Design and Experimental Study of the Steam Mining System for

Nov 6, 2012 - The mining technique for natural gas hydrates is an essential ... to enable natural gas hydrates to replace conventional energy sources...
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Design and Experimental Study of the Steam Mining System for Natural Gas Hydrates You-hong Sun,† Rui Jia,*,† Wei Guo,† Yong-qin Zhang,‡ You-hai Zhu,§ Bing Li,† and Kuan Li‡ †

College of Construction Engineering, Jilin University, Changchun 130026, People’s Republic of China The Institute of Exploration Techniques, Chinese Academy of Geological Science (CAGS), Langfang 065000, People’s Republic of China § Institute of Mineral Resources, Chinese Academy of Geological Science (CAGS), Beijing 100037, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: The mining technique for natural gas hydrates is an essential technology to enable natural gas hydrates to replace conventional energy sources. This paper presents a steam mining system for natural gas hydrates (SMSGH) for exploiting natural gas hydrates in terrestrial permafrost regions. The system comprises of the surface steam heating device, underground mining, sealing device, surface gas collection, and ignition device. The working principle, design and calculation process, and structure composition of the system are described. The heating effect of the system is verified by laboratory experiments. The optimal heating parameters are proposed; steam open/closed pressure is 1.0/0.5 MPa. A field experiment was conducted in drilling wells of natural gas hydrates in Qilian Shan permafrost in Qinghai province, China. A total of 3.28 m3 of natural gas was successfully exploited, and the ignition experiment was completed. This is the first study conducted in China that has successfully exploited natural gas hydrates in terrestrial permafrost.

1. INTRODUCTION Natural gas hydrate is a clean energy source with enormous potential. It is characterized by its wide distribution, large storage capacity, high energy density, and shallow underground burial. It is one of the ideal 21st century alternatives to coal, fossil oil, natural gas, and other conventional energy.1 Trial mining studies are necessary to test the suitability of natural gas hydrates for commercial exploitation. The mining method and technology for natural gas hydrates remain on the level of theoretical research and experiments.2,3 On the basis of the physical and chemical properties of natural gas hydrates, the main mining methods include the following: (1) Thermal stimulation is to directly heat the natural gas hydrate layer, so that the temperature of the natural gas hydrate layer exceeds its equilibrium temperature. Consequently, natural gas hydrates disassociate into water and natural gas (Figure 1). However, this method still cannot resolve the lower thermal use ratio and can also result in local heating. Thus, this method must be further improved. (2) Pressure drop

method is to decrease the pressure, so that the pressure of the natural gas hydrate is lower than its equilibrium pressure, resulting in dissociation of the natural gas hydrate. However, this method entails special demands for the properties of natural gas hydrates. The pressure drop method has economic viability only when natural gas hydrates are deposited near the temperature and pressure balance boundary. (3) Chemical reagent method is to add some chemical reagents, such as saline water, methyl alcohol, ethanol, ethylene glycol, and propanetriol, to alter the phase equilibrium condition of natural gas hydrates. This results in the disassociation of natural gas hydrates. However, the required chemical reagents are expensive, and the effect on the natural gas hydrate layer is achieved very slowly. In addition, this method will lead to some environmental problems.4−6 Fortunately, some new mining methods have been discovered, for example, CO2 replacement and solid mining methods. Trial mining studies of natural gas hydrates have been conducted in three places throughout the world only in terrestrial permafrost areas. In comparison to the marine environment, natural gas hydrates in the permafrost region can occur in conditions with lower temperature and pressure and are thus more easily exploited and studied during mining processes and construction operations. The Siberia Messoyakha field in Russia, which was discovered in the late 1960s, is a unique gas field for commercial exploitation of natural gas hydrates, where the pressure drop method and chemical reagent method have been jointly used to exploit natural gas hydrates.7 To date, the Messoyakha field has produced 12.6 billion m3 of natural gas. On the basis of theoretical calculation and analysis, approximately 6.9 Received: August 27, 2012 Revised: November 6, 2012 Published: November 6, 2012

Figure 1. Schematic illustration of the heating and pressure drop mining principle. © 2012 American Chemical Society

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billion m3 of natural gas was produced. In 2002, a trial study of natural gas hydrates was conducted in the Mackenzie Delta in Canada. The Mallik project used the pressure drop method and heating method to conduct an exploitation test in different layers of hydrates.8−10 In the field of pressure drop exploitation, the MDT logging instrument was used in six trial production layers from 974 to 1106.5 m of the Mallik 5L-38 well, in which pressure drop trial mining in MDT-1, MDT-2, MDT-3, and MDT-5 was a great success, as the pressure drop device. Three short-term pressure drops (for example, MDT-2 carried out three shortterm pressure drops in 0.6, 1.2, and 3.6 h) and pressure return test were carried out in each trial production layer. The test result showed that the pressure drop method individually can produce natural gas hydrate from hydrate layers of different degrees of saturation and different physical properties. In the field of heat mining, thermal fluid was added to the hydrate layer from 907 to 920 m of the Mallik5L-38 well to induce thermal activation. After 123.65 h of thermal exploitation, a total of 468 m3 of natural gas was obtained from Mallik 5L-38. In addition, 48 m3 of natural gas escaped from the end of trial production to the capping period. The total produced gas was 516 m3.11,12 In 2003, Hot Ice No. 1, a U.S. research project on natural gas hydrates in Alaska North Slope, conducted the first trial well of Alaska for studying natural gas hydrates and trial exploitation. The initial mining method was the pressure drop method. MDT was used to perform the pressure drop test of the hydrate layer. However, the trial mining study at Hot Ice No. 1 well did not achieve its intended goal.13−15 In 2009, hydrate samples were drilled out in the Muli Basin, Qinghai, China. This instance was the first time hydrates were discovered in middle and low latitudes with embedded depths of 133−396 m16−18 (Figure 2). The first trial mining study of the

inner tube of a dual-wall drill pipe (9); thus, the liquid level in the hole remains below the mined bed to decrease the liquid pressure in the mined bed. Therefore, hydrate is separated because of the decreased pressure of the natural gas hydrate bed. The working principle of steam heat mining is as follows: Running water is added to the water tank (1). A water pump (2) is used to transfer water from the tank (1) into the water treatment system (3). Water treated by the treatment system (3) is added to the soft water tank (4). A small pump (5) is used to transfer water from the soft water tank (4) to the electricity steam generator (6). Then, soft water will vaporize under high temperature and pressure from the heating effect of the electricity steam generator (6). When the temperature and pressure of vapor meet the requirements, the steam control valve (7) opens. Vapor from high temperature and pressure reach the mined bed of natural gas hydrates through the annular gap between the inner and outer tubes of the dualwall drill pipe. Natural gas hydrates will be separated under the heating effect of steam. When the pressure of the vapor is lower than the required pressure, the steam control valve (7) will close. When the pressure of the vapor reaches the required pressure again, the steam control valve will open. Thus, heat mining will be conducted in cycles. The working principle of the gas collection system is as follows: The obtained natural gas spills from the layer of earth through the floral tube in the mined bed (11) and will generate a high flow rate with the vapor and water mixture using the pump function of the air pump (13). Then, natural gas is separated into vapor and water by a multiple gas−liquid separating and circulating system comprising a gas−liquid separator (15) and water tank (14). After being filtered by the cartridge gas filter (16), gas enters the gas flow indicator (17) for flow measurement and recording. After separation and recording, part of the pure natural gas will enter the air storage tank (18) for gas collection and another part will enter the ignition unit (19) for an ignition test. 2.2. Key Parameter Design Calculations of SMSGH. 2.2.1. Calculation of Required Heat for Heating the Hydrate Layer. The heating process of steam for the hydrate layer is divided into three stages, including preheating of the hydrate layer, hydrate dissociation, and continuous heating of the hydrate layer. The volume of the design mining layer is V; the volume of hydrate is Vh; and the volume of the occurring bed is Vs = V − Vh.19 The amount of heat of all parts is calculated as follows: (1) In the pre-heating stage of the hydrate layer (temperature of earth layer T0 − T1):

Q 1 = Q s + Q h = ρs VsCs(T1 − T0) + ρh VhC h(T1 − T0) 1

(1)

(2) In the stage of hydrate dissociation:

Q2 = Q d = Figure 2. Gas hydrate core sample acquired at the depth of 139 m.

ρh Vh Mg + nM w

ΔHd

(2)

(3) In the stage of continuous heating of the hydrate layer (temperature of earth layer T1 − T2):

Muli Basin, Qinghai, China, was conducted in 2011 to develop the steam mining system for natural gas hydrates (SMSGH) and to be successfully used in the trial mining project of natural gas hydrates in the Muli Basin, Qinghai, China.

Q 3 = Q s + Q g = ρs VsCs(T2 − T1) + 2

ρh VhMg Mg + nM w

Cg(T2 − T1) (3)

2. DESIGN METHOD FOR SMSGH

(4) The required total heat of steam filled during hydrate dissociation:

On the basis of the occurring state of natural gas hydrates in the Muli Basin, Qinghai, China, and referring to hydrate mining data, this project was proposed, which focuses on steam mining combined with pressure drop mining to exploit natural gas hydrates. 2.1. Working Principle of SMSGH. The pressure drop mining method is used at the beginning of mining. When hydrates cannot be separated because the pressure does not meet the requirement from its decomposition and self-preservation, steam is injected into the hole to enable steam heat mining. The gas obtained from hydrate dissociation is collected by the surface collection system in the mining process (Figure 3). The working principle of pressure drop mining is as follows: In the hydrate mining hole, a submersible pump (12) is placed under the mined bed. The liquid in the hole is discharged to the surface by the

Q a = Q1 + Q2 + Q 3

(4)

When eqs 1, 2, and 3 are substituted in eq 4, Qa can be obtained. 2.2.2. Calculation of Pipe Lost Heat. Pipe lost heat primarily comprises surface pipe lost heat and in-well pipe lost heat. (1) Surface pipe heat loss:

⎛ 1 r r 1 1 ΦL = 2π(Ts − Te)/⎜ + ln 2 + ln 3 λ1 r1 λ2 r2 ⎝ r1α1 ⎞ 1 + ⎟ r3(α2 + αf ) ⎠ 7281

(5)

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Figure 3. Schematic diagram of the SMSGH: (1) water tank, (2) water pump, (3) water treatment system, (4) soft water tank, (5) small pump, (6) electricity steam generator, (7) steam control valve, (8) orifice device, (9) dual-wall drill pipe, (10) non-productive layer bushing, (11) floral tube in the mined bed, (12) submersible pump, (13) air pump, (14) water tank, (15) gas−liquid separator, (16) cartridge gas filter, (17) gas flow meter, (18) gas storage tank, and (19) ignition device.

Figure 4. Equipment drawing of the SMSGH: (1) surface steam heating device, (2) surface gas collection and ignition device, (3) sealing device, and (4) underground mining device. 2.2.3. Calculation of Power of the Electricity Steam Generator. The electricity steam generator is a key part of the entire mining system; it directly determines the mining capacity of the system. On the basis of the required heat of steam in the hydrate layer and pipeline heat loss, the power of the electricity steam generator can be calculated. The heat mining process is a continuous heating process. The heating time is represented by t. Considering the heat loss of connections, gap of insulating layer, and other factors, K1 is the loss coefficient and K2 is the safe coefficient if construction is performed in the plateau section. The power of the heater W is as follows:

(2) In-well heat loss: Heat loss in well primarily originates from the radial direction. The thermal flux of radial heat dissipation is as follows:

Φ′L = +

⎛ t − th dQ = f = (tf − th)/⎜R1 + R 2 + R3 dz U ⎝

⎞ 1 1 + R6 + ⎟ 1/R 4 + 1/R 5 1/R 7 + 1/R 8 ⎠

(6)

(3) Total pipe heat loss:

Q b = Q L + Q ′L = ΦLL1t + Φ′L L 2t

W=

(7) 7282

Q + Qb Q K1K 2 = a K1K 2 t t

(8)

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2.3. Composition of the SMSGH. The SMSGH is an independent system. This system includes mainly three parts: the surface steam heating device, the underground mining and sealing device, and the surface gas collection and ignition device (Figure 4). Among these parts, the surface steam heating device comprises the water tank, water pump, water treatment system, soft water tank, small pump, electricity steam generator, and steam control valve. The underground mining and sealing device comprises the orifice device, dual-wall drill pipe, nonproductive layer bushing, floral tube in the productive layer, and submersible pump. The surface gas collection and ignition device comprises the air pump, water tank, gas−liquid separator, cartridge gas filter, gas flow meter, air storage tank, and ignition device. Table 1 presents the main technical parameters of the system.

pressures are recorded. The optimal steam open and closed pressures can be obtained by comparison. 3.1.2. Results and Analysis of the Laboratory Experiment. The pressure in the pipe inlet was recorded as P1, and the pressure of the outlet was P2. The temperature at 0 m from the inlet was T1; the temperature at 6 m from inlet was T2; the temperature at 12 m was T3; the temperature at 18 m was T4; the temperature at 24 m was T5; and the temperature at 30 m was T6. The experiment was conducted on the basis of four groups of different open and closed pressures of the steam control valve. The first group was 1.0/0.5 MPa. The second group was 1.25/ 0.75 MPa. The third group was 1.5/1.0 MPa. The fourth group was 1.5/0.5 MP. Four air exhaust tests were conducted for each group, and the temperature and pressure data were recorded. The obtained steam temperature and pressure data are shown in Table 2. Steam pressure and temperature are merely part of the criteria for evaluating the steam heating device. In the steam heating process, exhaust and heat can be injected only when the set pressure is reached after heating for some time. The heating and exhaust times are also different because of different set open and closed pressures. The average heating and exhaust times are obtained through steam heating and exhaust times in each cycle to calculate the percentage of the exhaust time in total. The data comparison is shown in Figure 5.

Table 1. Main Technical Parameters of SMSGH total power (kW) maximum steam output (kg/h) maximum steam temperature (°C)

45 54.70 250

working voltage (V) maximum steam pressure (MPa) maximum sucking rate (m3/h)

380 4.0 27

3. EXPERIMENTS OF SMSGH 3.1. Laboratory Experiment of SMSGH. The laboratory experiment is conducted for the surface steam heating device for the following purposes: (1) to test the performance of the steam heating device to ensure that the system can run safely and effectively for a long time and (2) to choose the optimal steam open−close pressure to provide the basis for field experiment parameters. 3.1.1. Laboratory Experiment Project. The key instrument used for the experiment is the electricity steam generator. Its main parameters are as follows: heating power of 40 kW, maximum steam temperature at 270 °C, maximum steam pressure at 6.3 MPa, and working voltage of 380 V. During the experiment, a steel pipe 30 m in length is connected horizontally to the outlet of the electricity steam generator as the test channel of steam. The 50 mm thick polyurethane insulation coating envelops the steel pipe. The pressure sensor is installed at the inlet and outlet of the pipe to the conduct pressure test. Six temperature sensors are installed at 0, 6, 12, 18, 24, and 30 m from the inlet to the test temperature. Different open and closed pressures of the steam control valve are set. The pressure and temperature of the test channel in different open and closed

Figure 5. Exhaust, heating time, and percentage of exhaust time.

The laboratory experiment of the SMSGH was conducted for 35 h. During the entire period, the steam heating device ran smoothly and reached the design requirements. Thus, this device is able to meet field operation requirements. Table 2 shows the higher open temperature of the steam control valve and the

Table 2. Temperature and Pressure Record of the Laboratory Experiment on the Surface Steam Heating Device grouping number 1

number 2

number 3

number 4

data characteristics valve opens maximum temperature valve closes valve opens maximum temperature valve closes valve opens maximum temperature valve closes valve opens maximum temperature valve closes

T1 (°C)

T2 (°C)

T3 (°C)

T4 (°C)

T5 (°C)

T6 (°C)

temperature drop (°C/m)

P1 (MPa)

P2 (MPa)

pressure drop (MPa/m)

101.1 157.5

110.0 160.5

111.0 153.7

107.4 150.9

109.5 150.7

83.8 145.0

0.58 0.42

0.85 0.54

0.73 0.51

0.004 0.001

155.9 125.1 170.4

160.0 139.6 167.7

155.4 132.8 160.0

151.6 134.4 160.4

151.8 119.4 154.2

147.1 105.3 150.2

0.29 0.66 0.67

0.00 1.11 0.75

0.00 0.95 0.68

0.000 0.005 0.002

168.8 123.2 174.7

168.6 133.4 168.9

162.0 123.7 157.8

161.6 125.1 159.0

156.8 114.1 153.0

153.3 105.3 149.9

0.52 0.60 0.83

0.00 1.28 0.84

0.00 1.11 0.76

0.000 0.006 0.003

173.1 158.2 174.4

169.4 139.1 165.4

159.4 120.3 150.2

160.1 118.7 150.3

154.6 108.4 143.4

151.8 85.6 132.3

0.71 2.42 1.40

0.00 1.11 0.88

0.00 0.94 0.76

0.000 0.006 0.004

158.5

161.8

159.1

155.0

154.7

152.6

0.20

0.00

0.00

0.000

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higher steam temperature. Given the same open−close pressure difference, higher open−close pressure causes greater temperature and pressure loss of the steam pipe. The temperature and pressure loss in the first group is the lowest. Figure 3 shows that, given the same open−close pressure difference, a longer open− close pressure and steam open−close pressure result in a longer heating time. Given the same open pressure, a greater open− close pressure difference causes a longer exhaust time and heating time. Therefore, the percentage of exhaust time in heating time determines the heating efficiency. The exhaust percentage in the first group is the highest. Thus, the optimal steam open−close pressure obtained from the laboratory experiment is 1.0/0.5 MPa. 3.2. Field Experiment of SMSGH. 3.2.1. Field Experiment Project. A field experiment of the SMSGH was conducted in the number 1 trial well in the permafrost region located in the Muli Basin, Qinghai, China. Orifices were located near the scientific drilling wells DK-2, DK-3, and DK-4. This project commenced in May 2011 and was completed in October 2011. The leading exploration drill was 401.05 m deep. On the basis of the leading exploration hole, the hole was enlarged to 330 m using a Φ215 tricone bit. A Φ156 rock bit was then used to enlarge the hole to the bottom. A Φ168 tube was inserted 330 m deep. Following initial core observation, the 17th layer may contain hydrate. Further core observation revealed that the 17th layer might contain natural gas hydrates. The occurring bed of the hydrate was 110.4−304.28 m in depth. The lithologic characters of the formation where nature gas hydrate is mainly preserved are packsand, siltite, mudstone, oil shale, and a small quantity of medium sandstone. In the rock member of medium sandstone, packsand, and siltite, nature gas hydrate is mainly preserved in pore space and fracture of rock.20 On the basis of the thickness of the hydrate layer, occurrence of lithologic character, and abnormal features, a trial experiment of 10 hydrate layers was attempted after analyzing the distribution characteristics of hydrates. The floral tube was applied to 10 hydrate mining sections. As shown in Figure 6, hydrate abnormal from shallow to deep is in turn divided into 1− 17 layers. Red indicates that this layer drilled the hydrate sample, and yellow indicates hydrate abnormal, with a carried out test hole design to address this situation. To conduct the trial mining process correctly and record the temperature and pressure fluctuation in the heat mining process, sensors were installed to measure the steam intake temperature, steam intake pressure, natural gas outlet temperature, natural gas outlet pressure, and temperature and pressure of hole bottom and gas flow. First, pressure drop mining was executed.21 The submersible pump and dual-wall drill pipe were inserted into the well 320 m deep. The submersible pump was used to extract water from the well. The water level remained below 315 m. Water pressure disappeared, and therefore, natural gas hydrates separated because of the reduced pressure. No new gas was generated after some time. Then, steam heating mining was executed, comprising the heat injection stage and mining stage. In the heat injection stage, the electricity steam generator operated continuously. The pulse exhaust method was used to inject steam into the hole to trigger shock excitation reciprocating-type thermal activation to promote further hydrate dissociation. At the heat injection stage, the inlet of natural gas was closed to increase in-well pressure. When in-hole pressure reached a certain value, heat injection stopped. Then, the mining stage was initiated. The natural gas was extracted to the surface. The in-

Figure 6. Design diagram of the mining test hole.

hole pressure decreased, and therefore, hydrates were further dissociated. This operation was repeated several times to complete steam mining of natural gas hydrates. The obtained natural gas was collected and treated by the surface gas collection and ignition device. During the whole process of the steam heat method mining test, because of the dissociation of hydrate, the liquefy of steam, and the influence of groundwater, the water level in the test well will rise constantly. When the water level surpasses 315 m, the diving pump will extract the redundant water from the well along the inner tube of the dual-wall drill pipe and the diving pump will stop when the water level is below 320 m. In the test, the water level in the test well is always kept below 315 m and does not influence the effect of steam on hydrate. 3.2.2. Results and Analysis of the Field Experiment. 3.2.2.1. Heat Injection Stage. The data from the field experiment conducted on Oct 17 were recorded for analysis. First, the steam device was switched on. The open pressure and closed pressure of the steam control valve were set to 1.0 and 0.5 MPa, respectively. Some 40 min later, the surface steam heating 7284

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decalescence of hydrates near the hole wall lower the environmental temperature, which is referred to as the “induction period of dissociation”. The temperature of the hole bottom increased gradually after undergoing heat injection for 80 min; accordingly, the hydrate was dissociated slowly along the radial layer. The pressure of the hole bottom increased gradually, which is termed the “accelerated period of dissociation”. Thus, hydrates that cannot be separated by means of pressure drop can be further separated by the steam heating method. 3.2.2.2. Mining Stage. After the heat injection stage, the mining stage was executed. The air pump was used to extract natural gas obtained from the trial well. The pumping period lasted 42 min. Figure 9 shows the hole bottom pressure variation curve at the mining stage; the pressure is on a declining curve as a whole with

device began to release exhaust and inject into the hole. A total of eight time injections were made in the heat injection stage. The heating data of the surface steam heating device at the heat injection stage are shown in Table 3. The pressure and temperature change in the hole bottom are shown in Figures 7 and 8. Table 3. Data Record of the Surface Steam Heating Device in the Field Experiment of the Steam Mining System heat injection first heat injection second heat injection third heat injection forth heat injection fifth heat injection sixth heat injection seventh heat injection eighth heat injection total

heating time (min)

exhaust heat injection time (min)

intake pressure (MPa)

intake temperature (°C)

36.0

3.7

0.70

137.4

39.7

4.5

0.83

142.9

36.0

4.2

0.71

141.3

31.0

5.2

0.88

149.4

37.3

4.6

0.71

143.2

21.5

4.9

0.69

144.3

34.2

4.8

0.71

143.7

21.4

6.3

0.72

147.3

257.3

38.9

Figure 9. Hole bottom pressure variation curve at the mining stage.

steam extraction. However, the pressure decreases very slowly. In the wake of natural gas discharge, the declining hole bottom pressure promotes the hydrate dissociation further. Thus, new gas is produced. Figure 10 shows the natural gas flow variation

Figure 7. Pressure variation curve of the hole bottom at the heat injection stage.

Figure 10. Natural gas flow variation curve at the mining stage.

curve at the mining stage; the steam heating mining experiment for natural gas hydrates is successful. Hydrates continuously generated natural gas. The average flow is 4.21 m3/h, and accumulated flow is 3.28 m3. The ignition test was conducted for the natural gas obtained from mining, as shown in Figure 11. Restricted by experimental equipment and site working conditions, the entire mining process lasted only 42 min. During the process, the system ran for 300 min and total energy consumption was 720 MJ. The total obtained natural gas was 3.28 m3. The gas consisted mainly of C1 (approximately 70%), C2 (approximately 10%), and C3 (approximately 19%), and the total energy that it contained was 180 MJ. However, Figure 10 shows that the natural gas flow indicates an increasing trend and much more gas can be obtained. Thus, the known data comparison

Figure 8. Temperature variation curve of the hole bottom at the heat injection stage.

On the basis of the temperature and pressure variation curves, the hole bottom pressure increased continuously and slowly with steam injection. However, during the first 80 min, the hole bottom temperature decreased slightly. Therefore, the hydrate phase transition is an endothermic process. The dissociation and 7285

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Figure 11. Ignition test of SMSGH.

steam heating device for numbers 1−4. This material is available free of charge via the Internet at http://pubs.acs.org.

cannot correctly reflect the capacity usage ratio of the whole system. This aspect requires further study in future experiments.

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4. CONCLUSIONS AND RECOMMENDATIONS (1) The SMSGH has been used successfully in mine natural gas for number 1 trial well in the permafrost region of China. This study marks the first time that natural gas hydrates were mined in the Chinese terrestrial permafrost region. (2) On the basis of previous studies on the hydrate dissociation mechanism, this paper proposed a steam mining method combined with the occurrence conditions of natural gas hydrates in the Muli Basin, Qinghai, China. When the single pressure drop method was used, hydrates occurring in the hole or crack absorbed heat during the dissociation process. This method, thus, froze decomposed water and generated stoppage that affected further hydrate dissociation. The steam injection could not generate high pressure in the hydrate layer. Meanwhile, the carried heat triggered thermal activation to the hydrate layer to avoid self-preservation of hydrate dissociation and realized the combination exploitation of the pressure drop and thermal activation. The heat enlarged the mining scope and promoted further hydrate dissociation. (3) The mining principle was used as the basis for the design of the steam mining system. The key principle included the surface steam heating device, the underground mining and sealing device, and the surface gas collection and ignition device. The steam generator is essential in the function of the mining system. The energy balance equation was used to calculate and set the power of the heater as 40 kW. (4) On the basis of the laboratory experiment, the optimal open−close pressure was −1.0/0.5 MPa. (5) The results of the field experiment showed that, when hydrate dissociation was slower with the pressure drop method, the steam heating method could promote further hydrate dissociation. The method could lengthen stable time and increase gas production. The mining system is reasonably designed and exhibits reliable operation; the system can meet the mining requirements. (6) The experiment demonstrated that the steam mining method could be combined effectively with the pressurized mining method to increase the mined radius and gas production. However, the study on the capacity usage ratio of the whole system remains one-sided and, thus, is not conclusive. Complete experiments are necessary to obtain correct data.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-1394456964. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This paper is supported by the foundation item: China G e o l o g i c a l Su rv e y P ro j e ct ( G Z H L 20 1 10 32 0 a n d GZHL20110326).

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ASSOCIATED CONTENT

S Supporting Information *

Laboratory experiment of the SMSGH and temperature and pressure record of the laboratory experiment on the surface 7286

NOMENCLATURE Q1 = required heat in the first stage (kJ) Q2 = required heat in the second stage (kJ) Q3 = required heat in the third stage (kJ) Qs1 = required heat to increase the temperature of the occurring bed in the first stage (kJ) Qs2 = required heat to increase the temperature of the occurring bed in the third stage (kJ) Qh = required heat to increase the temperature of hydrate in the first stage (kJ) Qd = required heat for hydrate dissociation (kJ) Qg = required heat to increase the temperature of separated natural gas in the third stage (kJ) ρh = density of the natural gas hydrate (kg/m3) ρs = density of the occurring bed of the hydrate (kg/m3) ρg = density of separated natural gas (kg/m3) Ch = specific heat of the natural gas hydrate (kJ/kg) Cs = specific heat of the occurring bed of the hydrate (kJ/kg) Cg = specific heat of natural gas (kJ/kg) Vh = volume of the natural gas hydrate (m3) Vs = volume of the occurring bed of the hydrate (m3) Vg = volume of natural gas (m3) Mg = molecular weight of methane MW = molecular weight of water n = number of water molecules in the molecular formula of the natural gas hydrate ΔHd = decomposition heat of hydrate (kJ/mol) ΦL = thermal flux of the surface pipe Ts = steam temperature (°C) Te = surface temperature (°C) r1 = radius of the pipeline inner wall (m) r2 = radius of the pipeline outer wall (m) dx.doi.org/10.1021/ef3014019 | Energy Fuels 2012, 26, 7280−7287

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r3 = radius of the insulating layer (m) α1 = convection heat-transfer coefficient of steam and the inner wall of the pipeline (W m−2 °C−1) α2 = convection heat-transfer coefficient of atmosphere and the outer wall of the pipeline (W m−2 °C−1) αf = radiation heat-transfer coefficient (W m−2 °C−1) λ1 = heat conductivity coefficient of the pipeline (W m−1 °C−1) λ2 = heat conductivity coefficient of the insulating layer (W m−1 °C−1) Φ′L = thermal flux of the in-well pipeline dQ = radial heat loss of steam in section dz within unit time (kJ/h) U = entire thermal resistance while considering the outer wall surface of the drill pipe as a reference plane tf = temperature of steam (°C) th = temperature of the hydrate layer (°C) R1 = convection heat-transfer thermoresistance of steam condensation (°C/W) R2 = resistance of heat conduction of the drill pipe wall (°C/ W) R3 = resistance of heat conduction in the insulating layer (°C/ W) R4 = convection heat-transfer thermoresistance in the annular gap (°C/W) R5 = radiation thermoresistance in the annular gap (°C/W) R6 = resistance of heat conduction of bushing (°C/W) R7 = convection heat-transfer thermoresistance in annular clearance (°C/W) R8 = radiation thermoresistance in annular clearance (°C/W) Qb = heat loss of the total pipe (kJ) Q′L = heat loss of the surface (kJ) QL = heat loss of the in-well pipeline (kJ) L1 = length of the surface (m) L2 = length of the in-well pipeline (m) t = mined time (h)

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dx.doi.org/10.1021/ef3014019 | Energy Fuels 2012, 26, 7280−7287