6th International Conference on Gas Hydrates

DIRECT OBSERVATION OF CHARACTERISTIC DISSOCIATION BEHABIORS OF HYDRATE-BEARING CORES BY RAPID-SCANNING.. Ebinuma, Takao; Oyama, Hiroyuki; Utiumi, Takashi; Nagao, Jiro; Narita, Hideo 2008

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Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.  DIRECT OBSERVATION OF CHARACTERISTIC DISSOCIATION BEHABIORS OF HYDRATE-BEARING CORES BY RAPID-SCANNING X-RAY CT IMAGING Takao Ebinuma ∗ , Hiroyuki Oyama, Takashi Utiumi, Jiro Nagao, Hideo Narita Methane Hydrate Research Laboratory National Institute of Advanced Industrial Science and Technology (AIST) 2-17-2-1, Tsukisamu-Higashi, Toyohiraku, Sapporo 062-8517 JAPAN ABSTRACT Experiments involving the dissociation of artificial methane-hydrate-bearing sediments were performed using X-ray computed tomography (X-CT, 40 s scanning speed at 2 min intervals) to directly observe dissociation behavior in the sediments and the gas and water flows generated by dissociation. Dissociation by depressurization was performed using a backpressure regulator, and showed that the temperature reduction induced by depressurization depends on the phase equilibrium state of methane hydrate, and that preferential dissociation occurs along the periphery of the core. This behavior is caused by heat flux from the outside of the core, and this controls the dissociation rate. A heat exchanger was installed at one end of the core to simulate thermal stimulation, and propagation of a clear and unidirectional dissociation front was observed. Depending on the heating temperature, the dissociation rate was less than that observed for depressurization. Hot water was also injected at a constant rate from the bottom of the core, and CT images showed the movement of distinct accumulations of dissociated gas being pushed by the hot water. The gas production rate increased immediately after the gas accumulation reached the opposite end of the core where the gas and water flow out. Keywords: methane hydrate, dissociation, X-ray CT imaging, depressurization, thermal stimulation, hot water injection INTRODUCTION Methane hydrate has considerable potential as a new source of energy. Major considerations in developing production methods of methane from hydrates are the fundamental properties of hydrate-bearing sediments, and the dissociation behavior of methane hydrate and the gas and water flow generated by its dissociation in sediments. Marine methane hydrates occur in a variety of forms, several hundred meters below the sea floor. The form that is considered to be the most suited to exploitation is the pore-space filling-type, which is contained within the pore spaces of sandy sediments, as it has relatively larger gas ∗  permeability compared to other forms. Shallow sandy sediments are not usually consolidated, and methane hydrate is unstable at normal pressure and temperature; therefore, common methods are not suitable, and new experimental methods have been developed to study the fundamental properties of hydrate-bearing sediment and its dissociation process. X-ray computed tomography (X-CT), is one of the most effective methods to study both the fundamental properties of methane hydrate and its dissociation process. X-CT analyses have been used to observe undisturbed sedimentary texture and the occurrence of methane hydrate [1][2][3]. The  Corresponding author: Phone: +81-11-857-8950 Fax +81-11-857-8971 E-mail: t.ebinuma@aist.go.jp  porosity and volume fraction of methane hydrate in the pore spaces are important structural properties to be measured. A new method of structural analysis was developed, using a micro focus X-CT with a spatial resolution of approximately 0.03 mm [4]. Using high spatial resolution and measurement at -100 °C to prevent methane hydrate from dissociation, each sand particle is distinguished, and the porosity and volume fraction of the gas phase in the pore spaces are determined. This structural analysis was applied to assess the absolute permeability of hydrate-bearing sediment [5]. According to a previous observation of the dissociation process of methane hydrate in sediments, changes in the CT values indicated that pressure reductions were effectively transmitted through the core specimen, because of the fluid in the specimen [6]. The formation and dissociation of methane hydrate in the sand pack was observed by X-CT to validate numerical models of methane hydrate behavior in porous media [7]. We have developed an experimental method using a rapid scanning X-CT system and a tri-axial pressure vessel that is adequate for investigation of the dissociation processes of methane hydrate in sediments. Characteristic features of dissociation by depressurization and thermal recovery methods that were thought to be effective have been observed. EXPERIMENTAL METHODS Direct observation of the dissociation process in a hydrate-bearing core was conducted using a rapidscanning X-CT scanner, as shown schematically in Fig. 1(a). The core holder, which is a pressure vessel containing the core specimen, is located between an X-ray source with a maximum X-ray tube voltage and current of 130 kV and 20 mA, respectively, and a flat panel detector. A cone beam imaging method was applied by rotating the X-ray source and the detector around the core holder. The working field of view and the slice pitch were 100×100 mm and 0.25 mm, respectively. Rapid observation was performed at a scanning rate of 40 s with an interval of 2 min, which is adequate for observing fast dissociation of methane hydrate. Processed images display a distribution of brightness in a core specimen, which indicates the bulk density of the core specimen, and which is changed by the dissociation of methane hydrate releasing gas and water and their respective flows.  The core holder is made of duralumin, because of its low X-ray absorption properties. A core specimen is placed in the core holder and covered with a butadiene-acrylonitrile rubber sleeve, as shown in Fig. 1(b), for application of a confining pressure using antifreeze. An outer jacket is fitted to the core holder and the specimen temperature is controlled by the circulation of brine in the jacket from a constant-temperature bath. Methane gas or water is supplied to the specimen via 1 mm sintered metal plates installed at both end surfaces of the core specimen. Pressures and temperatures near both end surfaces are measured using pressure transducers and thermocouples, respectively. The confining pressure and pore water pressure are independently controlled using syringe pumps. During the dissociation of methane hydrate, the pore pressure is controlled using a back pressure control valve (BPCV). The flow rate of methane gas produced by the dissociation of methane hydrate was measured using a water trap and a cumulative gas flow meter connected to the BPCV, as shown in Fig. 2. The depressurization method was performed by adjusting the BPCV. Two thermal recovery methods were also performed using an end plug with an inner bath for circulating hot water; thermal stimulation and hot water injection. In the case of thermal stimulation, the end plug was used to heat the top surface of the core specimen, through which dissociated gas and water flowed out. This experiment simulates the production method tested in permafrost by hot water circulation in a production well [8]. The same end plug was used for the dissociation of methane hydrate by hot water injection from the bottom surface of the core specimen, in which dissociated gas and water are produced through the top surface. This experiment simulates a production method using dual wells. An artificial core specimen was prepared, based on the properties of a natural core recovered at a site in the Nankai Trough near Japan. The natural core consisted of typical sandy sediment deposited by turbidity currents, in which methane hydrate was formed in the pore spaces of the sand. The artificial core specimen was prepared in the core holder by pumping methane gas into a wet sand pack with a water content ranging from 10 to 15%, depending on the desired volume concentration of methane hydrate in the pore spaces. The water content is the mass ratio of water to dry sand, and the volume concentration of methane hydrate in the pore spaces is referred to as  EXPERIMENTAL RESULTS Depressurization process For the dissociation experiment of methane hydrate by depressurization, a Type II core specimen with a diameter of 28.7 mm, porosity of 41% and a methane hydrate saturation of 84% was used. The core specimen was kept at a temperature of 10.0 °C, a pore pressure of 8.0 MPa, and a confining pressure of 9.0 MPa prior to dissociation. Depressurization was performed by decreasing the  X-ray source  Core holder  Detector  (a)  Movable end plug Core specimen Rubber sleeve Fixed end plug with hot water circulation chamber  (b) Figure 1 Schematic diagram of rapid scanning Xray computed tomography scanner (a) and core holder with tri-axial structure (b).  Pressure gauge  Back pressure control valve  Gas flow meter  Constant temperature bath for thermal stimulation  Constant temperature bath for circulation in jacket  Gas - liquid separator Core holder  saturation of methane hydrate hereafter. In this procedure, methane hydrate first forms at the water surface, where the methane gas and water come into contact in the pore spaces, after a thin film of methane hydrate forms and becomes increasingly thicker at the water surface. Two artificial core specimens with different permeability were prepared. One specimen was made using silica sand with an average diameter of 0.22 mm and 97 wt% of particles between 0.15 mm and 0.30 mm, which is a commercial product known as Toyoura standard sand in Japan. The core specimen had an absolute permeability of 8 Darcy, which is referred to as a Type I core specimen hereafter. The other specimen with a smaller permeability, referred to as Type II, was made using fine silica sand (80 wt%), bentonite (2 wt%), mica (2 wt%) and chlorite (1 wt%), as well as Toyoura standard sand (15 wt%), to model the natural core specimen obtained near Japan, with an average particle diameter of 0.19 mm and 13 wt% of particles smaller than 0.10 mm. The Type II core specimen had an absolute permeability of 1×10-2 to 1×10-1 Darcy. The purity of methane gas was greater than 99.5%, and the core specimens used in the experiments measured 30 mm in diameter and from 100 mm to 120 mm in length. Subsequent to the formation of methane hydrate, distilled water was injected at a constant flow rate into the pore spaces of the core using a syringe pump to purge the methane gas. Pore pressure was kept constant during water injection using the BPCV. The dissociation of methane hydrate was performed after the injection of water by decreasing the pore pressure below the phase equilibrium pressure of methane hydrate, increasing the temperature of the end plug of the core holder, or injecting hot water. At all stages during the experiments, temperatures, pressures and cumulative gas and water productions were sampled at a frequency of 2 Hz using a digital data acquisition system.  Balance Differential pressure gauge Syringe pump  Pressure gauge Gas / Water  Figure 2 Experimental apparatus for dissociation of methane hydrate.  pore pressure to 4.0 MPa at a constant confining pressure, using the BPCV connected to the top surface of the core specimen, and gas and water were produced through it. The phase equilibrium temperature of methane hydrate is 4.3 °C at a dissociation pressure of 4.0 MPa. Figure 3 shows the variations of cumulative gas and water production over the elapsed time. Although the cumulative gas production increased monotonously for 45 min, the increasing rate of water production decreased after the initial large production rate. The thermocouple near to the top surface of the specimen, that is, the production side, indicated a rapid decrease of temperature to 5.2 °C, then increased gradually to the initial temperature. Because the thermocouple was installed in the end plug of the core holder, the minimum temperature was higher than the phase equilibrium temperature at the dissociation pressure. The temperature at the bottom end surface of the core specimen decreased 4 min later than that at the top, and the minimum temperature was 7.9 °C, which was higher than the value at the top surface. X-CT images obtained during this experiment are shown in Fig. 4. The images were processed by color, in order to visualize the distribution of brightness in the cross sections; a higher level of brightness is indicated by a warmer color. The duralumin core holder and rubber sleeve containing the core specimen are indicated by white and blue colors, respectively. The black space between the duralumin and the rubber sleeve was filled with antifreeze for application of the confining pressure to the core specimen. The sintered metal on the top of the core specimen is also shown as a white plate. Immediately prior to depressurization, the cross section of the core specimen had a homogeneous red color, which indicates that the pore spaces of the core specimen contained uniform distribution of methane hydrate and water. After 12 min from the beginning of depressurization, the color became light red and yellow, indicating a decrease of bulk density in the pore spaces, which was caused by an increase of gas saturation due to the dissociation of methane hydrate. This change in the distribution of color in the cross section continued over time. In particular, the color of the core specimen periphery became preferentially yellow, in addition to the upper part connected to the BPCV. The variation of the brightness over the elapsed time was studied by dividing the core  specimens into five disk-like layers, and calculating the volumetric average of brightness in each layer. Figure 5 shows the relation between the averaged brightness of each layer, numbered from the top of the core specimen, and the elapsed time. The brightness values deviated from the beginning of depressurization. The brightness of all the layers significantly decreased immediately after the beginning of depressurization, although the Type II core specimen had a small permeability. Layers 4 and 5, from the lower part of the core specimen, showed recovery of the brightness, indicating that part of the water produced flowed downward. After layers 4 and 5 recovered brightness, the brightness of all layers decreased gradually over time. The decreasing brightness of layers 3 and 4, from the middle part of the core specimen, were delayed compared with the layers from both ends. The dissociation of methane hydrate requires a large latent heat, which was measured at 54.19 kJ/mol, using a differential scanning calorimeter [9]. The endotherm caused by the latent heat of methane hydrate dissociation results in cooling of the sediments containing the methane hydrate, gas and water, and the temperature is decreased to the phase equilibrium temperature at the dissociation pressure [10]. After the decrease of temperature, the dissociation rate of methane hydrate is controlled by the heat transfer to the dissociation zone from the outside, depending on the temperature differential. Preferential dissociation at the periphery of the core specimen, observed by the CT images, coincides with dissociation behavior controlled by heat transfer. Thermal stimulation process Thermal stimulation was performed by connecting the end plug with the hot water circulation chamber and the top surface of the core specimen. Simulating the thermal stimulation conducted at a production well, the end plug is located at the top surface, where gas and water flow out. Figure 6 shows CT images of the dissociation process by thermal stimulation, in which a Type II core specimen with a diameter of 29.3 mm, porosity of 42% and a methane hydrate saturation of 72% was used. After the core specimen was kept at a temperature of 10.0 °C, pore pressure of 8.0 MPa and a confining pressure of 9.0 MPa, the dissociation of methane hydrate was caused by rapidly heating the top end surface of the core specimen up to 50.0 °C. After dissociation started,  each vertical cross section showed a clear planar boundary, which was caused at the front of the dissociation zone, and this front moved downward with time. Each vertical CT image was divided into five layers, and the volumetric average of brightness was calculated in each layer, similar to that for the depressurization experiment. The variation of the averaged brightness over the elapsed time is shown in Fig. 7. The brightness of each layer decreased significantly for a short duration from the top of the core specimen by turns, due to the passing of the planar front of the methane hydrate dissociation zone. Figure 8 shows the variations in the cumulative gas and water production over the elapsed time. The production of methane gas continued for 320 min, which coincides with the variation of brightness at the bottom of the core specimen shown in Fig. 7. According to other experiments performed at different temperatures, the rate of dissociation by thermal stimulation tended to be less than that by depressurization.  120  1.0  100 Gas  0.8  80  Water  0.6  60  0.4  40  0.2  20  0.0  0  10  20 30 Time (min.)  40  50  12 min.  40 min.  52 min.  Figure 4 X-CT images of vertical sections observed during the dissociation of methane hydrate by depressurization. Both reduction of pressure and production of gas and water are observed at the top surface of the core specimen. 0  layer layer layer layer layer  -20  0  Figure 3 Variation of cumulative gas and water production over time during dissociation of methane hydrate by depressurization.  1 2 3 4 5  -40 Brightness  1.2  Cumulative water production (mL)  Cumulative gas production (NL)  Hot water injection process Dissociation of methane hydrate by hot water injection was conducted using a Type I core specimen with a diameter of 30.0 mm, porosity of 41% and a methane hydrate saturation of 58%. Before the injection of hot water, the core specimen was kept at a temperature of 5.0 °C, a pore pressure of 5.0 MPa and a confining pressure of 9.0 MPa. Dissociation was initiated by injecting the hot water from the bottom of the core specimen at a temperature of 15.0 °C and a flow rate of 5.0 mL/min. Gas and water were produced  Initially  -60 -80 -100 -120 -140  0  10  20 30 Time (min.)  40  50  Figure 5 Variation of brightness with elapsed time during dissociation by depressurization. Brightness, obtained by volumetrically averaging each perpendicularly divided layer of the core specimen, indicates that the values deviate from the initial.  therefore, it is understood that the flow of injected water tends to converge, and form a preferential path. Figure 11 shows the variation of volumetrically averaged brightness over elapsed time, similar to the previous experiments. Although the brightness of each layer decreased by turns, from the bottom of the core specimen, all of the layers recovered, because of the sweep of residual gas in the core specimen by the injected water. The recovered brightness near the top side layer1 layer2 layer3 layer4 layer5  0 -20  Brightness  through the top surface. During the dissociation experiment, the confining pressure and the temperature of the brine circulated in the outer jacket of the core holder was kept constant. Cumulative gas and water production are shown in Fig. 9, where the hot water injection was started at a time of 4 min. The gas production was accelerated at 12 min from the beginning of the injection, and continued at a large rate for 29 min. The acceleration of gas production was understood by analyzing the CT images as follows. Figure 10 shows CT images of the vertical section of the core specimen. At 4 min from the beginning of injection, a yellow zone appeared immediately ahead of the injected hot water, which is shown as a red cone at the bottom of the core specimen. This indicates the accumulation of methane gas dissociated by the hot water. The injected hot water pushes the accumulated gas volume to the top of the core with time. At 12 min from the beginning of injection, the accumulated gas volume reaches the top of the core specimen, which results in acceleration of the gas production rate, as shown in Fig. 9. The red column shown in both CT images observed at 12 and 38 min from the beginning indicates the path of injected water between the bottom and the top of the core specimen. Other experiments using hot water injection showed similar gas and water flows;  -40 -60 -80 -100 -120  0  100  200 300 Time (min.)  Cumulative gas production (NL)  223 min.  Figure 6 X-CT images observed during dissociation of methane hydrate by heating and the production of gas and water at the top surface of the core specimen by simulation of thermal stimulation, such as hot water circulation in a production well. Two perpendicular cross sections are shown with a horizontal section at the middle height of the core specimen.  50  Gas  30 min.  40  0.8 0.6  Water  30  0.4  20  0.2  10  0.0  0  100  200 300 Time (min.)  400  Cumulative water production (mL)  100 min.  5 min.  500  Figure 7 Variation of brightness with elapsed time during dissociation by thermal stimulation. Brightness, obtained by volumetrically averaging each perpendicularly divided layer of the core specimen, indicates that the values deviate from the initial. 1.0  Initially  400  0 500  Figure 8 Variation of cumulative gas and water productions during dissociation of methane hydrate by thermal stimulation.  was less than that at the bottom, because the injected water converged into a flow path.  600  1.0  400 Gas Water  0.5  0.0  200  0  10  20  30 40 Time (min.)  50  60  Cumulative water production (mL)  Cumulative gas production (NL)  1.5  0  Figure 9 Variation of cumulative gas and water productions during dissociation of methane hydrate by hot water injection.  Initially  4 min.  12 min.  38 min.  Figure 10 X-CT images of vertical core sections observed during dissociation of methane hydrate by hot water injection from the bottom surface, and production of gas and water from the top surface of the core specimen. Simulation of a dual well system. layer 1 layer 2 layer 3  0  layer 4 layer 5  -20 Brightness  CONCLUSIONS The process for the dissociation of methane hydrate in sandy sediments was directly visualized using rapid-scanning X-ray CT imaging with a resolution of 0.25 mm and a scanning speed of 40 s at intervals of 2 min. Dissociation by depressurization showed that the temperature reduction induced by depressurization depends on the phase equilibrium state of methane hydrate, and that dissociation preferentially occurs at the periphery of the core. This behavior is due to the heat flux from the outside of the core; the heat flux controlled the dissociation rate. In the case of dissociation by thermal stimulation, a clear unidirectional dissociation front was observed to propagate. The dissociation rate depended on the heating temperature and was less than that observed for depressurization. Hot water injection showed movement of distinct accumulation of dissociated gas being pushed by the hot water. The gas production rate increased immediately after the gas accumulation reached the opposite end of the core, where the gas and water flowed out.  -40 -60 -80 -100  0  10  20  30 40 Time (min.)  50  60  Figure 11 Variation of brightness over elapsed time during dissociation by hot water injection. Brightness, obtained by volumetrically averaging each perpendicularly divided layer of the core specimen, indicates that the values deviate from the initial.  ACKNOWLEDGEMENTS The authors thank Drs. Y. Masuda and Y. Konno of University of Tokyo for valuable discussions regarding the dissociation behavior of methane hydrate. This work was financially supported by the Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium) on the National Methane Hydrate Exploitation Program planned by Ministry of Economy Trade and Industry (METI). REFERENCES [1] Uchida T, Dallimore S, Mikami J. Occurrences of natural gas hydrates beneath the permafrost zone in Mackenzie Delta - Visual and X-ray CT imagery. Gas Hydrates: Challenges for the Future, Annals of the New York Academy of Sciences 2000;912:1021-1033. [2] Uchida T, Lu H, Tomaru H, MITI Nakai Trough Shipbord Scientist. Subsurface occurrence of natural gas hydrate in the Nankai Trough area: Implication for gas hydrate concentration. Resource Geology 2004;54 (1):35-44. [3] Heeschen KU, Hohnberg HJ, Haeckel M, Abegg Drews FM, Bohrmann G. In site hydrocarbon concentrations from pressurized cores in surface sediments, Northern Gulf of Mexic. Marine Chemistry 2007;107:498-515. [4] JIn S, Takeya S, Hayashi J, Nagao J, Kamata Y, Ebinuma T, Narita H. Structural analyses of artificial methane hydrate sediments by microfocus X-ray computed tomography. Jpn. J. Appl. Phys. 2004;43 (8A):5673-5675. [5] Jin Y, Hayashi J, Nagao J, Suzuki K, Minagawa H, Ebinuma, T, Narita H. New method of assessing absolute permeability of natural methane hydrate sediments by microfocus X-ray computed tomography, Jpn. J. Appl. Phys. 2007;46 (5A):3159-3162. [6] Mikami J, Masuda Y, Uchida T, Satoh T, Takeda H. Dissociation of natural gas hydrates observed by X-ray CT scanner. Gas Hydrates: Challenges for the Future, Annals of the New York Academy of Sciences 2000;912:1011-1020. [7] Kneafsey TJ, Tomutsa L, Moridis GJ, Seol Y, Freifeld BM, Taylor CE, Gupta A. Methane hydrate formation and dissociation in a partially saturated core-scale sand sample. J. Pet. Sci. Eng. 2007; 56(1-3):108-126. [8] Takahashi H, Yonezawa T, Fercho E. Operation overview of the 2002 Mallik gas hydrate production research well program at the  Mackenzie Delta in the Canadian Arctic. OTC 15124, 2003. [9] Handa TP. Compositions, enthalpies of dissociation, and heat capacities in the range 85 to 270 K for clathrate hydrates of methane, ethane, and propane, and enthalpy of dissociation of isobutane hydrate, as determined by a heat-flow calorimeter. J. Chem. Thermodynamics 1986;18: 915-921. [10] Kamata Y, Ebinuma T, Ohmura R, Minagwa H, Narita H, Masuda Y, Konnno Y. Decomposition behavior of artificial methane hydrate sediment by depressurization method. In: Proceedings of the Fifth International Conference on Gas Hydrates, Trondheim, 2005.  

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