Experimental study on kinetic behaviors of natural gas hydrate

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Experimental study on kinetic behaviors of natural gas hydrate production via continuous simulated seawater injection DEXIANG LI, Shaoran Ren, Yan Xu, and Hongxing Rui Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01642 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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Experimental study on kinetic behaviors of natural gas hydrate production via continuous simulated seawater injection Dexiang Li,*,† Shaoran Ren,‡ Yan Xu,‡ Hongxing Rui† †School ‡School

of Mathematics, Shandong University, Jinan 250100, China

of Petroleum Engineering, China University of Petroleum (East China), Qingdao

KEYWORDS: Natural gas hydrate; Continuous seawater injection; Sand-pack model; Kinetic behavior; Threshold temperature

ABSTRACT: Natural gas hydrate (NGH) is a kind of potential energy with shallow buried depth, high energy density, huge reserves, and cleanliness. In this study, continuous seawater injection is adopted for NGH production experiments with applying a self-designed reactor which can simulate a NGH-bearing reservoir with a mini well, given that the surface seawater stores tremendous heat. Continuous seawater injection for NGH production can keep the balance between productivity and sand production through controlling production pressure with its thermodynamic and technical feasibilities. Kinetic behaviors of NGH production by continuous seawater injection are investigated using the simulated NGH-bearing reservoir. Meanwhile, the dissociation of NGH and its influence factors are analyzed. Threshold value of temperature for

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NGH dissociation is also discussed. The experimental results show that the changes in temperature and pressure keep constant at the initial status of continuous seawater injection process. However, temperature and pressure show obvious variation with injecting more seawater, which increase firstly and decrease subsequently. Especially, methane production rate shows a high level after the temperature exceeded threshold value for NGH dissociation. But the methane production rate drops quickly after short period of high level and keeps a low level until the end of experiment. Maximum value of methane production rate and cumulative methane production become higher with the presence of larger overheat and NGH saturation. Existence of threshold value of temperature for NGH dissociation is demonstrated by the experimental works. Minimum threshold values of temperature for NGH dissociation vary with the presence of different corresponding reservoir pressures.

1.INTRODUCTION Energy supply is an important pillar for development of global economy and society. However, environmental problems associated with consumption of traditional fossil fuels have aroused widespread concern. In recent years, the concept of sustainable development enjoys popular support. The development and application of clean energy have undoubtedly solved or alleviated the contradiction between economic development and environmental protection.1 Natural gas hydrate (NGH) is a kind of non-stoichiometric cage crystalline compound which is composed of natural gas and water. Under low temperature and high pressure, NGH can form with gas molecules (mainly methane) filling into the cage constructed by hydrogen bond action of water molecules through van der Waals force.2 1m3 of NGH can release approximately 164 m3 of natural gas and 0.8 m3 of water under standard conditions.3,4 The combustion of natural gas will release less CO2 than that of coal or oil for producing the same heat. The energy dependence


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transition from coal and oil to natural gas is also conducive to the mitigation of greenhouse gas emissions. The global NGH resource is about 2×1016 m3, which is about twice the total carbon content of conventional fossil energy (including petroleum, coal and natural gas).5 NGH is a kind of clean fossil fuel and widely distributed in seabed sediments and land permafrost with its characteristics of shallow buried depth, high energy density, huge reserves, and cleanliness.6 At present, the research and application of NGH experimental and pilot production methods mainly focus on the methods of depressurization,7-10 thermal stimulation,11,12 chemical inhibitor injection,13 CO2 replacement14-17 and direct flue gas injection.18 The relatively stable existence of NGH is due to the special temperature and pressure conditions of its structure. Hence, NGH will decompose into natural gas and water if the conditions are disturbed by the external stimulations which cannot meet the requirements for the stability of NGH. The production methods mentioned above are based on the stability conditions of NGH. By applying external disturbance on NGH-bearing layer, NGH is no longer stable and natural gas (mainly methane) can be achieved from the decomposition products of NGH. For example, the depressurization method can decompose NGH and obtain natural gas by lowering the pressure of NGH-bearing layer below the equilibrium pressure through extracting formation water or other fluids. Especially, it has thermodynamic feasibility that injecting CO2 or mixture of CO2 and N2 (flue gas) into NGHbearing layer to replace methane. The CO2 replacement method or direct flue gas injection for NGH production has both economic and environmental significance. The Paris Agreement was negotiated at the 2015 United Nations Climate Change Conference (COP21) and 174 countries signed the agreement in New York on 22 April 2016, which raises global awareness on greenhouse gas control once again.19 However, due to the requirements of the contact between


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CO2 and NGH during the replacement process, the contact area and the diffusion of CO2 in NGH-bearing layer restrict the replacement efficiency. The overview of field pilot NGH production projects around the world is listed in Table 1. It can be seen that depressurization method is a feasible way for NGH production. It is noteworthy that China in 2017 completed the pilot production of NGH in the South China Sea, which extracted more than 300,000 m3 after a 60 day non-stop production operation with the highest daily output at 35,000 m3. Some breakthroughs were made during the production process, ranging from production methods to environmental protection and no pollution to the environment or geological hazards occurred. Table 1. Overview of field pilot NGH production projects around the world

Year

Location

Reservoir Properties

Mackenzie Delta, Canada

Permafrost regions Sandy gravel NGH-bearing layer

2002 2007

2008

2012

2013

2017

North Slope, Alaska, United States

Permafrost regions Sandy gravel NGH-bearing layer

Nankai Trough, Japan

Ocean sediments Coarse sandy NGH-bearing layer

Shenhu area of the South China Sea, China

Ocean sediments Muddy siltstone NGHbearing layer

Method

Production Time

Cumulative Gas Production

Thermal stimulation

5 days

463 m3

Depressurization

12.5 hours

830 m3

Results

References

Low efficiency Termination with poor wellbore stability and sand production Highest daily output within 2000-4000 m3

14

Depressurization

6 days

13000 m3

Replacement + Depressurization

1 month

24000 m3

Low efficiency

14

119000 m3

About 20000 m3/d Termination with poor wellbore stability and sand production

20

300000 m3

Average production of more than 5000 m3/d Highest daily output at 35000 m3

21

Depressurization

Depressurization

6 days

60 days


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In conclusion, NGH production using depressurization method has good prospect of application. Nevertheless, the depressurization method depends on the propagation of pressure disturbance in NGH-bearing layer with the endothermic decomposition process, which can induce the reformation of NGH or the phenomenon of icing that will block near-wellbore area or the production pipeline.8 The dissociation of NGH will weaken formation consolidation at some extent and increase the risk of submarine geologic hazards.22 More importantly, the NGH production by depressurization method will inevitably face the problem caused by the contradiction and balance between productivity and sand production or formation stability. Although it is helpful to improve the productivity by increasing the production pressure difference, the excessive production pressure drop is bound to cause engineering problems such as sand production and wellbore collapse. The problem of sand production restricts the continuous production of NGH seriously. The projects especially in Mackenzie Delta, Canada and Nankai Trough, Japan show that sand production is a key factor limiting the long-term exploitation of NGH resources. 71% of the earth surface is covered by the ocean and seawater has a huge reserve. The sea surface temperature is relatively high with the average surface temperature exceeding 17 ℃. Furthermore, the average surface temperature of many sea areas exceed 20 ℃.23 Hence, the surface seawater stores tremendous heat. Although traditional thermal stimulation methods have energy-consumption problem that cannot be ignored during the process of heating and heat transport, this method is still widely concerned given its thermodynamic and technical feasibilities. Li et al.24 analyzed the energy efficiency of NGH dissociation based on thermal stimulation through experimental work using 2D sand-pack model. Their study showed that the energy efficiency can be influenced by geological parameters and thermal stimulation


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parameters. Wang et al.25 investigated the NGH dissociation process by thermal stimulation with different well-spacing using the Cubic Hydrate Simulator. The experimental results indicated that optimization of well-spacing can be obtained when it equals to the maximum range of NGH dissociation. Meanwhile, the maximum range for NGH dissociation using thermal stimulation was verified. Li et al.26 studied the dissociation driven force for NGH in porous media simulated by sand-pack model, and showed that the driving force will enhance with increasing the hotbrine injection rate and temperature when thermal stimulation method is applied. Yang et al.27 applied a 3D middle-size reactor to study the production behaviors from the simulated NHGbearing layer by hot-water cyclic injection. Their research exhibited that the energy efficiency ratio increases with the increase of the sand saturation and hydrate sample temperature but decreases with increasing the temperature of injection fluid and well pressure. Zhao et al.28 adopted an 2D axisymmetric model to analyze the heat transfer effects during the NGH production process by thermal stimulation with all the roles of sensible heat, conductive heat, and convective heat considered into the analysis. The results showed that the production rate can be inhibited by increasing the specific heat capacity of NGH-bearing porous media or initial water content with the relatively weak effects of the initial water content on the gas production. Fitzgerald and Castaldi12 investigated the dissociation behavior of NGH in sand sediment by thermal stimulation on a large lab-scale reactor with volume of 59.3 L, and showed that better production performance can be obtained with higher initial NGH saturations while higher peak efficiency rates displayed with the application of greater heating rates. Phirani and Mohanty29 simulated the NGH production process from confined NGH-bearing layer using warm water flooding and depressurization method. Their study demonstrated that warm water injection shows better performance than depressurization with the presence of high injection temperature


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and high production pressure. Improved injection pressure increases the heat influx to the NGHbearing layer accelerating the gas production. Traditional thermal stimulation methods such as injection of hot water and steam, can disturb the stability of NGH significantly via heating the injection fluids artificially. With their thermodynamic feasibility, researchers pay much attention to these NGH production methods and a lot of research works have been carried out. However, it will consume huge energy using the thermal stimulation methods for NGH production which increases its costs and restricts a large-scale application. Surface seawater stores tremendous heats which can provide the driving force for NGH dissociation potentially. It will improve the economic feasibility and weaken the limitation of heating injection fluids artificially with injecting surface seawater for NGH production directly. Hence, it is meaningful to investigate the kinetic behaviors of NGH production by injecting seawater directly. Large quantities of NGH are present in marine sediments along the coastlines of many countries as well as in arctic regions. It is undoubtedly an economic and environmentally friendly method to utilize the heat carried by seawater for producing the submarine NGH through continuous seawater injection into NGH-bearing reservoir. On the one hand, the heat of surface seawater can be fully utilized to disturb the NGH stable zone in order to produce natural gas with the dissociation of NGH. On the other hand, the production pressure drop will be reduced as low as possible by controlling the production pressure, which can decrease risk of sand production and contribute to continuous NGH production. In this paper, a thermal stimulation method for NGH production is adopted which is called continuous seawater injection. A lab-scale reactor is established to simulate the NGH-bearing reservoir with a mini well. Kinetic behaviors of NGH production by continuous seawater injection are investigated by experimental work using the simulated NGH-bearing reservoir. The


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typical changes of temperature, pressure and gas production rate are exhibited during the dissociation process using continuous simulated seawater injection. The dissociation of NGH and its influence factors are analyzed by continuous seawater injection. Threshold value of temperature for NGH dissociation is also discussed in this study. 2. EXPERIMENTS 2.1. Experimental apparatus. The schematic of the self-designed experimental apparatus is shown in Figure 1. A stainless reactor with natural quartz sand as solid filling materials is adopted to simulate the NGH-bearing reservoir which can exhibit the properties of porous media.

Figure 1. Schematic of the experimental apparatus for NGH production using continuous seawater injection


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The reactor is 200 mm in internal diameter, 400 mm long and has a volume of 12.51 L with the pressure tolerance up to 15 MPa. A water jacket connecting to the refrigerating circulator is used to refrigerate the reactor, which can provide temperature as low as -10 ℃. Mini well consists of injection tube and wellbore which is installed in the center of the reactor. The wellbore is slotted for the production of gas and water with 15 mm in diameter and 380 mm in length. The slotted holes are covered by meshes to control the sand production. The injection tube is 6 mm in diameter is placed inside the wellbore to provide the path for transporting the simulated seawater into the NGH-bearing reservoir. Pressure and temperature sensors are applied to capture the pressure and temperature inside the reactor with the precisions of 0.01MPa and 0.1 ℃ respectively. The injection power is supplied by a constant flow pump with maximum working pressure of 42 MPa and pumping flow range from 0~50 mL/min. Piston containers can transmit the power provided by the constant flow pump to inject seawater and methane into the simulated NGH-bearing reservoir. One of the three piston containers is set into the air bath for transmitting seawater with different temperatures. Back-pressure valve is connected to the production outlet of the simulated wellbore in order to control the production pressure during the continuous seawater injection process. Produced fluid is separated into gas and water by gas-liquid separator. The gas production rate and cumulative gas production are measured by gas flowmeter. Gas and liquid collectors are used to collect the waste gas and water respectively. All the data is collected by the data acquisition system. 2.2. Materials. Distilled water is prepared in the lab to serve as base fluid for simulated seawater. NaCl is supplied by Sinopharm Chemical Reagent Co., Ltd, China with purity of 99.5%. As the average salinity of the world's oceans is 3.5%, this salinity is selected to simulate the seawater with dissolving NaCl into distilled water throughout the experiments. Methane is


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purchased from Qingdao Tianyuan Gas Co., Ltd, China with the purity of 99.99%. Quartz sands is achieved from Jinshatan beach in Qingdao and sieve size range of 40-60 meshes is selected for making sand-pack model using high efficiency vibrating screen. Antifreezing solution is served as the refrigerating medium which is provided by Qingdao Compton Technology Co., Ltd, China. 2.3. Experimental procedures. A certain volume of methane is pumped into the reactor with the presence of simulated seawater for NGH formation. After completing NGH formation, continuous simulated seawater injection is conducted to dissociate the NGH with the seawater temperature higher than NGH equilibrium temperature. The details of experimental procedures are listed as follows. (1) Quartz sands are washed using distilled water and dried at high temperature before filling into the reactor. During the sand-pack model making process, sands are compressed into the vessel using relatively low pressure manually in order to simulate the loose deposits for bearing NGH. The sand-pack model is vacuumed for discharging air and simulated seawater is injected after conducting vacuumed process. When the pressure inside the reactor is up to 0.1MPa, the simulated seawater injection is terminated with recording the volume of seawater injection. The porosity of manual sand-pack model can be calculated through dividing the volume of simulated seawater injection by the volume of reactor and the resultant porosity is 0.4. More seawater is injected into the sand-pack model until getting the pressure of 10 MPa and the tightness of experimental reactor can be demonstrated by keeping this pressure for 2-3 hours. (2) Flowing line is opened and the pressure of back-pressure valve installed on flowing line is set to 1MPa with decreasing the pressure of the reactor to 1MPa. Methane is injected into the reactor with the flow rate of 10 ml/min at the pressure of 1MPa and temperature of 20 ℃ until


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getting the scheduled injection volume. The experimental parameters are chosen to simply the computational complexity on NGH saturation or other initial parameters. And then the flowing line is closed and more methane is pumped into the reactor continuously before the design pressure is obtained. The whole volume of methane injected into the reactor can be achieved by measuring the water volume collected by the liquid collector located in the end of flowing line. The NGH saturation in the sand-pack model can be adjusted through controlling the injected volume of methane. The methane distribution is optimized by means of rotating the reactor. (3) The temperature is set to the designed value (e.g. 4 ℃) for NGH formation after the pressure and temperature are stable. During the cooling process, the pressure inside the reactor decreases with deceasing the temperature of refrigerating medium. More simulated seawater is injected into the reactor to keep the designed pressure which can also promote forming more NGH. The reactor is rotated frequently after injecting more seawater to ensure the even distribution of methane and seawater. The NGH formation process usually lasts from 3 to 7 days. (4) The back-pressure of production outlet is adjusted equal to the pressure inside the reactor which can demonstrate the feasibility of continuous seawater injection for NGH production robustly without the influence of depressurization. Meanwhile, sand production can also be weakened with balancing the production pressure and pressure of NGH-bearing reservoir. Continuous simulated seawater injection with a constant rate of 10 ml/min is conducted through the injection pipe and simulated seawater is preheated by the air bath to the scheduled temperature. NGH dissociation is initiated with injecting preheated seawater whose temperature exceeds the NGH equilibrium temperature. Production fluid is separated by gas-liquid separator and gas production rate associated with cumulative gas production is measured by gas flowmeter in real time.


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2.4. Determination of NGH saturation. It is vital to calculate the NGH saturation in the simulated NGH-bearing formation for analyzing the NGH production behaviors using continuous seawater injection. The average NGH saturation in the simulated porous media can be evaluated by the following method. According to the equation of state for gas, following equations can be achieved:

PV i gi  nZ i RTi

(1)

PV s gs1  nRTs

(2)

where Vgi, Pi and Ti are the gas volume (mL), pressure (MPa) and temperature (K) in initial condition, respectively; Zi represents the compressive factor in initial condition, dimensionless; Vgs1, Ts and Ps are the gas volume (mL), temperature (273.15K) and pressure (0.1MPa) in standard condition, respectively. Based on eqs 1 and 2, gas volume in current condition can be expressed as:

Vgi 

Z iTV i gs1 Ps PT i s

(3)

Given the NGH formation process is isometric, the whole pore volume in the NGH-bearing reservoir is not changed and the following relationship can be obtained:

V  Vwi  Vgi  Vhi  Vwc  Vgc  Vhc

(4)

where V, Vwi and Vhi are the pore volume, water volume and NGH volume in initial condition with the units of mL, respectively. Vwc, Vgc and Vhc represent the water volume, gas volume and NGH volume in current condition with the units of mL, respectively. In order to decrease the calculation difficulty, water and NGH are regarded as incompressible fluids. It is also assumed that one volume of NGH can release 164 volume of methane gas at standard temperature and pressure with the hydration number of 5.75.3 Vgsh is used to express the


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volume of methane gas participates in the formation of NGH in standard condition with the unit of mL. The eq 4 can be reconstructed as:

Vwi 

Z iTV i gs1 Ps PT i s

 Vhi 

Vwi  w  Vgsh  h /164  Vgsh  g 

w



Z cTc Ps Vgs1  Vgsh  Vgsh  Ts Pc 164

(5)

where ρw, ρh and ρg are the density of water, NGH and methane (kg/m3), respectively. The NGH saturation is defined as the ratio of the volume of NGH and the effective pore volume. Therefore, the NGH saturation in the simulated NGH-bearing reservoir can be shown as:

Sngh 

Vhi  Vgsh /164 V

100%

(6)

It is should be noted that the value of Vhi is zero at the beginning of NGH formation process. 3. RESULTS AND DISSCUSSIONS 3.1. NGH dissociation process using continuous simulated seawater injection. In order to eliminate the influence of depressurization on methane and sand production, the back-pressure of production outlet is adjusted equal to the pressure inside the reactor before conducting continuous seawater injection. The changes on temperature and pressure during continuous seawater injection with simulated seawater in 25 ℃ are shown in Figure 2. As can be seen in Figure 2, the temperature and pressure keep constant at the preliminary status of continuous simulated seawater injection process, which indicates that NGH dissociation is negligible in this stage. With injecting more and more simulated seawater continuously, temperature and pressure show obvious variation, which increase firstly and decrease subsequently. Simulated seawater with relatively higher temperature will warm the NGH-bearing formation and disturb the NGH stability when the temperature is high enough to induce the dissociation of NGH. Meanwhile, the NGH dissociation is an endothermal process, which will


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consume heat to sustain the continuous reaction. Hence, the temperature increases with heat transferred by the injected seawater at the beginning stage. However, the temperature decreases with continuing NGH dissociation process that means the seawater cannot provide enough heats to promote the dissociation and additional heat is supplied by NGH-bearing formation, in view of this process consume a large amount of heat. Methane accumulation induces the increased pressure, and pressure decreases with the methane produces from the NGH-bearing formation when the pressure accumulated by the generated methane gas in NGH-bearing formation exceeds the back-pressure.

Figure 2. Changes of pressure and temperature during NGH production using continuous seawater injection As shown in Figure 3, methane production rate exhibits a high level after the temperature exceeded threshold value for NGH dissociation. The period for high methane production rate can


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sustain 32 minutes. The methane production rate drops quickly after short period of high production rate and keeps a low level until the end of experiment.

Figure 3. Variation of methane production rate with time The high production rate is caused by the relatively large temperature difference between simulated seawater and near wellbore area with enough supply of heat which can induce the NGH dissociation at the early stage of heat conduction. As exhibited in Figure 2, the highest temperature occurs at 124 min during continuous simulated seawater injection and the range of relative higher temperatures is located at 100-200 min, which indicates enhanced driving force for NGH dissociation can be achieved during this period. Seen from Figure 3, the highest methane production rate is achieved at the time of 126 min which is also located within 100-200 min. The postponed occurrence of highest methane production rate (delaying 2 minutes) is induced by the heat transfer process by comparing with the occurrence of highest temperature.


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The presence of higher temperature can provide lager driving force for NGH dissociation which can bring enhanced methane production rate during corresponding period. With injected seawater contacting more volume of reservoir, the increasing temperature of reservoir will consume a part of injected heat which transported by continuously injected seawater. Hence, the heat supply for NGH dissociation will decrease with constant heat transported by injected seawater. This unsteady heat conduction process induces the increase of reservoir temperature which weakens up the heat supply for NGH dissociation. Therefore, the changes of methane production rate curve are decided by the heat supply condition.3,10 3.2. NGH dissociation behaviors and it influencing factors. Continuous simulated seawater injection induced NGH dissociation is controlled by some key factors. Ahead of depicting NGH dissociation behaviors, the overheat is defined as the difference between NGH phase equilibrium temperature (Teq) at corresponding reservoir pressure (Peq) and the temperature of injected seawater (Tsw), which can be expressed as ΔT. For example, the overheat can be plotted in NGH phase equilibrium curve in Figure 4 with the presence of the injected seawater temperature of 17 ℃ at reservoir pressure of 6MPa. Meanwhile, the methane production potential is decided by the NGH saturation in reservoir. Therefore, a set of experimental schemes was designed to investigate the effects of seawater temperature and NGH saturation on methane production behaviors. The details of experimental schemes are shown in Table 2. Table 2. Experimental schemes of NGH dissociation Initial Case Initial Pressure, Temperature, ℃ No. MPa 1 9.4 4.3 2 6.9 3.9 3 7.7 3.5 4 5

5.3 6.1

4.2 3.9

Methane Volume, L 2.6 2.6 1.43

Seawater Temperature, ℃ 20 22 40

NGH Saturation, % 15 20 5

Overheat, ℃ 10 13.7 31

2.4 2.4

37 30

15 15

30 20


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6

8.7

7.9

3

15

30

7

7

8.9

4.1

1.5

35

10

30.9

8

10.0

4.0

2.6

25

20

15

Figure 4. Schematic diagram of overheat plotted in NGH phase equilibrium curve 3.2.1. Influence of overheat. Figures 5 and 6 present the methane production rates and the cumulative methane production in the sand-pack model with the presence of different overheats under the NGH saturation of 15 %. It is should be noted that the threshold values of temperature for NGH dissociation are different due to the specific conditions for NGH formation in different scenarios such as initial pressure, temperature and injected methane volume. Meanwhile, the differences in the start time could also be attributed to hydrate distribution around the wellbore, e.g., longer time was required to see gas production for less hydrate near to the wellbore because heat transfer needs time. As a result, the start time for methane production is not the same in selected experiments. Given this phenomenon, initial time for plotting the cumulative methane


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production is set at the start time for methane production in order to achieve the differences of production behaviors directly.

Figure 5. Methane production rate under different overheats with NGH saturation of 15 % Generally speaking, the curves of methane production rate vs. time under different overheats show similar trends in Figure 5, which accords with the descriptions on Figure 3 in section 3.1. All the methane production rates increase rapidly after the heat transported by injected seawater improved the inner temperature to overpass the threshold value of temperature for NGH dissociation. The methane production rates decrease dramatically and keep the state of low methane production rate after the short period of high methane production rate. Maximum value of methane production rate becomes higher with the presence of larger overheat. Maximum methane production rate can reach to 59.6 L/h at the overheat of 30 ℃. Moreover, the maximum methane production rates at the overheat of 10 ℃ and 20 ℃ are 15 L/h and 29 L/h, respectively.


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As can be seen in Figure 6, cumulative methane production improves with higher overheat. It is worth noting that cumulative methane production at the overheat of 20 ℃ is more than triple of that at the overheat of 10 ℃. However, the difference between cumulative methane production at the overheat of 30 ℃ and 20 ℃ is relatively smaller, especially in the first 150 minutes. Heat transfer efficiency can induce this phenomenon with small difference in heat supply for NGH dissociation at the overheat of 20 ℃ and 30 ℃. Meanwhile, quasi liquid lamellae can be formed during the process of NGH dissociation which limits the diffusion of methane molecules into gas phase.30 Both factors result in the disproportionate growth degree in cumulative methane production with the increase in overheat.

Figure 6. Cumulative methane production under different overheats with NGH saturation of 15 % 3.2.2. Influence of NGH saturation. Figure 7 and Figure 8 exhibit the kinetic behaviors of methane production rate and cumulative methane production with different NGH saturations (5


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%, 10 % and 15%) in porous media at the overheat of 30 ℃. As can be seen in Figure 7, maximum methane production rate becomes higher with the increase of NGH saturation. The maximum methane production rates are 30 L/h, 48 L/h and 59.4 L/h, respectively. Meanwhile, period of obvious methane production rate with higher NGH saturation lasts longer than that with lower NGH saturation. As mentioned before, the NGH saturation represents methane production potential to a certain extent.

Figure 7. Methane production rate under different NGH saturations at overheat of 30 ℃ As shown in Figure 8, enhanced cumulative methane production is achieved with higher NGH saturation. This phenomenon is more significant by contrasting the cumulative methane production under NGH saturation of 15 % with that under NGH saturation of 10 %. The content of NGH in near-wellbore zone decreases with the presence of lower NGH saturation. For continuous seawater injection, the NGH located in near-wellbore zone influences the methane


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production rate and cumulative methane production directly. Therefore, low NGH saturation worsens the NGH distribution in near-wellbore zone with limited amount of NGH for methane production when continuous seawater injection method is applied. With the increase of NGH saturation, the amount of NGH in near-wellbore zone becomes higher with enhanced methane production potential. Hence, the NGH saturation should be taken into consideration carefully before adopting continuous seawater injection.

Figure 8. Cumulative methane production under different NGH saturations at overheat of 30 ℃ 3.2.3. Threshold value of temperature for NGH dissociation. Generally speaking, NGH cannot dissociate immediately with the presence of NGH phase equilibrium temperature at corresponding environmental pressure. The dissociation process will be initiated only when the difference between NGH phase equilibrium temperature at corresponding reservoir pressure and the temperature of injected seawater exceeds a certain value which can be regarded as the


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threshold value of temperature for NGH dissociation. Threshold value of temperature represents the barrier for NGH dissociation using continuous seawater injection at some extent, which indicates that only when the barrier has already been overpassed could the methane produce successfully. Therefore, it can exhibit and analyze the minimum driving force needed for NGH dissociation with adopting continuous seawater injection by testing threshold value of temperature. It is meaningful to measure and study the threshold value of temperature for NGH dissociation. The feasibility of continuous seawater injection can be evaluated by achieving the threshold value of temperature for NGH dissociation at corresponding reservoir condition. At the same time, whether pre-heating technology should be applied or not can be decided by balancing thermodynamic feasibility and economic efficiency. The experimental procedures are as follows. (1) Before studying the threshold value of NGH dissociation temperature, NGH formation is needed to be completed according to the experimental procedures in section 2.3. (2) The continuous seawater injection with the temperature lower than NGH phase equilibrium temperature is conducted to sweep the free gas until no more methane is produced. (3) After the free gas is swept away, continuous seawater injection is conducted using the NGH phase equilibrium temperature at corresponding reservoir pressure. (4) Injection of seawater continues with increasing temperature 0.1 ℃ higher than NGH phase equilibrium temperature. (5) If methane production rate is low, the previous step to increase the temperature is needed to be repeated until achieving the threshold value of temperature for NGH dissociation with continuous increasing tendency of methane production.


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Energy & Fuels

It is notable that pressure change is not taking as a key factor for evaluating the threshold value of temperature for NGH dissociation because of a slight increase on pressure cannot be easily captured with the relatively large volume of sand-pack model. Existence of threshold value of temperature for NGH dissociation is demonstrated by the experimental works. Minimum threshold values of temperature for NGH dissociation vary with the presence of different corresponding reservoir pressures. As exhibited in Table 3, this threshold value of temperature changes from 0.6 ℃ to 2.4 ℃ with increasing the pressure from 5.9 MPa to 10 MPa. Taking Case 1 as an example, the pressure is stable at 5.9 MPa after the formation of NGH in sand-pack model with the corresponding phase equilibrium temperature of 6.4 ℃. Injection of seawater continues with increasing temperature 0.1 ℃ higher than previous temperature each time until achieving a sharp increase in methane production rate. Seen from Figure 9, the methane production rate increases sharply and keeps the tendency to rise when the temperature of injected seawater reaches to 7.0 ℃. Hence, 7.0 ℃ is regarded as initial temperature for NGH dissociation and the threshold value (0.6 ℃) can be achieved with subtracting NGH phase equilibrium temperature (6.4 ℃) from initial temperature for NGH dissociation (7.0 ℃). Table 3. Experimental conditions and results of threshold value of temperature for NGH dissociation

Case No.

Pressure, MPa

Phase Equilibrium Temperature ℃

Initial Temperature for Dissociation,℃

Threshold Value of Temperature, ℃

1

5.9

6.4

7.0

0.6

2

6.5

7.4

9.9

2.3

3

7.5

8.8

11.3

2.4


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4

8.7

10.2

12.0

1.8

5

9

10.5

12.0

1.5

6

10

11.5

12.6

1.1

Figure 9. Methane production rate variations with seawater temperature in Case 1 4. CONCLUSIONS NGH production provide an alternative way of clean energy supply with its characteristics of shallow buried depth, high energy density, huge reserves. Seawater is excellent thermal storage medium which can be continuously injected into NGH-bearing reservoir for methane production with its huge heat potential. Through controlling production pressure, the balance between methane production and NGH-bearing formation stability can be achieved using continuous seawater injection with its thermodynamic and technical feasibilities. In this study, kinetic behaviors of NGH production by continuous seawater injection are investigated using the


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Energy & Fuels

simulated NGH-bearing reservoir. At the same time, the NGH dissociation and its influence factors are analyzed during the continuous simulated seawater injection process. Threshold value of temperature for NGH dissociation is also discussed. The following conclusions can be drawn from the above study. (1) The temperature increases with heat transferred by the injected seawater at the beginning stage. However, the temperature decreases with continuing NGH dissociation endothermal process, which means the simulated seawater cannot provide enough heats to promote the dissociation. Methane accumulation induces the increased pressure, and pressure decreases with the methane producing from the simulated NGH-bearing formation. (2) The high production rate is induced by the relatively large temperature difference between injected seawater and near-wellbore zone with enough supply of heat at the early stage of heat conduction. With injected seawater contacting more volume of reservoir, a part of injected heat which transported by continuously injected seawater will be consumed to increase reservoir temperature and the heat supply for NGH dissociation will be weakened up with sharply drop on methane production rate. (3) Maximum methane production rates with NGH saturation of 15 % at the overheat of 10 ℃, 20 ℃ and 30 ℃ are 15 L/h, 29 L/h, and 59.6 L/h respectively. Cumulative methane production improves with higher overheat. With comparison to the difference between cumulative methane production at the overheat of 20 ℃ and 10 ℃, the difference between cumulative methane production at the overheat of 30 ℃ and 20 ℃ is relatively smaller. Both heat transfer efficiency and formed quasi liquid lamellae during NGH dissociation result in the disproportionate growth degree.


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(4) The maximum methane production rates at the overheat of 30 ℃ are 30 L/h, 48 L/h and 59.4 L/h with different NGH saturations of 5 %, 10 % and 15% respectively. Enhanced cumulative methane production is achieved with higher NGH saturation. The content of NGH in near-wellbore zone decreases with the presence of lower NGH saturation, which indicates that reduced amount of NGH can be provided for methane production during the process of continuous simulated seawater injection.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors would like to acknowledge the National Postdoctoral Program for Innovative Talents (Grant No. BX20180182) for the support of this research. This research is also partially financed by China Postdoctoral Science Foundation Funded Project (Grant No. 2018M640629) and the Shandong Natural Science Foundation (ZR2019BA004). REFERENCES (1) Khalili, N. R.; Duecker, S.; Ashton, W.; Chavez, F. From cleaner production to sustainable development: the role of academia. J. Cleaner Prod. 2015, 96, 30-43.


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CH4


Seawater

Experimental study on kinetic behaviors of natural gas hydrate

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