PUBLICATIONS. Geophysical Research Letters. Strengthening mechanism of cemented hydrate-bearing sand at microscales

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1 PUBLICATIONS RESEARCH LETTER Key Points: A microtriaxial testing apparatus was developed for analyzing hydrate-bearing sand via X-ray computed tomography Soil particles and hydrate in the shear band significantly move and rotate at the microscale The thickness of the shear band decreased with increasing hydrate saturation Supporting Information: Supporting Information S1 Movie S1 Movie S2 Movie S3 Movie S4 Correspondence to: J. Yoneda, jun.yoneda@aist.go.jp Citation: Yoneda, J., Y. Jin, J. Katagiri, and N. Tenma (2016), Strengthening mechanism of cemented hydrate-bearing sand at microscales, Geophys. Res. Lett., 43, , doi:. Received 10 JUN 2016 Accepted 2 JUL 2016 Accepted article online 6 JUL 2016 Published online 18 JUL American Geophysical Union. All Rights Reserved. Strengthening mechanism of cemented hydrate-bearing sand at microscales Jun Yoneda 1, Yusuke Jin 2, Jun Katagiri 1, and Norio Tenma 1 1 Methane Hydrate Geo-mechanics Research Group, Research Institute of Energy Frontier, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan, 2 Methane Hydrate Production Technology Research Group, Research Institute of Energy Frontier, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology, Sapporo, Japan Abstract On the basis of hypothetical particle-level mechanisms, several constitutive models of hydrate-bearing sediments have been proposed previously for gas production. However, to the best of our knowledge, the microstructural large-strain behaviors of hydrate-bearing sediments have not been reported to date because of the experimental challenges posed by the high-pressure and low-temperature testing conditions. Herein, a novel microtriaxial testing apparatus was developed, and the mechanical large-strain behavior of hydrate-bearing sediments with various hydrate saturation values (S h = 0%, 39%, and 62%) was analyzed using microfocus X-ray computed tomography. Patchy hydrates were observed in the sediments at S h = 39%. The obtained stress-strain relationships indicated strengthening with increasing hydrate saturation and a brittle failure mode of the hydrate-bearing sand. Localized deformations were quantified via image processing at the submillimeter and micrometer scale. Shear planes and particle deformation and/or rotation were detected, and the shear band thickness decreased with increasing hydrate saturation. 1. Introduction Methane hydrate (MH) is a potential next-generation energy resource, which is known to be stable only under certain temperature and pressure conditions, in particular, in permafrost layers and deep seabeds. MH disseminated in the pore space of permeable sediment particles is considered to have a high potential for economical gas production compared with nodular gas hydrates in fine-grained sediments and massive MH on seabeds [Boswell and Collett, 2006]. Currently, depressurizing pore water, heating reservoirs, and injecting inhibitors are being tested as methods for gas production from disseminated MH. Solid MH that exists in the pore spaces of sediments dissociates into gas and water during these production methods, and complex physical events may occur during the production process, including changes in sediment structure, a loss of cementation, pore fluid flow, and gas migration. These events may lead to a geomechanical instability in a gas hydrate reservoir; this, in turn, may cause differential settlement, borehole breakage, landslides, and gas leakage in the reservoir. On the basis of hypothetical particle-level mechanisms, several constitutive models of hydrate-bearing sediments have been proposed previously, and computational simulations have been employed to ensure safe and efficient exploitation of MH [Kimoto et al., 2007; Sakamoto et al., 2009; Klar et al., 2010; Kim et al., 2012]. Moreover, field-scale experiments were conducted to validate the feasibility and reliability of production methods onshore in 2007/2008 at Mallik, Canada, for ~6 days [Yamamoto and Dallimore, 2008] and in 2011/2012 at Ignik Sikumi, Alaska, for ~19 days [Schoderbek et al., 2013]. Similar field-scale experiments were conducted offshore in 2013 at the eastern Nankai Trough in Japan for ~6 days [Yamamoto et al., 2014]. Several projects have progressed to long-term field-scale tests [Yamamoto et al., 2014; Collett et al., 2012]. Longterm depressurization may lead to further geomechanical instabilities in the reservoir over a wide area and loss of well integrity. Therefore, precise investigations of the geomechanical and geotechnical problems are urgently required. In previous studies, mechanical tests of gas hydrate-bearing sediments were performed using both natural and synthetic samples; moreover, a sample preparation method using hydrate grains/ice seeding for synthetic MH-bearing sediments was proposed [Hyodo et al., 2005; Ebinuma et al., 2005; Clayton et al., 2005]. Another method for MH formation, known as the partially saturated method [Kneafsey et al., 2007], has been used, and the effect of hydrate saturation S h, effective confining stress, and density of the host sediment on the strength, stiffness, and deformation behavior of the hydrate-bearing sediment has been YONEDA ET AL. STRENGTHENING MECHANISM OF HYDRATE SAND 7442

2 reported [Priest et al., 2009; Miyazaki et al., 2011; Hyodo et al., 2013a]. Yun et al. [2007] investigated the strengthening of the hydrate-bearing sediment with increasing S h using tetrahydrofuran hydrate, which is stable at relatively low pressures and high temperatures. Mechanical tests using natural gas hydratebearing sediments were performed by Winters et al. [1999]. The sediments from Mallik 2 L-38, Mackenzie Delta, showed higher strength than similar samples recovered from a well that did not contain hydrates. Masui et al. [2007] recovered core samples in Nankai Trough via pressure coring, and triaxial compression tests were conducted onshore. The pore fluid pressure was maintained until the sample was on board and was then quenched using liquid nitrogen after rapid depressurized on board. This sample preparation process has the potential for hydrate dissociation during depressurization prior to quenching. This result has been used widely in several numerical simulations. In recent studies, pressure core triaxial testing was conducted while maintaining the pressure and temperature within the region of hydrate stability throughout the entire transfer process from the borehole in the deep seabed to the completion of the laboratory tests [Yoneda et al., 2015; Priest et al., 2005]. The measured values of the strength and stiffness of the synthetic sediments in previous studies using partially saturated (excess gas) method with water flushing and the results using THF were in agreement with those of the intact natural gas hydrate-bearing sediment sample [Yoneda et al., 2015]. Those studies simulated pore-filling or load-bearing types of hydrate morphologies. Strength and stiffness of cementing or grain-coating types of hydrate-bearing sediments which is formed by the partially saturated method were greater than pore-filling or load-bearing types [Priest et al., 2009; Hyodo et al., 2013b]. But recently, Chaouachi et al. [2015] showed that hydrate takes on a pore-filling morphology at the microscale even when the hydrate is formed using the partially saturated method. However, to the best of our knowledge, the actual mechanisms by which the strength and dilatancy of sediments change with S h formed by any method have not been investigated in any previous studies. We provide the first direct connection between the measured hydrate morphology and the resulting evolution of specimen deformation, stiffness, and strength. In this study, we form hydrate with the partially saturated method. This cementing method was chosen as a first test of this type of mechanistic study, so comparisons could be made with previous strength studies in hydrate-bearing sediment. This cementing morphology is also important in other geologic studies, such as in deep-sea sediment research, where mechanical property changes associated with a ductile-brittle transition have been reported in sediment containing silica cement, opal cement, and the others [Morgan et al., 2008; Spinelli et al., 2007]. Because an actual mechanism connecting morphology with stiffness and strength has not been demonstrated at the microscale, direct observations of the microscale behavior of hydrates in pore spaces are required for developing more accurate constitutive models. X-ray computed tomography (CT) has been used previously to investigate the mechanisms of deformation and the failure modes of soils [Viggiani et al., 2015]. Matsushima et al. [2006] visualized grain motion inside a microtriaxial specimen using X-ray CT. It was observed that the particles inside the shear band move and rotate significantly, while the changes in the particulate structures outside the shear band are small [Hall et al., 2010; Higo et al., 2013]. Jin et al. [2004] imaged artificial MH-bearing sediments at the pore scale, and a novel microscale imaging system was recently developed to investigate the hydrate morphology of subsampled pressure core sediment under pressure [Jin et al., 2014]. However, only a few studies on the mechanical and physical properties of hydrate-bearing sediments using CT [Rees et al., 2011; Seol et al., 2014] and the micromechanism of the behavior of hydrate-bearing sediments have not been observed because of experimental limitations such as the thick wall of the triaxial cell and the small density contrast between MH and water. In this study, we developed a microtriaxial testing apparatus with controllable pressure and temperature. Using microfocus X-ray CT, we observed a change in the inner structure of krypton hydrate (KrH)-bearing sand under triaxial compression at the submillimeter scale with the objective of investigating the mechanism of large-strain deformation and failure behavior. 2. Apparatus and Procedure The microtriaxial testing apparatus can apply 10 MPa of cell pressure and pore pressure and 60 MPa of axial stress. The sample size was 5 mm in diameter and 10 mm in height. The axial displacement of the sediment was measured using a laser displacement meter within an accuracy of ±0.05 mm (0.5% in axial strain). Images of the triaxial tests were obtained using a microfocus X-ray CT system (SMX225CTS-SV, Shimadzu Co., Japan). Additional details are provided in the supporting information (Figure S1). YONEDA ET AL. STRENGTHENING MECHANISM OF HYDRATE SAND 7443

3 Figure 1. (a) Stress-strain curves for hydrate-bearing and hydrate-free sediments. (b) Shear strength (maximum deviator stress) as a function of hydrate saturation for the effective confining pressures (1.0 MPa, 1.5 MPa, and 3.0 MPa) defined by Yun et al. [2007] and Yoneda et al. [2015]. The results for methane hydrate at σ c = 1.0 MPa and 3.0 MPa are from Miyazaki et al. [2011] and are shown as filled squares and triangles, respectively. The crosses show the results for the depressurized natural gas hydrate at σ c = 1.0 MPa [Masui et al., 2007], and the cross with a vertical bar shows the results for the pressure core At σ c = 1.5 MPa [Yoneda et al., 2015]. The results for the THF hydrate at σ c = 1.0 MPa are from Yun et al. [2007] and are shown as plus marks. The open rhombuses and triangles are the results for the saturated gas (excess gas) at σ c = 1.0 MPa and 3.0 MPa, respectively, and are from Hyodo et al. [2013b]. A sealing sleeve and small mold were set on the pedestal, and partially saturated Toyoura sand (average particle size D 50 = 0.2 mm, particle density G s = 2.65 g/cm 3 ) was tamped inside them. The water content of the Toyoura sand for the target S h and volume corresponded to the target initial bulk density. The porosity n of the host sediments was 39.1% ± 1.6% which is relatively dense for Toyoura sand. The pore pressure was vacuumed for the samples after the top cap was applied. A confining liquid was then injected into the triaxial cell, and 1 MPa of confining pressure was applied using a syringe pump that automatically controlled both the flow rate and the pore pressure. Gas hydrate was formed using the following procedure. First, pore pressure was applied via gas pressure using an effective confining pressure of 1 MPa at room temperature. To clearly visualize the results, krypton (Kr) was used as the guest gas molecule for the hydrates. Kr facilitates hydrate imaging because of the relative linearity of its X-ray mass attenuation coefficients [Hubbell and Seltzer, 2004]. The pore pressure and cell pressure were increased to 5 MPa and 6 MPa, respectively, and the temperature was decreased to 3 C using a chiller. Both the temperature and the pressure were maintained within the KrH stability region [Sugahara et al., 2005] for 3 days to allow the formation of KrH. Finally, axial loading was conducted with an axial strain rate of 1.0% min 1. The above mentioned method is the same as the excess gas method used by Priest et al. [2009] and Hyodo et al. [2013b] to prepare cemented-type hydrate-bearing sediments. 3. Results and Discussion The relationships between the deviator stress and the axial strain during compression for hydrate-free and hydrate-bearing sediments are shown in Figure 1a. The S h values derived from the mass balance were 39% and 62%. Here a hydration number of 6.1 [Handa, 1986] was used for the structure II-type KrH in the calculation. The peak and residual strength increased with increasing S h. This strengthening is in good agreement with the results of previous studies that used a partially saturated method for hydrate formation as shown in Figure 1b. This method promotes hydrate cementation at the menisci [Priest et al., 2009]. The results confirmed that the developed microtriaxial testing apparatus is capable of simulating the mechanical behavior of hydrate-bearing sediments. The stress relaxation periods (1) (7), (1 ) (7 ), and (1 ) (7 ) in each test ( shown in Figure 1a) are the scanning times; the CT images are shown in Figure 2. Animations of these images are provided in the supporting information Movies S1 S3. The grains visible in the images of the host sand (hydrate-free sediments) in Figure 2 are sand particles. The prepared sediment was dense sand with a porosity of 37.5%. The pedestal compressed the sediments from below. Sand particles moved upward with barreltype deformation and a ductile mode of failure during the compression process from (1) to (7). Next, we discuss the results for the sediment with S h = 39%. Figure 2 (1 ) shows the image after KrH formed in the host sand; the white parts of the image (high density) are KrH or iron sand grains. The KrH formed from the YONEDA ET AL. STRENGTHENING MECHANISM OF HYDRATE SAND 7444

4 Figure 2. Cross-sectional X-ray CT image under an effective confining pressure of 1 MPa during compression. capillary bridges of wet sand, and the sand particles and KrH formed in several clusters because of the high concentration of KrH. The patchy hydrate formed in the sand may be attributed to the patchy initial distribution of water in the sand particles. Liquids have been reported to distribute and form clusters of granular assemblies at the submillimeter scale [Scheel et al., 2008]. These water and granular assemblies might become a hydrate and sand particle clusters. The other sand particles were cemented by the low concentration KrH (shown as light gray). Based on the data, physical analyses, and simulations previously reported by Dai et al. [2012] and Jung et al. [2012], this patchy-type hydrate-bearing sediment is considered to reflect hydrate morphology. Our submillimeter-scale observations were able to confirm the presence of a patchytype hydrate-bearing sediment. Each cluster can be identified roughly during the compression process. Figure 2 (1 ) shows a CT image corresponding to S h = 62%; in this image, KrH is uniformly formed. The sediment shows a brittle failure mode with a clear shear plane. The significant change in the shearing may have occurred from (2 ) to(3 ), when the strength was observed to drop. Deformation was observed to localize after the shear plane appeared. Figure 3 shows the results of the image processing using particle image velocimetry (PIV) analysis [Taylor et al., 2010], which was conducted to visualize the local deformation of the sediment at the submillimeter scale. The YONEDA ET AL. STRENGTHENING MECHANISM OF HYDRATE SAND 7445

5 Figure 3. Local deformation of sediments for (a) hydrate-free sediment, (b) hydrate-bearing sediment with Sh = 39%, and (c) hydrate-bearing sediment with Sh = 62%. PIV analysis visualizes deformations via digital image correlation using the luminance values of the pixels before and after movement. The vector in Figure 3 expresses the tracked path between each scanning period, and the color bar shows the scalar value of the vector. The sediment (i.e., the host sand) was gradually deformed from the bottom to the top. An antisymmetric bifurcation mode, i.e., buckling, was observed. The shear plane did not form because of the ductile failure mode. The sediment with Sh = 39% displayed a downward sloping shear band from (2 ) to (3 ). The PIV analysis indicated a relatively large number of error vectors in the sediment from (3 ) to (6 ). There was a significant change in the two-dimensional arrayed luminance value of the pixels between each set of scanned data. This was due to the structural change in the soil particles that were weakly cemented by the hydrate and the hydrate morphology. In addition, a few changes in the cluster shape were observed. The sediment with Sh = 62% first underwent barrel-type deformation from (2 ) to (3 ). The mass of the upper sediment hardly moved, while the lower part of the specimen moved upward. Second, a clear shear plane appeared from (3 ) to (7 ), with deformation localized inside of the staircase-shaped shear band. This noisy shear band may promote and enhance the dilative behavior of hydrate-bearing sediments. Yun et al. [2007] and Waite et al. [2009] presented several hypothetical particlelevel mechanisms that might explain the dependence of shear strength on hydrate concentration. A shear plane is considered to develop through the hydrate mass when hydrate strength is smaller than the hydrate grain bonding strength. Conversely, failure occurs along the hydrate particle interface when the hydrate strength is greater than the hydrate grain bonding strength. Pinkert and Grozic [2014] extended this possible failure mechanism to cemented hydrate-bearing sands. In the fixed failure mechanism, the hydrate is debonded from the sand grains and then some hydrate breakage occurs during shearing. A combination of the above micromechanisms occurs during the actual failure. The shear bands observed in Figure 4 were YONEDA ET AL. STRENGTHENING MECHANISM OF HYDRATE SAND 7446

6 Figure 4. Micromechanism of the shear deformation of the hydrate-bearing sand. YONEDA ET AL. STRENGTHENING MECHANISM OF HYDRATE SAND 7447

7 drawn between (6 ) and (7 ) for S h = 39% and between (6 ) and (7 ) for S h = 62%. The thickness of the shear band was 1.45 mm for S h = 39% and 0.54 mm for S h = 62%; these values correspond to 6.3 times and 2.3 times the average diameter of the Toyoura sand (D 50 = 0.23 mm), respectively. The shear band for S h = 39% exhibited a stepwise plane between the interfaces of the strain localization due to the movement of the clusters as one aggregate. Conversely, the shear band for S h = 62% exhibited a relatively sharp plane, and the band s thickness indicated that it could contain only two or three sand particles. We believe that the structure of the hydrate-bearing sand markedly changed because of the dilation of sand and hydrate particles and the destruction of the hydrate; however, the inside of the shear band cannot be tracked efficiently via the PIV analysis. The inclinations of the shear bands were 47.6 for S h = 39% and 51.3 for S h = 62%. The inclination angle of the shear band (θ), which is expressed by the Mohr-Coulomb failure criteria as θ = π/4 + φ/2, where φ is the internal friction angle of the material, increased with increasing S h. This tendency of the internal friction angle is likely because of the increase in the bulk density; this increase is caused by the hydrate particles and clusters, which behave like solid sand particles. An additional triaxial compression test was performed to investigate the mechanism of particle deformation inside the shear band for sediments with n = 39.6% and S h = 65%. The stress-strain curve for this test is shown in the supporting information Figure S2. A zoomed-in cross-sectional CT image is shown in Figure 4a. The images are perpendicular to the shear plane. Figure 4b shows the CT image before and after hydrate formation and at the peak strength appeared. The image at the critical state, i.e., an axial strain of ε a = 22% and 24%, is also shown in the figure. An animation of these images was prepared and is shown in the supporting information Movie S4. Soil grains and pore spaces, or krypton hydrate, can be captured. Both grain-coating and load-bearing type of hydrates were identified, and a hydrate that completely filled the void space between the particles was observed. This type of hydrate potentially moves as a cluster. During compression, the change in the morphology of the hydrate and the displacement of the particles were significant. In addition, particle crushing, which makes it difficult to obtain correlations via digital image processing, was observed. Therefore, the microstructural changes at the critical state in the shear band (Figures 4b and 4c) have been discussed in this study. Hand-drawn sketches of the grains and the hydrates are illustrated in Figure 4b on the basis of the grain shape of the posthydrate dissociation. The particles inside the shear band were tracked as the particles moved and rotated; however, the changes in the particulate structures outside the shear band are small, as shown in Figure 4c. Particles that are supported by the hydrate rotate, and the surrounding hydrate is sheared (numbers 5, 7, and 8 in Figure 4c). Particles that are located next to a vacant space are displaced and occupy this void without any rotation, even in the shear band (numbers 1, 2, and 3 in Figure 4c). Chaouachi et al. [2015] observed that a water film of up to several microns in thickness exists between the gas hydrates and the surfaces of the quartz grains, even under stable conditions. This water film must exist in the current study, and it may have no bonding force at meniscus of sand grains. However, the strengthening with increasing S h was observed, as discussed previously in Figure 1b. This is due to the hydrate that forms at the meniscus and overlaps the particle grains, which may act as a bonding force. In addition, hydrate masses in pore spaces can resist deformation. This resistance and bonding by the hydrate lead to strengthening, strain localization, and particle crushing. 4. Conclusion We observed the triaxial compression of cemented hydrate-bearing sands at submillimeter and micrometer scales using a novel high-pressure microtriaxial testing apparatus. The deformation of the inner soil and the hydrate distribution were visualized via microfocus X-ray CT, and the stress-strain relationships for sediments with S h = 0%, 39%, and 62% were successfully measured. The results suggest that the hydrate-bearing sand exhibits a brittle failure mode. The localized deformations were quantified via image processing on the submillimeter scale. In the sediment with S h = 39%, heterogeneous hydrate nucleation was observed, and the hydrate formed and filled in the pore spaces of soil particles to generate aggregates of sand particles and a highly concentrated hydrate. Conversely, in the sediment with S h = 62%, the hydrate uniformly formed strong bonds between the soil particles, and the thickness of the shear band decreased with increasing S h. Only two or three sand particles were located in the shear band for the sediment with S h = 62%. Particle displacement was proven based on microscale observations of the S h = 65% sediment. The structure of the sediment in the shear band YONEDA ET AL. STRENGTHENING MECHANISM OF HYDRATE SAND 7448

8 significantly changed due to the movement and rotation of the soil particles and the hydrate. The inclination angle of the shear band increased with S h. The hydrates at the menisci that overlapped the grain particles governed the shear resistance as a bonding force, and the hydrates in the pore spaces behaved as solid materials and may have increased the angle of the failure envelope, i.e., the friction angle. Acknowledgments This study was conducted as part of the activity of the Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium) as planned by the Ministry of Economy, Trade, and Industry (METI), Japan. We would like to express our sincere thanks for the support. We also wish to thank Shigenori Nagase, Sayuri Kumagai, and Junko Hayashi for their technical support during the experiments. We extend our thanks to Yoshihiro Konno, Masato Kida, Motoi Oshima, William Waite, and reviewers for their valuable insights, which improved the manuscript. Data used in the preparation of this work are available from the corresponding author ( jun.yoneda@aist.go.jp). The data are archived at the Methane Hydrate Project Unit in AIST. References Boswell, R., and T. Collett (2006), The gas hydrates resource pyramid, Fire in the Ice, U.S. Department of Energy, Office of Fossil Energy, Natl. Energy Technol. Lab., 6(3), 5 7. Chaouachi, M., A. Falenty, K. Sell, F. Enzmann, M. Kersten, D. Haberthur, and W. F. Kuhs (2015), Microstructural evolution of gas hydrates in sedimentary matrices observed with synchrotron X-ray computed tomographic microscopy, Geochem. Geophys. Geosyst., 16, , doi: /2015gc Clayton, C. R. I., J. A. Priest, and A. I. Best (2005), The effects of disseminated methane hydrate on the dynamic stiffness and damping of a sand, Geotechnique, 55(6), , doi: /geot Collett, S. T., R. Boswell, M. W. Lee, B. J. Anderson, K. Rose, and K. A. Lewis (2012), Evaluation of long-term gas-hydrate-production testing locations on the Alaska North Slope, SPE Res. Eval & Eng., 15(2), SPE PA. Dai, S., J. C. Santamarina, W. F. Waite, and T. J. Kneafsey (2012), Hydrate morphology: Physical properties of sands with patchy hydrate saturation, J. Geophys. Res., 117, B11205, doi: /2012jb Ebinuma, T., Y. Kamata, H. Minagawa, R. Ohmura, J. Nagao, and H. Narita (2005), Mechanical properties of sandy sediment containing methane hydrate, in Fifth International Conference on Gas Hydrates, pp , Tapir Acad., Trondheim, Norway. Hall, S. A., M. Bornert, J. Desrues, Y. Pannier, N. Lenoir, G. Viggiani, and P. Bésuelle (2010), Discrete and continuum analysis of localized deformation in sand using X-ray micro CT and volumetric digital image correlation, Geotechnique, 60(5), Handa, Y. P. (1986), Calorimetric determinations of the compositions, enthalpies of dissociation, and heat capacities in the range 85 to 270 K for clathrate hydrates of xenon and krypton, J. Chem. Thermodynam., 18, Higo, Y., F. Oka, T. Sato, Y. Matsushima, and S. Kimoto (2013), Investigation of localized deformation in partially saturated sand under triaxial compression using microfocus X-ray CT with digital image correlation, Soils Found., 53(2), , doi: /j.sandf Hubbell, J. H., and S. M. Seltzer (2004), Tables of X-Ray Mass Attenuation Coefficients and Mass Energy-Absorption Coefficients (version 1.4). [Available at (National Institute of Standards and Technology, Gaithersburg).] Hyodo, M., Y. Nakata, N. Yoshimoto, and T. Ebinuma (2005), Basic research on the mechanical behavior of methane hydrate-sediments mixture, Soils Found., 45(1), Hyodo, M., J. Yoneda, N. Yoshimoto, and Y. Nakata (2013a), Mechanical and dissociation properties of methane hydrate-bearing sand in deep seabed, Soils Found., 53(2), , doi: /j.sandf Hyodo, M., Y. Li, J. Yoneda, Y. Nakata, N. Yoshimoto, A. Nishimura, and Y. Song (2013b), Mechanical behavior of gas-saturated methane hydrate-bearing sediments, J. Geophys. Res. Solid Earth, 118, , doi: /2013jb Jin, S., S. Takeya, J. Hayashi, J. Nagao, Y. Kamata, T. Ebinuma, and H. Narita (2004), Structure analysis of artificial methane hydrate sediments by microfocus X-ray computed tomography, Jpn. J. Appl. Phys., 43(8A), , doi: /jjap Jin, Y., Y. Konno, and J. Nagao (2014), Pressurized subsampling system for pressured gas-hydrate-bearing sediment: Microscale imaging using X-ray computed tomography, Rev. Sci. Instrum., 85, , doi: / Jung, J.-W., J. C. Santamarina, and K. Soga (2012), Stress-strain response of hydrate-bearing sands: Numerical study using discrete element method simulations, J. Geophys. Res., 117, B04202, doi: /2011jb Kim, J., G. J. Moridis, D. Yang, and J. Rutqvist (2012), Numerical studies on coupled flow and geomechanics in hydrate deposits, Paper SPE , SPE J., 17(2), , doi: / pa. Kimoto, S., F. Oka, T. Fushia, and M. Fujiwaki (2007), A chemo-thermo-mechanically coupled numerical simulation of the subsurface ground deformations due to methane hydrate dissociation, Comput. Geotech., 34(4), , doi: /j.compgeo Klar, A., K. Soga, and M. Y. A. Ng (2010), Coupled deformation-flow analysis for methane hydrate extraction, Geotechnique, 60(10), , doi: /geot.9.p Kneafsey, T. J., L. Tomutsa, G. J. Moridis, Y. Seol, B. M. Freifeld, C. E. Taylor, and A. Gupta (2007), Methane hydrate formation and dissociation in a partially saturated core-scale sand sample, J. Petrol. Sci. Eng., 56, , doi: /j.petrol Masui, A., H. Haneda, Y. Ogata, and K. Aoki (2007), Mechanical properties of sandy sediment containing marine gas hydrates in deep sea offshore Japan, Proc. 7th ISOPE-OMS, pp , International Society of Offshore and Polar Engineers, Lisbon. Matsushima, T., K. Uesugi, T. Nakano, and A. Tsuchiyama (2006), Visualization of grain motion inside a triaxial specimen by micro X-ray CT at SPring-8, in Advances in X-Ray Tomography for Geomaterials, edited by J. Desrues, G. Viggiani, and P. Bésuelle, pp , ISTE Ltd., London, U. K. Miyazaki, K., A. Masui, Y. Sakamoto, K. Aoki, N. Tenma, and T. Yamaguchi (2011), Triaxial compressive properties of artificial methane-hydratebearing sediment, J. Geophys. Res., 116, B06102, doi: /2010jb Morgan, J. K., E. B. Ramsey, and M. V. S. Ask (2008), Deformation and mechanical strength of sediments at the Nankai subduction zone, in The Seismogenic Zone of Subduction Thrust Faults, edited by T. H. Dixon and J. C. Moore, pp , Columbia Univ. Press, New York. Pinkert, S., and J. L. H. Grozic (2014), Failure mechanisms in cemented hydrate-bearing sands, J. Chem. Eng. Data, 60, , doi: / je500638c. Priest, J. A., A. I. Best, and C. R. I. Clayton (2005), A laboratory investigation into the seismic velocities of methane gas hydrate-bearing sand, J. Geophys. Res., 110, B04102, doi: /2004jb Priest, J. A., E. V. L. Rees, and C. R. I. Clayton (2009), Influence of gas hydrate morphology on the seismic velocities of sands, J. Geophys. Res., 114, B11205, doi: /2009jb Rees, E. V. L., T. J. Kneafsey, and S. Nakagawa (2011), Geomechanical properties of synthetic hydrate bearing sediments, in Proceedings of the 7th International Conference on Gas Hydrates, Shell, Edinburgh, Scotland, United Kingdom, Paper 501, 11. Sakamoto, Y., K. Masayo, K. Miyazaki, N. Tenma, T. Komai, T. Yamaguchi, M. Shimokawara, and K. Ohga (2009), Numerical study on dissociation of methane hydrate and gas production behavior in laboratory-scale experiments for depressurization: Part 3 Numerical study on estimation of permeability in methane hydrate reservoir, Int. J. Offs. Pol. Eng., 19(2), Scheel, M., R. Seemann, M. Brinkmann, M. D. Michiel, A. Sheppard, and S. Herminghaus (2008), Liquid distribution and cohesion in wet granular assemblies beyond the capillary bridge regime, J. Phys. Condens. Matter, 20, , doi: / /20/49/ YONEDA ET AL. STRENGTHENING MECHANISM OF HYDRATE SAND 7449

9 Schoderbek, D., H. Farrell, K. Hester, J. Howard, K. Raterman, S. Silpngarmlert, K. L. Martin, B. Smith, and P. Klein (2013), Oil & Natural gas Technology, ConocoPhillips Gas Hydrate Production Test Final Technical Report. Seol, Y., J. H. Choi, and S. Dai (2014), Multi-property characterization chamber for geophysical-hydrological investigations of hydrate bearing sediments, Rev. Sci. Instrum., 85, , doi: / Spinelli, G. A., P. S. Mozley, H. J. Tobin, M. B. Underwood, N. W. Hoffman, and G. M. Bellew (2007), Diagenesis, sediment strength, and pore collapse in sediment approaching the Nankai Trough subduction zone, Geol. Soc. Am. Bull., 119(3 4), Sugahara, K., T. Sugahara, and K. Ohgaki (2005), Thermodynamic and Raman spectroscopic studies of Xe and Kr hydrates, J. Chem. Eng. Data, 50, Taylor, Z. J., Gurka, R., Kopp, G. A., Liberzon, A. (2010), Long-duration time-resolved PIV to study unsteady aerodynamics: Instrumentation and measurement, IEEE Trans., 59(12), , doi: /tim Viggiani, G., E. Ando, D. Takano, and J. C. Santamarina (2015), Laboratory X-ray tomography: A valuable experimental tool for revealing processes in soils, Geotech. Test. J., 38(1), 61 71, doi: /gtj Waite, W. F., et al. (2009), Physical properties of hydrate-bearing sediments, Rev. Geophys., 47, RG4003, doi: /2008rg Winters, W. J., S. R. Dallimore, T. S. Collett, T. J. Katsube, K. A. Jenner, R. E. Cranston, J. F. Wright, F. M. Nixon, and T. Uchida (1999), Physical properties of sediments from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well, Geol. Surv. Can. Bull., 544, Yamamoto, K., and S. Dallimore (2008), Aurora-JOGMEC-NRCan Mallik gas hydrate research project progress: Fire in the ice, NETL Methane Hydrate Newslett., 8(3), 1 5. Yamamoto, K., Y. Terao, T. Fujii, T. Ikawa, M. Seki, M. Matsuzawa, and T. Kanno (2014), Operational overview of the first offshore production test of methane hydrates in the eastern Nankai Trough, OTC In: 2014 Offshore Technology Conference, Houston, Texas, USA, 5e8 May Yoneda, J., A. Masui, Y. Konno, Y. Jin, K. Egawa, M. Kida, T. Ito, J. Nagao, and N. Tenma (2015), Mechanical behavior of hydrate-bearing pressure-core sediments visualized under triaxial compression, Mar. Petrol. Geol., doi: /j.marpetgeo , in press. Yun, T. S., J. C. Santamarina, and C. Ruppel (2007), Mechanical properties of sand, silt, and clay containing tetrahydrofran hydrate, J. Geophys. Res., 112, B04106, doi: /2006jb YONEDA ET AL. STRENGTHENING MECHANISM OF HYDRATE SAND 7450