X‐ray computed tomography (CT) has become a critical technique in the study of porous media. It has attracted growing attention for analyzing hydrate‐bearing sediment, but this has been done using surrogates (Xe/Kr) only due to difficulties in distinguishing methane hydrate from water. This study presents the successful imaging of methane hydrate coexisting with pore liquid, gas, and sediments. We used potassium iodide (KI) solutions and in‐line propagation‐based phase‐contrast CT analysis of X‐ray attenuation and diffraction to distinguish the four materials. Thus, consideration for CT‐related X‐ray physics was necessary to optimize KI concentrations, improve material separation with X‐ray propagation, and properly interpret artifacts within the images. The images clearly show methane hydrate in the pore space of sand (~250 μm) coexisting with KI solution and gas. Following this, X‐ray CT can now be used to visualize pore habits of natural methane hydrate in sediment cores.

Fine‐grained sediments limit hydrate nucleation, shift the phase boundary, and hinder gas supply. Laboratory experiments in this study explore different strategies to overcome these challenges, including the use of a more soluble guest molecule rather than methane, grain‐scale gas‐storage within porous diatoms, ice‐to‐hydrate transformation to grow lenses at predefined locations, forced gas injection into water saturated sediments, and long‐term guest molecule transport. Tomographic images and thermal and pressure data provide rich information on hydrate formation and morphology. Results show that hydrate formation is inherently displacive in fine‐grained sediments; lenses are thicker and closer to each other in compressible, high specific surface area sediments subjected to low effective stress. Temperature and pressure trajectories follow a shifted phase boundary that is consistent with capillary effects. Exo‐pore growth results in freshly formed hydrate with a striped and porous structure; this open structure becomes an effective pathway for gas transport to the growing hydrate front. Ice‐to‐hydrate transformation goes through a liquid stage at premelt temperatures; then, capillarity and cryogenic suction compete, and some water becomes imbibed into the sediment faster than hydrate reformation. The geometry of hydrate lenses and the internal hydrate structure continue evolving long after the exothermal response to hydrate formation has completely decayed. Multiple time‐dependent processes occur during hydrate formation, including gas, water and heat transport, sediment compressibility, reaction rate, and the stochastic nucleation process. Hydrate formation strategies conceived for this study highlight the inherent difficulties in emulating hydrate formation in fine‐grained sediments within the relatively short time scale available for laboratory experiments.

Mechanical behavior of hydrate‐bearing sediments is critical for well stability, reservoir deformation, and sea floor settlement during gas production. Current understanding on the mechanism of hydrate strengthening hosting sediments relies on conceptual models based on idealistic pore‐scale assumptions. Yet pore‐scale study is rare and limited to hydrate‐bearing sediments formed with surrogate guest molecules rather than methane. We present for the first time pore‐scale triaxial test results of methane hydrate‐bearing sediments. Besides the traditional stress‐strain relationship, we further explored (1) sand particle crushing and (2) pressure‐temperature dependent strength variations and (3) creep of hydrate‐bearing sediments. Results show that as hydrate enables the sand skeleton to bear additional loads, the potential of sand crushing upon hydrate dissociation also increases. Strength of hydrate‐bearing sediments decreases as pressure‐temperature condition approaches hydrate phase boundary. Hydrate‐bearing sediments creep and heal with time. The new observations suggest additional complications to be considered during gas production.

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 VF_h 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.

Physical properties of hydrate‐bearing sediments are often correlated with hydrate saturation with little or no information on hydrate distribution uniformity in the specimens. This study focuses on water redistribution and sediment skeleton shift depending on various hydrate formation conditions in unsaturated systems, as well as on the resulting hydrate distribution patterns. Using X‐ray computed tomography, we investigate the factors such as fines content and the pressure‐temperature path on mass migration during carbon dioxide hydrate formation. The experiments show water migration, preferential hydrate formation toward the core periphery, localized patchy hydrate distribution, and sediment particle movement toward the core center. Sediment particle movement can be impeded in densely packed specimens. The overall mass migration due to hydrate formation can be significantly suppressed by adding 5% by mass of kaolinite. Hydrate formation initiated by pressurization and then cooling causes less mass migration than the cases where hydrate is formed using cooling followed by pressurization or pressurizing frozen cores followed by heating methods. Freezing can induce water migration and particle pushing in a similar manner as hydrate formation. Image analyses show that the pressure‐temperature path and the rates of heat transfer during hydrate nucleation and growth govern the uniformity of hydrate distribution in sediments.