Dissociation Behavior of Methane Hydrate in Sandy Porous Media

Article pubs.acs.org/EF

Dissociation Behavior of Methane Hydrate in Sandy Porous Media below the Quadruple Point Yoshihiro Konno,*,† Takashi Uchiumi,† Hiroyuki Oyama,† Yusuke Jin,† Jiro Nagao,*,† Yoshihiro Masuda,‡ and Hisanao Ouchi§ †

Production Technology Team, Methane Hydrate Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 2-17-2-1 Tsukisamu-Higashi, Toyohira-Ku, Sapporo 062-8517, Japan ‡ Frontier Research Center for Energy and Resources (FRCER), Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-Ku, Tokyo 113-8656, Japan § Petroleum Engineering & Consulting Department, Japan Oil Engineering Co. Ltd., Kachidoki Sun-Square, 1-7-3 Kachidoki, Chuo-Ku, Tokyo 104-0054, Japan ABSTRACT: To investigate the effect of ice formation on gas production from gas-hydrate-bearing sandy porous media, we conducted dissociation experiments using artificial methane hydrate cores by depressurizing them to below the quadruple point. We prepared water- and gas-saturated hydrate cores to evaluate the influence of water content on ice formation. The experiments showed that gas production under the ice formation regime had a unique high-rate period in the early stage of production, whereas under the water generation regime, the high-rate period was not observed. During ice formation, the gas production rates of the water-saturated cores exhibited greater acceleration than those of the gas-saturated cores. We conducted numerical simulations using the hydrate reservoir simulator MH21-HYDRES for quantitative analyses. The results showed that ice forms faster in a water-saturated core because of the availability of pore water for ice formation. This further enhances the gas production rate of a water-saturated core. Sensitivity analyses indicated that the rate of ice formation and the permeability reduction by ice formation are key model parameters affecting gas production behavior. From the experimental and numerical investigations, we conclude that depressurization-induced gas production can be accelerated by ice formation during hydrate dissociation at a pressure below the quadruple point.

1. INTRODUCTION Gas hydrates are clathrate compounds in which gas molecules are trapped inside a cage of hydrogen-bonded water molecules.1 In oceanic sediments and permafrost regions, significant amounts of hydrocarbons (mainly methane) are stored as gas hydrates. The global estimate of methane trapped in oceanic gas hydrate deposits is about 1−5 × 1015 m3, which is approximately 2−10 times larger than the ultimately recoverable reserves of conventional natural gas.2 Hence, hydratebound methane represents a future energy resource.3 Methane recovery from methane hydrate deposits has been an active area of research. One method of methane production is depressurization, which dissociates gas hydrates by lowering the wellbore pressure below the hydrate stability pressure.4−8 This method is considered to be the most promising because it can achieve the highest energy profit ratio.5 The temperature of hydrate deposits decreases during depressurization-induced production because of an endothermic reaction of hydrate dissociation. When the system pressure is decreased below the quadruple point of the gas hydrates, the temperature drops below the freezing point of water, and the gas hydrates then dissociate into gas and ice instead of gas and liquid water. Ice formation during gas hydrate dissociation potentially has both positive and negative effects on gas production. In terms of thermodynamics, ice formation is desirable because the dissociation heat under the ice formation regime is smaller than that under the water generation regime.9,10 At the freezing point of water, the latent heat of hydrate dissociation under the © 2012 American Chemical Society

ice formation regime is about one-third of that under the water generation regime.11 Therefore, gas hydrate dissociation requires less energy under the ice formation regime. On the other hand, according to fluid dynamics, the presence of ice in the pore space may reduce the permeability of sediments, reducing gas productivity.12 To assess the effect of ice formation on gas productivity, the gas production behavior under the ice formation regime has been studied theoretically and experimentally. Tsypkin constructed a mathematical model of hydrate decomposition by considering ice formation and predicted that an anomalous increase in gas may be produced in the transition of the hydrate decomposition regime at which water and ice are formed simultaneously.9 In contrast, Moridis et al.12 conducted numerical simulations and showed that ice clogging decreased the gas production rate; however, under the right well configuration involving warm water injection to destroy the secondary hydrate or ice around the wellbore, Moridis and Reagan13 showed that rapid dissociation of the remaining hydrate in the reservoir was caused by ice formation due to the pressure dropping below the quadruple point. In laboratory experiments, Zhou et al.10 observed a rapid increase in gas production under the transition decomposition regime from water to ice during pressure reduction process. Haligva et al.14 Received: September 2, 2011 Revised: June 21, 2012 Published: June 26, 2012 4310

dx.doi.org/10.1021/ef300628c | Energy Fuels 2012, 26, 4310−4320

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independently observed increased gas production below the freezing point of water at a constant pressure of 2.3 MPa. As described above, the gas production behavior under the ice formation regime is a relevant topic; however, the effect of ice formation on gas productivity remains unclear. In this study, we report the first experimental and numerical attempts to quantitatively analyze the effect of ice formation on gas production from methane hydrates in sandy porous media. We conducted depressurization experiments using two types of artificial methane hydrate cores: water-saturated and gassaturated. In addition, we conducted numerical simulations to support the experimental interpretation. We evaluated the influence of water content on ice formation and discussed the key model parameters affecting gas production during ice formation.

2. METHODS 2.1. Laboratory Experiments. A schematic of the core holder used in this study, which is the same as that used in Oyama et al.,15 is

Figure 2. Schematic drawing of core with pressure/temperature sensors. was used for the matrix sediments of the cores. The host sediment porosity was 38.1%−40.5%, and the absolute permeability was approximately 15 μm2 (15 Darcy). The porosity and the absolute permeability were determined by the mass balance calculation and the water flooding test after dissociation experiment, respectively. Methane gas of more than 99.5% grade purity was used. Methane hydrate was formed by the excess gas method.16 First, moist sand was packed inside the rubber sleeve. Then, methane gas was injected into the wet core and pressurized to 10 MPa, which is more than the equilibrium pressure of methane hydrate at the system temperature of 275.2 K. The pressure was maintained under hydratestable conditions for a couple of days, allowing hydrate to form. This method is the water limited method in which the core contains hydrate and gas (and unconverted water). We prepared two types of cores: gas-saturated and water-saturated. The gas-saturated core was created by the above-mentioned method. The water-saturated core was obtained by flooding a gas-saturated core with distilled/deionized water to remove the excess gas. Water flooding was conducted under equilibrium conditions. The core properties and experimental conditions are summarized in Table 1. The hydrate, water, and gas saturations were estimated by the material balance. A system of equations for the volume of each phase is expressed as

Figure 1. Schematic of core holder. shown in Figure 1. Triaxial confining pressure was applied to the core by using a syringe pump (Teledyne Isco, 500D). A movable end plug and a rubber sleeve were pressurized through a brine solution pumped out the syringe to control the axial and confining pressures, respectively. The temperature of the core was controlled by circulating a brine solution (Haake, CT50L). The pressure at the production line was maintained using a back pressure regulator valve (TESCOM, 261700 Series 1500 PSI). The core holder was positioned vertically, and the fluid was produced from the top of the core. The volume of gas and the quantity of water produced were measured and recorded (SHINAGAWA, Wet Gas Meter W-NK-1; sartorius, FC6CCE-HX explosion proof scale). The temperatures inside the core were measured using type T thermocouples (CHINO, Type T JIS Class 1), and the pressure inside the core was measured using a fiber optic sensor (FISO Technologies, FOP-M). Figure 2 shows a schematic drawing of the core with the sensors. Thermocouples were inserted into the core center and positioned 25, 75, and 125 mm from the core bottom (opposite to the production end). The fiber optic sensor for pressure measurement was inserted into the core center and positioned 25 mm from the core bottom. The pressures at both ends of the core were also measured using strain gauge pressure sensors (KYOWA, PG-300KU). Cylindrical cores 51 mm in diameter and 150 mm long were prepared. Toyoura standard sand with an average diameter of 220 μm

Vp = VH + Vw + Vg

(1)

VH = NHnH M w /ρH

(2)

Vw = (nw − NHnH )M w /ρw

(3)

Vg = (ng − nH )zRT /P

(4)

Vp, VH, Vw, and Vg are the volumes of pore space, hydrate phase, water phase, and gas phase, respectively. NH is the hydration number of methane hydrate. nw, ng, and nH are the amounts of material of total water, total gas, and methane in hydrate phase, respectively. ρH and ρw are the densities of empty hydrate and water phase, and Mw is the molar weight of water. P, T, R, and z are pressure and temperature of the system, molar gas constant, and compressibility factor, respectively. VH, Vw, Vg, and nH are the independent variables of the system of equations and can be solved when the other properties are determined experimentally or theoretically.17 The residual gas saturation of less than 3% in the water-saturated cores is in contrast with the residual gas saturation of more than 20% in the gas-saturated cores. In Runs 1, 1b, 1c, 2, 2b, and 2c, the initial pressure and temperature were 3.6 MPa and 275.2 K, respectively. To form ice during dissociation, the production pressure was set to 2.1 MPa, which is below the quadruple pressure of methane hydrate. The production 4311


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Table 1. Core Properties and Experimental Conditions saturation (%) Run Run Run Run Run Run Run

1 1b 1c 2 2b 2c 3

initial condition

hydrate

water

gas

porosity (%)

pressure (MPa)

temperature (K)

production pressure (MPa)

69.9 59.1 63.4 68.1 71.4 76.4 63.4

26.8 40.9 36.6 5.2 7.8 1.3 33.6

3.3 0.0 0.0 26.7 20.8 22.3 3.0

39.4 40.0 40.5 39.4 38.1 38.5 39.9

3.6 3.6 3.6 3.6 3.6 3.6 4.2

275.2 275.2 275.2 275.2 275.2 275.2 275.6

2.1 2.1 2.1 2.1 2.1 2.1 2.9

pressure and ambient temperature (temperature at a brine solution for confining pressure) were kept constant. For comparison, an experiment under the water generation condition (above the ice point) was conducted at an initial pressure and temperature of 4.2 MPa and 275.6 K, respectively (referred to as Run 3). The production pressure was set to 2.9 MPa, which is above the quadruple pressure of methane hydrate. The confining pressure was set to 11.5 MPa and kept constant during dissociation in all runs. 2.2. Numerical Simulations. We conducted numerical simulations to quantitatively evaluate the effects of fluid flow and heat flow as well as the kinetics of hydrate dissociation and ice formation. We used the numerical simulator MH21-HYDRES to investigate the effect of ice formation on gas production. MH21-HYDRES is a state-of-the-art hydrate reservoir simulator developed by the University of Tokyo; Japan Oil Engineering Co., Ltd.; and the National Institute of Advanced Industrial Science and Technology.18 Various models are incorporated in this code to describe the phenomena related to hydrate dissociation and formation, ice formation and melting, and fluid flow in sediments, including heat transfer by conduction and convection. MH21-HYDRES has been used for many studies such as analyses of core-scale experiments19,20 and the predictions of reservoirscale production tests.5,7,21 The predictions were validated by comparison studies with the laboratory experiments20 as well as through a comparative study of the world’s leading hydrate reservoir simulators.6 The detailed theories on which the models are based are mentioned in our previous studies.5,7 The cylindrical coordinate system was used for the analyses in this study. The core section was divided into 10 × 60 discrete grids in (r, z). The diameter and length of the core were 50.0 mm and 150.0 mm, respectively, and the core was uniformly divided along the r−z axis. The rubber sleeve (4 × 60 grids) was placed at the periphery of the core to simulate accurate heat flow from the surroundings. The thickness of the rubber sleeve was 10.0 mm. Gas and water were produced from the top of the core, and the surface at the other end and the periphery of the rubber sleeve were set as a no-flow boundary. A heat-transfer coefficient was applied as the thermal boundary condition to all boundary grid blocks for allowing heat exchange from the environment. The input data and model parameters used in this study are summarized in Table 2 with some reference studies.22−25 The initial data reflected the representative settings of laboratory experiments. The model parameters were determined on the basis of previous history matching simulations of laboratory experiments using artificial and natural hydrate cores.17,20 The rate of ice formation and the index of permeability reduction by ice formation were varied for sensitivity analyses. The heterogeneities of the porosity and saturation may affect the dissociation behavior; however, we do not consider the heterogeneities in this simulation study in order to obtain a basic understanding of the effect of ice formation on hydrate dissociation.

decrease after the stop valve was opened to the set pressure of 2.1 MPa. The pressure reached the hydrate equilibrium curve calculated using the CSMGem program of Sloan and Koh1 and then followed it. The temperature decreased to around 272.5 K and then increased to the freezing point of water as a result of ice formation. The pressure and temperature stayed at the quadruple point during hydrate dissociation. After hydrate dissociation, the pressure decreased to 2.1 MPa, and then the temperature increased to the ambient temperature after ice melting. The P−T change in Run 2 (gas-saturated core) showed features similar to those in Run 1, except that the temperature was lower than the equilibrium temperature during depressurization. This is attributed to the Joule−Thompson effect of exiting gas. The temperature reached the quadruple point after the residual gas expanded. Note that the pressure during hydrate dissociation under the ice formation regime stayed at the quadruple point in both Runs 1 and 2, although the setting pressure was maintained at 2.1 MPa. In contrast, the pressure under the water generation regime (Run 3) decreased to the set pressure of 2.9 MPa during hydrate dissociation. This indicates that gas transport was limited to some extent under the ice formation regime. 3.2. Characteristics of Hydrate Dissociation under the Ice Formation Regime. Figure 4 shows the gas production rate with the changes in temperature and pressure in Run 1. The unit N of gas production rate is the normal condition (273.15K, 1 atm). The results show that a rapid temperature decrease occurred in the first 10 min from the top of the core toward the bottom. The temperature decreased below the freezing point of water and then showed a rapid increase as a result of ice formation (from 20 to 60 min). After 60 min, the temperature stayed at the freezing point of water and then gradually increased to the ambient condition because hydrate dissociation and ice melting ended. This trend indicates that the sensible heat of the core and the latent heat of ice formation were supplied in the period between 10 and 60 min. The gas production rate for this period is obviously higher than that for the period after 60 min. This indicates that the in situ heat supplied by ice formation accelerated hydrate dissociation. In the ice formation period, the core pressures decreased and became constant around the quadruple point. Then, the pressures decreased to the production pressure from the bottom. These behaviors suggest that ice formed simultaneously after depressurization and melted from the bottom as a result of heat influx. Figure 5 shows the gas production rate with the changes in temperature and pressure in Run 2. The results show features essentially similar to Run 1. However, the temperature decreased at three points almost simultaneously, indicating that the pressure reduction propagated rapidly. One possible reason is that the low water content in Run 2 yielded a high

3. RESULTS AND DISCUSSION 3.1. Pressure and Temperature Change during Hydrate Dissociation Experiments. Figure 3 shows the pressure−temperature (P−T) change during hydrate dissociation by depressurization. The P−T pathways are indicated by arrows. In Run 1 (water-saturated core), the pressure started to 4312


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Table 2. Input Data and Model Parameters parameter initial pressure (MPa) initial temperature (K) production pressure (MPa) initial saturation, S porosity, φ effective thermal conductivity, λ thermal conductivity of each phase (W/m/K) specific heat (J/kg/K) density (kg/m3) absolute permeability, kD0 (10−3 μm 2 = mD) permeability of hydrate-bearing media, kD reduction index of Masuda model, N permeability of hydrate and icebearing media, kD reduction index of ice formation, N2 relative water/gas permeability, krw, krg normalized water saturation, Se end-point relative permeability, krw0, krg0 irreducible saturation, Siw, Sig relative permeability index for water, Nw relative permeability index for gas, Ng capillary pressure, Pc0

value 3.6 275.15 2.1 water-saturated core: SH:0.7, Sw:0.3 gas-saturated core: SH:0.7, Sg:0.3 0.4 estimated by the parallel model: λ = (1−φ)λR + φ(SHλH+Sgλg+Swλw+SIλI) sand, λR: 4.0, MH, λH: 0.49, ice, λI: 2.2 gas, λg: 0.0335, water, λw: 0.5564 sand: 800, MH: 2010, ice: 2000 sand: 2650, ice: 917 15000 estimated by the Masuda model:22 kD = kD0(1 − SH)N 4

Figure 3. Pressure−temperature changes during hydrate dissociation due to depressurization. Sensors were inserted into the core center and positioned 25 mm from the core bottom. Arrows show P−T pathways.

kD = kD0(1 − SH)N(1 − (SI/(1−SH))N2 base case: 16, in the case study: 14/16/18 estimated by the Hirasaki model:23 krw = krw0(Se)Nw, krg = krg0(1-Se)Ng Se = (Swm−Siw)/(1−Sig−Siw), Swm = Sw/ (1−SH−SI) each 1 each 0.1 2 3

estimated by the Brooks-Corey model:24 Pc0 = Pe(Se)−nc capillary entry pressure, Pe (kPa) 5 pore-size distribution index, nc 0.25 capillary pressure for hydrate and PcH = Pc0/(1−SH−SI)0.5 ice-bearing media, PcH hydrate formation/dissociation estimated by the Clarke and Bishnoi kinetics model25 formation/dissociation rate, Kd Kd = kd0 exp(−ΔE/RT) (mol/m2/Pa/s) intrinsic rate constant, kd0 (mol/ 3.6 × 104 m2/Pa/s) activation energy, ΔE (kJ/mol) 81 water consumption/generation nw = Kice × MH2O × ΔTice, where by ice formation/melting, MH2O: water/ice content (kg/m3), and nw (mol/m3/s) ΔTice: degree of supercooling (K) rate constant of ice formation/ base case: 0.05, in the case study: melting, Kice (mol/kg/K/s) 0.05/0.2/0.5 hydration number 6.0

effective gas permeability under the initial conditions. Temperature changes related to ice formation were observed from 5 to 40 min. The pressure during this period stayed around the quadruple point; however, the pressure at the core bottom reached that at the quadruple point after decreasing to the production pressure. This suggests that ice formed gradually after depressurization, which is the other reason for the rapid propagation of depressurization. A high gas production rate was observed during ice formation. Figure 6 shows the gas production rate with the changes in temperature and pressure in Run 3. The behaviors are

Figure 4. Gas production rate, temperature, and pressure changes in Run 1.

completely different from the results in Runs 1 and 2. The gas production rate was constant during dissociation. The 4313


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temperature and pressure decreased at three points almost simultaneously. A high gas production rate and an increase of pressure during hydrate dissociation were not observed. The result indicates that high gas production rates in Runs 1 and 2 were caused by ice formation, and the difference of pressure propagation between Runs 1 and 2 was caused by ice formation than a high effective gas permeability. 3.3. Comparison of Gas Production Rates. Figure 7 compares the gas production bahaviors of Runs 1, 2, and 3. The gas production by hydrate dissociation in Run 1 reached >200 N mL/min at the peak in 20 min and then decreased with time. For the gas-saturated core (Run 2), the gas production rate was high in the first few minutes owing to the production of residual gas in the pore spaces. Thereafter, the dissociation behavior of Run 2 was similar to that of Run 1; however, the gas production rate was lower than that of Run 1 in the period between 10 and 50 min. In contrast, the gas production rate of Run 3 was constant during dissociation (approximately 35 N mL/min) and showed values comparable to those of Runs 1 and 2 after 60 min. These results indicate that the dissociation regime of Runs 1 and 2 obviously differs from that of Run 3 until 60 min. To confirm the reliability of the experimental results, repeat experiments were conducted for the water- and gas-saturated

cores under the ice formation regime. Figures 8 and 9 compare the gas production rates for the water- and gas-saturated cores, respectively. The repeat experiments showed the same tendency described above. The experimental results showed that gas production under the ice formation regime had a unique high-rate period in the early stage of production. The gas production rates of the water-saturated cores exhibited greater acceleration than those of the gas-saturated cores, except just after depressurization. The observed differences are attributed to differences in the amount of ice formed during dissociation. Recent studies have shown that gas hydrate initially dissociates into supercooled liquid water below the freezing point of water,26,27 and then a portion of the supercooled liquid water forms ice.28 These reports indicate that the presence of liquid water is essential for ice formation. From this viewpoint, gas-saturated cores lack liquid water in the early stage of production. In contrast, watersaturated cores contain not only water released by hydrate dissociation but also pore water. In fact, the amount of pore water in Run 1 is five times larger than that in Run 2. The high gas production rates observed in the water-saturated cores can be explained by ice formation from some of the pore water. 3.4. Numerical Simulations of Hydrate Dissociation under the Ice Formation Regime. To confirm the 4314


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Figure 7. Comparison of gas production bahaviors of water-saturated cores (Runs 1 and 3) and gas-saturated core (Run 2). Runs 1 and 2 were conducted below the quadruple point, and Run 3 was conducted above the quadruple point.

Figure 10. Comparisons of gas production rates, core temperatures, and core-end pressures of water-and gas-saturated cores with and without ice formation.

gas production rates, temperatures at the core center, and pressures at the core end opposite to the production surface. In the simulations with no ice formation, the ice formation model was set to nonfunctional. The gas production rate in each case showed the peak rate just after depressurization owing to rapid dissociation using sensible heat. The initial free gas also contributed to the high production rates for the gas-saturated cores. The temperatures decreased rapidly with exhaustion of sensible heat for hydrate dissociation. The temperatures of the gas-saturated cores decreased more rapidly to a lower level than those of the water-saturated cores because of the Joule− Thompson effect of exiting gas. After several minutes, the gas production rates without ice formation slowed because of sensible heat exhaustion, and the temperatures stayed at the equilibrium point of the production pressure. The gas production behaviors of the water- and gas-saturated cores without ice formation were very similar in this stage. This similarity shows that differences in properties such as thermal conductivity, which is a function of saturation, have little effect on gas production behavior in a core-scale experiment. In contrast, the gas production behaviors with ice formation showed obvious differences between the water- and gas-

Figure 8. Comparison of gas production rates of water-saturated cores.

experimental interpretation, we conducted four simulations comparing the behavior of the water- and gas-saturated cores with and without ice formation. Figure 10 shows the resulting

Figure 9. Comparison of gas production rates of gas-saturated cores. 4315


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Figure 11. Hydrate saturation, ice saturation, temperature, and pressure distributions of water-saturated core.

saturated cores. The gas production rate of the water-saturated core was higher than that of the gas-saturated core. The temperature of the water-saturated core recovered to the ice point faster than that of the gas-saturated core. Although the pressures of the gas-saturated cores with and without ice formation and the water-saturated core without ice formation

remained at the production pressure after depressurization, the pressure of the water-saturated core with ice formation showed a unique increase to the quadruple point. Figures 11 and 12 show the distributions of hydrate saturation, ice saturation, temperature, and pressure for the water- and gas-saturated cores with ice formation, respectively. 4316


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Figure 12. Hydrate saturation, ice saturation, temperature, and pressure distributions of gas-saturated core.

In the water-saturated core, ice formed across the core concurrently with hydrate dissociation. The hydrate- and icebearing area shrank from both ends and the periphery of the core because of heat flow from the surroundings. However, ice saturation of the core center increased with time. Because ice in the core center blocked the flow paths, the pressure at the

bottom increased with time. In the gas-saturated core, the evolution of ice was obviously slower than in the watersaturated core. Ice started to form in the boundary area where hydrate has dissociated. This is because the gas-saturated core had a shortage of liquid water available for ice formation under the initial conditions. As a result, the pressure at the bottom did 4317


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not increase, so the flow paths were not blocked by ice in the early stages of active hydrate dissociation. The results indicate that the initial pore water content plays a predominant role in ice formation behavior. The high gas rate and the pressure increase in the water-saturated core are found to be caused by rapid ice formation using the initial pore water. The results of numerical simulations exhibited the same tendency as those of the laboratory experiments. That is, the water-saturated core showed a higher gas production rate than the gas-saturated core, except just after depressurization. The numerical simulations indicate that the results obtained by laboratory experiments are caused by the difference in the initial pore water content, which resulted in differences in the ice forming behavior. 3.5. Sensitivity Analyses of Ice Formation Rate and Permeability Reduction by Ice Formation. Sensitivity analyses were conducted to investigate the effects of the ice formation rate and permeability reduction by ice formation on gas production. Figure 13 compares the gas production rates, temperatures, and pressures for varying ice formation rates (Kice) in the watersaturated core. The production rate increases with increasing

ice formation rate, especially in the early stage of production. The reason is that ice forms and supplies heat rapidly for hydrate dissociation with increasing ice formation rate. At a higher ice formation rate, the temperature shows a faster recovery to the ice point owing to the rapid supply of heat. As a result of the increment in ice saturation, the flow paths were rapidly obstructed, and the pressure at the bottom increased rapidly. The simulation with an ice formation rate of 0.05 mol/ kg/K/s showed similar trends to laboratory experiments; however, further study will be necessary to define the value of ice formation rate. Figure 14 compares the gas production rates, temperatures, and pressures in the water-saturated cores for varying indexes of

Figure 14. Effect of ice-induced permeability reduction (watersaturated core).

permeability reduction by ice formation (N2). The permeability decreased with increasing index of permeability reduction. As a result, gas production rates slowed and pressures were increased further by the increase in the index. The heat consumption rate was slowed by deceleration of hydrate dissociation, which caused a rapid recovery of the temperature to the ice point. Although the permeability reduction had a

Figure 13. Effect of ice formation rate (water-saturated core). 4318


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Reference case were set to the values mentioned in Table 2. The initial effective thermal conductivity in Case HT (high thermal conductivity) was 3.22 W/m/K which is higher than the value of 2.62 W/m/K in the Reference case. The results of Case HT were exactly similar to those of the Reference case. This similarity shows that the effective thermal conductivity has a low sensitivity to dissociation behaviors in this experiment. By contrast, the reduction index of permeability and the intrinsic rate constant of hydrate dissociation greatly influenced dissociation behaviors. The gas production rate of Case LP (low permeability) slowed in the early stage because the pressure reduction propagated slowly. As a result, the temperature and pressure decreased slowly after depressurization, and the pressure stayed around the quadruple point without reaching to the production pressure. In Case LR (low rate constant), the gas production rate under the ice formation regime remarkably decreased, whereas in Case HR (high rate constant), the gas production rate during this period showed a large increase. Rapid hydrate dissociation causes a rapid and high amount of ice formation, which accelerates hydrate dissociation further. Conversely, slow hydrate dissociation causes a slow and low amount of ice formation, in which case an enhancement of gas production would be limited. These behaviors affected the temperature and pressure changes. The temperature decreased lower and the pressure increased higher in Case HR due to rapid dissociation and a high amount of ice formation. The simulations showed that the gas production rates in Reference case or Case HR were close to the experimental data. However, Case LP showed relatively close predictions for the temperature and pressure behaviors just after depressurization. On the other hand, the temperature increase and the pressure drop after the high-rate period showed some differences between the simulations and experiments. These discrepancies can be attributed to experimental factors such as heterogeneities in the saturations, porosities, and flow paths. That is, flow paths were clogged by ice formed in the early stage; however, some paths were opened during hydrate dissociation, and also, modeling factors such as the neglect of the metastable condition below the quadruple point and neglect of the stochastic process of ice formation may affect ice formation behaviors. Additional experimental and numerical investigations are needed to obtain complete history matching simulations.

negative effect on gas production, the results indicate that the gas production rates with ice formation were higher than those without ice formation. 3.6. History Matching Simulations of Run 1. History matching simulations of Run 1 were conducted to investigate key model parameters not directly involving ice formation but affecting whole dissociation behaviors. The core properties such as saturations and porosity were based on the experimental data mentioned in Table 1. The pressure measured at the production line was used as the boundary pressure condition. The values are time-series data as shown in Figure 4 (Production Press). The temperature of the brine solution for confining pressure was used as the boundary temperature condition. The ice formation rate and the permeability reduction index of ice formation were set to 0.05 mol/kg/K/ s and 16, respectively. Figure 15 compares the gas production rates, temperatures, and pressures in Run 1 for varying the thermal conductivity of sand (λR = 5.0, Case HT), the index of permeability reduction of Masuda model22 (N = 7, Case LP), and the intrinsic rate constant of hydrate dissociation (kd0 = 1.8 × 104 in Case LR, and kd0 = 5.4 × 104 in Case HR). The model parameters in the

4. CONCLUSIONS We conducted laboratory experiments and numerical simulations of depressurization-induced gas production from methane hydrate in sandy porous media under the ice formation regime. We analyzed the effects of ice formation and initial water content on ice formation. We conducted sensitivity analyses by varying model parameters related to the rate of ice formation and the permeability reduction by ice formation, and also, history matching simulations were conducted for Run 1 to investigate key model parameters. The following findings were obtained: (1) Laboratory experiments showed that gas production under the ice formation regime had a unique high-rate period in the early stage of production. This behavior was not observed under the water generation regime. The gas production rates of the water-saturated cores exhibited greater acceleration than those of the gas-saturated cores, except just after depressurization.

Figure 15. History matching simulations of Run 1. 4319


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(2) The results of numerical simulations showed the same tendency as in the laboratory experiments. The simulations indicated that ice forms faster in a watersaturated core because pore water is available for ice formation. This results in a further enhancement of gas production in a water-saturated core. (3) The rate of ice formation and the permeability reduction by ice formation are key model parameters affecting gas production behavior. Sensitivity analyses showed that the rate of ice formation affects the gas production rate in the early stage. The gas production rate increases with the ice formation rate. Although permeability is rapidly reduced by ice formation, resulting in deceleration of hydrate dissociation, the gas production rate with ice formation is larger than that without ice formation. (4) Gas production rates predicted in the Reference case and Case HR (high rate constant) were close to the experimental data. However, Case LP (low permeability) showed relatively close predictions for the temperature and pressure behaviors just after depressurization. Some differences between the simulations and experiments can be attributed to experimental factors such as heterogeneities in the saturations, porosities, and flow paths. The experimental and numerical findings of this study indicate that the depressurization-induced gas production can be accelerated by ice formation during hydrate dissociation at a pressure below the quadruple point. Although the gas production mechanism of field reservoirs is more complex than that of core experiments, the findings of this study represent an important contribution to the development of optimum production schemes for application to methane hydrate deposits. Thus, evaluations of the gas productivity of field reservoirs under the ice formation regime will be an important subject in the future.

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AUTHOR INFORMATION

Corresponding Author

*E-mail Y.K.: [email protected], J.N. jiro.nagao@aist. go.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium) to carry out Japan’s Methane Hydrate R&D Program conducted by the Ministry of Economy, Trade and Industry (METI). The authors gratefully acknowledge them for the financial support and permission to present this paper. The authors thank Prof. M. Kurihara of Waseda University and Drs. T. Ebinuma and H. Ohno of AIST for their fruitful discussions.

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REFERENCES

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