Tetrahydrofuran Hydrate in Clayey SedimentsLaboratory Formation, Morphology, and Wave Characterization

摘要

Fine-grained sediments host more than 90% of gas hydrates on Earth. However, the fundamental properties of hydrate-bearing silty and clayey sediments are much less understood than those of hydrate-bearing sands, mainly due to the experimental challenges in synthesizing gas hydrate in fine-grained sediments in the laboratory as the way they form in nature. This study forms tetrahydrofuran (THF) hydrate in kaolinite, visualizes the hydrate distribution and morphology using X-ray computed tomography, and uses P and S waves to characterize the formed hydrate-bearing clayey sediments. The results show that THF hydrate formed in clay is innately segregated and heterogeneous, no longer as a pore constituent as in sandy sediments. Hydrate nucleation, growth, and distribution in clays are dominated by the thermal condition and constrained by water activity and mass transport, which can result in low stoichiometric-solution-to-hydrate conversion ratios of approximately 0.4-0.7. Thus, the estimation of hydrate volume in clays based on the mass of stoichiometric solution can be erroneous. The heterogeneity in hydrate-bearing clays imposes challenges in wave velocity based characterization. The bulk elastic properties of hydrate-bearing clays can be well predicted using the self-consistent model. Specimens with higher hydrate volume fraction VFh show higher wave attenuations Q(-1), highlighting the dominant role of THF hydrate in the wave attenuation of hydrate-bearing clays. The results suggest that Q(p)(-1)=0.08+0.4VF(h)=2Q(s)(-1), which underlines the potential of using wave attenuation based methods to quantify the hydrate volume fraction in clayey sediments.