The mass of carbon trapped in methane hydrates exceeds that in conventional fossil fuel reservoirs. While methane in coarse-grained hydrate-bearing sediments is technically recoverable, most methane hydrates are found in fine-grained marine sediments where gas recovery is inherently impeded by very low gas permeability. Using experimental methods and analyses, this thesis advances the understanding of fine-grained sediments in view of gas production from methane hydrates. The research scope includes: a new approach for the classification of fines in terms of electrical sensitivity, the estimation of the sediment volume contraction during hydrate dissociation, a pore-scale study of gas migration in sediments and the self-regulation effect of surfactants, the formation of preferential gas migration pathways at interfaces during gas production, pressure core technology for the characterization of hydrate bearing sediments without causing hydrate dissociation, and the deployment of a bio-sub-sampling chamber in Japan.

Small-scale morphology of gas hydrate is important for understanding the formation of gas hydrate deposits, for estimating the concentrations of gas hydrate from geophysical data, and for predicting their response to climate change or commercial production. The recent use of borehole pressure coring tools has allowed marine gas-hydrate-bearing sediments to be recovered with centimeter to sub-millimeter gas hydrate structures preserved in their in situ condition. Once these sediment samples are recovered at in situ temperature and pressure, nondestructive analyses, including gamma density, P-wave velocity, and X-ray imaging, are used to examine the character of the gas hydrate relative to the structure of the surrounding sediment. Gas hydrate morphology from pressure core data is summarized from the recent national gas hydrate expeditions of India, China, and Korea, as well as from Ocean Drilling Program Leg 204, Integrated Ocean Drilling Program Expedition 311, and the Gulf of Mexico Chevron-Texaco Joint Industry Project. The most striking result is the variability of gas hydrate morphology in clay, ranging from complex vein structures to an invisible pore-filling matrix. Both of these morphologies have been observed in clay sediments at gas hydrate saturations equivalent to 30-40% of pore volume. A clear knowledge of detailed gas hydrate morphology will provide important data to help determine the mechanisms of gas hydrate deposit formation and also provide crucial data for modeling the kinetics of deposit dissociation, from both natural and artificial causes. The morphology also has large effects on sedimentary physical properties, from seismic velocities on a large scale to borehole electrical resistivities on a smaller scale, and gas hydrate morphology will therefore impact estimation of gas hydrate saturation from geophysical data. The detailed morphology of gas hydrate is an essential component for a full understanding of the past, present, and future of any gas hydrate environment.

Gas hydrate is a naturally occurring crystalline compound formed by water molecules and encapsulated gas molecules. The interest in gas hydrate reflects scientific, energy and safety concerns - climate change, future energy resources and seafloor stability. Gas hydrates form in the pore space of sediments, under high pressure and low temperature conditions. This research focuses on the fundamental understanding of hydrate bearing sediments, with emphasis on mechanical behavior, thermal properties and lens formation. Load-induced cementation and decementation effects are explored with lightly cemented loose and dense soil specimens subjected to ko-loading; the small-strain stiffness evolution inferred from shear wave velocity measurement denounces stiffness loss prior to structural collapse upon loading. Systematic triaxial tests address the intermediate and large strain response of hydrate bearing sediments for different mean particle size, applied pressure and hydrate concentration in the pore space; hydrate concentration determines elastic stiffness and undrained strength when Shyd>45%. A unique sequence of particle-level and macro-scale experiments provide new insight into the role of interparticle contact area, coordination number and pore fluid on heat transfer in particulate materials. Micro-mechanisms and necessary boundary conditions are experimentally analyzed to gain an enhanced understanding of hydrate lens formation in sediments; high specific surface soils and tensile stress fields facilitate lens formation. Finally, a new instrumented high-pressure chamber is designed, constructed and field tested. It permits measuring the mechanical and electrical properties of methane hydrate bearing sediments recovered from pressure cores without losing in situ pressure (~20MPa).

Gas hydrate investigations are converging on a suite of common techniques for hydrate observation and quantification. Samples retrieved and analyzed at full in situ pressures are the ”gold standard” with which the physical and chemical analysis of conventional cores, as well as the interpretation of geophysical data, are calibrated and groundtruthed. Methane mass balance calculations from depressurization of pressure cores provide the benchmark for gas hydrate concentration assessment. Nondestructive measurements of pressure cores have removed errors in the estimation of pore volume, making this methane mass balance technique accurate and robust. Data from methane mass balance used to confirm chlorinity baselines makes porewater freshening analysis more accurate. High-resolution nondestructive analysis of gas-hydratebearing cores at in situ pressures and temperatures also provides detailed information on the in situ nature and morphology of gas hydrate in sediments, allowing better interpretation of conventional core thermal images as well as downhole electrical resistivity logs. The detailed profiles of density and Vp, together with spot measurements of Vs, electrical resistivity, and hardness, provide background data essential for modeling the behavior of the formation on a larger scale. X-ray images show the detailed hydrate morphology, which provides clues to the mechanism of deposit formation and data for modeling the kinetics of deposit dissociation. Gashydrate- bearing pressure cores subjected to X-ray tomographic reconstruction provide evidence that gas hydrate morphology in many natural sedimentary environments is particularly complex and impossible to replicate in the laboratory. Even when only a small percentage of the sediment column is sampled with pressure cores, these detailed measurements greatly enhance the understanding and interpretation of the more continuous data sets collected by conventional coring and downhole logging. Pressure core analysis has become the keystone that links these data sets together and is an essential component of modern gas hydrate investigations.

An extensive amount of natural gas trapped in the subsurface is found as methane hydrate. A fundamental understanding of natural hydrate-bearing sediments is required to engineer production strategies and to assess the risks hydrates pose to global climate change and large-scale seafloor destabilization. This thesis reports fundamental studies on hydrate nucleation, morphology and the evolution of unsaturation during dissociation, followed by additional studies on sampling and pressure core testing. Hydrate nucleation is favored on mineral surfaces and it is often triggered by mechanical vibration. Continued hydrate crystal growth within sediments is governed by capillary and skeletal forces; hence, the characteristic particle size d10 and the sediment burial depth determine hydrate morphologies in natural sediments. In aged hydrate-bearing sand, Ostwald ripening leads to patchy hydrate formation; the stiffness approaches to the lower bound at low hydrate saturation and the upper bound at high hydrate saturation. Hydrate saturation and pore habit alter the pore size variability and interconnectivity, and change the water retention curve in hydrate-bearing sediments. The physical properties of hydrate-bearing sediments are determined by the state of stress, porosity, and hydrate saturation. Furthermore, hydrate stability requires sampling, handling, and testing under in situ pressure, temperature, and stress conditions. Therefore, the laboratory characterization of natural hydrate-bearing sediments faces inherent sampling disturbances caused by changes in stress and strain as well as transient pressure and temperature changes that affect hydrate stability. While pressure core technology offers unprecedented opportunities for the study of hydrate-bearing sediments, careful data interpretation must recognize its inherent limitations.