Evaluation of gas production from methane hydrates

Applied Energy 145 (2015) 265–277

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Review

Evaluation of gas production from methane hydrates using depressurization, thermal stimulation and combined methods Yongchen Song, Chuanxiao Cheng, Jiafei Zhao ⇑, Zihao Zhu, Weiguo Liu, Mingjun Yang, Kaihua Xue Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, School of Energy and Power Engineering, Dalian University of Technology, Dalian 116024, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

Three gas production methods were

The schematic diagram of hydrate decomposition process and the gas production by using different methods.

evaluated with different hydrate saturations. The roles of temperature, pressure, sensible heat and heat transfer were analyzed. The driving force of hydrate dissociation at different stages was analyzed. The combined method effectively improved the gas production and energy efficiency.

a r t i c l e

i n f o

Article history: Received 20 April 2014 Received in revised form 5 February 2015 Accepted 9 February 2015 Available online 3 March 2015 Keywords: Methane hydrate-bearing sediments Energy efficiency Depressurization Thermal injection Combination production Buffer effect

a b s t r a c t To investigate the gas production from methane hydrate-bearing sediments, the gas production processes from methane hydrate in porous media using depressurization, two-cycle warm-water injection and a combination of the two methods were characterized in this study. The methane hydrates were formed in porous media with various initial hydrate saturation (Shi) in a pressure vessel. The percentage of gas production, rate of gas production, and energy efficiency were obtained and compared using the three methods. The driving force of the hydrate dissociation at different stages of depressurization was analyzed and ice formation during the gas production was observed. For the two-cycle warm-water-injection method, the percentage of gas production and the energy efficiency increased with increasing of Shi. However, due to the large amount of warm water needed to heat the porous media at the dissociation site, the percentage of gas production was lower than the other two methods under the same experimental conditions. The experimental results proved that the combined method had obvious advantages for hydrate exploitation over the depressurization and warm-water-injection method in terms of the energy efficiency, percentage of gas production and average rate of gas production, and with increasing of Shi, the advantages are enhanced. For the Shi of 51.61%, the percentage of gas production reaches 74.87%, which had increments of 18.63% and 31.19% compared with the depressurization and warm-water-injection

⇑ Corresponding authors. Tel./fax: +86 411 84706722. E-mail address: [email protected] (J. Zhao). http://dx.doi.org/10.1016/j.apenergy.2015.02.040 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.

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Y. Song et al. / Applied Energy 145 (2015) 265–277

methods. The energy efficiency for the combined method were 31.47, 49.93 and 68.13 for Shi of 31.90%, 41.31% and 51.61%, respectively. Ó 2015 Elsevier Ltd. All rights reserved.

Contents 1. 2.

3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Materials and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Apparatus and materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Hydrate formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. Hydrate dissociation using the depressurization method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3. Hydrate dissociation using the two-cycle warm-water-injection method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4. Hydrate dissociation using a combination of the depressurization method and the warm-water-injection method . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. The gas production under different hydrate saturations using the depressurization method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. The gas production at different hydrate saturations using the two-cycle warm-water-injection method . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Gas production using a combination of the depressurization and warm-water-injection methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. A comparison of the gas production from the three methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Gas hydrates are known to occur worldwide in locations such as the permafrost regions and beneath the sea [1,2]. They have important impacts on flow assurance, safety issues, energy recovery, transportation and climate change [3,4]. Due to this potential resource, the gas production technologies of natural gas from gas hydrate have become of great interest. Currently, various methods of gas production from hydrate reservoirs have been proposed, and most methods are based on breaking the phase equilibrium of gas hydrate, mainly through depressurization method, thermal stimulation method, inhibitor injection method, carbon dioxide replacement method, etc. [5–7]. The obvious gas production approaches involve depressurization, heating and combined methods [8]. The approaches and production methodologies that have been investigated cover a wide range of alternatives. However, there are some salient limitations in the state of knowledge [9–11]. It has been found that the least energy-intensive method suggested is the depressurization technique, where the heat of decomposition is provided by the surrounding formation [12]. Field tests at Mackenzie Delta, North Slope, Alaska and Nankai Trough along the Pacific coast of Japan also revealed that depressurization is a promising gas production method from the perspectives of energy efficiency and productivity [13–16]. At 2013, Japan Oil, Gas and Metals National Corporation conducted a flow test from March 12 until March 18 in the first offshore production test off the coasts of the Atsumi and Shima peninsulas using a depressurization method, which had a gas production duration of 6 days and an average gas production volume of 20,000 m3/day [17]. In Canada, Aurora Mallik, a similar field test performed for onshore production was also conducted using a depressurization method in 2007–2008 [18]. However, some key problems are still not clear. The depletion of sensible heat, the low rate of gas production, the formation of ice and the blocking effect are the key problems, which should be further researched to improve the gas recovery efficiency. More details of the depressurization method have being studied in the laboratory. Yousif et al. used a three-phase 1D model

266 267 267 269 269 269 269 269 269 269 272 273 274 275 276 276

implemented using experimental results to describe the dissociation process of methane hydrate in Berea sandstone via depressurization. They predicted the volume of gas and dissociation front location and proved that the resistance of gas production increased during the dissociation process [19]. Sun et al. [20] measured the kinetic data for methane hydrate dissociation at various temperatures and pressures in a sapphire cell apparatus via the depressurizing method. They concluded that when the system temperature was lower than 0 °C the hydrate dissociation was controlled by gas diffusion because of the formation of ice, and the hydrate dissociation process was then treated as a moving boundary problem. Konno et al. use a large reservoir simulator, the Highpressure Giant Unit for Methane-hydrate Analyses to simulate field-like gas production behavior through laboratory experiments. They proved that more in-place methane could be produced when the production pressure was decreased to 2.1 MPa, which is below the quadruple point [21]. For the decomposition of hydrate using the depressurization process, the gas production rate is obviously restricted when there is no heat input due to the strong endothermic effect and small natural heat flux of the hydrate sediments [22]. The sensible heat is insufficient for dissociating all the existing methane hydrate. After exhausting the sensible heat, the gas production rate turns downward because of the lack of hydrate dissociation heat. The gas production rate at this stage stabilizes at a very low level, which would not be economically viable [21]. In addition, the formation of ice and the reformation of hydrate during the decomposition process also have an impact on the gas production [23]. Macrocosmic numerical results also show that for depressurization of gas hydrate sediment in the South China Sea, the hydrate deposit exploitation by depressurization only is not a good method because of the secondary hydrate formation and ice formation at the wellhead during the hydrate dissociation [17,24,25]. Therefore, based on depressurization, the application of thermal stimulation in certain stages of depressurization is one of the effective methods worthy of study [8,25,26]. The thermal injection method could effectively improve the problems that emerged in the depressurization process, such as


the low rate of gas production, the secondary hydrate formation or ice formation, the blocking effect, and the depletion of sensible heat [13,25,27,28]. In the field of thermal stimulation or heating studies, numerical simulations and experiments have been researched. McGuire et al. developed two models and proposed that if the hydrate reservoir has a high permeability or if the reservoir was beneath a body of salt water in an aquifer, then thermal stimulation was the most attractive method of gas production [29]. This method has been researched by Holder et al. [30–32] from a thermodynamic viewpoint and may be viable. They were concerned that such thermal recovery techniques may be problematic in severe environments. Selim and Sloan studied a physical model that described hydrate dissociation under thermal stimulation in porous media. Its results showed that the energy efficiency is dependent on time and is dependent only on the parameters of the system and the boundary and initial conditions. They predicted that higher porosities should yield larger values for the energy efficiency [32]. For the experimental studies, Kamath et al. applied the thermal recovery method to dissociate methane hydrate from the hydrate sediment sample using hot-water injection. They used a warm-water flow over the top surface of the hydrate for the hydrate dissociation and studied the heat transfer of dissociation for the hydrates of different gases [33]. The results from this previous study proved that hydrate dissociation is a heat-transfer-limited process, and the water generated from hydrate dissociation continually forms a thin liquid film on the surface of the remaining hydrates, resulting in a resistance to heat transfer [34]. Tang et al. [10] studied the temperature distributions and flow characteristics of the dissociated gas and water from the hydrate under thermal stimulation in an unconsolidated sediment. The thermal stimulation was usually performed using a single-cycle hot-water-injection process in the literature, and the experiments proved that multiple-cycle warmwater injections have advantages for the energy utilization efficiency over a single cycle [35]. Recently, Wang and Li [36–38] evaluated the thermal stimulation methods (five-spot thermal huff and puff, five-spot water flooding) of producing gas from the gas hydrate reservoir by calculating the energy efficiency in a cubic hydrate simulator. They proposed that the injected warm water effectively removed the secondary hydrate or ice through the pipe connecting to the production well during the depressurization period. They also proved that the energy efficiency changed with the hydrate saturations. In addition, an onshore production test of a hot-water-circulation method (a type of heating method) was selected for producing methane gas from methane hydrate at the Mallik site. In their studies, 50 °C hot water was fed into test wells to heat methane hydrate layers that exist approximately 1100 m below ground. This test succeeded in producing only 470 m3 of methane gas over the five-day production period. However, the hot water circulation method and the heating method need to use another form of energy as a means of producing the energy resource, which is the methane hydrate. As you can easily imagine, the energy efficiency of these approaches is poor. Kurihara et al. [25] compared the theoretical energy efficiency of different production methods, such as depressurizing, depressurizing and heating the wellbore, hot-water huff and puff, hot-water and methanol huff and puff, and hot-water flooding by numerical simulations. They also concluded that, on the basis of the boundary conditions of the eastern Nankai Trough, the economics of additional wellbore heating at a constant 50 °C are almost the same as for depressurization only. Overall, the problems of poor energy efficiency, the unclear relationships between the hydrate saturations and the production methods, and a shortage of measurements using large scale reactors mean that investment in laboratory-based experiments is still necessary [8,25]. The field test proved that using only the depressurization or thermal methods leads to poor efficiency. T Using the combination of these two methods may be more economically feasible

267

[13,24,27,39]. To exploit gas hydrates more effectively and economically and avoid the limitations and disadvantages of using a single method (such as the lack of a heat source in the depressurization method and the low efficiency of the thermal stimulation method), methods that use a combination of multiple methods have been developed in recent years. Demirbas [8] noted that as a production method for natural gas from methane hydrate-bearing layers, depressurization with well-wall heating seems to be effective and economical. Jiang et al. developed a 3D numerical model for gas production from hydrate reservoirs, and the sensitivity analysis of the factors was performed. They proposed that for the Class I hydrate reservoir, when initial hydrate reservoir temperature is very low, gas production from hydrates is recommended using the combination of heat injection and depressurization [40]. Moridis et al. proved that the most promising production strategy for Class 2 hydrates involves a combination of depressurization and thermal stimulation, and it is clearly enhanced by multi-well production-injection systems [41]. Bai and Li et al. proposed a gas production method from hydrate reservoirs using the combination of warm-water injection and depressurization to overcome the deficiency of a single production method. Feng et al. also conducted depressurization and thermal stimulation experiments in porous media using one-dimensional systems [10,42–45]. Based on the combined production methods, physical and mathematical models were developed to simulate hydrate dissociation [22,46,47]. Li et al. employed TOUGH + HYDRATE simulator based on the geological data of the SH2, SH3 and SH7 site in the Shen Hu Area in South China Sea to simulate hydrate dissociation and water/gas production processes with only depressurization and with depressurization combined with thermal stimulation [48]. Moridis et al. [49] also showed that a new horizontal well design using thermal stimulation coupled with mild depressurization yields production rates that appear modest and insufficient for commercially viable production levels. Multiple studies have explored the beneficial effects of combining heating and depressurization. However, experiments using a combined method to exploit gas hydrates with different saturations were rarely reported. More parameters of the experimental investigations need to be characterized to compare the combined methods with other gas production methods. Under these circumstances, laboratory tests continue to play an important role in the investigation of gas production methods. To identify the characteristics of different production methods, this article focus on gas production from hydrate reservoirs in porous media using depressurization and two-cycle warm-water injection and a combinations of these methods in a 5-L reactor with different saturations. The temperature, pressure, gas production rate, and percentage of gas production were characterized in each method, and the relationship between the production methods and the saturations was also discussed in this paper. In addition, the three methods were first evaluated separately. Then, the three methods were compared, and suggestions were provided.

2. Materials and methods 2.1. Apparatus and materials The details of the experimental system have been introduced in the previous studies [50–52]. A schematic diagram of the experimental device is illustrated in Fig. 1. The primary components of the apparatus are a stainless steel reactor, an air bath system and a data collection system. The reactor is 300 mm in internal diameter, 70 mm in inside height and has a volume of 5 L (liter). It is made of stainless steel, which can sustain pressures of up to 20 MPa. There is a movable piston in the reactor that was used

268


Fig. 1. Schematic diagram of the experimental apparatus.

Fig. 2. The distribution of thermal detectors and wells in the reactor.

to press the porous media and change the effective volume of the reactor. The effective volume of the reactor was obtained by using a displacement sensor that is attached to the piston. A gas flow meter (Seven Star Company) that has a precision of ±7.99 102 SL/M (Stand Liter per Minute at 0 °C, 101,325 Pa) was used to obtain the amount of gas injected. The gas could be steadily injected into the reactor at a constant pressure difference, which was used to make the gas flow meter more accurate by using the PID system. The pressure of N2 (Dalian DATE special gas, 99.99%) was used to provide the power for the PID system and back pressure regulator. Uniform quartz glass beads BZ04(AS-ONE corporation; Japan; Diameter, D, 0.4 mm; porosity, U, 0.361) were used to simulate the porous media. The reactor was enclosed in an air bath, which could be maintained at a constant temperature ranging from 20 to 50 °C with a precision of

±0.1 °C. The temperature and pressure profiles were measured by sixteen Pt. 1000 thermal resistance thermometers and three pressure sensors, which have precisions of ±0.1 °C and ±0.1 MPa, respectively, placed on top of the reactor at different positions, as shown in Fig. 2. The temperature of the warm water was controlled using a high-precision thermostatic water bath (F25-ME, Julabo Inc., Germany) with a temperature controlled range from 28 °C to 200 °C and a precision of ±0.01 °C. The warm water was pumped into the reactor by a metering pump. The pipe for the warm water flow was capped with thermal insulation foam to avoid heat loss. Before the warm water flowed into the reactor, there was a thermal resistance thermometer to detect the effective temperature of the water injected in the reactor. Fine holes are distributed of the surface of the wells, which avoids sand clogging the pipe’s annulus. The produced gas in the porous media could flow into


the well through the holes uniformly. The details of the position of the thermal resistance thermometers and wells are shown in Fig. 2.

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0.2 SL/M and the pressure of the reactor returns to the back pressure, we assumed that the depressurization process finished at this time.

2.2. Procedures 2.2.1. Hydrate formation Prior to all experiments the reactor was cleaned with deionized water. The dry quartz sand (BZ-04) was tightly filled in the reactor and the quartz sand in the reactor was saturated with deionized water. Then, the reactor was evacuated for 10 min to remove air using a vacuum pump. To keep the same initial experimental conditions, the air bath system was first turned on and the temperature in the reactor was kept at 18 °C. The CH4 was charged into the reactor and removed three times to displace any remaining air in the reactor. Next, CH4 was injected into the reactor until the pressure increased to the given pressure values, which was higher than the hydrate equilibrium pressure corresponding to 3 °C (the working temperature) based on the fugacity model of Li et al. [53]. Finally, the air bath system was kept at 3 °C. During the experiment, the total amount of injected gas was recorded using the gas flow meter. The hydrate was formed at the constant temperature of 3 °C. The materials used in the experiment are shown in Table 1, and the experimental conditions and parameters (Mass of sand, MS; Volume of deionized water, VW; Initial pressure, Pi; Initial temperature, Ti; Back pressure, Pb; Final temperature, Tf; and Volume of injected gas, Vg) were given in Table 2. To compare the three methods (depressurization, two-cycle warm-water injection and a combination of these methods), the same three hydrate saturated samples that formed in the reactor were used. 2.2.2. Hydrate dissociation using the depressurization method In this paper, the percentage of gas production is defined as the volume ratio of produced gas and the total injected gas [12,54]. And the methane gas was produced under the experimental back pressure which is 2 MPa. The average rate of gas production is obtained from the ratio of the volume of methane gas production and the consumed time. Three groups (Runs 1–3) of different saturated hydrate decompositions were conducted using the depressurization method. After the hydrate formation, the temperature of the air bath was kept at 3 °C. The back pressure was set at 2 MPa. Then, the well-0 was opened. The dissociated gas initially flows through a drying bottle and then into the gas flow meter, which was used to detect the rate of gas production and accumulated gas. When the gas production rate decreased to Table 1 Experimental materials. Sample

Specification

Supply

CH4 Water Quartz sand

99.99% Deionized water D – 0.4 mm, U – 0.361, q – 2.6 g/cm3

Dalian DATE special gas Laboratory AS-ONE corporation (Japan)

Table 2 Experimental conditions and parameters. Run.

MS (g)

Vw (ml)

Pi (MPa)

Ti (°C)

Pb (MPa)

Tf (°C)

Vg (SL)

1 2 3 4 5 6 7 8 9

4020 4020 4020 4020 4020 4020 4020 4020 4020

920 920 920 920 920 920 920 920 920

6.02 7.01 8.02 6.03 7.02 8.1 6.03 7.03 8.01

18 18 18 18 18 18 18 18 18

2.0 2.0 2.0 3.2 3.2 3.2 2.0 2.0 2.0

3. 3 3 3 3 3 3 3 3

163.21 204.92 231.52 163.21 204.92 231.52 163.21 204.92 231.52

2.2.3. Hydrate dissociation using the two-cycle warm-water-injection method Decomposition tests for three groups (Runs 4–6) of different saturated hydrates were conducted using the two-cycle warm-water-injection method. There are three stages, which are water injection, closing of the well and gas production [37]. The three stages were repeated for two cycles. After the hydrate formation, the temperature of the air bath was kept at 3 °C. The back pressure was set to 3.2 MPa. The temperature of the injected warm water was 60 °C. The rate of the water flow and the cyclic injection time were 20 ml/min and 10 min, respectively. The first cycle of warm water was injected from well-1. After the warm water injection, the reactor was kept closed, which lasted 10 min. Then, the valve of the production well-0 was opened. The gas flowed out from the back pressure. The gas also first flowed through a drying bottle, then into the gas flow meter. For the second warm-water injection, the first warm-water-injection process was repeated. After the rate of gas production decreased to 0.2 SLM/min, the first gas production cycle finished. Then, the first warm-water cycle was repeated from well-2 for the second cycle. When the gas production rate decreased to 0.2 SL/M and the pressure of reactor returned to the back pressure, the warm-water-injection process finished. Fine holes are distributed over the wells and the wells are inserted into the hydrate sediment. Therefore, the warm water could permeate into the hydrate sediment uniformly around the well. 2.2.4. Hydrate dissociation using a combination of the depressurization method and the warm-water-injection method Decompositions tests on three groups (Runs 7–9) of different saturated hydrates were conducted using a combination of the depressurization method and the warm-water-injection method. To compare with the two-cycle warm-water-injection method, the total amount of warm water and temperature were the same as in the two-cycle warm-water-injection method. The velocity of the flow was 10 ml/min. and the total injection time was 40 min. After the hydrate formation finished, the temperature of air bath was kept at 3 °C. The back pressure was set at 2 MPa. The temperature of the warm water was 60 °C. Well-0 was used to produce gas, and the warm water was pumped into well-1. The gas flowed out though the drying bottle and the gas flow meter under the setting back pressure (2 MPa). When the gas production rate decreased to 0.2 SL/M and the pressure of reactor approached to the back pressure, the combined production process was assumed to be finished. 3. Results and discussion Three different saturated hydrate samples were formed by controlling the amount of methane injected into the reactor. In this study, the initial hydrate saturations (Shi) of the hydrates were calculated using the model of Linga et al. [37,55]. The parameter value of density of the hydrate, porosity of the BZ-04 and hydration number is 0.918 g/cm3, 0.361, and 6.1 respectively [56]. The Shi in porous media are shown in Table 3. The hydrate distribution in the porous media was assumed to be homogeneous. 3.1. The gas production under different hydrate saturations using the depressurization method The experimental parameters and results of hydrate dissociation using depressurization are shown in Table 4 (Runs 1–3).

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Table 3 The different Shi sample in this study. No.

Mass of BZ04 and water (g)

Moles of methane in hydrate (mol)

Volume of hydrate (cm3)

Volume of interspace (cm3)

Hydrate saturation (%)

A B C

4940 4940 4940

2.14 2.77 3.46

278.64 360.84 450.81

873.49 873.49 873.49

31.90 41.31 51.61

Fig. 3 shows the percentage of gas produced from hydrates with different Shi using the depressurization method. The results indicate that the percentage of gas production increased dramatically in the early stages. However, at the end of the gas production stage, due to the decrease in driving force of the phase equilibrium, the rate of gas production decreased, and the percentage of gas production stopped increasing and remained constant. Note that at the different Shi, it is observed that the percentage of gas production decreased with increased of Shi at the earlier stage, which was marked in Fig. 3 (approximately 0–40 min). This result is mainly induced by the sensible heat in the free water and the permeability of the hydrate sediments. In the depressurization method, the sensible heat in the hydrate, free water and the associated porous rock provides the heat required for the decomposition [57]. In the hydrate sediments at low Shi, the amount of free water is greater than in the highly saturated sediment. Therefore, the hydrate sediment at low Shi required more heat to compensate for the endothermic effect of the hydrate dissociation. In addition, the high Shi would result in a low permeability. Then, the low permeability would suppress the flow of decomposed gas, which can also decrease the rate of gas production. The percentage of gas production and pressure at high Shi using the depressurization method (Run 3) are shown in Fig. 3, and the average temperature in the hydrate dissociation process using depressurization is shown in Fig. 4. Similar to the pressure, the gas production from the hydrate dissociation process shows four different stages, as shown in Fig. 3. The free gas and water were discharged first, and the pressure decreased at the beginning of gas production process (AB stage). According to the phase equilibrium data, the hydrate had not decomposed and free gas in the reactor were produced at this stage. During stage BC, the pressure sharply decreased from 3.2 MPa to 2.3 MPa. The hydrate began to decompose quickly, and the temperature decreased quickly from 1.5 °C to 0.5 °C. As shown in Figs. 3 and 4, the sharp decrease in the temperature and the increase in the percentage of gas production prove that large amounts of CH4 were released from the porous media hydrate during stage BC, and the sensible heat of hydrate sediments supplied the majority of the energy for the heat decomposition of the hydrate. During stage CD, the pressure stops decreasing and remained steady around 2.2 MPa. Because the sensible heat of the hydrate sediments could not compensate for the heat consumed by the endothermic effect of hydrates dissociation,

Fig. 3. The percentage of gas production and pressure of depressurization at different Shi.

Fig. 4. The average temperature of the hydrate dissociation process using depressurization at different Shi.

the rate of hydrate decomposition decreased. The temperature difference between the dissociation area and the surroundings resulted a low rate of heat flow from surroundings. Note that, as the sensible heat in the hydrate sediments had been mainly consumed during stage BC, the temperature decreased below the freezing

Table 4 Experimental results of the hydrate dissociation using depressurization, two-cycle warm-water injection and a combination of the two methods. Run

Saturation (%)

Production time (min)

Total gas production (SL)

Energy efficiency

Average rate of gas production (SL/M)

Percentage of gas production (%)

1 2 3 4 5 6 7 8 9

31.90 41.31 51.61 31.90 41.31 51.61 31.90 41.31 51.61

62.7 84.8 93.9 84.1 129.2 158.1 52.5 82.5 89.7

73.77 106.56 130.21 46.17 68.28 101.13 80.07 127.02 173.33

– – – 18.15 26.84 39.75 31.47 49.93 68.13

1.18 1.26 1.39 0.83 0.71 0.70 1.53 1.54 1.93

45.20 52.00 56.24 28.29 33.32 43.68 49.06 61.99 74.87


point (0.14 °C at 2 MPa) [58], as shown in Fig. 4; Ice would form at this stage, which would also result in a decrease in the rate of gas production during stage CD. During stage DE, heat transferred to the reactor from the outer environment (Air bath, 3 °C), the heat consumed by the decomposition of hydrate decreased, and the temperature of the hydrate sediment increased slowly, as shown in Fig. 4. The continuous input of energy compensated for the consumption of heat by the hydrate dissociation, and the remnant hydrate continued to decompose slowly. Overall, at the beginning of the depressurization production process, the free gas and water were discharged. Then, during the second stage (BC), due to the heat transfer (sensible heat of hydrate sediment) and pressure driving forces, the hydrate decomposed quickly. As the temperature decreased during the third stage (CD), however, the sensible heat in the hydrate sediment could not compensate for the consumption of heat due to the hydrate dissociation, and ice formed. The pressure became the main driving force for the hydrate dissociation, and the rate of gas production decreased. As heat transferred from the environment, the temperature increased, but the hydrate still decomposed slowly during the last stage (DE). Note that Oyama et al. [59] constructed a depressurization-induced dissociation model that takes into account the heat and mass transfer effect on the hydrate dissociation characteristics. Their model also proposed four different stages in the process of depressurization at the pressure and temperature trajectory. In the first stage, the hydrate sample is depressurized and free gas is produced, but the methane hydrates are maintained in a stable condition. In the second stage, the methane hydrate becomes unstable and begins to decompose with the continuing depressurization. In addition, the core temperature decreases because of the endothermic hydrate dissociation along the equilibrium curve. In the next stage, the continued hydrate dissociation progresses because of the heat supplied from the area surrounding the core. In the fourth stage, the pressure does not change and the methane hydrate is completely dissociated. Moreover, heat transfer from the area surrounding the core raises the core temperature. It is clear that all the four stages agree with our experimental results. As shown in Fig. 4, the temperature quickly decreased at the beginning due to the hydrate dissociation and reached a minimum, then increased and achieved a stable value. For the different Shi, it is clear that the temperatures decreased below the freezing point, which induced the formation of ice during the production period. The formation of ice blocked the flow of gas and suppressed the hydrate dissociation and gas production, which was also observed by Pang et al. [23,60]. The experimental results suggested that auxiliary heat transfer measures were necessary at this stage and were added during the depressurization method to improve the rate of gas production. Moreover, as shown in Fig. 4, as the Shi increased, the temperature gradient curves decreased at earlier stages. The temperature of the low Shi decreased faster than the highly saturated hydrate. This result also proved that a large amount of the hydrate decomposed quickly in the low Shi, and the percentage of gas produced during the earlier stage increased with an increase in the Shi. Moreover, it also can be observed in Fig. 4 that the temperature for the low Shi is higher than the other two and the temperature was also higher than the water freezing point at the end of gas production. These results proved that the depressurization method with low Shi could effectively improve the production temperature and avoid ice formation, which is advantageous for real engineering applications. In addition, Fig. 5 shows the temperature, rate and average rate of gas production at different Shi using the depressurization method. Fluctuations in the rate of gas production were observed. In contrast to the temperature, the fluctuations appeared after the temperature decreased to the freezing point. The fluctuations in the rate of gas production were closely related to the local

271

Fig. 5. The temperature, rate and average rate of gas production at different Shi using the depressurization method.

formation of ice in the hydrate-bearing sediment. As the temperature decreased, ice formed, and the hydrate decomposition was suppressed, which resulted in a decrease in the rate of gas production. Due to the heat transfer from the hydrate sediments, the hydrate would decompose slowly. Additionally, the production times increased with increasing of Shi, which were 62.7, 84.8 and 93.9 min for Runs 1 to 3, respectively. The average rates of gas production were 1.18, 1.26 and 1.39 SL/M. Under the same environmental temperature and production pressure driving force conditions, the gas production percentage increased with increasing of Shi. However, as the rate of gas production decreased, more time was consumed, which is the main reason that the average rate of gas production decreased as the Shi increased. Konno et al. [21,61,62] conducted numerical simulations using hypothetical methane hydrate reservoir models to investigate gas production behavior from oceanic methane hydrate deposits. In their study, the back pressure of 2.1 MPa was also employed, which is similar to ours [21,62]. Their results also proved that more inplace methane could be produced when the production pressure was decreased to 2.1 MPa, which is below the quadruple point, because the latent heat of ice formation was efficiently used for hydrate dissociation [21]. Based on this conclusion, our study further pointed out that the latent heat of ice formation could partly be used for the hydrate dissociation; however, it also results in a blocking effect, which is proved by the fluctuation of the gas production as shown in Fig. 5. Their simulation results, which our experiments demonstrated as well, showed that the sensible heat is insufficient for dissociating all the existing methane hydrate, especially for highly hydrate-saturated and lower temperature reservoirs. In addition, similar to our results, Konno et al. proposed that after exhausting the sensible heat, the gas production rate turns downward because of the lack of hydrate dissociation heat [21,61,62]. However, hydrate dissociation continues by heat conduction from over- and under-burden layers, and the gas production rate at this stage stabilizes at a very low level, which would not be economically viable. Therefore, we can conclude that at the initial stage, the rate of gas production is higher due to the consumption of sensible heat and no other energy is needed in this period. Because the gas production worsened after the depletion of the sensible heat, other measures need to be taken to improve the gas production.

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3.2. The gas production at different hydrate saturations using the twocycle warm-water-injection method The energy efficiency was used to value the two-cycle warmwater-injection method. The energy efficiency (n) is defined as follows [32,35]:

n¼

V t Mgas C w M w ðT w T o Þ

ð1Þ

where Vt is the cumulative volume of the methane gas production at standard temperature (0 °C) and pressure (101,328 Pa), Mgas is the heating value of the methane gas at standard temperature (273.15 K) and pressure (101,328 Pa) and is equal to 37.64 MJ/m3, Mw is the mass of the injected warm water, Tw is the maximum temperature of the injected water (60 °C), T0 = 3 °C is the temperature of the air bath, and Cw is the specific heat of water and is equal to 4.2 103 J/(kg °C). The energy efficiency is the ratio of the combustion heat of the produced gas to the total input heat. The details of the energy efficiency for the thermal-injection method are shown in Table 4. The percentage of gas production and the average rate of gas production at different Shi using two-cycle warm-water-injection are shown in Fig. 6. The results show that there are two stages of gas production and the total percentage of gas production increased as the Shi increased. Because the energy consumed by the dissociation heat of the hydrate is mainly supplied by the injected warm water in the thermal-injection method, more heat is absorbed from the injected water than from the sensible heat and free water in the low Shi sample, and a smaller amount of heat is consumed by the decomposition of the hydrate than in the highly saturated hydrates. The energy efficiency of the three different Shi samples in the two-cycle warm-water-injection method were calculated using Eq. (1). The energy efficiency of the first cycle were 23.49, 38.95 and 42.85 for Runs 4–6, and the energy efficiency at the second cycle for Runs 4–6 were 13.11, 14.72 and 36.65, respectively. It is clear that the energy efficiency during the first cycle is higher than during the second cycle, and this observation also proved that a high Shi yields much more favorable gas production than a low Shi when using the thermal-injection method. Note that Li et al. [38,63] employed a five-spot water flooding method in a 5.8 L cubic reactor, which is similar to ours. They concluded that the energy efficiency is approximately 20.6, which indicates the

Fig. 6. The percentage of gas production and average rate of gas production at different Shi using the two-cycle warm-water-injection method.

multiple warm-water injection would be a promising gas producing method for the hydrate reservoir. Meanwhile, Fitzgerald et al. [64] calculated the net efficiency of gas production to analyze the energy efficiency of gas production from hydrate-bearing porous media using the thermal method. They also found that the energy efficiency is closely tied to the Shi, as well as to the sensible heat of hydrate sediments. As the hydrate saturation increases, it will result in a concurrent rise in energy efficiency. Both their findings are coincident with our results. Because the saturation of the hydrate during the first warm-water-injection cycle was higher than the second cycle, the first cycle yielded a much more favorable gas production than the second cycle for Runs 4–6, as shown in Table 4. As the Shi increased (Runs 4–6), the accumulated gas production for the first and second cycle are 29.49 and 16.68, 49.55 and 18.73, 54.51 and 46.62 SL (Stand liter at 0 °C, 101,325 Pa) respectively. And the amount of gas produced in the second cycle and the total gas production were significantly enhanced. The result indicated that two cycles of warm-water injection could improve the gas production for the highly saturated hydrates. However, the highly saturated, hydrate-bearing sediment consumes a much longer dissociation time; thus, the average rate of gas production decreased from 0.83 to 0.70 SL/M with increasing Shi. As discussed in the depressurization section, there is a sharp decrease of temperature at the initial stage, which might induce the formation of ice or hydrate and suppress the hydrate dissociation [25,65]. The warm water injection method could effectively improve this problem. However, the concentrated injection of warm water results in the low energy efficiency of the hydrate recovery due to the large energy consumption of the porous media in the hydrate sediments. Li et al. [38] used the TOUGH + HYDRATE simulator to evaluate the methane hydrate dissociation process using the inverted five-spot water flooding method, and the energy efficiency was calculated and compared. Their numerical simulation proved that the remnant hydrate saturation decreased quickly, and then the continued injection of warm water was only wasted on the sensible heat of the porous media, which is in accordance to our results [22]. Moreover, Kurihara et al. [25] developed the state-of-the-art numerical simulator (MH21-HYDRES) for rigorously predicting hydrate dissociation and production behaviors, both at the core and field scales. Their model proposed that the multiple water injection and soaking stage can improve the efficiency of gas production. Based on this, in our study, the warm water was

Fig. 7. The temperature of hydrate dissociation using two-cycle warm-waterinjection method at different Shi.


injected into the reactor in two cycles during the gas production process, and a stage of soaking occurred between the two cycles. Compared with the one-time thermal water injection method, proposed by Wang et al. [66], the average energy efficiency of our studies has an increase of more than 12% for the same Shi and hot water injection rates. This high value of energy efficiency indicated that two-cycle warm water injection and a soaking period could effectively improve the energy efficiency. It is also expected that multiple cycles and soaking periods should be employed in large-scale hydrate sediments. Moreover, the temperature for hydrate production using the thermal-injection method is shown in Fig. 7. The results show that the warm-water injection induced an increase in the local temperature. Then, as the hydrate dissociated and consumed heat, the temperature decreased gradually. During the warm-water injection, the heat of the warm water and the sensible heat in the hydrate formation were consumed mainly by the endothermic effect of hydrate dissociation. Therefore, with the Shi increasing, the temperature of the hydrate sediment during the dissociation process generally decreases. After the first cycle of warm-water injection, the temperatures of the hydrate sediment samples decrease with the Shi (Run 4–6), as shown in Fig. 7. This result indicated that more hydrate decomposed in the highly saturated hydrate sample, which also proved that the gas production increased as the Shi increased. Moreover, note that the temperatures of the entire hydrate dissociation process are above the freezing point (0.24 °C at 3.2 MPa) [58] for the two-cycle warm-water injection. Thus, in contrast to the depressurization method, the warm water injection avoids the formation of ice. In actual applications, the problems that arise from the blockage of rock pores and wellbores because of the formation of hydrates during gas production also would be avoided by the warm-water injection method.

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Fig. 8 shows the percentage of gas production using a combination of the depressurization and warm-water-injection methods. As depicted in Fig. 8, the percentage of gas production increased as the Shi increased and gave values of 49.06%, 61.99% and 74.87%, respectively. Due to the use of warm-water injection, the percentage of gas production increased faster during the period

of warm-water injection (0–40 min). In the low Shi sample (Run 7), the majority of the hydrate quickly decomposed, and the rate of gas production began to decrease after 30 min, which suggested that further injection of warm water would have little effect on the gas production from the hydrate-bearing sediment with low Shi. Moreover, the energy efficiency were calculated and were 31.47, 49.93 and 68.13 for Runs 7–9, respectively. Eq. (1) reveals that as the amount of gas production increases, it will result in a concurrent rise in energy efficiency. For the combined method, the value of energy efficiency obtained in this method is overall (effective) energy efficiency. Moreover, the low back pressure decreased the pressure of hydrate sediments and increased the pressure driving force which could effectively improve the rate of gas production and increase the amount of gas production during the gas production process. Therefore, the driving force of back pressure markedly increased the overall energy efficiency of the gas production from hydrate sediments [25,37]. Wang and Li [36–38] also evaluated the efficiency of producing gas from the gas hydrate reservoir by calculating the energy efficiency. The energy efficiency during the process of gas production was obtained using different production methods. During the experiment with the huff and puff in conjunction with the depressurization method, their results also show that the value of energy efficiency ranges from 15.6 to 41 at the steady process of gas production, which is lower than our results due to the high temperature of injection water (130 °C). Therefore, the injection water with high temperature would yield higher rate of gas production; however, more energy would be consumed by the sensible heat of porous media, which would decrease the energy efficiency [21,28,38,39,67]. The results of the combined method show an obvious advantage in the energy efficiency, especially for the highly saturated hydrate-bearing sediments [21,28,38]. Compared with the warm-water injection method (Run 4–6), the energy efficiency for the combined method (Run 7–9) had increments of 13.32, 23.09 and 28.38, for Shi of 31.9%, 41.31% and 51.61%, respectively. The results of the combined method show an obvious advantage in the energy efficiency, especially for the highly saturated hydrate-bearing sediments. Fig. 9 shows the rate of gas production and the average rate of gas production for hydrates with different Shi using a combination of the depressurization and warm-water-injection methods. Fig. 10 depicts the temperature of hydrates with different Shi using a combination of the depressurization and warm-water-injection methods. As shown

Fig. 8. The percentage of gas production using the combined method at different Shi.

Fig. 9. The rate of gas production from hydrates using the combined method at different Shi.

3.3. Gas production using a combination of the depressurization and warm-water-injection methods

274


Fig. 10. The temperature of the hydrate dissociation using the combined method at different Shi.

in Figs. 9 and 10, at the beginning of gas production, the rate of gas production grew, and the temperature correspondingly decreased quickly. The decrease in the temperature is mainly caused by the concentrated dissociation of methane hydrate in the earlier stage. However, because the heat in the warm water compensated for the heat consumption from hydrate dissociation, the warm-water injection method avoided the sharp decrease in the temperature of the hydrate sediments that the single depressurization method experience. The heat of the injection water also suppressed the formation of ice and the blockage of gas production. The rate of gas production still had fluctuations, but the magnitude of the fluctuations decreased, and the rate of gas production remained higher than the other methods used in the dissociation process. Moreover, as the Shi increased, the average rate of gas production and the percentage of gas production also increased. As shown in Table 4, the average rates of gas production were 1.53, 1.54 and 1.93 SL/M, which are much higher than the other two methods. Because the injected warm water has enough energy to compensate for the dissociation heat, the injection of warm water has a significant effect on the average rate of gas production when combined with the depressurization method, especially for the highly saturated hydrate samples. Furthermore, Falser et al. [22] built a numerical model in one radial dimension, using a fully coupled thermo-hydromechanical code for hydrate-bearing sediments and analyzed the gas production process from hydrates by combining depressurization with heating of the wellbore. Their simulation results indicated that the combined method could supply a more sustained dissociation drive compared to only depressurization or thermal stimulation. In our results, for the high Shi sample (Run 9), as shown in Figs. 8 and 9, the combined method of thermal injection and depressurization resulted in a sustained high rate of gas production even after warm water injection. In addition, for the combined method, 33.1% more gas was produced than for the depressurization method with the same hydrate saturation sample, which indicated that smaller remnant hydrates remained and a higher hydrate recovery ratio was obtained. These results experimentally proved the numerical simulation conclusions of Falser [22]. On the other hand, for the low Shi sample (Run 7), the rate of gas production decreased quickly after the warm water injection period, which was caused by the low remnant hydrate saturation. The continued thermal injection

or depressurization did not promise a larger gas production. Both of these results also coincided with Li’s studies [68]. Therefore, for the low Shi sample because most hydrates have been dissociated at the earlier stage, the sustained dissociation drive has a weak effect on the gas production efficiency, which also can be proved by the low energy efficiency as shown in Table 4. Moreover, as depicted in Fig. 10, the temperature of the reactor remained above the freezing point for most of the dissociation process, which inhibits the formation of ice and hydrates. Moridis et al. [69] also proposed that the short-term application of thermal stimulation, involving the injection of warm water could effectively remove the flow blockages phenomenon caused by the formation of secondary hydrates or ice. Additionally, the slope of the temperature curves decreased as the Shi decreased. The time that the temperature remains below the freezing point is shorter for the more highly saturated samples. This result is mainly caused by the free water content and the porous structure of the hydrate-bearing sediment. Because the sediments with low Shi are much more porous, they have more free water and a higher permeability. A large amount of the hydrates in the reactor could decompose homogenously under the diving force of pressure. The free water and hydrae sediment also consumed heat from the injected warm water, which induced the drastic decrease in the temperature. The experimental results suggested that the combined method could effectively avoid the formation of ice and provide significant advantages for the dissociation of highly saturated hydrates. Moreover, when using samples with identical Shi, the combined method could collect more gas from the hydrate and obtain high values for the average rate of gas production. These advantages are greater for highly saturated hydrates. These experimental results show a promising gas production method from the economic point of view, which can also be used for reference in real engineering applications. 3.4. A comparison of the gas production from the three methods Fig. 11 shows the percentages and average rates of gas production using the depressurization, warm-water-injection and combined methods (Runs 3, 6 and 9). As shown in Fig. 11, compared with only the depressurization or warm-water-injection method, the combined method has significant improvements of 18.63% and 31.19% over the percentage of gas production at the same Shi. As shown in Table 4, the improvements of percentage of gas

Fig. 11. The percentage and average rate of hydrate dissociation using the different production methods.


production decreased to 9.99% and 28.67% for Shi of 41.31%. And the improvements of percentage of gas production came to 3.86% and 20.77% for the Shi of 31.90%. These results indicated that the advantages of the combined method were greater in the high saturated hydrate sample. Because the combined method has a higher percentage of gas production and a shorter production time, the combined method also achieves a better average rate of gas production than the other two methods; these values are 1.39, 0.7 and 1.93 SL/M for the depressurization, warm-water injection, and combined methods, respectively. The high rate of gas production of the combined method has important practical significance for future field studies. The results also clearly show that the percentage and average rates of gas production for the depressurization method are higher than the warm-water injection method, because in the decomposition of hydrates via depressurization, the driving force permeates into the entire hydrate sample and more hydrate could decompose due to heat transfer from the outer environment into the hydrate sediment sample. However, in the thermal-injection method, only the hydrates located near the injection well decomposed rapidly when the warm water was injected into the reactor, and the decomposition ended shortly after the completion of the warmwater injection. In the MH21 program, the first Onshore Production Test was carried out at the Mallik site, and a heating production method test succeeded in producing approximately 470 m3 of methane gas over the five-day production period. The temperature of the hot water was estimated to be approximately 50 °C when it approached the methane hydrate layers. In 2008, methane gas was also collected from methane hydrates using the depressurization method. In this test, MH21 achieved continuous production for approximately 5.5 days. The amount of methane gas produced during the test period was approximately 13,000 m3, much greater than the approximately 470 m3 in the First Onshore Gas Hydrate Production Test, which demonstrated that the depressurization method is effective for producing methane hydrate [70]. Our experimental results also agree with these test results. Moreover, Fig. 12 shows the temperature of gas production in the different production methods. As shown in Fig. 12, the combined method can prevent the formation of ice, which allows a high rate of gas production and total gas production. Additionally, even though the percentage and rate of gas production in the combined method are higher than the other two methods, the

Fig. 12. The temperature of the hydrate dissociation process using the different production methods.

275

temperature of the gas production in the combined method is still between the depressurization and thermal-injection method. Therefore, a combination of the depressurization and warmwater-injection methods could increase the gas production, which also improves the efficiency of the energy utilization of the warm water. Therefore, compared with the depressurization and warmwater-injection methods, a combination of depressurization and thermal injection is a more effective production method. Through the experimental results, it is concluded that for the low Shi reservoirs with sufficient sensible heat, the depressurization method may provide a feasible hydrate dissociation and gas production from an economic point of view. For the thermal injection method, the energy efficiency is the key evaluation parameter to determine the gas production efficiency. The combined method is more efficient especially for high Shi reservoirs. Moreover, the results presented here are supported by the numerical simulation studies of others [25,65,71]. In a word, the choice between the different hydrate gas production methods should be based on the combined evaluation of the production efficiency and energy efficiency according to the hydrate reservoir conditions. 4. Conclusions This paper investigated the gas production from hydrate-bearing sediments using depressurization, two-cycle warm-water injection and a combination of the depressurization and warmwater-injection methods in a 5-L reactor. The roles of the temperature, pressure, sensible heat and heat transfer from the surrounding environment on the gas production were analyzed during the hydrate dissociation process. The characteristics of the different production methods were identified and compared. Based on the experimental study, the following conclusions can be drawn: 1. The experimental results for the depressurization method proved that the depressurization method was efficient in the earlier stage of gas production. However, due to the limitations of sensible heat in the hydrate sediments, the rate of gas production decreased as the temperature decreased. Additionally, a buffer effect was observed during the gas production process that could suppress the decomposition of the hydrate, and heat input improved the production efficiency during the buffer effect stage. 2. Hydrate dissociation via the depressurization process was initially driven by the combination of heat transfer and pressure, and the hydrate decomposed quickly. Then, as the temperature of the hydrate sediments decreased, the sensible heat in the hydrate sediments could not compensate for the heat consumed by the endothermic hydrates dissociation process; the back pressure became the main driving force. Finally, as heat transferred from the surrounding environment, the temperature of the hydrate sediment increased, and the hydrate decomposed slowly. 3. In the two-cycle warm-water-injection method, as the Shi increased, the energy efficiency increased from 18.15 to 39.75, and the energy efficiency during the first cycle was higher than the second cycle. In the low Shi sample, a large amount of the heat from the warm water was used to increase the sensible heat in the porous media. In contrast to the depressurization method, the warm-water-injection method effectively avoided the formation of ice. The blockage of rock pores and wellbores due to the formation of secondary hydrates during gas production can be avoided. However, the percentage and average rate of gas production for the thermal-injection method was lower than the depressurization method.

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4. As the Shi increased, the percentage of gas production and the average rate of gas production in the combined method increased and were much higher than the other two methods. Moreover, the combined method had an obvious advantage in the energy efficiency, especially for the highly saturated hydrate samples. During the production process, the combined method can effectively suppress ice formation. As the Shi increased, the advantages of the combined method were enhanced. The experimental results proved that the combined method is more effective and useful for methane hydrate gas production than the depressurization or two-cycle warm-water-injection methods. This paper provided experimental evaluations of the methane gas production from porous media using depressurization, two-cycle warm-water-injection and combined methods based on different Shi. The experimental results proved that a combination of depressurization and thermal injection is a useful and practical method for gas production from methane hydrate-bearing sediments. In future work, the production pressure and temperature of the injected warm water should be investigated to suppress the formation of ice and secondary hydrates, and to improve the rate of gas production and the energy efficiency.

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