Microstructure of Gas Hydrates in Sedimentary Matrices

Abstract

Gas hydrates (GH) are found worldwide in marine sediments and permafrost regions. The detection and quantification of gas hydrates present in marine sediments is crucial for safe oil and gas extraction, seafloor stability assessments and for quantifying the impact of GH in climatic change. Therefore, there is a considerable interest in studying the microstructure of gas hydrate-bearing sediments. Although a large amount of research on gas hydrates has been carried out over the years, the micro-structural aspects of gas hydrate growth and the crystallite sizes of hydrate in sediments are still poorly known and understood. The formation process of gas hydrates in sedimentary matrices is of crucial importance for the physical and transport properties of the resulting aggregates. This process has never been observed in-situ at sub-micron resolution. In this study, the nucleation and growth processes of GH were observed using synchrotron X-ray micro-computed tomography at 276 K in various sedimentary matrices such as natural quartz (with and without admixtures of clay minerals) or glass beads at varying water saturation. The process was observed on a timescale of a few minutes to many hours. Both, juvenile water as well as gas-enriched water obtained from gas hydrate decomposition was used in the experiments. Xenon gas was employed to enhance the density contrast between gas hydrate and the fluid phases involved. The nucleation sites can be easily identified and the various growth patterns are clearly established. In sediments under-saturated with juvenile water the nucleation starts at the water-gas interface resulting in an initially several micrometer thick gas hydrate film; the further growth proceeds to form chains of predominantly isometric single crystals. The growth of gas hydrate from gas-enriched water clearly follows a different pattern, via the nucleation in the bulk of liquid producing polyhedral single crystals. A striking feature in both cases is the systematic appearance of a fluid phase film of up to several µm thickness between gas hydrates and the surface of the quartz grains. It appears that the initially different growth morphologies of gas hydrate tend to move towards more similar arrangements with coarsening of gas hydrate crystals. The initial microstructural findings obtained in laboratory experiments cannot be compared to the real situation in natural environments as the p-T conditions are different. However, the final microstructure of the system is comparable for both cases and experimental studies can permit to better understand the microstructures of GH aggregates and to tighten their physical properties required for physical detection and quantification. Several intrinsic properties of gas hydrates e.g. low affinity between gas hydrate and sediment grains revealed by laboratory experiments can be expected to remain unchanged in natural settings. New micro-structural models were developed based on these findings in order to explain the so-called seismic anomalies of high wave velocity and high wave attenuation characterizing GH-bearing sediments. The developed models are relevant for future efforts of quantitative rock physics modeling of effective elastic properties and electromagnetic properties of gas hydrates in sedimentary matrices. The imaging of gas hydrate microstructures was complemented by a quantitative analysis of the in-situ evolution of the crystallites size distribution of synthetic xenon hydrates and of natural gas hydrate-bearing sediments retrieved from Mallik 5L-38 research well. This study revealed that the crystal sizes of gas hydrate increase rapidly with time during the nucleation and initial growth processes. After completion of the hydrate formation process, an Ostwald ripening or normal grain growth process takes place; the number of gas hydrate crystals decreases and their intensities increase indicating the agglomeration of GH crystals into larger masses to reduce the interfacial energies of the system.