Experimental Study On The Relationship Between Gas Hydrate Saturation And Acoustic Velocities In Porous Media

Marine gas hydrate, potential fossil energy resources, has attracted more and more attention. As an economic and effective technique, marine geophysical survey is still the primary explorative technology at present. Relationship between gas hydrate saturation and acoustic characteristics in marine sediments is the foundation of geophysical exploration. However, it's unclear how gas hydrate affect seismic wave.There are many existing models to describe the relationship between acoustic velocities and gas hydrate saturation. However, different models used in the same area may bring on different results. It's necessary to confirm the legitimacy of the models and get an approach to use them correctly.Interpretation of seismic data, afterward locating of gas hydrate and estimation of resource require to understand the relationship between physical properties and saturation in gas hydrate-bearing sediments.An experimental apparatus was developed in the thesis. The apparatus combined high precision ultrasonic detection with Time Domain Reflectometry (TDR) in the same experimental apparatus to study the relationship between gas hydrate saturation and acoustic characteristics. Gas hydrate was formed and dissociated in artificial core and unconsolidated sediments for 58 experimental runs. Ultrasonic and TDR can detect the formation and dissociation of gas hydrate in porous media sensitively. The results show the experimental method is very effective.Relationship between gas hydrate saturation and acoustic velocities (including Vp and Vs) in consolidated artificial core was gained. Observed data in experiments was compared with the calculated data by time-average equation, Wood's equation, Lee's weighted equation and Biot-Gassmann Theory by Lee (BGTL). It suggests that the Lee's weighted equation and BGTL are more applicable in our experiments and various sediments. The thesis also gave some instructive advices about the selection of W, n and G, n values in Lee's weighted equation and BGTL.Experimental results revealed that the relationship between gas hydrate saturation and acoustic characteristics in sediments is quite complicated, and single model or parameter can't be applicable for all situations. It is indicated that the micromodels of gas hydrate formed in the artificial core are grain contact model or cementation model in our experiments. However gas hydrate is formed firstly on the skeleton and cement the granules in unconsolidated sediments experiments, afterward they grow suspending in the pore space.Although the physical characteristics of experimental materials are different from them of actual marine sediments, the experimental results can direct practical application in some sense. Actual marine sediment cores will be used in similar experiments to study the relationship between gas hydrate saturation and acoustic parameters. Parameters of the models determined by experiments will be useful in practical application.