Experimental Study On Hydrate Saturation And Mechanical Properties Of Gas Hydrate-Bearing Sediments

Methane hydrate, which is considered as a new energy to substitute for the oil in thefuture, exists in permafrost layers and in deep ocean floors. However, during the process ofdrilling for exploration and production, the hydrate dissociation may reduce hydratesaturation and shear strength of geological formations with consequent of the reduction infoundation capacity, which affects the stability of surface equipment and even triggers atsunami. Therefore, it is important to study hydrate saturation and the mechanical propertiesof gas hydrate-bearing sediments.TDR was used to determine dielectric constant of THF aqueous solution with differentTHF/H2O mole ratios before and after THF hydrate formation. As a result, the empiricalformula was built to measure the water content of the system of “THF aqueous solution+THF hydrate”. Meanwhile TDR was used to measure THF and methane hydrate saturation insediments respectively. Through the comparison between TDR measurement and theoreticalvalue of THF hydrate saturation and the comparison between TDR measurement and pressuredrop calculation of methane hydrate saturation, the errors are shown extremely small, whichmeans that TDR measurement is reliable.With the integrated apparatus of gas hydrate deposit syntheses and triaxial test, themethane hydrate in the skeleton of natural sand is synthesized and then a series of un-drainedtriaxial tests were carried out. The results indicate that the damage of methanehydrate-bearing sediments is brittle failure. The shear strength and elasticity modulusincrease with the increases of effective confining pressure and the hydrate saturation underthe same type of sediments; The increasing amplitude of shear strength and elasticitymodulus is smaller when the hydrate saturation is low (less than about35%), but theincreasing amplitude is bigger when the hydrate saturation is high (about35%to about55%);The internal friction angles is not sensitive to hydrate saturation, but the cohesion shows a high hydrate saturation dependency; Based on the results, a model relating shear strength tohydrate saturation and effective confining pressure has been presented. Meanwhile, theresults showed that the shear strength of methane hydrate-bearing fine-grained sediments isgreater than methane hydrate-bearing coarse-grained sediments under the same hydratesaturation and effective confining pressure; The shear strength of methane hydrate-bearingsediments is greater than THF hydrate-bearing sediments under the same type of sediments,hydrate saturation, and confining pressure.