistance thermometers placed at 5, 75, 145, and 215 mm from the inlet, used to measure the resistances and temperatures at different places of the reactor. Because the real hydrate reservoir is a three-dimensional (3D) reservoir, it is very important to study the hydrate dissociation behaviors in the three-dimensional experimental apparatus. In this study, the main focus is to investigate the effect of hydrate saturation on the characteristics and productivity of dissociated gas by depressurization process in a cubic hydrate simulator (CHS). The experiments using the depressurization scheme in sediments were carried out at the dissociation pressure of 5.0 MPa and the environmental temperature of 281.15 K. The hydrate saturation is 43.2%, 31.5%, 26.6%, and 17.0%, respectively. The experimental conditions are based on currently available data from site measurements in the Shenhu Area, South China Sea. 2628

DOI: 10.1021/ie5042885 Ind. Eng. Chem. Res. 2015, 54, 2627−2637

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Figure 2. Distributions of temperature and resistance measuring points and production wellhead of each layer within the three-dimensional reactor.

2. EXPERIMENTAL SECTION

and between Layers B−B and C−C are both a quarter of the internal length of the CHS (60 mm), while Layer B−B is in the middle of the CHS. Figure 2b shows the schematic of production wellhead, and the resistance and temperature measuring points of each layer in the CHS. There are 12 electrode probes and 25 thermometers evenly distributed on each layer. There are a total of 36 resistance measuring points and 75 temperature measuring points in the CHS to measure the electrical resistances and in the reactor. As shown in Figure 2, in the experiments in this work, the inlet for the gas or liquid injection is the 13C wellhead in the bottom layer (C), and the

2.1. Experimental Apparatus. The experimental apparatus is described in detail by Li et al.20 Figure 1 is a schematic of the experimental apparatus. Briefly, the CHS is a pressure reactor fabricated from stainless steel with a 5.8 L internal volume. The whole apparatus are placed inside the water bath (−8 to 30 °C, ±0.1 °C) to obtain the experimental temperatures. Figure 2 gives the schematic drawing of the layers and the well design of the CHS. As shown in Figure 2a, three horizontal layers named Layers A−A, B−B, and C−C inside the reactor equally divide the inner room of the reactor into four regions. The distances between Layers A−A and B−B 2629


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Industrial & Engineering Chemistry Research Table 1. Experimental Conditions and Results run 1

depressurizing period

steady-pressure period

dissociation pressure (MPa) bath temperature (K) hydrate saturation water saturation gas saturation Hydrate Dissociation cumulative gas production (L) gas production rate (L/min) depressurization rate (MPa/min) cumulative gas production (L) gas production rate (L/min)

run 3

run 4

31.5% 26.2% 43.3%

26.6% 29.8% 43.6%

17.0% 39.0% 44.0%

53.5 9.727 0.209 83.5 0.253

62.5 6.823 0.125 70.0 0.238

50.5 5.611 0.127 45.9 0.177

5.0 281.15 43.2% 13.5% 43.3% Period 58.9 3.569 0.070 104.2 0.225

using the Peng−Robinson equation. The hydrate saturation (the volume ratio of the hydrate and available pore space), water saturation, and gas saturation in the reactor can be calculated from Vh2, Vw2, and Vg2, respectively. The formation was finished when the pressure dropped to the value, corresponding to the desired hydrate saturation. After the completion of each hydrate formation experiment, the hydrate dissociation via depressurization was carried out in the following procedures. First, the backpressure regulator was set to the desired dissociation pressure value (5.0 MPa) for the hydrate dissociation. The outlet valve was then opened and the temperature in the water bath was allowed to stabilize in the whole experimental process. Subsequently, the pressure decreased gradually to the dissociation pressure, and the gas and water were produced from the production well. The experiments were finished when there is no further gas release. During the experiments, the data were recorded by the data acquisition system in real time.

outlet for the gas and water production is the 13A wellhead in the top layer (A). A pressure transducer at the center of the bottom of the reactor and another pressure transducer installed at the production well are used to measure the pressure in the hydrate-bearing sediments and the outlet pressure, respectively. A vacuum pump and a safety valve are connected to the CHS. The gas injection rate and gas production rate are monitored by two gas flow meters (D07-11CM, 0−10 L/min, from Seven Star Company) with accuracy of ±2%. The water was injected into the reactor using a metering pump with a range of 0−50 mL/min. A back-pressure regulator with the pressure range of 0 to 30 MPa and an uncertainty of ±0.02 MPa is connected to the outlet of the CHS to control the pressure of the production well. A balance, used to measure the water production, is Sartorius BS 2202S, 0-2200 g, 0.01 g. A data acquisition system is used to record the temperature, the pressure, the gas production, and the water production. In the experiments, the methane with the purity of 99.9% obtained from Guangzhou Hua Te Gas Co., Ltd., is used. 2.3. Experimental Procedure. During the experiments, the quartz sand with a quality of 8162 g and porosity of approximately 48%, was tightly packed into the reactor. The particle size of the quartz sand ranges from 300 to 450 μm. The quartz sand was supplied by Haiqi Trading Co., Ltd. The reactor was pressurized with methane at approximately 1.0 MPa and depressurized three times to eliminate the presence of air in the system. The quartz sand in the reactor was wetted to saturation by injecting the distilled water using a metering pump. By measurement, 1537 g deionized water was injected into the reactor. Thereafter, the water bath was turned on and kept at the predetermined temperature for the hydrate formation, which was 281.15 K in the work. The reactor was pressurized to approximately 20 MPa by introducing methane gas into the reactor. The inlet and outlet valves of the CHS were closed and the system was kept at a constant volume condition. The hydrate saturation can be calculated by the pressure change in the hydrate formation. The total volume of the water and gas before hydrate formation was equal to the total volume of water, gas, and hydrate after formation:24 Vw1 + Vg1 = Vw2 + Vg2 + Vh2

run 2

3. RESULTS AND DISCUSSION In this work, a total of four experimental runs were carried out. The experimental conditions for methane hydrate dissociation and corresponding results are summarized in Table 1. The initial hydrate saturation for experimental runs 1, 2, 3, and 4 is 43.2%, 31.5%, 26.6%, and 17.0%, respectively. 3.1. Dissociation Process. One example of the experimental results is shown in Figure 3, which shows the results of the experiment with the hydrate saturation of 31.5%. In Figure 3, two parameters are plotted against the elapsed time: (a) the system pressure; (b) the system temperature. As shown in Figure 3, TE is the average system temperature of all the temperature measurement points in the reactor, Teq is the equilibrium hydrate dissociation temperature corresponding to the system pressure, which is calculated using the fugacity model given by Li et al.25 T(1B) and T(13B) are the temperatures at the measurement points 1B and 13B, respectively. In all the experiments, the pressures at the different measuring points in the CHS have little discrepancy due to the high porosity and permeability of the sediments. Thus, the pressure at any point in the CHS can be taken as the system pressure. As shown in Figure 3, the hydrate dissociation process can be divided into two periods: the depressurizing period and the steady-pressure period. Before the hydrate dissociation, the pressure in the CHS is simultaneously reduced when the outlet is opened during the depressurization process, as explained in the Experimental Section. The free gas in the CHS is released and the pressure in the CHS decreases rapidly. In this period, the pressure in the reactor is higher than the

(1)

In the calculation, it was assumed that there is 6.1 mol of water in 1 mol of methane hydrate, the density of methane hydrate is 0.94 g/cm3, and the density of methane hydrate and water remain constant at different pressure and temperature. The gas consumption in the hydrate formation was calculated by the changes of the pressure and temperature in the reactor 2630


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K. It is found that the calculated value is in excellent agreement with the experimental data. The error may be due to the thermometer accuracy. This indicates that the temperature from point A in the system becomes to be higher than the equilibrium hydrate dissociation temperature corresponding to the system pressure and the hydrate starts to dissociate. Thus, the point A is set as the starting time of the hydrate dissociation process, as shown in Figure 3. The same method is used and the hydrate dissociation behaviors of the hydrate dissociation are discussed in the following analysis. The depressurizing period is between 0.0 and 5.3 min. In the depressurizing period, the temperature in the reactor declines rapidly due to the quick phase transformation. The equilibrium hydrate dissociation temperature corresponding to the pressure in the reactor also decreases and is nearly equal to the average system temperature. In the steady-pressure period of the hydrate dissociation (from Point B to the end of the experiment), the pressure in the system keeps constant and is the same as the setting dissociation pressure. The gas is mainly produced from the hydrate dissociation in this period. Because the heat supplied from the ambient is almost consumed by the hydrate dissociation, the temperature at 13B basically remains around the steady lowest value. As the hydrate dissociation at 13B becomes slow and trends to end, the heat transferred from the ambient is more than the heat required for hydrate dissociation. Therefore, the temperature gradually rises to the temperature of the water bath. The temperature at point 1B rises immediately after dropping to the lowest point. In this period, the heat conduction is the main driving force for the hydrate dissociation, and the heat supplied from the ambient (water bath) is successively transferred from the inner wall to the center of the reactor.19,26 Because most of the hydrate at point 1B has been dissociated at the end of the depressurizing period, the temperature at this place increases immediately after dropping to the lowest value. The similar phenomena are observed from other measuring points in each layer. 3.2. Water Production. Figure 4 gives the cumulative water production in the hydrate dissociation process versus

Figure 3. Example of the depressurization process. Changes in elapsed time at a hydrate saturation of 31.2%: (a) pressure change and (b) temperature change.

equilibrium hydrate dissociation pressure, and there is no hydrate dissociation, as discussed in Li et al.19,20 Thus, the average system temperature in the CHS only decreases slightly due to the Joule−Thomson effect at first and increases again due to the hydrate reformation.19,20 From Point A, the system pressure continuously decreases. The pressure and the average system temperature corresponding to point A are 6.15 MPa and 281.67 K, respectively. The equilibrium hydrate dissociation temperature corresponding to the pressure at point A is 281.82

Figure 4. Cumulative water production in the hydrate dissociation process of the experiments. 2631


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Figure 5. Cumulative gas production in the hydrate dissociation process for different experiments.

time for the experimental runs 1, 2, 3, and 4. In the depressurizing period, the gas volume expands as the depressurization progresses, the expansion gas pushes pore water and the water flows from the pore space to the outside. The water flows from the bottom to the upper of the reactor on account of the effect of the driving force (the difference between the pressure in the reactor and the outlet). It has been explained in the previous work.20 In this work, the water production behavior in the hydrate dissociation process with different hydrate saturation is investigated. As shown in Figure 4, for runs 1, 2, 3, and 4, the total amount of the water production during the depressurizing period is 116.9, 95.3, 18.4, and 84.7 g, respectively. Before the pressure in the reactor decreasing to the hydrate dissociation pressure (Point A in Figure 3), some water has been produced. The water production before Point A for runs 1, 2, 3, and 4 is 52.1, 78.9, 343.4, and 303.3 g, respectively. Therefore, the water production before the steady-pressure period (Point B in Figure 3) for runs 1, 2, 3, and 4 is 169.0, 174.2, 361.8, and 388.0 g, respectively. It can be noted that the water production during the whole depressurization process increases with the decrease of the hydrate saturation. In the steady-pressure period, there is little water production for runs 3 and 4. In this period, the amount of water in the reactor is limited, and most of the pore spaces in sediments are filled continuously with the expanded gas dissociated from the hydrate. In this case, as water enters a few pore spaces, the gas follows the path that it generates in the pore spaces. Meanwhile, the force driving for the water production (the difference between the pressure in the reactor and the outlet) becomes small in the steady-pressure period. Therefore, the dissociated water remains in the pore spaces because the gas flows more easily from it. There is high hydrate saturation in experimental runs 1 and 2, thus the hydrate dissociation generates enough water to enter the pore spaces and forms a water path, causing the water to flow out. Therefore, there is some water production during the steadypressure period for runs 1 and 2. The water production in this period for runs 1 and 2 are 100.1 and 39.4 g, respectively, which increases as the hydrate saturation increases. For run 3, the

hydrate saturation and the water dissociated from hydrate is lower than those of run 4. Just when the dissociated water accumulated to a certain amount, the water can be produced out from the well. Therefore, the water production in the steady-pressure period has a stepwise production pattern. The water production for run 1 mainly occurs in the starting stage of the steady-pressure period. It may be due to the fact that a huge mass of water is dissociated from hydrate in the steady-pressure period and flows out continuously with the gas production. 3.3. Gas Production. Figure 5 shows the cumulative gas production in the two different hydrate dissociation periods in the experiments. It can be seen that the final gas production and the gas production rate is slightly related to the hydrate saturation in the depressurizing period. Table 1 gives the depressurization rates in the depressurizing period for different experiments. For runs 1, 2, 3, and 4, the depressurization rates are 0.070, 0.209, 0.125, and 0.127 MPa/min, respectively. The gas production rate in the depressurizing period increases with the increase of the depressurization rate on the whole. For run 3, because the water production is lower than that of run 4, even the depressurization rate is lower, the gas production rate is higher than that of run 4. Therefore, in the depressurizing period, the gas production is affected by the depressurizing rate and the difference of the water production. In order to analyze the hydrate dissociation rate in the steady-pressure period, the hydrate dissociation processes in the steady-pressure periods for different experiments are given in Figure 6. Recall that time zero coincides with the time when the pressure in the reactor reaches the dissociation pressure (the start of the steady-pressure period). It can be seen that the cumulative gas production in the steady-pressure increases as the hydrate saturation increases. However, in the starting stage of the steady-pressure period, the gas production rates for the different experiments are similar. For example, in the first 133.5 min, the gas production rates for experiments of runs 3 and 4 are similar. The gas production rate for run 1 is even lower than those of runs 2 and 3 in the first 80.0 min. It may be due to the fact that more water dissociated from the hydrate is produced from the reactor in the steady-pressure period for run 1, and 2632


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is mainly produced from the hydrate dissociation. Thus, the cumulative gas production increases with the increase of the hydrate saturation. It also can be noted from Figure 7 that, in the depressurizing period, the average gas production rate has a slight relationship to the hydrate saturation. Because the pressure drops in this period for different experiments are similar, the discrepancy of the gas production rate may be caused by the difference of the pressure reduction rate, as analyzed by Li et al.20 In the steadypressure period, the average gas production is mainly controlled by the hydrate dissociation rate. Therefore, the gas production rate presents the hydrate dissociation rate. It can be noted that the average gas production rate first increases with the increase of the hydrate saturation and then decreases at the hydrate saturation of 43.2% in this period. It may be because the hydrate dissociation duration is very long for the hydrate saturation of 43.2%. This demonstrates that, even for the higher hydrate saturation, complete hydrate exploitation may be not economical. It is also noted that the average gas production rate in the steady-pressure period is much lower than that in the depressurizing period, which inhibits the efficiency in the whole hydrate dissociation process. Thus, the enhancement of the hydrate dissociation rate in the steady-pressure period is very important to obtain the high hydrate dissociation efficiency by depressurization. In the steady-pressure period, the heat needed for the hydrate dissociation is transferred from the ambient, and the hydrate dissociation rate is limited by the heat transfer rate in the hydrate reservoir. Thus, the enhancement of the heat supply can obtain the high hydrate dissociation efficiency in the steady-pressure period. The hydrate dissociation in this period can use the method of hot injection conjunction with depressurization. 3.4. Temperature Profiles and Spatial Distributions. Figure 8 shows the temperature spatial distributions in the hydrate reservoir during the hydrate dissociation for run 2. Figure 8a gives the temperature spatial distribution at 0.0 min,

Figure 6. Cumulative gas production in the steady-pressure period of the hydrate dissociation for different experiments.

some gas dissociated from the hydrate stays in the reactor, resulting in the lower gas production. Figure 7 gives the cumulative gas production and the average gas production rates in the two hydrate dissociation periods in

Figure 7. Cumulative gas production and gas production rates in the hydrate dissociation process for different experiments.

the experiments. The results are also given in Table 1. As shown in Figure 7 and Table 1, in the depressurizing period, the cumulative gas production for different experiments is similar. In this period, the gas production consists of the free gas (from the gas phase) during quick depressurization and the gas from hydrate dissociation. The pressure at the beginning of the depressurizing period is same as the equilibrium dissociation pressure corresponding to the system temperature,20 which is similar for the experimental runs 1, 2, 3, and 4. The pressure at the end of the depressurizing period is the setting dissociation pressure (5.0 MPa). Therefore, the pressure drop in the depressurizing period is similar for the different experiments in this work, resulting in the similar free gas production in the depressurizing period. Meanwhile, the temperature drop in the depressurizing period is also similar for different experiments. The gas dissociated from the hydrate is also similar because the sensible heat of the reservoir is supplied to dissociate the hydrate. Therefore, the gas production in the depressurizing period has little difference for different experiments. In the steady-pressure period, the gas

Figure 8. Temperature spatial distributions in the hydrate reservoir over time for run 2. 2633


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corresponding to the setting dissociation pressure. Thus, the lowest temperatures of point 13B are similar for different experimental runs. It also can be noted from Figure 9 that the temperature of point 13B increases earlier from the lowest point for the experiment with the lower hydrate saturation. The reason is that the dissociation process ends earlier with the lower hydrate saturation. Figure 10 shows the temperature spatial distributions in the hydrate reservoir at the end of the depressurizing period of the

when the hydrate in the reactor begins to be dissociated. It can be found that the temperatures at different spots are similar because no hydrate has been dissociated. As shown in Figure 8b, at 5.3 min, which is the initial time of the steady-pressure period of the hydrate dissociation, the temperatures in the reactor decrease significantly compared to those at 0.0 min because of the endothermic reaction of the hydrate dissociation. The temperatures change at similar degrees in most of the regions in the reactor, demonstrating that the temperatures at different measuring points change nearly in-step in the depressurizing period, as discussed above. The hydrate dissociation in the depressurizing period occurs in all the regions in the reactor. Because the time in Figure 8b is at the end of the depressurizing period, the temperatures in the innerwall region are some higher than those in the center region. This is due to the fact that at the end of the depressurizing period, a large amount of hydrate in the inner-wall region has been dissociated, resulting in the temperature increasing earlier than that in the center region. Figure 8c gives the temperature spatial distribution at 161.7 min, which is at the 80% of the gas production in the steadypressure period of the hydrate dissociation. Figure 8d gives the spatial distribution of temperature at 328.9 min, when the temperature at 13B begins to increase. As shown in Figure 8b− d, the temperatures gradually increase from the inner-wall region to the center region due to that the heat of the water bath is successively transferred from the inner-wall region to the center of the reactor. It also can be noted that, different from that in the depressurizing period, the hydrate is gradually dissociated from the inner-wall region to the center region in the steady-pressure period. The similar phenomena for the temperature changes and the temperature distributions can be seen in the other experiments with different hydrate saturation. Figure 9 shows the temperature changes of point 13B vs time for runs 1, 2, 3, and 4. It can be seen from Figure 9 that the

Figure 10. Temperature spatial distributions at the end of the depressurizing period for different experiments.

hydrate dissociation for different experimental runs. As shown, the temperatures change at similar degrees in most of the regions in the reactor for different experiment. It can also be seen that the temperatures in the center region are some higher than those in the inner-wall region. For the experiment with the low hydrate saturation, the temperatures in the inner-wall region are significantly high. It is because that at this time, the temperature in the reactor has decreased significantly due to the hydrate dissociation, and the temperature in the inner-wall region starts to increase caused by the heat transfer from the ambient. The temperature increase is higher in the experiment with the lower hydrate saturation due to the lower latent heat of the hydrate dissociation. As analyzed in section 3.3, the gas production rates in the starting stage of the steady-pressure period for the four experimental runs are similar. In this period, the rate of the hydrate dissociation is controlled by the heat transfer. Because the temperature in the inner-wall region is similar due to the temperatures in the reactor decreasing to the similar lowest temperature, as shown in Figure 9, the temperature difference between the water bath and the inner-wall of the reactor is similar, which controls the heat transfer rate at the beginning of the depressurizing period. Therefore, the gas production rate is also similar for different experiments. As the hydrate dissociation processes, for the lower hydrate saturation, the nondissociated region of hydrate in the reactor reduces more quickly, and distance of the heat transfer is longer, resulting in the lower hydrate dissociation rate.

Figure 9. Temperature changes of point 13B vs time for runs 1, 2, 3, and 4.

temperatures of point 13B for different experiments have a similar change. It has to be pointed out that, in the steadypressure period, the lowest temperatures of point 13B are similar for different experimental runs. This is due to the fact that the dissociation pressures for different experiments are same. As analyzed by Li et al.,20 the lowest temperatures at various points are same to the equilibrium temperature 2634


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Industrial & Engineering Chemistry Research 3.5. Reservoir Resistance. The electrical resistance in the hydrate-bearing sediments is closely related to water and hydrate saturation.20,27,28 It has been known that the electrical resistances in the hydrate reservoir change with the hydrate dissociation and the flow of the gas and water, and the resistances increase as the hydrate saturation increases and the water saturation decreases.19,20,28 In the hydrate dissociation, the hydrate saturation decreases with hydrate gradually being dissociated into water and gas, and the water flowing and production cause the water saturation changing, resulting in the changes of the resistances in the reactor. In order to further analyze the effect of the hydrate dissociation on the resistance change, the resistance ratio spatial distributions at the end of the steady-pressure period for different experiments are shown in Figure 11. In Figure 11, the resistance ratio is ratio of the

regions have slight difference. Because there is some water produced in the hydrate dissociation process in the experimental runs 1 and 2, the resistance changes in the upper region and the bottom region in the reactor are different, illustrating that the resistance change is also affected by both of the water flow and the hydrate dissociation. 3.6. Analysis of Dissociation Heat. In the hydrate exploitation, the hydrate dissociation needs to absorb a huge amount of heat. As discussed above, in the depressurizing period, the sensible heat is consumed to dissociate the hydrate, resulting in the dramatical temperature drop in the reactor. To further quantify the ratio of the sensible heat to the latent heat of the hydrate dissociation, energy balance calculations of the sensible heat of the hydrate-bearing core and the latent heat of the dissociation heat were performed. Because the gas production during the depressurizing period is a combination of dissociation and free gas release, the ratio of the methane gas dissociated from hydrate in the depressurizing period and all the gas in the hydrate is calculated by the following:

R g = Vd /Vtotal

(2)

Where, Vd is the total volume of the dissociated gas in the depressurizing period; Vtotal is the total volume of the methane gas formed in hydrate before the hydrate dissociation. Because the gas production contains both free gas and dissociated gas in the depressurizing period, the value of Vd is also calculated by the following equation: Vw1 + Vg1 + Vh1 = Vw2 + Vg2 + Vh2

(3)

Vtotal is calculated by the hydrate saturation in the reactor before the dissociation experiments. In the calculation, the density of the water and hydrate phases is assumed to remain constant. Oyama et al.10 calculated the ratio of sensible heat to dissociation latent heat. The sensible heat of a methane hydrate-bearing core Qsen is estimated by the following Q sen = (CSMS + CwM w + CMHMMH)ΔT

(4)

where Cx is specific heat, Mx is sample mass, and the subscript x refers to the components S, W, and MH representing sand, water, and methane hydrate, respectively. The values of CS and CW are from the previous study,29 and CMH is given by Handa.30 In this work, eq 4 is also used to calculate the sensible heat in hydrate dissociation process. In the calculation, ΔT is the difference of the average temperature in the system at Points A and B. The experimental values of Ms, MW, and MMH are calculated from the initial conditions and the gas and water production data in real time. The dissociation latent heat of the methane hydrate was calculated by the following:

Figure 11. Resistance ratio spatial distributions at the end of the steady-pressure period for different experiments.

resistance at the end of the steady-pressure period to the resistance at the starting time of the steady-pressure period (Point B in Figure 3). At the beginning of the steady-pressure period, for runs 3 and 4, the water production is nearly over. Therefore, the resistances are mainly influenced by the hydrate dissociation. As shown in Figure 11, the resistances in most of the regions in the reactor decrease due to the hydrate dissociation in all reservoir and the resistance ratios in different

Table 2. Ratios of the Sensible Heat to the Dissociation Latent Heat in the Depressurizing Period runs

dissociation pressure (MPa)

hydrate saturation (%)

dissociated gas (L)

gas production (L)

Qsen/Qdis

Rg

1 2 3 4 5 6 7 8

5.0 5.0 5.0 5.0 4.5 5.0 5.6 4.7

43.2 31.5 26.6 17.0 33.1 33.1 33.1 30.0

57.4 45.1 52.7 41.0 69.9 40.3 14.6 428.8

58.9 53.5 62.5 50.5 69.9 48.5 17.8 586.3

0.060 0.081 0.087 0.135 0.127 0.075 0.043 0.122

0.293 0.310 0.428 0.523 0.431 0.248 0.090 0.222

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Q dis =

Vtotal ΔH(T ) 22.4

the experiments in the larger volume reactor is significantly lower. In the experiment, the heat transfer area per volume of the hydrate reservoir will decrease with the increase of size of the in reactor. Therefore, the heat transfer from the ambient per volume of the hydrate reservoir in the same time will also decrease, resulting in the less gas dissociated from hydrate in the depressurizing period. The size of the nature hydrate reservoir is much bigger than that in the experiment in laboratory. Therefore, during the depressurization process toward the setting dissociation pressure, the heat transfer in nature hydrate reservoir is slower than that in the experiment. Therefore, the ratio of Rg in the depressurization process will be significantly lower than that in these experiments.

(5)

The latent heat of hydrate dissociation per mol ΔH(T) is described by Holder31 as follows: ΔH(T ) = 4.184(13521.0 − 4.0T )

(6)

The ratios of the sensible heat to the dissociation latent heat (Qsen/Qdis) are calculated from eqs 4 and 5 and given in Table 2. Li et al.19,20 gave the experimental results of the hydrate production using depressurization method with the different dissociation pressure and reactor volume. In this work, the heat consumptions are also calculated. The results are shown in Table 2 and Figure 12. As shown in Table 2, for runs 5, 6, and 7, the dissociation pressure is 4.5, 5.0,

4. SUMMARY AND CONCLUSIONS In this work, we used a cubic hydrate simulator (CHS), a 5.8-L three-dimensional apparatus, to investigate the three-dimensional dissociation behaviors of methane hydrate in sediments by depressurization. The effects of the hydrate saturation in the hydrate reservoir on the hydrate dissociation behaviors have been investigated. The following conclusions are drawn: (1) The hydrate dissociation in sediments by depressurization in the experiments consists of two periods: the depressurizing period and the steady-pressure period. The cumulative water production increases with the decrease of the hydrate saturation in the whole depressurization process. In the steady-pressure period, the cumulative gas production increases with the increase of the hydrate saturation, and the average gas production rate first increases with the increase of the hydrate saturation and decreases at the hydrate saturation of 43.2%. (2) The cumulative water production is slightly related to the hydrate saturation and is mainly controlled by the gas release and depressurization rate in the depressurizing period. The water production during the steady-pressure period only occurs in the experiments with the higher hydrate saturation and increases with the increase of the hydrate saturation. (3) In the depressurizing period, the temperature decreases quickly because of the hydrate dissociation, and the temperatures in different regions change at similar degrees. In the steady-pressure period, the temperatures increase gradually from the inner-wall region to the center region. The temperatures in the reactor have the similar lowest values for different experiments and increase earlier from the lowest points in the experiment with the lower hydrate saturation. (4) The resistances in the hydrate reservoirs are mainly affected by the flow of water and gas in the depressurizing period and by the hydrate dissociation in the steady-pressure period. (5) On the basis of the calculation of the sensible heat, it is found that the ratio of the sensible heat of the reservoir to the latent heat of the hydrate dissociation decreases with the increases of the hydrate saturation and the dissociation pressure. The amount of the dissociated hydrate in the depressurizing period should be significantly lower if there is no heat transferred from the ambient.

Figure 12. Ratios of sensible heat to dissociation heat and the methane dissociated in the depressurizing period to the total dissociated gas.

and 5.6 MPa, respectively, and Qsen/Qdis is 0.127, 0.075, and 0.043, respectively. The sensible heat contribution to dissociation increases exponentially with decreasing dissociation pressure. This is due to the fact that the lowest temperature will decrease as the dissociation pressure decreases, resulting in the increase of ΔT. The sensible heat contribution to dissociation decreases with the increase of the hydrate saturation. This effect reflects the advantage of the depressurization method, particularly for the experiments with the low hydrate saturation. However, as the Qsen/Qdis ratio is less than unity, the sensible heat is not supplied in quantities sufficient to dissociate all methane hydrate. It can be seen that the ratio of Qsen/Qd is significantly less than Rg. This demonstrates that the sensible heat of the reservoir is consumed in a very short period and not the only heat source for the hydrate dissociation. In the depressurizing period, after consuming the sensible heat, surrounding closely contacted materials supplies heat to methane hydrate quickly because of the geometrical arrangement. As shown in Figure 8, at the end of the depressurizing period, the temperatures in different regions have dropped to the individual lowest values. However, the temperatures in various regions have some differences. The temperatures in the inner-wall region are higher than those in the center region because the hydrate in the center region is dissociated quickly by absorbing the heat transferred from the ambient due to the huge temperature drop. Compared with runs 5 and 8, the two experiments have similar dissociation pressure and hydrate saturation. The Qsen/Qdis ratios are also similar. However, Rg for 2636


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(14) Haligva, C.; Linga, P.; Ripmeester, J. A.; Englezos, P. Recovery of methane from a variable-volume bed of silica/hydrate by depressurization. Energy Fuels 2010, 24, 2947. (15) Mekala, P.; Babu, P.; Sangwai, J. S.; Linga, P. Formation and dissociation kinetics of methane hydrates in seawater and slica sand. Energy Fuels 2014, 28, 2708. (16) Zhou, Y.; Castaldi, M. C.; Yegulalp, T. M. Experimental investigation of methane gas production from methane hydrate. Ind. Eng. Chem. Res. 2009, 48 (6), 3142. (17) 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. (18) Zhao, J. F.; Ye, C. C.; Song, Y. C.; Liu, W. G.; Cheng, C. X.; Liu, Y.; Zhang, Y.; Wang, D. Y.; Ruan, X. K. Numerical simulation and analysis of water phase effect on methane hydrate dissociation by depressurization. Ind. Eng. Chem. Res. 2012, 51, 3108. (19) Li, X. S.; Yang, B.; Zhang, Y.; Li, G.; Chen, Z. Y.; Wu, H. J. Experimental investigation into gas production from methane hydrate in sediment by depressurization in a novel pilot-scale hydrate simulator. Appl. Energy 2012, 93, 722. (20) Li, X. S.; Zhang, Y.; Li, G.; Chen, Z. Y.; Wu, H. J. Experimental investigation into the production behavior of methane hydrate in porous sediment by depressurization with a novel three-dimensional cubic hydrate simulator. Energy Fuels 2011, 25, 4497. (21) Wu, N. Y.; Zhang, H. Q.; Yang, S. X.; Liang, J. Q.; Wang, H. B. Preliminary Discussion on Natural Gas Hydrate (NGH) Reservoir System of Shenhu Area, North Slope of South China Sea. Nat. Gas Ind. 2007, 27 (9), 1. (22) Guan, J. A.; Liang, D. Q.; Wu, N. Y.; Fan, S. S. The methane hydrate formation and the resource estimate resulting from free gas migration in seeping seafloor hydrate stability zone. Journal of Asian Earth Sciences 2009, 36, 277. (23) Xiong, L. J.; Li, X. S.; Wang, Y.; Xu, C. G. Experimental study on methane hydrate dissociation by depressurization in porous sediments. Energies 2012, 5, 518. (24) 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, 3388. (25) Li, X. S.; Zhang, Y.; Li, G.; Chen, Z. Y.; Yan, K. F.; Li, Q. P. Gas hydrate equilibrium dissociation conditions in porous media using two thermodynamic approaches. J. Chem. Thermodyn. 2008, 40, 1464. (26) Konno, Y.; Masuda, Y.; Hariguchi, Y.; Kurihara, M.; Ouchi, H. Key factor for depressurization-induced gas production from oceanic methane hydrate. Energy Fuels 2010, 24, 1736. (27) Hyndman, R. D.; Yuan, T.; Moran, K. The concentration of deep sea gas hydrates from downhole electrical resistivity logs and laboratory data. Ear. Plan. Sci. Lett. 1999, 172 (1−2), 167. (28) Zhang, Y.; Li, X. S.; Chen, Z. Y.; Li, G.; Wang, Y. Experimental Investigation into Gas Hydrate Formation in Sediments with Cooling Method in Three-Dimensional Simulator. Ind. Eng. Chem. Res. 2014, 53 (37), 14208. (29) Rika-Nenpyo (Chronological Scientific Tables) (in Japanese); Maruzen Co., Ltd., 2007. (30) Handa, Y. J. Compositions, enthalpies of dissociation, and heatcapacities in the range 85-K to 270-K for clathrate hydrates of methane, ethane, and propane, and enthalpy of dissociation of isobutane hydrate, as determined by a heat-flow calorimeter. Chem. Thermodyn. 1986, 18, 915. (31) Holder, G.; Zetts, S.; Pradhan, N. Phase-behavior in systems containing clathrate hydrate - A review. Rev. Chem. Eng. 1988, 5 (1− 4), 1.

ASSOCIATED CONTENT

S Supporting Information *

Resistance ratio spatial distributions at different time in the steady-pressure period for different experimental runs. This material is available free of charge via the Internet at http:// pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 20 87057037. Fax: +86 20 87034664. E-mail address: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Fund for Distinguished Young Scholars of China (51225603), the National Natural Science Foundation of China (51476174, 51276182 and 51306188), Key Arrangement Programs of the Chinese Academy of Sciences (Grants KGZD-EW-301-2), and National Marine Geology Special program (GHZ2012006003), which are gratefully acknowledged.

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REFERENCES

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