Evaluation of Gas Production from Methane Hydrate Sediments with

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Evaluation of Gas Production from Methane Hydrate Sediments with Heat Transfer from Over-Underburden Layers Chuanxiao Cheng, Jiafei Zhao,* Mingjun Yang, Weiguo Liu, Bin Wang, and Yongchen Song* 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, P. R. China ABSTRACT: To clarify the dissociation characteristics by depressurization with heat flow rate from over-underburden layers (Qov), the effects of different Qov levers on gas production by depressurization were analyzed with various initial hydrate saturations in a 5 L pressure vessel. The ratio of sensible heat of the hydrate sediments to hydrate dissociation latent heat (ΔHSen/ΔHL), the accumulated volume of gas production, the percentage of gas production, and the rate of gas production were obtained and compared. The effects of ΔHSen and Qov on gas production in the fast depressurization stage and the stable temperature stage were analyzed separately during the gas production process. A sharp increase of temperature and pressure was observed which was caused by the latent heat of ice formation during the fast depressurization stage. It is concluded that the Qov has a positive influence on gas production during the stable temperature stage after the total consumption of ΔHSen. The Qov effectively increased the production temperature, rate of gas production, and percentage of gas production under these experimental conditions. With increased Qov, the promotion effects are different depending on ΔHSen and Shi. High Qov had a remarkable influence on the rate of gas production and the percentage of gas production for the high Shi sample. In this experiment, va increased from 1.33 to 2.24 SL/M depending on the Qov, an increase of 68.42%. In addition, with high Qov, the upward migration of free water decreased the thermal conductivity of the hydrate sediments, which would decrease the rate heat flow from Qov. endothermic effect and the small natural heat flux of hydrate sediments. Sun et al. obtained 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.13,14 In addition, the formation of ice and the reformation of hydrate during the decomposition process also have impacts on gas production.15 Therefore, based on depressurization, the application of thermal stimulation in certain stages of depressurization is one of the effective methods worthy of study. In the decomposition of hydrates using the depressurization process, the heat of dissociation is provided by the surrounding sediments.16 As the sensible heat of the dissociation area is consumed in a very short period during methane hydrate dissociation, compensation from over-underburden has a significant effect on gas production. Baghel studied the endothermic decomposition process with microcanonical ensemble molecular dynamics simulations of methane hydrate, and he proved that the hydrate dissociation process is heat transfer limited. In the absence of a continuous supply of heat from the environment, the hydrate decomposition process is more like an adiabatic system, and further hydrate dissociation ceases once the temperature of hydrate sediments reaches the equilibrium temperature of the gas hydrate.17 Kamath18 proved that hydrates dissociate by means of heat conduction from the

1. INTRODUCTION Gas hydrates are solid crystalline compounds in which gas molecules are lodged within the lattice of ice crystal structures, which are stable at high pressure and low temperature conditions.1,2 It is estimated that twice the volume of energy presently stored in conventional hydrocarbons is preserved in the form of natural gas hydrates.3,4 Because of this potential resource, technologies for production natural gas from gas hydrates 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 the gas hydrate, mainly through such methods as depressurization, thermal stimulation, inhibitor injection, and carbon dioxide replacement. The obvious gas production approaches involve depressurization, heating, and combined methods.5−7 Depressurization is considered a promising technique of producing gas from methane hydrate reservoirs.8 The depressurization technique is the least energy intensive and most economically viable method.9 In depressurization, methane hydrates dissociate when the pressure in the production well is maintained below its equilibrium value. Recently, 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, and the duration of gas production was 6 days with an average gas production volume of 20 000 m3/day.4,10,11 In Canada, Aurora Mallik conducted a similar field test performed for onshore production using a depressurization method in 2007−2008.12 However, it has been proven that the gas production rate is obviously restricted when there is no heat input due to the strong © XXXX American Chemical Society

Received: October 29, 2014 Revised: December 27, 2014

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Figure 1. Schematic diagram of the experimental apparatus and the distribution of thermal detectors and wells in the reactor.

underburden layers. Therefore, experimental studies on the heat transfer from over-underburden in the depressurization method are necessary to assess and enhance the understanding of gas production from methane hydrate sediments. To identify the characteristics of heat transfer from overunderburden in the depressurization method, this article focuses on gas production from hydrate reservoirs in porous media using depressurization for different heat flow rates from the top and bottom of a 5-L reactor. The temperature, pressure, accumulated volume of gas production, gas production rate, percentage of gas production, and the ratio of sensible heat to dissociation latent heat were characterized. The effects of different initial hydrate saturations (Shi) and Qov on gas production are also discussed in this paper. In addition, the relationships of gas production, Qov, and the ratio of sensible heat to dissociation latent heat were evaluated in different stages of gas production.

surrounding formations in the depressurization method. The rate of dissociation, however, is controlled by the thermal conductivity of the surrounding formations. Selim and Hong et al.19,20 have proposed that depressurization be used when the thermal conditions of the reservoirs favor heat transfer. The heat flow from overlay can affect the temperature and pressure distribution and the hydrate permeability in porous media, thereby affecting hydrate dissociation. At the eighth International Conference on Gas Hydrates in 2014, Yamamoto made a keynote address on a milestone on the path to real energy resource, and he noted that even if depressurization or inhibitor injection techniques are used, some sort of heat supply is necessary to dissociate hydrate crystals.21 In the case of simple depressurization, the explicit heat of methane hydrate crystals in the usual temperature conditions is not enough to dissociate the entire volume of hydrates, but additional explicit heat from rock and pore fluid mass may dissociate the hydrate. Additionally, heat supply from outside of the methane hydrate concentration by thermal conduction, convection, and advection can continue the dissociation process. Oyama et al.9,22 also proposed that the sensible heat of the dissociation area is consumed quickly, and after consuming the sensible heat the dissociation process progresses at a stable production pressure. The driving force of dissociation is the heat transfer from the surrounding area (heat from top, bottom, and radial formations). In addition, the ratio of sensible heat to dissociation latent heat also has an influence on the depressurization method, as does the heat transfer effect from surrounding formations. Moridis et al. also found effects of heat flow through the over-underburden layers on gas production, and they proved that this heat flow can induce a temperature increase in the hydrate dissociation region.23,24 However, the heat transfer from overburden and underburden layers in the depressurization method has not been fully investigated, and few studies have focused on the aforementioned heat transfer from over-

2. MATERIALS AND METHODS 2.1. Apparatus and Materials. The details of the experimental system have been introduced in previous studies.25 A schematic diagram of the experimental device is illustrated in Figure 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. The radial surface of the reactor was capped with thermal insulation foam to avoid heat transfer with the air bath, as shown in Figure 1. A gas flow meter (Seven Star Company) that has a precision of ±7.99 × 10−2 SL/M (stand liters per minute at 0 °C, 101 325 Pa) was used to obtain the volume of gas injected. The gas could be steadily injected into the reactor at a stable 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 BZ-04 (AS-ONE corporation; Japan; Diameter, D, 0.4 mm; porosity, Φ, 0.361) were used to simulate porous media. The reactor was enclosed in an air bath, which could be B

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Energy & Fuels maintained at a stable temperature ranging from −20 to 50 °C with a precision of ±0.1 °C. The temperature and pressure profiles were measured by 16 Pt. 1000 resistance temperature detector (RTD) and three pressure sensors, which have a precision of ±0.1 °C and ±0.1 MPa, respectively, placed on top of the reactor at different positions, as shown in Figure 2. Fine holes distribute on the surface of the wells,

reactor.6,25 In addition, after the hydrate formation, the reactor was opened to examine the hydrate distribution in the porous media. This similar hydrate formation method has been used in the previous study.25 Therefore, hydrate distribution in the porous media was assumed to be homogeneous. Finally, the air bath system was kept at 3 °C. During the experiment, the total volume of injected gas was recorded using the gas flow meter. The hydrate was formed at the stable temperature of 3 °C. During the hydrate formation stages, the initial pressure was determined by the total injected gas, which is shown in Table 2. On the basis of the increased amount of injected gas, the four different initial pressures are 6, 7, 8, and 9 MPa, respectively. For the same saturation runs, the initial pressures were the same due to the constant amount of gas injection. The formation time of hydrate was around 72 h in the experiments.27 Moreover, after the hydrate formation, the temperature was maintained at 3 °C in 24 h in order to make sure the steady state of hydrate sediments sample.28 The species properties used in this study are shown in Table 1. The experimental conditions and parameters are given in Table 2. To compare the three methods (depressurization, two-cycle warm-water injection, and a combination of these methods), the same four hydrate saturated samples that formed in the reactor were used. 2.2.2. Hydrate Dissociation Process. In this paper, the percentage of gas production is defined as the volume ratio of the produced gas to the total injected gas, and the hydrate recovery is defined as the percentage of the original hydrate that has decomposed. Methane gas was produced under the experimental back pressure which was 2 MPa. The average rate of gas production is obtained from the ratio of the volume of methane gas production and the consumed time. Four different saturated hydrate decompositions were conducted using the depressurization method. During the process of gas production, this study used the temperature of the air bath to control the Qov from the top and bottom surface of the reactor; thus the temperature of the air bath was set at 3, 15, 20, and 25 °C considering the low convective heat transfer coefficient of the air bath. In this study, the Qov was determined by the temperature difference between the air bath and the top and bottom surface of the reactor.1 At the fast depressurization stage, the temperatures of the hydrate sediments and the top and bottom surface of the reactor decreased, and the Qov also changed. After the fast depressurization stages, the temperatures of the hydrate sediments and the top and bottom surface of the reactor became stable, and the temperature difference between the air bath and overunderburden layers was constant at the stable temperature stage. The Qov then was calculated by combining this temperature difference and the convective heat transfer coefficient for forced convective flow over the top and bottom surface of the reactor. The average convective heat transfer coefficient for forced convective air flow over the surface of the top and bottom surface of the reactor was obtained by Newton’s law of cooling at the temperature stable stage, which was 70 w/m2· k.1,29 Thus, for the four different temperatures of the air bath (3, 15, 20, 25 °C), the Qov was 29.67, 148.37, 197.82, and 247.28 W at the stable temperature stage, respectively.29 These different Qov values are referable at the practical field research.1,6,30,31 In addition, the initial temperature of top and bottom of reactor is 3 °C, and this temperature first decreased slightly due to the fast temperature drop of hydrate sediments in the reactor in stage A and then kept stable at stage B. Therefore, the temperatures of the reactor’s top and bottom surfaces were around 3 °C during the gas production process. And the actual temperature difference of hydrate sediments and the top and bottom of reactor was around 3−5 °C during the gas production process. The radial surface of the reactor was capped with thermal insulation foam to prevent heat transfer with the air bath. It is assumed that the heat of

Figure 2. Pressure, temperature, and rate of gas production by depressurization with Shi of 30% (Tair = 3 °C). which prevents sand from clogging the pipe’s annulus. The gas produced 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 Figure 1. 2.2. Procedures. 2.2.1. Hydrate Formation. Prior to all experiments, the reactor was cleaned with deionized water. Then porous media (BZ-04) was partly saturated with water. Before the hydrate formation, the water and the porous media were first mixed homogeneously, and the water was distributed in the porous media homogeneously. The mixed water and porous media were carefully placed within the reactor to ensure a homogeneous sample. 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 15 °C. CH4 was injected into the reactor and removed three times to displace any remaining air. Next, CH4 was injected into the reactor until the pressure increased to a predetermined pressure value, which was higher than the hydrate equilibrium pressure corresponding to 3 °C (the working temperature) based on the fugacity model of Li et al.26 And then the temperature of the air bath decreased slowly to prevent the migration of water in the porous media during the cooling period. During the formation stage, the temperature history curves of 16 temperature detectors in the different positions of the reactor were observed. The phenomenon of the temperature increase and the pressure decrease indicated the formation of hydrate. And it was observed that the onset time of the temperature increase for the 16 temperature detectors, along with the temperature history curves of the same detectors, were consistent in the hydrate formation process, which indicated that the temperatures in the different positions of the reactor were uniform and the hydrate formed uniformly in the

Table 1. Species Properties Used in This Study density, ρ(kg/m3) thermal conductivity, K(W/(m·K)) specific heat capacity, Cp(J/(kg·K))

water

CH4

sands

hydrate

1000 0.58 4190

16 g/mol 0.032 39

2588 1.35 742

913 0.62 2010

C

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Tair (°C)

time (min)

Shi

Vin (SL)

Vt (SL)

Vre (SL)

va (SL/M)

ΔHSen/ΔHL (%)

Td (°C)

ΔHSen (J)

ΔH L(J)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

3.00 15.00 20.00 25.00 3.00 15.00 20.00 25.00 3.00 15.00 20.00 25.00 3.00 15.00 20.00 25.00

43.76 36.34 35.34 34.42 56.76 51.19 49.47 47.38 58.70 52.52 50.05 48.02 83.43 72.11 60.64 57.21

0.20 0.20 0.20 0.20 0.30 0.30 0.30 0.30 0.38 0.38 0.38 0.38 0.48 0.48 0.48 0.48

106.93 106.74 106.87 106.80 126.97 126.81 126.01 126.50 148.30 148.30 148.61 148.45 168.77 168.66 168.25 168.75

60.56 61.74 67.95 68.34 72.77 76.59 78.91 86.56 91.65 95.48 98.06 106.42 111.18 123.03 125.99 128.18

34.94 36.35 34.86 35.17 35.17 34.58 36.16 34.24 36.34 35.64 36.73 35.41 33.48 32.81 36.22 37.04

1.38 1.70 1.92 1.98 1.28 1.52 1.59 1.69 1.45 1.68 1.81 1.90 1.33 1.71 2.08 2.24

21.8 21.8 21.8 21.8 14.7 14.7 14.7 14.7 11.6 11.6 11.6 11.6 8.9 8.9 8.9 8.9

−0.03 −0.02 0.16 0.22 −0.11 −0.01 0.04 0.20 −0.19 −0.14 0.00 0.09 −0.21 −0.07 0.04 0.18

23926 23926 23926 23926 23878 23878 23878 23878 24003 24003 24003 24003 23938 23938 23938 23938

109841 109841 109841 109841 162357 162357 162357 162357 207378 207378 207378 207378 268778 268778 268778 268778

the air bath only transfers from the top and bottom of hydrate sediments. 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 a gas flow meter, which was used to detect the rate of gas production and the accumulated gas. When the gas production rate decreased to 0.2 SL/M and the pressure of the reactor returned to the back pressure, we assumed that the depressurization process was finished at this time. In this study, the total gas produced is lower than the injection volume mainly due to the remnant gas in the reactor. The remnant gas was calculated based on the volume of each component in the reactor, and the volumes of remnant gas are shown in Table 2. It can be found that the combination of the gas remaining in the reactor and the gas produced is equal to the gas injected, which could indicate that the hydrate was totally dissociated in the experiments.32

stopped decreasing and began to increase at the end of stage A, when the rate of gas production remained stable. This result indicated that the Joule-Thomson cooling effect was not obvious after stage A. As the time is short in stage A, the Qov was ignored. So the sensible heat consumed for hydrate dissociation is equal to the heat consumed for hydrate dissociation. Therefore, the rate of hydrate dissociation is related to the rate of sensible heat consumed. Additionally, the temperature drop is related to the drop of the equilibrium temperature, which is dependent on the system’s initial pressure and the back pressure. Thus, the rate of hydrate dissociation is dependent on the rate of pressure drop in stage A. There was a significant temperature increase at the end of stage A, which indicated the formation of ice. We also carried out comparison experiments on the hydrate dissociation process over the freezing point with the same four Shi and a back pressure of 2.5 MPa as shown in Figure 3. The temperature decreased during the hydrate dissociation process; however, the phenomenon of a sharp temperature increase did not appear. These comparative results indicated that the sharp

3. RESULTS AND DISCUSSION Four different saturated hydrate samples were formed by controlling the volumes of methane injected into the reactor. In this study, the cumulative amount of methane gas consumed by the hydrate formation was calculated from the temperature and pressure data, and the Shi was calculated using the model of Linga et al.27,33 The parameter value of the density of the hydrate, the porosity of the BZ-04, and the hydration number are 0.918 g/cm3, 0.361, and 6.1 respectively. The initial saturations of the methane hydrates in porous media are shown in Table 2. 3.1. The Effects of Different ΔHsen/ΔHL Values on Gas Production. Figure 2 shows the temperature, pressure, and rate of gas production in Run 5. According to the consuming conditions of sensible heat of hydrate sediments by dissociation heat, there are two main stages which are marked in Figure 2.34 In stage A, the temperature decreases sharply with the drop of the equilibrium temperature, which indicates that the hydrate is dissociating by consuming the sensible heat in the sediments and surroundings. The Joule-Thomson cooling also induces the temperature to decrease during the fast depressurization stage.6,15,35,36 Both the Joule-Thomson effect and the endothermic reaction of hydrate decomposition contributed to the fast temperature decrease during stage A. To avoid the influence of the Joule-Thomson effect, the rate of gas discharged needs to be slowed down. Note that the JouleThomson effect became weaker gradually due to the increase of the porosity of the hydrate sediments when the hydrate began to decompose. It is found that the temperature and pressure

Figure 3. Temperature of gas production without ice formation for different saturations of hydrate (Tair = 3 °C). D

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8.9%, as shown in Table 2. This means that gas production from highly saturated hydrate sediments requires more additional energy compensation to sustain the continued decomposition of the hydrate and avoid further temperature decline, which would induce the formation of ice and hydrate. In addition, as the ΔHSen was less than ΔHL in this study, sensible heat was not supplied in quantities sufficient to dissociate all the methane hydrate. Therefore, the decomposition of hydrate requires supplementation of energy from environment and heat transfer from over-underburden such as cap and base rock, which has the advantage of gas production from Classic 1 hydrate reservoirs in offshore and onshore environments.6 This effect reflects an advantage of the heat transfer from over-underburden for highly saturated hydrate sediments. As shown in Figure 2, in the depressurization scheme, pressure reduction causes destabilization of hydrates. As the hydrates dissociate, they absorb heat from the surrounding formations and the temperature decreases sharply. The hydrates continue to dissociate and the temperature is under the freezing point. Then, ice forms in the reactor. The latent heat released from the ice formation process increases the temperature of hydrate sediments significantly. At the same time, the rate of gas production remained at a high value, which is approximately 7.2 SL/M, indicating that the rate of hydrate dissociation had not decreased. The formation of ice releases the latent heat of phase change, which increases the pressure of the reactor as shown in Figure 2. Konno also indicated that the depressurization-induced gas production can be accelerated by ice formation during hydrate dissociation at a pressure below the quadruple point at the early stage of gas production.32 As the temperature in the hydrate sediments is lower than the original value, a temperature gradient is thus generated between the hydrates and the over-underburden, and heat flows to the hydrates sample. The rate of dissociation of the hydrate, however, is controlled by the rate of heat flux from the surrounding media and by the thermal conductivity of the surrounding rock matrix. Then, the hydrate decomposed slowly and the ice began to melt gradually with the increase of temperature. Figure 4 shows the volumes and percentages of gas production with various ΔHSen/ΔHL values (Runs 1, 5, 9, and 13). In the stage of fast depressurization, the sensible heat of the hydrate sediments supplied the energy for the heat of hydrate dissociation. At higher values of ΔHSen/ΔHL, less heats is needed from the environment. Therefore, at this stage (about 0−10 min) the percentage of gas production increased with the ΔHSen/ΔHL as shown in Figure 4. These results proved that high ΔHSen/ΔHL can improve gas production at the fast depressurization stage. The temperature during the gas production process is shown in Figure 5. In the gas production process, the temperature of the hydrate sediments quickly decreased to the freezing point, and the sensible heat of the hydrate sediments was consumed first. Then, the temperature of the hydrate sediments increased owing to the formation of ice. The latent heat released from the ice partly compensates for the heat of hydrate dissociation and also partly supports the sensible heat of the hydrate sediments for the temperature increase. In addition, the increase of temperature induces corresponding increase in the pressure and the rate of gas production as shown in Figure 6. Especially for low ΔHSen/ ΔHL, the formation of ice significantly increases the pressure of the system. After that, there is a period in which the

temperature increase stage is ice formation. Because of the temperature decrease, a temperature gradient is thus generated between the hydrates and the overunder burden layers and heat must flow to the hydrates. However, the value of heat flow from over-underburden layers is small because of the low temperature of air bath (3 °C). By keeping the pressure low by removing gas, the hydrates continue to dissociate, thus staying cold. Thus, the temperature gradient and heat flux to the hydrates can be maintained. At the start of the stage B, the temperatures remained stable. Therefore, the hydrate sediments stop supplying sensible heat for hydrate dissociation. The heat for hydrate dissociation in this stage is transferred from air bath. Thus, the model of heat transfer in this stage is different from that in stage A, and the main driving force of dissociation has changed. In this experiment, the difference between the ambient temperature (3 °C) and the hydrate undissociated zone (−0.1 °C) is the heat transfer driving force. Hydrate dissociation is a slow and stable process in stage B. In this stage, the heat for hydrate dissociation transfers though the walls of the reactor, which includes a steel tube and top and bottom plugs. To analyze the effect of sensible heat on gas production, ΔHSen/ΔHL was calculated by the following method. The bulk density is obtained by a volume weighted average in eq 1:9,37 ρb = ρs (1 − φ) + φ(ρw Sw + ρh φS h + ρg Sg)

(1)

where the subscripts b, s, w, h, and g stand for bulk, sand, water, hydrate, and gas. The bulk’s specific heat capacity is modeled by a mass weighted average in eq 2 and the ΔHSen is calculated in eq 3:6 CP ,b =

1 [CP ,sρs (1 − φ) + CP ,wρw φSw + CP ,hρh φS h ρb + CP ,gρg φSg ]

ΔHSen = Cp ,b*mp ,b*(To − Td)

(2) (3)

In this study, the value of Td was defined as the temperature of the hydrate sediments at the stable gas production stage. The value of Td was obtained by the mean value of the temperature history between the stage of the fast depressurization and the end moment of the gas production with the 16 temperature detectors. And then the ΔHSen was calculated based on the mean value of Td. The Td for different runs is given in Table 2. The latent heat of hydrate dissociation per mole ΔH(T) is described by Holder as in eq 4 and the ΔHL is calculated in eq 5:38 ΔH(T ) = 4.184(13521 − 4.0T )

(4)

ΔHL = Vtotal*ΔH(T )

(5)

where Vtotal refers to the total volume of methane including the hydrate sediments sample. The experimental values of Ms, Mw, and MMH were calculated from the sample weight and saturation for each sample, as shown in Table 1. The values of ΔHSen/ΔHL in this study are shown in Table 2. ΔHSen/ΔHL reflects the degree of sensible heat contribution to dissociation. For high values of ΔHSen/ΔHL, the sensible heat of a hydrate sediments sample supplies the most of the energy consumed by the hydrate decomposition, indicating little dependence on energy supply from outside and good economic efficiency. It is apparent that ΔHSen/ΔHL decreases with Shi from 21.8% to E

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Figure 5. Temperature of gas production with various ΔHSen/ΔHL (Tair = 3 °C).

temperature of the air bath was kept at 3, 15, 20, or 25 °C to obtain different rates of heat flow from the over-underburden layers (Qov). It is noted that the rate of heat transfer through the tube is much lower than that through the top and bottom because the tube is encircled by thermal insulation foam, but the top and bottom are in the air of the cold room. With the temperature increase of the hydrate sediments, Qov decreases gradually. Figure 7 shows the temperature during the gas production process. First, the endothermic effect of hydrate decomposition mainly contributes to a temperature decrease. During the period of rapid depressurization (0−7 min), the sensible heat of the hydrate sediments is rapidly consumed by hydrate dissociation. The rate of hydrate decomposition mainly depends on the consumption of sensible heat. During this period, as the total amount of sensible heat is the same, higher Qov could promote more hydrate to decompose. Therefore, the temperature for the high rate of heat transfer decreased more quickly than the small one, which indicated that Qov can effectively promote the rate of hydrate decomposition. Thereafter, low temperature induced the formation of ice, and the latent heat released from the ice increased the whole temperature of the hydrate sediments. Then, it can be observed that the temperature remains almost stable after rapid depressurization. At this stage, the residual hydrate saturation was low and the hydrate decomposed slowly. The stable temperature indicated that the heat flow from Qov equals the heat of hydrate dissociation. Without heat supply from sensible heat, at this period the rate of hydrate dissociation solely depends on the heat transfer from Qov. In addition, it is apparent that the temperature of the hydrate sediments increased with Qov at a stable temperature stage. As shown in Figure 7, after the fast depressurization period, the temperature is above the freezing point especially for the high Qov. Therefore, high values of Qov can increase the production temperature which also can suppress the formation of secondary hydrate and ice. Finally, after the steady temperature period, the hydrate decomposed completely and the temperature of the reactor increased quickly until reaching the temperature of the air bath.

Figure 4. Percentage (top) and volume (bottom) of gas production with various ΔHSen/ΔHL (Tair = 3 °C).

temperature remains steady around of the freezing point. At this period, the residual hydrate dissociated slowly and the rate of gas production was very low. The heat transferred from the environment equals the heat consumed by hydrate dissociation. It is apparent that the temperature during this period decreases with the ΔHSen/ΔHL. Moreover, after the fast depressurization period, the temperature was mostly below the freezing point except for the high ΔHSen/ΔHL (Run 1), and the low temperature can induce secondary hydrate formation and ice formation. Lastly, the hydrate dissociated completely and heat transfer from outside continued to increase the temperature gradually until reaching the initial temperature (3 °C). In addition, the rate of temperature increase decreased with the Shi. These results also proved that the rate of heat transfer from the environments is too slow to promote gas production with low Qov. 3.2. Effects of Varying Heat Flow from Over-Underburden on Gas Production. To research the effect of heat transfer from over-underburden of hydrate sediments on gas production during the decomposition process, four experiments (runs 1−4) were carried out with same hydrate saturation. The F

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Figure 7. Temperature of hydrate dissociation processes with different Qov at Shi of 20%.

Figure 6. Rate (top) and pressure (bottom) of gas production from methane hydrates with different Shi values (Tair = 3 °C).

Figure 8 shows the volume of gas production and percentage gas production. The experimental results proved that high Qov could effectively improve the gas production. Compared with low Qov, the high Qov allowed more volumes of gas production and a higher percentage of gas production. The rate of gas production at the rapid depressurization stage (0−7 min) also increased with Qov, and these results indicated that the higher Qov could improve the rate of gas production. As shown in Figure 7, the sharper decrease in temperature also indicates the increased hydrate dissociation at this stage, which also shows the positive effect of Qov on the hydrate decomposition. In addition, as shown in Table 2, the average increased from 1.38 to 1.98 SL/M as Qov increased, which also proves the promotion effect of Qov. However, it also can be found that with the increase of Qov, the average rate of gas production, volume of gas production, and percentage of gas production increased slowly, which indicates that with the continued increase of Qov, the promotion effect on gas production weaken based on these experimental conditions. Zhao et al. also noted that heat flow from the cap- and base-sediment clearly increases

Figure 8. Volume (top) and percentage (bottom) of gas production from hydrates with different Qov at Shi of 20%.

gas hydrate dissociation, but this effect is diminished as the heat flow is further increased.39 Generally, during the initial G

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increase was mainly induced by the increase of temperature and the rate of hydrate decomposition. The latent heat from ice promoted hydrate decomposition and increased the temperature of the hydrate sediments simultaneously. From the rate of gas production, it can also be concluded that the hydrate largely decomposed at the stage of rapid depressurization with the consumption of sensible heat from the hydrate sediments and the Qov. After that, the residual hydrate decomposed solely from the influence of Qov, and then the rate of gas production decreased gradually. Therefore, it is concluded that Qov can effectively improve the rate of gas production and the total volume of gas production during the rapid depressurization stage and then increase the production temperature. However, as Q ov increases, the influence of Qov on gas production decreases due to the low residual hydrate saturation and the low conductivity of hydrate sediments. 3.3. Effects of Heat Transfer from Over-Underburden on Different Shi. To further research the effects of heat transfer from over-underburden on hydrates with different Shi, four samples different Shi were studied based on the same Qov. The air bath was first set at 15 °C when the depressurization experiments began to produce methane from hydrate and four hydrate saturations levels were investigated (Runs 5−8). The temperature, volume of gas production, and percentage of gas production for the different saturations levels are shown in Figures 10 and 11. At the rapid depressurization stages (0−7

dissociation period, the sensible heat is consumed preferentially, and the hydrate largely decomposes in a short time. Because of the low conductivity of hydrate sediments (0.48 W/ m·k), the heat flow was small in this short time, and the Qov consumed by the hydrate dissociation was also small. After that, the saturation of the hydrate decreased and then the hydrate decomposed slowly, especially with the low initial saturation. The low residual hydrate saturation resulted in a weak effect of high Qov on gas production. In addition, the Qov was partly used to support the heat of the hydrate dissociation and partly consumed by the sensible heat of the hydrate sediments, which also decreased the influence of Qov. Figure 9 shows the pressure and rate of gas production for the different Qov values with Shi of 20%. As shown in Figure 9,

Figure 10. Temperature of the gas production process for various Shi with Qov (Tair = 15 °C).

min), it is clear that the slope of the temperature decreases with the increase of Shi. As shown in Figures 11, the volume of gas production is similar in the initial 10 min, which indicates that the four experiments have the same rate of gas production. Given the same Qov and rate of gas production, the temperature decreased with the Shi due to the increased heat consumed by hydrate dissociation. In addition, at this stage, the low Shi sample yielded the same amount of methane form hydrate due to the combined effect of Qov. The slope of the percentage of gas production also decreased with the Shi. After the rapid depressurization stage, the phenomenon of ice formation also appeared in the four runs. Because of the low conductivity of hydrate sediments, Qov has a weak influence on the inhibition of

Figure 9. Pressure (top) and rate (bottom) of gas production from hydrate with different Qov and Shi of 20%.

the slope of the pressure decreased with the Qov during the rapid depressurization period, and this trend corresponded to the volume of gas production. Then, the phenomenon of pressure increase was also observed after the rapid depressurization period. It is noted that the pressure began to increase at around 2.2 MPa when the temperature began to increase, and the pressure had an increment of 0.1 MPa. The pressure H

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Figure 12. Temperature of the gas production process for various Shi with Qov (Tair = 20 °C).

Figure 11. Percentage (top) and volume (bottom) of gas production from hydrate for different Shi with Qov (Tair = 15 °C).

ice formation, so that the dissociation temperature of the hydrate sediments is mainly determined by the rate of hydrate decomposition. As shown in Figure 10, during the stable temperature period (11−40 min), the temperature remained steadily around −0.2 °C and decreased slightly with the Shi. After the total dissociation of the hydrate, the temperature increased quickly and the temperature also increased with the Shi. As shown in Figure 11, the accumulated gas production and percentage of gas production both increased with the Shi. Then, to increase the effect of Qov on the various Shi, the air bath was set at 20 °C (Runs 9−12) and 25 °C (Runs 13−16). The temperature, volume of gas production, and percentage of gas production for Runs 9−12 are shown in Figures 12 and 13. The changing trend of temperature in runs 9−12 is similar to that in Runs 5−8. It can be observed that the temperature difference between Shi (20%) and Shi (30%, 40%, and 50%) increased. This result was mainly induced by the rate of hydrate decomposition. As shown in Figure 12, the Shi (30%, 40%, and 50%) spent more time in the stable temperature period, which indicates that more time was consumed by the hydrate dissociation. In addition, as shown in Figure 12, after the fast depressurization period, the production temperature was all above the freezing point due to the higher Qov. Moreover, the Qov significantly improved the gas production and the

Figure 13. Percentage (top) and volume (bottom) of gas production from hydrate with different Shi at Qov (Tair = 20 °C). I

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Energy & Fuels percentage of gas production, especially for the low Shi as shown in Figure 13. The percentage of gas production for Run 4 increased to 63.95%, an increase of 7.31% compared with Run 1 due to the improvement of Qov. The temperature, volumes of gas production, and percentage of gas production for Run 13−16 are shown in Figures 14 and

Figure 15. Temperature of the gas production process for various Shi at Qov (Tair = 25 °C).

hydrate sediments increased with Shi after the hydrate dissociation, which could be caused by the migration of residual gas and free water in the sediments. Because the large temperature difference between the top and inside of sediments, the natural convection effect exists, and the free water would move to the top of the sediments. With the low Shi, the free water was transported to the top of the sediments due to the effect of natural convection. The temperatures in the different positions of the reactor were uniform during the hydrate formation stage, which indicated that the hydrate formed uniformly in the reactor, and the free water was distributed in the hydrate sediments homogeneously. However, after the hydrate dissociation with the Qov, it was memorably observed that a large amount of free water gathered at the surface of the porous media when the reactor was opened. Moreover, the free water was produced during the gas production process, which also indicated the phenomenon of the upward migration of the free water. The migration of free water may decrease the effective thermal conductivity of the sediments. However, with the high Shi, the amount of free water is less than the low Shi, and the migration of free water has a weaker impact on the effective thermal conductivity of the sediments, which is also proven by Zhao et al.39 Kneafsey41 noted out that thermal conductivity would be changed due to the changing water saturation during the course of dissociation. Moreover, the hydrate yielded even more gas, and the gas production time dramatically decreased with high Qov. The average rate of gas production (va) was also calculated and compared. Qov has a positive effect on the rate of gas production, especially for the high Shi. On the basis of the same Shi, the va increased with Qov, and this effect was enhanced as Shi increased. As shown in Table 2 (Runs 13, 14, 15, and 16), va increased from 1.33 to 2.24 SL/M, an increase of 68.42%. The results proved significant promotion by Qov on the high Shi. The rate of gas production for various Qov with Shi (48%) is shown in Figure 16. For the high Shi, the ΔHSen/ΔHL of the sediments is small, indicating that additional energy from the environment is needed. Without enough Qov, the rate of gas production quickly decreased after the fast depressurization period, and the residual hydrate decomposed slowly after that. However, with

Figure 14. Percentage (top) and volume (bottom) of gas production from hydrates with different Shi at Qov (Tair = 25 °C).

15. As shown in Figure 14, for the low Shi, the percentage of gas production stops increasing and maintains at around of 63.99%, whereas the percentages of gas production for the high Shi (Runs 8, 12, 16) kept on increasing with the increase of Qov. The percentage of gas production for Run 16 increased to 75.96%, an increase of 10.08% compared with Run 13 due to the improved Qov. It is also can be observed that the production temperatures for the four different Shi values were above the freezing point after the fast depressurization period. The total production time decreased as Qov increased for the same Shi. After the hydrate dissociation, the slope of the temperature curve increased with Shi, which is different from the above phenomenon. This result indicates that the conductivity of sediments changed after the hydrate dissociation, which is also shown by Sparrow et al.40 The thermal conductivity of the J

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the gas production for low Shi samples during the fast depressurization stage. (2) After the fast depressurization period, ice formed and the latent heat released from ice could sharply increase the temperature of sediments. The pressure and the rate of gas production increased correspondingly. (3) Qov effectively increased the production temperature, rate of gas production, and percentage of gas production under these experimental conditions. After the total consumption of ΔHSen, the residual hydrate decomposed solely by the influence of Qov. The rate of gas production revealed that Qov could effectively increase gas production during the stable temperature stage. However, as Qov increases, the influence of Qov on gas production decreases due to the low residual hydrate saturation and the low conductivity of hydrate sediments. (4) With increased Qov, the promotion effect became weaker due to the high ΔHSen/QL of the low Shi sample. However, high Qov markedly increased the rate of gas production and percentage of gas production for the high Shi sample. At high Qov, the upward migration of free water decreases the thermal conductivity of hydrate sediments, which would decrease the rate of heat flow from Qov. In these experiments, the va increased from 1.33 to 2.24 SL/M depending on the Qov, an increase of 68.42%. Moreover, the percentage of gas production for Run 16 increased to 75.96%, an increase of 10.08% compared with Run 13 dependent on the high Qov.

Figure 16. Rate of gas production process for various Qov with Shi of 48%.

heat compensation from Qov the rate of gas production was maintained at a high level for a long time, which also resulted in a high average rate of gas production. Recently, Misyura42 showed that an increase in the density of heat flux from 255 to 13700 W/m2 results in a 9-fold increase in the dissociation rate of methane hydrate. It can be expected that depending on the Qov, various options for hydrate dissociation may exist. Thus, the efficiency of dissociation of methane gas hydrate in sediments depends on the heat flux from Qov, especially when Shi is high. Therefore, it is clear that the Qov effectively increased the production temperature, rate of gas production, and percentage of gas production under these experimental conditions. However, as Qov increased, the promotion became weaker with low Shi due to its high ΔHSen/ΔHL. High Qov has remarkable advantages for the rate of gas production and percentage of gas production for the high Shi sample. In addition, the upward migration of residual free water would decrease the thermal conductivity of hydrate sediments, which would decrease the rate of heat flow from Qov.

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AUTHOR INFORMATION

Corresponding Authors

*(J.Z.) Telephone: +86 411 84706722. Fax: +86 411 84706722. E-mail: [email protected]. *(Y.S.) Telephone: +86 411 84706608. Fax: +86 411 84706608. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Natural Science Foundation of China (Grant Nos. 51006017, 1276028), the Major National S&T Program (No. 2011ZX05026-004), the National Hightech Research and Development Projects (No. 2013AA09250302), and a candidate of Liaoning Province Colleges and Universities Outstanding Young Researcher Foundation (No. LJQ2013009).

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4. CONCLUSIONS In this work, systematic experiments were carried out to clarify the dissociation characteristics by depressurization with heat transfer from over-underburden layers. The effects of different Qov and ΔHSen values on gas production by depressurization were analyzed with various Shi levels. ΔHSen/ΔHL, the accumulated volume of gas production, the percentage of gas production, and the rate of gas production were all evaluated separately in the fast depressurization stage and the stable temperature stage. The following conclusions can be drawn from this study: (1) The sensible heat of hydrate sediments is first consumed for hydrate dissociation by depressurization, which is equal to the heat consumed for hydrate dissociation in the fast depressurization stage. The experimental results proved that high ΔHSen/ΔHL can promote the accumulated gas production and the percentage of gas production and can increase the production temperature. ΔHSen played a positive influence on K

NOMENCLATURE CP,b = specific heat capacity of the bulk hydrate sediments (J /(kg·°C)) ΔHSen = sensible heat of the hydrate sediments (J) ΔHL = hydrate dissociation latent heat (J) mP,b = mass of the bulk hydrate sediments (kg) Qov = heat flow rate from over-underburden layers (W) Shi = the initial hydrate saturation Sw = the pore volume saturation of water in hydrate sediments Sg = the pore volume saturation of gas in hydrate sediments Tair = the temperature of air bath during the gas production process (°C) To = the temperature at the 0 time (°C) Td = temperature of hydrate sediments at the stable gas production stage (°C) va = the average rate of gas production (SL/M) DOI: 10.1021/ef502429n Energy Fuels XXXX, XXX, XXX−XXX

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VW = volume of free water (L) Vg = volume of injected gas (SL) Vtotal = total volume of methane including the hydrate sediments sample (SL) Vt = volume of gas decomposed from hydrate (SL) Greek letters

ρ = density (kg/m3) φ = porosity

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