Mechanical Properties of Natural Gas Hydrate Bearing Sediments Retrieved from Eastern Nankai Trough

Abstract

MH21 research consortium of Japan conducted off-shore drillings to survey gas hydrate in Tokai-oki and Kumano-nada areas of Eastern Nankai Trough in early 2004 and recovered drill cores of gas hydrate bearing sediment successfully. Marine gas hydrate was confirmed mainly in sandy drill cores retrieved from mud and sand alternation strata. Tri-axial compression test on those core samples has been conducted to ascertain mechanical properties of natural hydrate bearing sediments for the first time. The results from the tests show that with increase in pore saturation of gas hydrate, shear strength and elastic modulus tend to arise, whereas no significant tendency was found in Poisson's ratio in relation to hydrate saturation. IntroductionPublic attention has been drawn recently to gas hydrate or methane hydrate in common in Japan as potential energy resources since gas hydrate was located offshore Japan. In general, clathlate compounds trapping gas molecules in the cage of water molecules are called gas hydrate, which is stable at ceratin temperature and pressure in phase equilibrium. Gas hydrate exists stably, therefore, in a permafrost region and/or under sea floor in deep-water along continental margins where the equilibrium state can be met in natural. In the development of gas hydrate resources, conventional methods used for a current gas and oil production are not necessarily applicable, because gas hydrates should be dissociated before an accumulation of methane from a hydrate reservoir. Moreover, the endothermic reaction of gas hydrate in dissociation reducing temperature at surroundings will cause regeneration of gas hydrate and restrain the extension of a gasification path, which will be pecurior to the gas production from gas hydrate. Considering the unique characteristics of gas hydrate, pressure decrease and/or temperature increase are needed to destabilize the equilibrium state of gas hydrates. Field-scale trial tests by MH21 research consortium of Japan were conducted and more onshore field tests have been scheduled employing depressurization method 1) . At present, dissociation by depressurization is a promising application to a future production compared with thermal stimulation and inhibitors in addition. Meanwhile, dissociation of gas hydrate may reduce a mechanical strength of sediment strata, which will cause wellbore instability directly affecting a safety and stable gas production from gas hydrates in case. Since test samples of drill cores containing natural gas hydrate are quite limited in number, Toyoura sand containing methane hydrate which was formed synthetically in a tri-axial compression test has been used so far to ascertain the mechanical properties of gas hydrate bearing sediments 2) . MH21 research consortium conducted offshore drilling survey in Tokai-oki and Kumano-nada areas of Eastern Nankai Trough region in early 2004 as shown in Fig.1, and confirmed marine natural gas hydrates in drill cores recovered from mud and sand alternation strata. In this paper, experimental results f...

Experimental study on mechanical properties of gas hydrate-bearing sediments using kaolin clay

“…However, it is vital to study the mechanical properties of gas hydrate under various conditions such as temperature, confining pressure, strain rate and saturation. Researchers used triaxial testing apparatus to measure mechanical properties of actual and synthetic gas hydrate or gas hydrate-bearing sediments with limited success (Hyodo et al, 2002(Hyodo et al, , 2005Masui et al, 2007Masui et al, , 2008aMasui et al, , 2008b. Further exploration of mechanical properties is required in order to assess the stability of gas hydrate-bearing sediments.…”

Coupled flow and geomechanical analysis for gas production in the Prudhoe Bay Unit L-106 well Unit C gas hydrate deposit in Alaska

“…The geomechanical properties of the Mallik samples are similar to those of Toyoura Sand (Winters et al, 1999(Winters et al, , 2008. Then, from the results of Masui et al (2005) and Masui et al (2008), For plasticity, according to results of Miyazaki et al (2010b), the shear strength for the Mohr-Coulomb plastic model is a function of the effective confining volumetric (mean) stress and hydrate saturation. The cohesion increases with the hydrate saturation, but the friction angle is almost independent of the hydrate saturation (Miyazaki et al, 2010c,a,b).…”

“…Reservoir compaction greater than 5% is a typical and consistent indicator for potential casing failure . Note that the simulation results in this study are based on the generalized reservoir model that contains several assumptions such as the failure model and geomechanical properties, which are approximated from other experimental and field data (Masui et al, 2005(Masui et al, , 2008Moridis et al, 2010;Rutqvist et al, 2011). Thus, a specific reservoir model for the PBU-L 106 unit C site with accurately measured geomechanical data and failure models is required for higher accuracy in geomechanical analysis.…”

“…The drained bulk and shear moduli are 95 MPa and 87 MPa for zero solid saturation (i.e., ice plus hydrate saturation), respectively, while drained bulk and shear moduli are 670 MPa and 612 MPa for full solid saturation, respectively. There are several experiments to estimate geomechanical properties for gas hydrate bearing sediments (Masui et al, 2005(Masui et al, , 2008Miyazaki et al, 2010c,a,b). The experiments and their estimates are based on Toyoura sand, which consists of silicon dioxide SO 2 as a major component.…”

DEM simulations of methane hydrate exploitation by thermal recovery and depressurization methods

“…It is not practical to carry out laboratory tests either, because the occurrence conditions of MH are too difficult to maintain and samples are easily disturbed during transportation. Therefore undisturbed samples are difficult to attain for laboratory tests [25,26] and many previous laboratory tests were performed with artificial samples. For example, Lee et al [27] studied the effect of MH saturation on volumetric changes during MH dissociation.…”