Research On The Strength Characteristics Of Gas Hydrate-bearing Sediments In The Permafrost

Gas hydrate has become a kind of worth commercial new energy because of its huge carbon, easing the global energy crisis. Chinese onshore permafrost zone which covers a vast area is always the potential of gas hydrates. However, gas hydrates in permafrost regions have enormous environmental risks, hydrate exploration will result in permafrost hydrate-bearing strength decreases, deformation; even cause landslides and ground subsidence and other natural disasters. Therefore, methane hydrate mechanical characteristics research and the effect of different mining methods on the mechanical properties of methane hydrate-bearing sediments have guiding significance for commercial exploitation.This paper was carried out by the following experiments:Firstly, this study uses the low temperature high hydrate triaxial statics experiment instrument developed by Dalian university of technology, analyzing and contrasting the strength characteristics of methane hydrate sediments in the skeleton of different granular media (quartz sand, kaolin clay and the mixture of quartz sand and kaolin clay) synthesized in laboratory. The results show that the fracture strength of methane hydrate sediments increased with the increase of confining pressure and sand content, and increased with temperature decrease. We found that cohesion is less affected by sand content, internal friction Angle increased significantly with the increase of the sand content, using Mohr coulomb criterion analysis.In addition, this paper also analysis the effect of the different sand grain diameter on the mechanical properties of methane hydrate-bearing sediments. The experimental results show that sand particles, the more thick, the greater the breakdown strength of methane hydrate sandy sediments. Using Mohr coulomb criterion analysis we found that cohesion is less affected by sand grain diameter, sand particles, the thicker, the greater the Angle of internal friction.The mining of methane hydrate can cause the decrease of strength of hydrate permafrost, softening and deformation, therefore, development of methane hydrate need to understand the effect of different mining methods on the mechanical properties of methane hydrate-bearing sediments. In this paper, with two kinds of hydrate dissociation way (depressurization and heating) for hydrate dissociation in triaxial pressure chamber, the mechanical properties of methane hydrate-bearing sediments after dissociation were measured by a low-temperature and high-pressure triaxial compression apparatus under consolidated exhaust shear test and consolidated non-exhaust shear test. In addition, triaxial shear test of the CO2hydrate-bearing sediments and CH4hydrate-bearing sediments before dissociation were also carried out in the same conditions. The experimental results show that the strength of the methane hydrate-bearing sediments after dissociation is less than that of the methane hydrate-bearing sediments before dissociation under the two kinds of dissociation way. The shear strength under consolidated exhaust shear test and consolidated non-exhaust shear test is also quite different. Furthermore, combining the strength differences between the CO2and CH4hydrate-bearing sediments, this paper compares the mining safety of three different methods, namely, the CH4-CO2replacement method, depressurization and heating.