Experimental Investigation into the Production Behavior of Methane

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Experimental Investigation into the Production Behavior of Methane Hydrate in Porous Sediment by Depressurization with a Novel Three-Dimensional Cubic Hydrate Simulator Xiao-Sen Li,*,†,‡ Yu Zhang,†,‡ Gang Li,†,‡ Zhao-Yang Chen,†,‡ and Hui-Jie Wu†,‡ † ‡

Key Laboratory of Renewable Energy and Gas Hydrate, Guangzhou Institute of Energy Conversion, and Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China ABSTRACT: The gas production behavior from methane hydrate in a porous sediment by depressurization was investigated in a three-dimensional (3D) cubic hydrate simulator (CHS) at 281.15 K, hydrate saturation of 33.1%, and a production pressure range of 4.5
5.6 MPa. The results show that the gas production process consists of three periods: free gas production, mixed gas (free gas and gas dissociated from the hydrate) production, and gas production from the hydrate dissociation. The temperature in the nearwell region in the 3D hydrate reservoir changes during gas production in five stages. In the first period, the free gas in the system is released and the temperature change is not significant. In the second period, the temperature increases because of the reformation of the hydrate. In the third period, the temperature at each measuring point decreases quickly to the lowest value because of the considerable dissociation of the hydrate. The fourth period is the thermostatic hydrate dissociation period. During this period, the temperature at each point remains constant. In the fifth period, the hydrate has almost dissociated completely and the temperatures gradually increase to the environmental temperature of 281.15 K. There is no thermostatic dissociation period in the far-from-well region, in which the temperature at each measuring point gradually increases after it reaches the lowest value. In the third period of gas production, the temperatures in the near-well region are lower than those in the far-from-well region. In the gas production process, the resistances in the hydrate reservoir change with the hydrate dissociation and the flow of the gas and water. It can also be found that the gas production rate and the cumulative gas production increase with the decrease of the pressure. The gas hydrate dissociation in the gas production process is mainly controlled by the rate of the pressure reduction in the system and the heat supplied from the ambient. There is significant water production in the free gas production process. However, there is little water production in the hydrate dissociation process.

1. INTRODUCTION Natural gas hydrates are solid, non-stoichiometric compounds formed from small gas molecules and water at high pressures and low temperatures. The formation of gas hydrate requires favorable thermodynamic conditions and physical contact of natural gas and water. One volume of hydrate may contain as much as 164 volumes of gas (primarily methane). Thus, these solids contain a large amount of energy that is buried in deep marine sediments or permafrost regions and have been recognized as a potential future energy resource.1
3 At present, the production of natural gas from hydrate reservoirs is attracting a great amount of attention. To exploit this energy resource, a variety of methods have been proposed, i.e., (1) thermal stimulation,4 to heat the reservoir above the hydrate dissociation temperature with hot water, steam, or hot brine injection, (2) depressurization,5 to decrease the reservoir pressure below the equilibrium hydrate dissociation pressure at a specified temperature, and (3) chemical inhibitor stimulation,6,7 to inject the chemicals, such as methanol or ethylene glycol (EG), to shift the pressure
temperature equilibrium conditions for hydrate. Among these methods, the depressurization is one of the earliest methods to be proposed and has a particular merit.8 To produce natural gas from hydrate by depressurization, it is important to be able to characterize the hydrate. Because the natural gas hydrate mainly exists in sediment, it is important to be able to mimic natural conditions as r 2011 American Chemical Society

much as possible in a laboratory study of the kinetics of methane hydrate dissociation. It is noted that most of the studies on the hydrate in sediments were mainly focused on the phase equilibrium of the gas hydrate.9
12 Yousif et al.13
15 developed a one-dimensional apparatus to measure the gas production from the hydrate in Berea sandstone cores using the depressurization method and suggested a moving boundary model, which provided a satisfactory fit to hydrate dissociation measurements. It was found that more hydrate can form in the consolidated porous media through an annealing process. They also found that the rapid gas production from the hydrate near the freezing conditions could cause enough temperature reduction to stop the dissociation process. Kono et al.16 measured the dissociation rate of methane gas hydrate in various custom-designed porous sediments by the depressurizing method and derived the kinetic dissociation rate equation and the order of reaction. It was found that the dissociation rate can be adjusted by the control of sediment properties. Elwood Madden et al.17 studied the hydrate formation and dissociation in both the simulated and actual field samples via depressurization. The experimental results showed that free methane gas within the sediment greatly increases the possibility of nucleating hydrate Received: May 22, 2011 Revised: August 23, 2011 Published: August 29, 2011 4497

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Figure 1. Schematic of the 3D experimental apparatus.

crystals. However, they did not perform any experiments involving hydrate dissociation based on the temperature change in the system. Tang et al.18 studied the methane gas production behavior from an experimental-scale hydrate reservoir by depressurization and used the hydrate reservoir simulator Tough-Fx/Hydrate to predict the experimental results. The results suggested that the hydrate dissociation kinetics has a great effect on the gas production behavior for the laboratory-scale hydrate-bearing core. However, for a field-scale hydrate reservoir, the flow ability dominates the gas production behavior and the effect of hydrate dissociation kinetics on the gas production behavior can be neglected. Du et al.19 used a set of two-dimensional experimental apparatus to investigate the hydrate dissociation in porous media by depressurization. The temperature, pressure, and capacitance were monitored during the process. It was found that the change of the amount of water in the hydrate dissociation process is the main factor resulting in the capacitance change in the system. Zhou et al.20 used a cylindrical reactor vessel to investigate the gas production from methane gas hydrate. The results confirmed that the hydrate formation occurred not only just at the top of the sediment but also at various locations. A transition regime below 0 °C was observed and was characterized by a sharp increase in the gas production during the dissociation. Thus far, the experimental studies on methane dissociation and gas production by depressurization are carried out using a one- or two-dimensional experimental apparatus.13
19 Actually, the real hydrate reservoir is a three-dimensional (3D) reservoir. To investigate the gas production characteristics in a 3D reservoir, it is very important to simulate the hydrate dissociation behaviors in the 3D experimental apparatus. However, thus far, there are few reports on this aspect. In this work, a 3D experimental apparatus has been developed to investigate the gas production behavior from a methane hydrate reservoir in a unconsolidated sediment under depressurization. The experiments were performed at the hydrate saturation of 33.1%, environmental temperature of 281.15 K, and initial pressure of 11.8 MPa at the beginning of the hydrate dissociation. These conditions simulate the ones of the hydrate reservoir in the Shenhu

Area, South China Sea. The gas production pressures are 4.5, 5.0, and 5.6 MPa, respectively. With the temperature, pressure, and resistance changes at different locations in the 3D hydrate reservoir, the kinetic characteristics in the dissociation process were studied. In addition, the production of gas and water under depressurization was monitored.

2. EXPERIMENTAL SECTION 2.1. Experimental Apparatus. Figure 1 is a schematic of the apparatus. It consists of a reactor, a water bath, a back-pressure regulator, an aqueous solution injection system, a gas injection system, a water/gas separator, some measurement units, and a data acquisition system. The main part of the apparatus is a 3D high-pressure reactor made of stainless-steel 316, with a pressure range up to 25 MPa. The inside of the reactor is cubic, with the edge length of 180 mm and effective volume of 5.8 L. The 3D reactor, called the cubic hydrate simulator (CHS), is placed in a water bath at a constant temperature. The reactor is equipped with multiple sets of the measuring points and wellheads, and the layout of the measuring points and the wellheads is divided into three layers, namely, top (A), middle (B), and bottom (C), with each layer having a total of 25 points. Panels a and b in Figure 2 show the distributions of the temperature and resistance measuring points in the different layers and production wellheads within the 3D reactor. It can be seen that there are a total of 25  3 temperature measuring points, 12  3 resistance measuring points, and 1  3 central vertical wells along the centerline of the reactor, and the measuring points are evenly distributed in the reactor. In this experiment, the inlet for the gas or liquid injection is the 13C wellhead in the bottom layer (C), and the outlet for the gas and water production is the 13A wellhead in the top layer (A). Pressure is measured by the Trafag NAT8251.7425-type pressure sensor, with a measuring range of 0
25 MPa and an accuracy of (0.02 MPa. A pressure transducer at the outlet for the gas and water production is used to measure the outlet pressure, and another pressure transducer at the center of the bottom of the reactor is used to measure the pressure in the bottom of the hydrate-bearing layer. The temperature is measured by a Pt100 platinum resistor temperature sensor, with a measuring range from
20 to 200 °C and an accuracy of (0.1 °C. Resistance is measured by a H28 resistivity meter. Two D07-11CM (Seven Star Co.) gas flow 4498

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Figure 3. Pressure change versus time for runs 1, 2, and 3.

Figure 2. Distributions of temperature and resistance measuring points and production wellhead of each layer within the 3D reactor. meters are used to measure the cumulative gas injected into the CHS, the gas production rate, and the cumulative gas produced from the vessel. The thermometers, pressure transducers, and gas flow meters were calibrated using a mercury thermometer with a tolerance of (0.01 °C, a pressure test gauge with an error of (0.05%, and a wet gas meter with an accuracy of (10 mL/min, respectively. A metering pump “Beijing Chuangxintongheng” HPLC P3000A with a range of 50 mL/min, which can withstand pressures up to 30 MPa, is used to inject the liquid into the reactor. An inlet liquid container with an inner volume of 10 L containing the deionized water is used in the experiments. To protect the metering pump from corrosion by the hot brine or the chemicals, three middle containers are used for the solution injection, and the inner volume of each container is 4 L. A back-pressure regulator (pressure range of 0
30 ( 0.02 MPa) connected to the outlet of the CHS is used to control the pressure of the production well. A gas cylinder is used to provide the driving force of the back-pressure regulator. A balance, used to measure the mass of liquid produced from the CHS, is Sartorius BS 2202S (0
2200 ( 0.01 g). The data acquisition system records the temperature, pressure, amount of the cumulative gas produced from the vessel, gas production rate, and liquid production rate. The liquid injection rate can be controlled by the metering pump, which is connected to the data acquisition system. In this work, the methane gas with its purity of 99.99% is used. 2.2. Experimental Procedure. The 8162 g of quartz sand with particle sizes from 300 to 450 μm and porosity of approximately 48% is first tightly packed in the CHS as porous media. Then, the system is evacuated twice to remove the residual air with a vacuum pump. Subsequently, the quartz sand in the reactor is wetted to saturation with distilled water using a metering pump. The sand sediment was considered at saturation when the rate of water produced from the reactor is equal to that of water injected. The water injected into the reactor is 1537 mL. The temperature of the water bath is then set to the predetermined temperature required for the gas hydrate synthesis, which is 281.15 K in the work. Then, the methane gas is injected to pressurize the CHS to 20 MPa. The inlet and outlet valves of the CHS are closed to keep the system in a constant volume condition. When the

hydrate formation starts, the pressure inside the CHS decreases. When the pressure drops to the desired value (11.8 MPa), the production experiment is carried out by depressurization. In the gas production process, first, the back-pressure regulator is set to the desired pressure value for the gas production. Then, the outlet valve is opened to make the pressure in the reactor decrease to the desired value, which is the gas production pressure. Subsequently, the hydrate begins to dissociate, and the gas and water are produced from the reactor through the outlet of the production well. When there is little gas release, it is considered the end of the gas production.

3. RESULTS AND DISCUSSION In this work, a total of three experimental runs were carried out to investigate the dissociation behavior of methane hydrate in porous media by depressurization. In the experiments, at the beginning of the hydrate dissociation, the pressure in the CHS is 11.8 MPa, the corresponding hydrate, water, and gas saturations are 33.1, 28.3, and 38.6%, respectively, which are calculated by the method given by Li et al.,21 and the temperature at each measuring point is almost equal to 281.15 K. The ambient temperature is 281.15 K, and the pressures for gas production are 4.5, 5.0, and 5.6 MPa for runs 1, 2, and 3, respectively. 3.1. Pressure Change. Because of the high porosity and permeability of the sediment, the pressures at the different measuring points in the CHS have little difference. Thus, the pressure at any one point in the CHS can be represented as the system pressure. Figure 3 gives the system pressure in the reactor versus time for runs 1, 2, and 3. As a typical example of the experiment with the production pressure of 4.5 MPa, as shown in Figure 3, the pressure change consists of three sections. The first section is between 0 and 22.5 min, which represents the process of the free gas release. In this section, the free gas in the reactor is released and the pressure in the reactor decreases quickly. The pressure is still higher than the equilibrium hydrate dissociation pressure. The second section is between 22.5 and 37.0 min. In this section, the system pressure continuously decreases and is lower than the equilibrium hydrate dissociation pressure. Therefore, the hydrate begins to dissociate, and the rate of the pressure drop reduces during this period, compared to that in the first section. It is noted that, as shown in Figure 3, the initial points of the second section are points A, A0 , and A00 for runs 1, 2, and 3, respectively. The pressures corresponding to points A, A0 , and A00 are 6.14, 4499

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Figure 4. Cumulative water production versus time for runs 1, 2, and 3.

6.08, and 6.11 MPa, respectively. The average temperatures corresponding to points A, A0 , and A00 are 281.85, 281.71, and 281.71 K, respectively. Using the fugacity model by Li et al.,22 the equilibrium hydrate dissociation pressures in the sediment at these temperatures at points A, A0 , and A00 are calculated. The calculated values are 6.15, 6.06, and 6.06 MPa, respectively. It is found that the calculated values are in excellent agreement with the above experimental data. This illustrates that the pressures corresponding to points A, A0 , and A00 are almost equal to the equilibrium hydrate dissociation pressures at the corresponding average temperatures at points A, A0 , and A00 , respectively. However, the starting time of the second section is slightly different for the three experimental runs. It is due to the fact that the free gas release rates are not completely identical at the different gas production pressures. The third section for run 1 is from approximately 37.0 min to the end of the experiment. In this section, the pressure in the reactor remains constant and is the same with the set production pressure. In the period, the hydrate dissociates continuously until the dissociation is completed. 3.2. Water Production. Figure 4 gives the cumulative water production change versus time at the different gas production pressures. As shown in Figure 4, during the free gas production period (0
26.7 min for run 1), there is a large amount of water production instantaneously. The starting time of the water production for runs 1, 2, and 3 is 20.0, 20.0, and 23.3 min, respectively, and is within the first section of the individual pressure change. In the process of water production for each run, the water moves from the bottom to the wellhead close to the top of the reactor. There is a difference between the pressure in the reactor and the set pressure from the back regulator (the gas production pressure), which acts as a driving force. The water reaches the production wellhead after a few minutes, and the water production begins. It can also be seen from Figure 4 that the amount of water produced increases with the decrease of the gas production pressure. For runs 1, 2, and 3, the final amounts of the water production are 355, 385, and 953 g, respectively. As the driving force increases with the production pressure lowering, more water is produced. As shown in Figure 4, there is little water production during the hydrate dissociation (the second section plus the third section in Figure 3). This may be due to the fact that the amount of water in the reactor is limited and the driving force for the water production is not enough to remove the large

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Figure 5. Instantaneous gas production rate versus time for run 1.

Figure 6. Cumulative gas production versus time for runs 1, 2, and 3.

amount of water out of the reactor and gradually lowers and approaches zero with the hydrate dissociation progressing. 3.3. Gas Production. Figure 5 gives the instantaneous rate of the gas production for run 1. Corresponding to the pressure change in the reactor, the gas production process can also be divided into three periods. The first period represents the free gas release. It can be seen from Figure 5 that the instantaneous rate of the gas production is relatively high and stable in the first period. The second period is the mixed gas production step. In this period, the pressure in the reactor has declined below the equilibrium hydrate pressure and the hydrate begins to dissociate. The rate of the gas production considerably increases and then decreases to a relatively low value. In the third period, the pressure remains constant, the hydrate continuously dissociates, and the gas production rate continuously declines until zero. Figure 6 shows the cumulative gas productions for runs 1, 2, and 3. It can be seen that the gas production rate increases with the drop of the gas production pressure. In the hydrate dissociation process, with the lower set gas production pressure, the rate of the pressure reduction is higher, and thus, the higher rate of the pressure reduction results in the higher rate of gas production at the fixed time. Because the hydrate saturations and system pressures 4500

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Figure 7. Temperature change versus time at points 1B and 13B for run 1.

before the production experiments for runs 1, 2, and 3 are equivalent, the total amount of methane in the system is almost the same for the three runs. However, the amount of the methane staying in the reactor reduces with the drop of the production pressure at the end of the production experiment. Thus, the final cumulative gas production increases with the drop of the production pressure, as seen from Figure 6. 3.4. Temperature Change. Figure 7 gives the temperature change profiles at points 1B and 13B for run 1. As shown in Figure 7, the temperature change at point 13B in the reactor consists of five periods. The first two periods of the temperature change correspond to the first period of the pressure change in Figure 3. The first period of the temperature change (0
15 min) is the free gas release. Because there is no hydrate dissociation, the temperature does not change significantly. The second period is between 15 and 22.5 min, in which the free gas continuously releases and the water in the sediment begins to flow toward the outlet (the head) of the production well. Because the pressure in the reactor is still higher than the equilibrium hydrate dissociation pressure during this period, the hydrate forms again as a result of the water flow. It is attributed to the fact that, in the formation process before the beginning of the hydrate dissociation, as the forming hydrate gradually increases, the formerly forming hydrate adsorbs and encases some water, and thus, this hinders the contact of methane gas with the enclosed water for the further hydrate formation. This phenomenon is called the “loricae effect”.23 However, in the depressurizing process for the hydrate dissociation, the water encased in the hydrate is driven out to be the free water flowing toward the outlet under the effect of the driving force. This flowing free water contacts methane gas in the sediment to form the hydrate again, causing the rise of the system temperature as a result of the effect of the heat released from the hydrate formation. The third period corresponding to the second section of the pressure change in Figure 3 is between 22.5 and 37.0 min. In this period, the temperature immediately decreases remarkably. It is because the system pressure has been lower than the hydrate equilibrium dissociation pressure and the hydrate rapidly dissociates in the period. The quick phase transformation process requires absorbing plenty of heat and, thus, causes the temperature in the system to decline rapidly to the lowest point. The fourth plus fifth periods of the temperature change correspond to the third section of the pressure change in Figure 3.

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The fourth period is between 37.0 and 228.0 min. In this period, the hydrate dissociation process continues and the temperature in the reactor is basically kept around this steady lowest value. It illustrates that the heat needed for the hydrate dissociation is equivalent to that supplied from the ambient in this period. The fifth period is from 228.0 min to the end of the experiment. In the fifth period, the hydrate dissociation becomes slow and trends to end and the temperature in the reactor gradually rises from the lowest temperature point to the environmental temperature. In this period, the heat transferred from the ambient is more than the heat required for hydrate dissociation. Similar phenomena can be seen from other points in the near-well region (points 7
9, 12, 14, and 17
19). For point 1B, the temperature change is similar to that of 13B in the first three periods. It is due to the fact that the hydrate reformation and dissociation are completed within a short time in the first three periods, and the temperature change in the reactor is mainly due to the heat released from the phase transformation. However, the temperature at point 1B gradually increases after dropping to the lowest point. There is no constant temperature period. It is because point 1B is nearest to the inner wall of the reactor, and therefore, the effect of the heat transferred from the ambient is the largest. In addition, at the end of the third period, the large amount of the hydrate has dissociated and there is only a small amount of non-dissociating hydrate. Thus, the heat needed for the hydrate dissociation at point 1B is less than that supplied from the ambient after the third period. Similar observations can be made at the other points approaching the inner wall (points 1
6, 10, 11, 15, 16, and 20
25). Figure 8 shows the temperature spatial distribution in the hydrate reservoir over time for run 1. In the work, we use the temperature difference between the real-time temperature in the production process and the temperature at the starting time of the hydrate production experiment, to describe the temperature characteristics. Figure 8a gives the temperature distribution at the 5.0th min, which is during the free gas release period. It can be seen that the temperatures at various measuring points in the reactor do not change, which can also be seen in Figure 7. Figure 8b gives the temperature spatial distribution at the 22.5th min, which is the end time of the second period of the temperature change (see Figure 7). It can be seen from the above analysis that the temperatures of all of the measuring points in the reactor reach their individual own highest values at the 22.5th min, as shown from Figure 7 as a typical example. As seen from Figure 8b, generally, the temperatures in most regions in the reactor have some increases in comparison to those at the starting time of the gas production and the temperature increases are almost same and approximately equal to 0.5 °C. This is due to the hydrate reformation caused by the flowing water liberated from the hydrate in contact with methane gas, as analyzed above. The behavior of the temperature rise at the same degree in most regions illustrates that the water flowing almost happens in the whole hydrate reservoir bearing the sediment in the reactor and results in the hydrate reformation. As shown in Figure 8c, at the 30th min, the temperatures decrease compared to those at the 22.5th min because of the endothermic reaction of the hydrate dissociation in the hydrate reservoir, resulting in the temperature drop in the system. Figure 8d shows the spatial distribution of the temperature at the time when the pressure in the reactor reaches the set gas production pressure, that is at the starting time of the third section of the pressure change in the system, as shown in Figure 3. 4501

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Figure 8. Temperature distributions over time for run 1.

Figure 9. Temperature change of point 13A versus time for runs 1, 2, and 3.

Figure 10. Equilibrium dissociation pressure at mean temperature, temperature of 13B in the reactor, and ambient temperature and outlet pressure versus time for run 1.

At this time, the temperatures in the different regions have reached the individual lowest values. However, the values in the various regions have some differences. The temperatures in the near-well region are lower than those in the far-from-well region. Figure 8e gives the temperature spatial distribution at the 100th min for run 1, which is within the fourth period of the temperature change. At this time, the hydrate continues dissociating. As shown in Figure 8e, the temperatures in the far-from-well region increase and are obviously higher than those in the near-well region. It is due to the fact that the heat supplied from the water bath is successively transferred from the inner wall to the center of the reactor. Figure 8f gives the spatial distribution of the temperature at the 300th min. As shown, the temperatures in all regions have increased from their individual lowest values and the temperatures in the near-well region are still lower than those in the far-fromwell region. Similar phenomena for the temperature changes and the temperature distributions can be seen for runs 2 and 3. Figure 9 shows the temperature change of point 13A versus time for runs 1, 2, and 3. It can be seen from Figure 9 that the

starting time of the temperature rise (corresponding to points A, A0 , and A00 for runs 1, 2, and 3, respectively) in the system with the lower gas production pressure is earlier than that with the higher gas production pressure in the second period of the temperature change. It is because the driving force of the pressure difference is bigger with a lower production pressure. Thus, the water encaged in the hydrate can be more easily driven out and flows earlier in the system, resulting in the hydrate reformation earlier. It also can be seen from Figure 9 that, in the fourth period (between points C and D, C0 and D0 , and C00 and D00 for runs 1, 2, and 3, respectively), the lowest temperature decreases with the decrease of the gas production pressure. It is because the lower production pressure creates the higher rate of the pressure reduction in the hydrate dissociation process. Thus, with the higher rate of the pressure reduction, the hydrate dissociates more intensively, resulting in adsorbing more heat for the dissociation, which causes the decrease of the lowest temperature. It can also be seen from Figure 9 that the starting time for the fourth period of the temperature change (corresponding to points D, D0 , and D00 for 4502

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Figure 12. Resistance ratio distribution for run 1 at the 20th min. Figure 11. Equilibrium dissociation pressure at mean temperature, temperature of 13B in the reactor, and ambient temperature and outlet pressure versus time for run 3.

runs 1, 2, and 3, respectively) is earlier for the lower production pressure. The reason is that, with the lower production pressure, the dissociation rate is higher and the dissociation process is shorter. In the gas production process, the hydrate dissociation leads to an obvious drop of the temperature in the system and, furthermore, causes the decrease of the corresponding equilibrium dissociation pressure. Thus, the effectiveness of the depressurizing dissociation is lowered. To investigate the effect of the temperature change on the gas production by depressurization, we give the changes of the equilibrium hydrate dissociation pressures corresponding to the mean temperature in the system, the temperature at point 13B, and the ambient temperature with time for run 1, as shown in Figure 10. As a comparison, the pressure change of the outlet (the gas production wellhead) is also presented in Figure 10. The equilibrium hydrate dissociation pressures were calculated using the fugacity equation given by Li et al.22 In Figure 10, TE is the mean temperature in the system, TC is the ambient temperature, and PE is the equilibrium hydrate dissociation pressure at the corresponding temperature. It can be seen from Figure 10 that, with the decrease of the pressure of the outlet in the early stage of the hydrate dissociation (the second section of the pressure change), the equilibrium hydrate dissociation pressures at the mean temperature in the system and the temperature at point 13B also decrease and the pressure values are almost equal to that of the outlet. Therefore, it is found that the driving force for the hydrate dissociation (the difference between the equilibrium hydrate dissociation pressure and the system pressure) is quite small during this period and much less than the difference between the pressure of the outlet and the equilibrium pressure at the ambient temperature. Later, the equilibrium pressure corresponding to the mean temperature in the reactor gradually increases. It is due to the fact that, with the reduction of the amount of the hydrate, the heat required for the hydrate dissociation is gradually less than that supplied from the ambient. However, the equilibrium pressure corresponding to the temperature at point 13B is consistent with the outlet pressure for a long time. It is because point 13B is at the center of the reactor, where the supplied heat reaches last and, hence, the temperature rise is slowest.

Figure 11 gives the changes of the equilibrium hydrate dissociation pressures corresponding to the mean temperature in the system, the temperature at point 13B, and the ambient temperature, as well as the pressure change of the outlet with time for run 3. As seen, the change trends are similar to those in Figure 10. However, the equilibrium pressure at the mean temperature condition has no significant change, compared to that in run 1 after the outlet pressure drop to the gas production pressure (the third section of the pressure change, as shown in Figure 3), and at the 670th min, it starts to increase gradually to approach the equilibrium pressure at the ambient temperature. This is attributed to the fact that the system mean temperature for run 3 has little change during the period from the 40th to the 670th min. Thus, because the ambient temperature is kept constant, the heat-transfer rate has little change during this dissociation period, which explains the reason that the rate of gas production remains nearly constant for a long time (approximately 630 min) with the gas production pressure of 5.6 MPa, as in Figure 6. In the second period of the gas production, the system pressure declines to be lower than the equilibrium pressure at the system temperature, resulting in the hydrate dissociation. Subsequently, the system temperature is dropped on account of the hydrate dissociation. Thus, the equilibrium hydrate dissociation pressure corresponding to the system temperature is also decreased. It can be seen from Figures 10 and 11 that the pressure driving force (ΔP) for the dissociation approaches zero. This demonstrates that the sensible heat of the hydrate reservoir is spent by the hydrate dissociation, and the hydrate dissociation is mainly controlled by the rate of the pressure reduction during a very short period. In the third period of the gas production, the pressure in the reactor decreases to the set production pressure, almost all the sensible heat in the reservoir is spent in the second stage, and gas production stabilizes at a low rate. Because the hydrate system studied in this work is not isolated and the circumstance temperature remains constant, in the period, the heat conduction from the ambient (water bath) is the main driving force to dissociate hydrate. The results are similar to those by Kono et al.24 The lower production pressure causes the larger temperature drop with the hydrate dissociation in the system, leading to the higher rates of the pressure reduction and heat transfer, which results in the higher hydrate dissociation rate. 3.5. Resistance Change. Generally, the resistivity of gas is higher than that of the hydrate, and the resistivity of the hydrate is 4503

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Figure 13. Resistance ratio distribution for run 1 at the 100th min.

higher than that of water in the hydrate/gas/water system in the sediment. In the hydrate dissociation process, the hydrate is gradually transferred into water and gas, resulting in the change of the resistance in the sediment with time. Therefore, the resistance can be used to characterize the change of the gas hydrate in the porous sediment in the process of hydrate dissociation.25
27 In the work, we use the resistance ratio, which is the ratio of the resistance in the gas production process to the resistance at the starting time of the hydrate dissociation experiment, to describe the resistance change in the reactor in the gas production process. Figure 12 shows the resistance ratio spatial distribution in the hydrate reservoir for run 1 at the 20th min, which is in the free gas releasing process and before the water production. As seen, the resistance ratios in the bottom region are higher than those in the top region. In the free gas release process, the water starts to move up from the bottom and the contents of water and gas change in the different regions in the reactor. There are higher resistances in the bottom region because the water flows toward the top of the reactor, resulting in the increase of the gas content and the decrease of the water content in the bottom region. On the contrary, the lower resistances in the upper region result from the decrease of the gas content and the increase of the water in the upper region at the time. Figure 13 gives the resistance ratio distribution for run 1 at the 100th min, which is in the hydrate dissociation period (see Figures 3 and 5). As shown, the resistance ratios in the bottom region are still higher than those in the top region. However, the discrepancies of the resistance ratios between the bottom and top regions reduce. It may be due to the fact that, after the water production, some water moving to the top region of the reactor falls back to the bottom region because of gravity. Generally, the resistances in the reactor reduce at the 100th min, compared to those at the 20th min, which are shown in Figure 12. It is because most of the hydrate has dissociated at the time, resulting in the water generated from the hydrate dissociation filling in the pore space in the sediment instead of the hydrate. Thus, because the resistance of water is lower than that of the hydrate, the resistance value at each measuring point in the reactor at the 100th min is relatively lower than that at the 20th min. Same phenomena were observed in the other experiments.

4. CONCLUSION In this work, the dissociation behavior of methane hydrate in the CHS by depressurization was investigated. The following conclusions are made with the experimental results: (1) The gas production process consists of three periods: the free gas production,

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mixed gas (free gas and gas dissociated from the hydrate) production, and gas production from the hydrate dissociation. The gas production rate and the cumulative gas production increase with the decrease of the gas production pressure. The gas hydrate dissociation in the gas production process is mainly controlled by the rate of the pressure reduction in the system and the heat supplied from the ambient. (2) There is a large amount of water production in the free gas release process. However, there is little water production in the hydrate dissociation process. The amount of water increases with the decrease of the gas production pressure. (3) The temperature changes in the near-well region during the gas production from the hydrate consist of five periods. The first period represents the free gas releasing, which is practically isothermal. In the second period, the temperatures increase because of the hydrate reformation. In the third period, the system pressure is lower than the equilibrium dissociation pressure and the hydrate begins to dissociate, resulting in the quick and remarkable decrease of the system temperature. In the fourth period, which is corresponding to the third period of the gas production, the temperature at each measuring point remains constant. In the fifth period, which is also in the third period of gas production, the hydrate has almost dissociated completely and the temperatures gradually increase to the environmental temperature. The temperatures in the far-from-well region have no stable period and gradually increase after reaching the lowest values. (4) The resistances in the hydrate reservoir change with the water flow and the hydrate dissociation in the gas production process, and the resistances in the bottom region are higher than those in the top region. Generally, the resistances in the free gas release process are higher than those in the dissociation process. In the work, the investigations are carried out with the conditions simulating the conditions of the hydrate reservoir in the Shenhu Area, South China Sea. Thus, it is expected that there are some similar characteristics between the gas production behaviors of hydrate deposits examined here and the expected behaviors of hydrate deposits in the Shenhu Area, South China Sea. In the early stage, the gas production rate is high, the hydrate dissociates by absorbing the sensible heat of the reservoir, and the gas production is mainly controlled by the rate of the pressure reduction. In the late stage, the gas production is mainly driven by the heat conduction from the ambient. In addition, some production behaviors in the work, such as the water and gas production rates, the production time, and the pressure reduction rate, can be enlarged by the theory of similarity to predict those in the Shenhu Area, South China Sea, which is our next work. Therefore, the results in this work provide significant data and support for the gas hydrate production in the Shenhu Area, South China Sea, in the future.

’ AUTHOR INFORMATION Corresponding Author

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

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grants 51076155 and 51004089), the Chinese Academy of Sciences (CAS) Knowledge Innovation Program (Grant KGCX2-YW-3X6), and the Science and Technology Program of Guangdong Province (Grant 2009B050600006), which are gratefully acknowledged. 4504

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