Experimental Investigations into Gas Production Behaviors from

Article pubs.acs.org/EF

Experimental Investigations into Gas Production Behaviors from Methane Hydrate with Different Methods in a Cubic Hydrate Simulator Xiao-Sen Li,*,†,‡ Yi Wang,†,‡,§ Gang Li,†,‡ and Yu Zhang†,‡ †

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 § Graduate University of Chinese Academy of Sciences, Beijing 100083, People’s Republic of China ABSTRACT: In this work, the gas production behaviors of methane hydrate in porous media were investigated in the threedimensional cubic hydrate simulator (CHS) with a single well using the different methods, depressurization (DP), regular huff and puff (HP), huff and puff in conjunction with depressurization (HP-D), and huff and puff with no-soaking (HP-NS), at 281.15 K and hydrate saturation of 33.5%. The temperature spatial distributions and the hydrate spatial distributions in the hydrate reservoirs, the amount of gas and water produced, and the gas and water production rates in the gas production process were measured. In addition, the gas production efficiencies with the four methods were evaluated by calculating the gas recoveries, average production rates, thermal efficiencies, and energy efficiencies. It was found that HP-D is the optimal method for gas hydrate recovery. recovery.15−17 The hot water, hot brine, or steam huff and puff method is a special form of the combination of depressurization and thermal stimulation methods for gas production from the hydrate deposit. The Mallik 2002 well demonstrated the proof of the concept that it is possible to recover energy from permafrost hydrates combining the dissociation techniques of the depressurization and the thermal stimulation.18−20 Experimental investigations of the hydrate dissociation behaviors under the depressurization or thermal stimulation in the sediments using one-dimensional,8,21 two-dimensional,22 and three-dimensional23 apparatuses have been reported. In all of the experiments, an accordant evaluation method of the production efficiency for evaluating the advantages and disadvantages of the different hydrate production methods is lacking. In this work, the gas production behaviors of methane hydrate in porous media were investigated in the threedimensional cubic hydrate simulator (CHS) with a single well using the different methods, which include depressurization (DP), regular huff and puff (HP), huff and puff in conjunction with depressurization (HP-D), and huff and puff without soaking (HP-NS). The experiments were performed at the hydrate saturation of 33.5%, environmental temperature of 281.15 K, and initial pressure of 13.5 MPa at the beginning of the hydrate dissociation. These conditions simulate the conditions of the hydrate reservoir in the Shenhu area.24 In addition, we studied the energy efficiencies, the thermal efficiencies, the gas recoveries, and the production rates for the hydrate production with the four methods. Furthermore, we evaluated the advantages and disadvantages of the four

1. INTRODUCTION Natural gas hydrates are non-stoichiometric crystalline, inclusion compounds formed from the reaction between natural gas and water under a certain temperature and pressure. Gas hydrate formation requires relatively low temperature and high pressure, and 1 m3 of nature gas hydrate can release 160− 180 m3 of natural gas under standard conditions.1,2 Estimates of the world hydrate reserves are considerable and vary from 0.2 × 1015 to 120 × 1015 m3 of methane at standard temperature and pressure (STP). It is clear that the energy in these hydrate deposits is likely to be significant compared to all other fossil fuel deposits and was considered to be a potential strategic energy resource.3−6 To exploit this large energy resource, the researchers have proposed many methods, such as (1) the thermal stimulation method,7−9 in which the hydrate reservoirs are heated above the equilibrium hydrate decomposition temperature using in situ combustion or injecting hot water, steam, or hot salt water to decompose the hydrates, (2) the depressurization method,10 in which the hydrate reservoir pressure is reduced below the equilibrium decomposition pressure to decompose the hydrates, and (3) the chemical injection method,11 in which the chemicals (such as methanol or ethylene glycol) are injected to change the equilibrium hydrate decomposition conditions and, thus, to decompose the hydrates. The other two recent ideas, which need experimental and field confirmation, include CO2 replacement,12 to inject liquid CO2 into offshore hydrate reservoirs by forming CO2 hydrate and replacing the methane gas, and gas lift,13 to lift the hydrate particle as a solid from the sea bottom. Gas production strategies often involve combinations of these dissociation methods.14 The huff and puff method, also known as the cyclic steam stimulation (CSS), was accidentally discovered by the Shell Oil Company in 1960 during a Venezuela recovery project and is widely used in the oil industry to enhance oil © 2011 American Chemical Society

Received: October 27, 2011 Revised: November 30, 2011 Published: December 5, 2011 1124

dx.doi.org/10.1021/ef201641v | Energy Fuels 2012, 26, 1124−1134

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

production methods and determined the optimal production method.

2. EXPERIMENTAL SECTION 2.1. Experimental Apparatus. Figure 1 shows the schematic of the experimental apparatus. The experimental apparatus mainly consists of a reactor, a water bath, a back-pressure regulator and an aqueous solution injection system, a gas injection system, a water/gas separator, some measurement units, and a data acquisition system. The core component of the apparatus is a three-dimensional 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 three-dimensional reactor, called the CHS, is placed in a water bath with 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 of Figure 2 show the distributions of the temperature and resistance measuring points of the different layers and production wellheads within the three-dimensional 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 heat injection is the B wellhead in the middle layer (B) along the centerline of the reactor, and the outlet for the gas and water production is the A wellhead in the top layer (A). Pressure is measured by the Trafag NAT8251.7425type 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 of the bottom of the hydrate-bearing layer. The temperature is measured by the 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 gas flow meters, which are used to measure the cumulative gas injected into the CHS, the gas production rate, and the cumulative gas produced from the vessel, are both D07-11CM, 0−10 L/min, with the accuracy of ±2%, from the “Seven Star Company”. The thermometers, pressure transducers, and gas flow meters were calibrated using a mercury thermometer with the tolerance of ±0.01 °C, a pressure test gauge with the error of ±0.05%, and a wet gas

Figure 2. Distributions of temperature and resistance measuring points and production wellhead of each layer within the threedimensional reactor. meter with the accuracy of ±10 mL/min, respectively. A metering pump “Beijing Chuangxintongheng” HPLC P3000A with the range of 50 mL/min can withstand pressures of up to 30 MPa. An inlet liquid container with the inner volume of 10 L is used to contain the deionized water used in the experiments. To protect the metering pump from corrosion by the hot brine or the chemicals, three middle 1125


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Figure 3. Changes of the system pressures with time in the experiments with different production methods. containers are used for the solution injection, and the inner volume of each container is 4 L. A back-pressure regulator (the pressure range of 0−30 MPa, with the accuracy of ±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 g, with the accuracy of ±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 Methods. 2.2.1. Regular Huff and Puff (HP). The regular huff and puff method consists of three stages: the heat injection, soaking, and production.25 The heat injection starts by injecting hot water at a constant rate for some time. The initial temperature of the water injected is constant. The inlet valve is closed after the heat injection, and the soaking starts, during which the system pressure rises slowly. When the pressure stops rising, the soaking stage comes to an end and the gas production stage starts. The outlet valve of the well is opened, and when the system pressure drops to the set production pressure, the outlet valve is turned off. At this point, the current cycle of the gas production finishes, and the next cycle of the thermal huff and puff experiment starts. When the number of the thermal huff and puff cycles increases to a certain value, the increase of the pressure during the soaking stage in this cycle is close to 0. Thus, we believe that there has been no more hydrate decomposition in the CHS. The entire experimental run ends when the cycle is completed. For gas production with the following

production methods, the determination of the gas production end is the same with that discussed above. 2.2.2. Huff and Puff in Conjunction with Depressurization (HPD). The huff and puff method in conjunction with depressurization consists of four stages: the pre-depressurization, injection, soaking, and production. In the pre-depressurization stage, the pressure of the production well is set to be lower than the equilibrium hydrate dissociation pressure at the working temperature. First, the outlet valve of the well is opened. Then, the outlet valve is turned off when the system pressure drops to the set production pressure. At that time, the pre-depressurization stage comes to an end. After that, the injection, soaking, and production stages with the HP-D method are similar to those with the HP method. A unique difference is that the set production pressure is below the equilibrium hydrate dissociation pressure, which is the same with that in the pre-depressurization stage. 2.2.3. Huff and Puff with No Soaking (HP-NS). The huff and puff method without soaking consists of two stages: the heat injection and production. This method removes the soaking process in the HP method, and only the injection and production processes are carried out. 2.2.4. Depressurization (DP). The depressurization method consists of two stages: the depressurization period and steady-pressure period. In the depressurization period, the back-pressure regulator is set to the production pressure, which is lower than the equilibrium hydrate dissociation pressure at the working temperature. Then, the outlet valve of the well is opened to make the system pressure decrease to the set production pressure. In the process, the hydrate is dissociated. When the system pressure is equal to the set production pressure, the depressurization period ends and the steady-pressure period starts. Subsequently, the hydrate is continuously dissociated 1126


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under the condition of the constant pressure. When there is little gas release, it is considered as the end of the gas production. 2.3. Experimental Procedure. Quartz sands with the quantity of 8162 g, the particle size from 300 to 450 μm, and porosity of approximately 48% are tightly packed in the CHS as porous media. The system is emptied twice to remove the residual air in the system. The deionized water of 1537 mL is injected to the CHS by the metering pump. The temperature of the water bath is set to a predetermined temperature required for the gas hydrate synthesis, which is 8.0 °C in the current research. Methane gas of 13.4 mol is then 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 hydrate formation starts, the pressure inside the CHS decreases. The hydrate formation process lasts for 10−14 days, and then the pressure in the vessel is reduced to 13.5 MPa. Using the model by Linga et al.,26 the hydrate saturation (the volume ratio of hydrate and available pore space) is calculated as approximately 33.5% before hydrate dissociation. While the water bath is maintained at a constant temperature (8 °C), the water in the preheater is heated to the injection temperature of the hot water (Tinj). The rate of hot water injection is set as 40 mL/min, and then the bypass valve is opened to preheat the pipelines with hot water. The gas production pressure controlled by the back-pressure regulator is set as 6.5 MPa. Before the heat injection starts, the system temperature is 8.0 °C. Using the fugacity model by Li et al.,27 the equilibrium hydrate dissociation pressure in the sediment at the working temperature of 8.0 °C is calculated and the calculated value is 5.7 MPa. Therefore, the set production pressure is higher than the equilibrium pressure, and the hydrate cannot dissociate at this time. Then, the experiments for hydrate dissociation with the HP, HP-NS, DP, and HP-D methods are carried out. Finally, after the end of the experiment, the system pressure is released to atmospheric pressure gradually. The temperatures and pressures in the vessel, the gas production rates, and the water injection and production rates are recorded at 10 s intervals during the hydrate dissociation.

puff cycle starts from 10 to 330 min with the production pressure of 5.6 MPa, and this experiment includes 11 cycles of the huff and puff process. With the fourth cycle of huff and puff as a typical case, the huff and puff process still consists of the injection stage (from e to f), during which 200 mL of hot water with the temperature of 130 °C at 40 mL/min is injected into the CHS and the system pressure increases from 5.6 to 6.3 MPa, the soaking stage (from f to g), during which the system pressure increases from 6.3 to 6.5 MPa, and the gas production stage (from g to h), during which the system pressure decreases from 6.5 to 5.6 MPa. Figure 3c shows the change of the system pressure with time during the experiment with the HP-NS method. As seen in Figure 3c, in the process, the thermal huff and puff cycle starts from time 0 and the production pressure is 6.5 MPa. This experiment includes 14 cycles of the huff and puff process. With the fourth cycle of huff and puff as a typical case, the huff and puff cycle includes the injection stage (from i to j), during which hot water of 200 mL with the temperature of 130 °C at 40 mL/min is injected into the CHS and the system pressure increases from 6.5 to 7.4 MPa, and the gas production stage (from g to h), during which the system pressure decreases from 7.4 to 6.5 MPa. Figure 3d shows the change of the system pressure with time during the DP process. As seen in Figure 3d, the depressurization stage starts from 0 min and lasts for 12 min and the system pressure decreases from 6.5 to 5.6 MPa, which is lower than the equilibrium pressure. The process from 12 to 2380 min is the gas production stage (from m to o) with the steady production pressure, which is equal to the set gas production pressure of 5.6 MPa. In these four gas production experiments, the total time of gas production with the HP, HP-D, HP-NS, and DP methods is 625, 330, 140, and 2370 min, respectively. Obviously, the time with the DP method is the longest, because in the gas production process by depressurization, the gas production is mainly controlled by the pressure reduction rate in the depressurization period and the heat conduction from the ambient is the main driving force to dissociate the hydrate in the steady-pressure period.28 However, the low rate of the heat conduction results in the long period for gas production. Whereas the high injection temperature in the HP experiment causes the high driving force for the hydrate dissociation, resulting in the higher dissociation rate and less time consumption compared to the DP method. Because the HPD method provides a dual driving function of the depressurization and thermal huff and puff for the hydrate dissociation, the hydrate dissociation rate is higher and the time consumption is less than those with the HP method. Because the soaking stage is removed in the HP-NS method, the time consumption is the least. 3.1.2. Temperature Distribution. As a typical case, Figure 4 shows the three-dimensional spatial temperature distributions at the fourth cycle over time in the experiments with the four different methods. Panels a−o of Figure 4 are the temperature distributions corresponding to time points a−o marked in panels a−d of Figure 3. Among them, panels a−d of Figure 4 gives the temperature distributions at the beginning of the heat injection, end of the heat injection, end of soaking, and end of the production during the fourth huff and puff cycle in the HP experiment, respectively. At the beginning of the injection (Figure 4a), the temperature at all points within the system remains almost the same and is close to the environmental

3. RESULTS AND DISCUSSION 3.1. Production Process. 3.1.1. Pressure Change. Figure 3 shows the changes of the system pressures in the experiments using the different methods for hydrate dissociation. Figure 3a shows the change of the system pressure with time during the HP process. As seen in Figure 3a, in the HP experiment, the thermal huff and puff process starts at time 0 and the production pressure is 6.5 MPa. This experiment includes 15 cycles of the huff and puff process. With the fourth cycle of the huff and puff as a typical case, in the injection stage (from a to b), the heat injection starts by injecting hot water of 130 °C at 40 mL/min for 5 min. The system pressure increases from 6.5 to 7.2 MPa. It is attributed to the fact that the gas in the CHS is compressed by the injected hot water. Meanwhile, the heat injection leads to the hydrate dissociation and dissociated gas production. In the soaking stage (from b to c), the increase of the system pressure is from approximately 7.2 to 7.3 MPa, because continuous heat diffusion leads to the continuous decomposition of the hydrate. In the gas production stage, the outlet is opened for gas and water production and the pressure decreases rapidly to approximately 6.5 MPa (the set production pressure). As the number of cycles increases to a certain value, the pressure increase during soaking is close to 0. At that time, the thermal huff and puff production of the gas hydrate reaches the maximum production range.23 Figure 3b shows the change of the system pressure with time during the HP-D method experiment. As seen in Figure 3b, the pre-depressurization stage lasts from 0 to 10 min, and in this stage, the system pressure decreases from 6.5 to 5.6 MPa, which is lower than the equilibrium pressure. The thermal huff and 1127


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temperature. At the end of the injection (Figure 4b), the temperature in the near-well region has raised, the heat has diffused to the surroundings, and the temperature at the central point has reached the maximum as a result of the injection of 200 mL of hot water with the temperature of 130 °C. At the end of the soaking stage (Figure 4c), the temperature of the surroundings continues to rise but the central temperature drops, which indicates that the heat continuously diffuses outward and finally dissipates through the boundary of the CHS. At the end of the production stage (Figure 4d), the temperature in all of the regions in the system decreases. This is because the heat of the system is almost entirely consumed. Panels e−h of Figure 4 show the temperature distributions at the beginning of the injection, end of the injection, end of soaking, and end of the production during the fourth huff and puff cycle in the HP-D experiment, respectively. When panels e−h of Figure 4 are compared to panels a−d of Figure 4, it is noted that the change trend of the temperature distribution from panels e to h of Figure 4 is similar to that from panels a to d of Figure 4. However, at the same stage, for example, as shown in panels e and a of Figure 4, the temperature of all of the regions in Figure 4e is lower than that in Figure 4a. It is due to the fact that the production pressure in the experiment with the HP-D method is 5.6 MPa, which is lower than the equilibrium pressure. Thus, the driving forces for the hydrate dissociation with the HP-D method consist of the pressure driving force and the thermal driving force, which is bigger than the single thermal driving force with the HP method. This

Figure 4. Three-dimensional spatial temperature distributions during the hydrate dissociation with different production methods.

Figure 5. Cumulative volumes of produced gas and water and water injected during hydrate dissociation with different methods. 1128


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number of cycles increases, because the hydrate located around the central well has been completely decomposed and the hydrate in the far-from-well region is gradually decomposed. However, the rate of the gas production gradually decreases because of the heat loss through the boundary of the reactor, which leaves less capability to progress to the surroundings and to decompose hydrate. The final cumulative volume of the produced gas is 167.6 L. There is the low water production rate during the first 4 huff and puff cycles, approximately 50−100 g/ cycle. However, after that, the water production rate gradually increases and tends to be steady. Eventually, the volume of the produced water per cycle is similar to the volume of the injection water per cycle (200 g/cycle). It may be due to the fact that there are a lot of pores in the reservoir in the CHS before the beginning of the experiment and the hydrate decomposition in the CHS during the first 4 huff and puff cycles causes the increase of the porosity in the reservoir. Thus, some of the injected water remains in the CHS during the first 4 cycles. However, after this period, the space of the pores in the reservoir is filled by the injected water gradually, the amount of hydrate decomposition gradually decreases, and eventually, the amount of the water injected is equal to that of the water produced during the remaining huff and puff cycles. Figure 5b shows the cumulative volumes of the produced gas and water and the water injected during the hydrate dissociation with the HP-D method. As seen in Figure 5b, during the pre-depressurization stage (0−10 min), the cumulative volume of the produced gas is 50 L because of the free methane release from the CHS and the hydrate dissociation caused by depressurization. During the huff and puff process with a total of 11 cycles (10−335 min), the injection and soaking stages are similar to those with the HP method (Figure 5a). The gas production rate is relative high during the first 5 cycles (10−155 min), approximately 20 L/ cycle. After that, the gas production rate gradually decreases to 10 L/cycle, which is higher than 5 L/cycle in the remaining period with the HP method, because the double driving forces of the thermal stimulation and the depressurization cause the increase of the hydrate decomposition rate. Eventually, the cumulative volume of the produced gas is 198.6 L, which is higher than that with the HP method, and the gas production time is less than half of that with the HP method. The volume of water production in the pre-depressurization stage is small, approximately 50 g. However, after that, the volume of the produced water is approximately 200 g/cycle, which is the same as the volume of the injected water for each cycle. Figure 5c shows the cumulative volumes of the produced gas and water and the water injected during the hydrate dissociation using the HP-NS method. As seen in Figure 5c, during the thermal huff and puff process with a total of 14 cycles, the injection stage is similar to that with the HP method (Figure 5a). However, after the injection stage, the following process is the production stage without the soaking period in each cycle. As shown in Figure 5c, the volume of the produced gas in the first cycle (0−10 min) is approximately 10 L and the volume increases to 20 L in the second cycle (10−20 min). This is because, in the first cycle, the heat injected from the well at the center point does not have enough time to expend to surroundings on account of removing the soaking stage. Thus, a small amount of the hydrate is dissociated in the period. The heat expends continuously out in the second cycle and causes more hydrate located around the central well to be dissociated. After that, the rate of gas production gradually decreases to 1.8

causes the hydrate to be more intensively dissociated and more heat to be absorbed from the surroundings, resulting in the bigger magnitude of the temperature drop in the hydrate reservoir in the hydrate decomposition process. Similar behaviors can be observed with panels f and b of Figure 4, panels g and c of Figure 4, and panels h and d of Figure 4. Panels i−k of Figure 4 give the temperature distributions at the beginning of the injection, end of the injection, and end of the production during the fourth huff and puff cycle in the HPNS experiment, respectively. At the beginning of the injection stage (Figure 4i), the temperature in the central region in the CHS has been very high, because the heat from the last cycle of the huff and puff still remains in the CHS. At the end of the injection stage (Figure 4j), the volume affected by the heat gradually increases with the heat injection. At the end of the production stage (Figure 4h), because the injected heat flow has little time to expand continuously on account of no soaking process, the heat accompanying gas and water production is removed. Thus, the region affected by the heat at the end of the production is similar to that at the beginning of the injection stage. Panels l−o of Figure 4 show the temperature distributions at the beginning of the depressurization, the end of the depressurization, at a certain time at the steady-pressure period, and the end of the production during the DP experiment, respectively. At the beginning of the depressurization (Figure 4l), the temperatures in the system are identical to the environmental temperature. At the end of the depressurization (Figure 4m), the temperatures at all points in the CHS drop to the individual lowest values and the magnitudes of the temperature drops are the same. The reason is that, because of the high porosity and permeability of the sediment, the pressures at the different measuring points in the CHS have little discrepancy. Thus, in the depressurization process, the pressure reduction rate corresponding to the rate of the endothermic reaction of the hydrate dissociation is the same at any point, resulting in the rate of the temperature drop being the same at any point. Subsequently, at the production stage (panels n and o of Figure 4), the system pressure drops to the set production pressure and remains constant. Therefore, the pressure reduction rate is equal to zero. During that period, the heat conduction from the ambient is the main driving force for the hydrate dissociation. The heat transfers from the surroundings to the central of the CHS constantly until the temperatures of the entire hydrate reservoir are the same as that of the ambient. 3.1.3. Production Behavior. Figure 5a shows the cumulative volumes of the produced gas and water and the water injected during the hydrate dissociation with the HP method. As seen in Figure 5a, each cycle of the huff and puff includes the injection stage, in which hot water of 200 mL with the temperature of 130 °C at 40 mL/min is injected into the CHS, the soaking stage, in which the cumulative volume of the produced gas and water and the water injected do not change, and the production stage, in which the gas and water are released from the CHS. Obviously, during the 15 cycles of thermal huff and puff, there is the relative high gas production rate during the first 4 huff and puff cycles (0−130 min), approximately 20 L/cycle. It is due to the fact that most of the injected heat quickly contacts the hydrate reservoir in the near-well region in the first 4 cycles and is mainly used for the hydrate decomposition in the nearwell region, resulting in the high gas production rate. After that, the gas production rate gradually decreases to 5 L/cycle as the 1129


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L/cycle. Finally, the cumulative volume of the produced gas is only 131.6 L. Because the heat expends incompletely without the soaking stage, little hydrate is decomposed in the far-fromwell region, resulting in the low cumulative volume of the produced gas. The volume of water production during the first 2 cycles of huff and puff is small (50 g/cycle). However, after that, the volume gradually increases to approximately 200 g/ cycle, which is similar to the volume of the injected water in each cycle. The reason is similar to that explained in Figure 5a. Figure 5d shows the cumulative volumes of the produced gas and water and the water injected during the hydrate dissociation with the DP method. As seen in Figure 5d, in the depressurization stage (0−12 min), the volume of gas production from free gas release and hydrate decomposition is approximately 65 L and the volume of water production is 45 g. In the period, the gas and water production rates are relatively high. The period from 12 to 2380 min is the production stage with the steady production pressure of 5.6 MPa, in which the rate of gas production is low and gradually deceases to 0. The final cumulative volume of the produced gas is 184.6, and the gas production time is much longer than that with the above three production methods. In that period, the rate of the water production is also very low and no more water is produced form the CHS after 100 min. Eventually, the cumulative volume of the produced water is only 80 g. It is attributed to the fact that there is no water injected into the CHS during the gas production by depressurization, resulting in the small amount of water being produced. 3.2. Energy Efficiency and Thermal Efficiency. In this work, the energy efficiency and thermal efficiency are both employed to evaluate the efficiency of producing gas from the gas hydrate reservoir.29 The energy efficiency is defined as the ratio of the combustion heat of the produced gas to the total input heat. The thermal efficiency is defined as the ratio of the heat used for the hydrate dissociation to the total input heat. The energy efficiency can be calculated with the following equation:

ξ=

Figure 6. Changes of energy efficiencies with time with different production methods.

fourth cycle (125 min). After 125 min, the energy efficiency shows a drop trend over time and, eventually, declines to 9.44 at the end of the last huff and puff cycle. During the experiment with the HP-D method, the volume of gas production is large but the volume of water production is small in the predepressurization stage (0−10 min). Thus, the energy efficiency increases immediately to approximately 6000. In the first huff and puff cycle, the energy efficiency declines rapidly to 41 with the hot water injection (12−17 min). During the remaining 11 cycles, the energy efficiency gradually reduces to 15.6. During the experiment with the HP-NS method, the energy efficiency reaches its maximum of 10.5 at the end of the production stage in the fourth huff and puff cycle. After that, the energy efficiency decreases gradually to 7.94. During the experiment with the DP method, the high rate of gas production causes the rapid increase of the energy efficiency during the period from 0 to 50 min. However, because the rate of gas production decreases and the water production increases from 50 to 100 min, the energy efficiency slightly decreases. After that, the water production stops but the gas is continuously produced from the system. Thus, the energy efficiency increases to 11000 slowly. The highest energy efficiency can be obtained using the DP method in the experiments with these four methods. The reason is that no energy is injected, and the volume of water produced is small in the depressurization method. In the experiment with the HP-D method, the additional depressurization driving force is supplied for the gas production, resulting in the increase of the rate of gas production and the decrease of the number of huff and puff cycles. Thus, the energy efficiency of the HP-D method is higher than the HP method. Whereas the lowest energy efficiencies are with the HP-NS method, because the heat injected is not applied fully, resulting in the small volume of gas produced and the large number of huff and puff cycles. The thermal efficiency can be calculated with the following equation:

Q tMgas C wM w (T0 − T ) + W

(1)

where Qt is the cumulative volume of the produced gas, Mw is the mass of the injected hot water, T0 is the initial temperature of the injected hot water, T is the ambient temperature (8 °C), the combustion heat of natural gas (Mgas) is 37.6 MJ/m3, the specific heat of water (Cw) is 4.2 × 103 J kg−1 K−1, and W is the pump work and can be expressed as follows:

W = Winj + Wpro

(2)

Here, Winj is the injection pump work, which is defined as the energy required for injecting the water into the CHS, and Wpro is the production pump work, which is defined as the energy required for water production from the CHS. Figure 6 shows the changes of the energy efficiencies with time in four different experiments. As seen in Figure 6, during the experiment with the HP method, in the injection and soaking processes of the first huff and puff cycle (0−13 min), the energy efficiency is 0 because no methane is released from the vessel. Then, the energy efficiency increases rapidly with the gas release from the system during the production stage in the first cycle (13−18 min), After that, during the remaining 14 cycles (18−650 min), the energy efficiency reaches its maximum of 16.58 at the end of the production stage in the

η=

mdissMhyd C wM w (T0 − T )

(3)

where mdiss is the moles of the hydrate, which have been dissociated, and the dissociation heat of the hydrate (Mhyd) is taken as 54.1 kJ/mol. 1130


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dissociated completely and there is a maximum hydrate dissociation region impacted by the heat diffusion with the thermal stimulation driving force.23 However, the combination of the HP or HP-NS with the depressurization can make up for the disadvantage. Thus, the gas recovery can be increased to 99.2% with the HP-D method. In addition, as shown in Figure 7, the gas recovery with the HP-NS method is lower than that with the HP method. It indicates that the soaking process can enhance the gas recovery effectively. The resistance can be used to characterize the change of the gas hydrate reservoir.30,31 Because the resistivity of gas hydrate is greater than water, generally the resistance decreases as the hydrate decomposes. In this work, we use the ratio between the real-time resistance during the experiment and the resistance before the experiment (referred to as the resistance ratio) as a characterization parameter. Figure 8 shows the three-dimen-

The thermal efficiency with the HP method is 0.58, because of the heat loss through the boundary of the reactor. The thermal efficiency of the HP-D method is 0.90. It may be because the addition of the depressurization driving force improves the hydrate dissociation and, thus, causes the increase of the porosity in the whole reservoir on account of the hydrate dissociation. Therefore, the hot water injected more easily expands to the surroundings, which enhances the heat use for the hydrate dissociation. The thermal efficiency of the HP-NS method is only 0.47. it is attributed to the fact that the heat flow does not have enough time to expand for the hydrate dissociation on account of no soaking stage. Thus, most of the heat is wasted. Therefore, the thermal efficiency with this method is the lowest. The thermal efficiency with the DP method does not exist. It is due to the fact that there is no heat injected into the system in the entire gas production process. 3.3. Gas Recovery. Gas recovery is significant data for evaluating the capacity of the gas production. In this work, it is defined as the ratio of the amount of hydrate decomposed to the total amount of hydrate in the CHS. The gas recovery can be calculated with the following equation: m φ = diss mhyd (4) where mhyd is the total moles of hydrate in the CHS. Figure 7 shows the changes of the gas recoveries with time in the experiments with the above four methods. As seen in Figure

Figure 8. Three-dimensional spatial resistance ratio distributions at the ends of the experiments with four different production methods.

sional spatial resistance ratio distributions at the ends of the experiments with the above four methods. As seen in panels a and c of Figure 8, the resistance ratios in the near-well region are remarkably lower than those in the far-from-well region at the ends of the experiments with the HP and HP-NS methods. It indicates that the hydrate in the near-well region is decomposed completely, while little hydrate is decomposed in far-from-well region. In addition, it can also be seen from Figure 8 that the decomposition region with the HP-NS method is smaller than that with the HP method, which is corresponding to that discussed in Figure 7. On the other hand, the values of the resistance ratios are almost the same in the whole hydrate reservoir in the vessel at the ends of the gas production with the HP-D and DP methods, as shown in panels b and d of Figure 8, which indicates that the hydrate is almost completely decomposed in the whole hydrate reservoir. This is consistent with that discussed in Figure 7. 3.4. Average Production Rate. The average production rate is defined as the ratio of the cumulative volume of the produced gas to the time. The average production rate is given by

Figure 7. Changes of gas recoveries with time with different production methods.

7, the gas recoveries with the four methods all increase with time and eventually reach their individual maximum values. The final gas recoveries with the HP, HP-D, HP-NS, and DP methods are 79.7, 99.2, 60.0, and 97.9%, respectively. There is a maximum of the region impacted by heat diffusion in the experiment with the HP method, and thus, the hydrate in the CHS cannot be decomposed completely by this method.23 The gas recoveries with the HP-D and DP methods are both close to 100%. The reason is that the depressurization driving force is used with these two methods, which leads to the hydrate decomposition in the whole hydrate reservoir in the CHS. The disadvantages for the HP and HP-NS methods with a single production well are that the hydrate in the reservoir cannot be

Vt = 1131

Qt t

(5) dx.doi.org/10.1021/ef201641v | Energy Fuels 2012, 26, 1124−1134

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Figure 9 shows the changes of the production rate with time in the experiments with the four methods. As seen in Figure 9, the

Table 1. Final Thermal Efficiencies, Energy Efficiencies, Gas Recoveries, and Average Production Rates for Hydrate Dissociation with Different Methods energy efficiency thermal efficiency gas recovery (%) average production rate (L/min)

HP

HP-D

HP-NS

DP

9.44 0.58 79.7 0.26

15.6 0.90 99.2 0.55

7.94 0.47 60.0 0.96

11000 97.9 0.08

The thermal efficiency, the energy efficiency, the gas recovery, and the production rate are the significant parameters for evaluating the economic efficiency for gas production from the hydrate, and the higher parameter values can create the higher economical efficiencies. If any one of the parameters for each gas production method is remarkably low, it is hard to be a valuable commercial production method. Obviously, the average production rate is too low with the DP method, and the gas recovery is also very low with the HP-NS method compared to the HP and HP-D methods. In addition, each parameter with the HP-D method is better than that with the HP method. Thus, in general, the HP-D method is the optimal method in all of these methods for gas production from hydrate reservoir. In addition, some production behaviors and efficiency items in the work, such as the water and gas production rates, the production time, the thermal efficiency, the energy efficiency, the recovery rate, and the average production rate, can be enlarged by the theory of similarity32,33 to predict those in the hydrate reservoir field. Therefore, the results in this work provide significant data and support for the gas hydrate production in the hydrate reservoir field.

Figure 9. Changes of average production rates with four different production methods.

average production rate with the HP method reaches its maximum of 0.68 L/min at the end of the production stage in the second huff and puff cycle (48th min). After that, the production rate shows a drop trend and, at last, it decreases to 0.26 L/min. During the experiment with the HP-D method, the average production rate increases rapidly to 2.7 L/min in the pre-depressurization stage (0−10 min) and then declines gradually to 0.55 L/min during the remaining 11 huff and puff cycles. During the experiment with the HP-NS method, at the end of the production stage in the second huff and puff cycle, the average production rate reaches its maximum of 1.19 L/min and, subsequently, gradually decreases to 0.96 L/min. The average production rate of the DP experiment reaches its maximum of 3.2 L/min at the 12th min. After that, it drops gradually to 0.4 L/min during the period from 12 to 200 min and, subsequently, continuously drops slowly. The final average production rate is 0.08 L/min at the 2380th min. Generally, the average production rate at the 12th min in the DP experiment is the highest in all of the experiments with the four methods. However, the average gas production rate in the later period of gas production is very low, and thus, the total time for gas production is quite long (approximately 2380 min). Thus, the time efficiency is quite low with the method. During the experiment with the HP-D method, the average production rate in the pre-depressurization stage is close to that in the depressurization stage of the DP experiment and the heat injected into the system makes the time of the production reduce remarkably. Thus, the final production rate with the HPD method is higher than that with the DP and HP methods. In addition, the production time for the HP-NS method is the shortest, while the final average production rate is the highest. 3.5. Efficiency Evaluation. Table 1 shows the final thermal efficiencies, energy efficiencies, gas recoveries, and the production rates with the different methods for hydrate dissociation. As seen in Table 1, each method has its advantage and disadvantage. For example, the highest energy efficiency can be obtained with the DP method, while the average production rate is the lowest. The highest average production rate is with the HP-NS method, while the other parameters are relatively low.

4. CONCLUSION In this work, the gas production behaviors from methane hydrate in porous media are investigated in the threedimensional CHS with a single well using four different methods, which include DP, HP, HP-D, and HP-NS, at 281.15 K and the hydrate saturation of 33.5%. The following conclusions are made. The pressure changes, temperature distributions, and production behaviors of the four different methods have been shown in this work. First, the time with the DP method is the longest. The time consumption of the HP-D method is less than those with the HP method because of the dual driving force for the hydrate dissociation. The time consumption of the HP-NS method is the least because the soaking stage is removed. Second, the heat continuously diffuses outward and finally dissipates through the boundary of the CHS in the HP method. The temperature drop in the HP-D method is bigger than the HP method because the dual driving force for the hydrate dissociation. In the HP-NS method, the injected heat flow has little time to expand continuously on account of no soaking process. In the DP method, the heat transfers from the surroundings to the center constantly until the temperatures of the entire hydrate reservoir are the same as that of the ambient. Third, the gas production rate in the HP method decreases from 20 to 5 L/cycle, and the water production rate gradually increases from 50 to 200 g/cycle and tends to be steady. The gas production rate in the HP-D method decreases from 20 to 10 L/cycle, and the water production rate is the same as the volume of the injected water for each cycle (200 g/cycle). The gas production rate in the HP-NS method finally decreases to 1132


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1.8 L/cycle. During the DP method, the gas production in the depressurization stage is approximately 65 L and the volume of water production is 45 g. After that, the rate of gas production gradually deceases to 0, the rate of the water production is also very low, and no more water is produced from the CHS after 100 min. The energy efficiency is the highest with the DP method, while the production time is the longest and the production rate is the lowest. The soaking process can improve the thermal efficiency, energy efficiency, and gas recovery, while it reduces the production rate. With the HP-D method, the addition of the depressurization driving force results in the higher thermal efficiency, energy efficiency, gas recovery, and production rate compared to those with the HP method. In comparison to the four methods, the HP-D method is optimal for hydrate decomposition.

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

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ACKNOWLEDGMENTS The authors gratefully appreciate the financial support from the National Natural Science Foundation of China (51076155, 51106160, and 51004089) and the Science and Technology Program of Guangdong Province (2009B050600006).

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