Effect of Confining Pressure on Triaxial Compressive Properties of Artificial Methane Hydrate Bearing Sediments
Authors: Miyazaki Kuniyuki , Masui Akira , Sakamoto Yasuhide , Tenma Norio , Yamaguchi Tsutomu 
Published: July 2010

Abstract
It is essential to consider the mechanical behavior of natural gas hydrate reservoir to have sustainable production of natural gas from the reservoir under the seafloor. Although the mechanical properties of marine sediments containing natural gas hydrates are essential to simulate the geo-mechanical response to gas production from the reservoir, they are not fully understood. In this study, effect of confining pressure on mechanical properties of artificial methane hydrate bearing sediment was experimentally examined and discussed. Drained triaxial compression tests were conducted for artificial sediment specimens containing Toyoura sand (average particle size D50 = 230 × 10-6 m and fine fraction content Fc = 0 %) and synthetic methane hydrate (hydrate-sand specimen), following the testing methods of Masui et al. (2005). During axial loading process in the tests, the specimen was compressed axially at 0.1 %/min of strain-rate under hydrate-stable condition, with 278 K of temperature and 8 MPa of pore water pressure. The cell pressure, or total confining pressure, was kept constant at 8.5 MPa, 9 MPa, 10 MPa or 11 MPa. Axial displacement and lateral displacement were measured with 25 mm and 5 mm linear variable differential transformers. Axial load was measured with a 50 kN load cell. The findings can be summarized as follows; (1) the hydrate-sand specimen becomes ductile by increase of confining pressure like many other geological materials; (2) the hydrate-sand specimen appears to be restrained from expanding diametrically by confining pressure; (3) strength and stiffness of the hydrate-sand specimen increases with confining pressure; (4) strength of No.8 silica sand containing methane hydrate (Masui et al. 2008) does not greatly differ from that of hydrate-sand specimen in this study; and (5) cohesion and internal friction angle of the hydrate-sand specimen, or the Mohr-Coulomb failure criterion, can be formulated as a function of methane hydrate saturation. These findings concerning the effect of confining pressure will be of considerable help not only in full understanding of the deformation mechanism of methane hydrate bearing sediments, but also in proposal of constitutive equation and numerical simulation in the future.

Introduction
Gas hydrates consist of cage-like crystal structures made up of hydrogen-bonded water molecules surrounding a guest molecule of such gases as methane, ethane, propane, carbon dioxide or hydrogen sulfide. Natural gas hydrates existing in marine sediments or in permafrost regions worldwide are anticipated to be a promising energy resource of natural gas (Makogon 1982; Kvenvolden 1988; Kvenvolden et al. 1993).
It is essential to consider the mechanical behavior of gas hydrate reservoir in order to have sustainable production, because it may have some influence on the stability of the wellbore or the other subsea structures, geo-hazards (e.g., subsea landslides and seafloor subsidence) or gas productivity (Collett and Dallimore 2002; Bugge et al. 1988; Kleinberg 2005; Sakamoto et al. 2009). Although the mechanical properties of marine sediments containing natural gas hydrates are essential to simulate the geo-mechanical response to gas production from the reservoir, they are not fully understood.