Microstructural evolution of gas hydrates in sedimentary matrices observed with synchrotron X‐ray computed tomographic microscopy

Abstract

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 submicron resolution. Here we report on synchrotron-based microtomographic studies by which the nucleation and growth processes of gas hydrate were observed at 276 K in various sedimentary matrices such as natural quartz (with and without admixtures of montmorillonite type clay) or glass beads with different surface properties, at varying water saturation. Both juvenile water and metastably gas-enriched water obtained from gas hydrate decomposition was used. 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, nucleation starts at the water-gas interface resulting in an initially several micrometer thick gas hydrate film; further growth proceeds to form isometric single crystals of 10-20 mm size. The growth of gas hydrate from gas-enriched water 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 micron thickness between gas hydrates and the surface of the quartz grains. These microstructural findings are relevant for future efforts of quantitative rock physics modeling of gas hydrates in sedimentary matrices and explain the anomalous attenuation of seismic/sonic waves.

On the path to the digital rock physics of gas hydrate-bearing sediments – processing of in situ synchrotron-tomography data

“…This paper complements the paper of Chaouachi et al 2015 with a sufficient workflow to process 3-D image data derived from synchrotronbased tomography to gain digitized results that are ready for subsequent use as a direct numerical model input. The data processing requires a pre-analysis to aim for best possible image quality enhancement.…”

“…In the following section the in situ experiment and scanning procedure will be described briefly as details are given in Chaouachi et al (2015) and Falenty et al (2015). As the nucleation and formation of hydrates within the pore space occurs on the nano-to microscale, high-resolution synchrotron radiation X-ray tomography (SRXCT) was the best experimental choice.…”

“…The primary challenge is found in the acquisition of highquality 3-D models that are capable of capturing fine features such as micro-cracks, fine porosity and grain-to-grain contacts that could potentially influence the computed properties (Kerkar et al, 2014;Chaouachi et al, 2015). Another critical point specific to the seismic anomaly in GH sediments is a clear identification of the at most a few-micrometers-thick water film separating mineral frame and GHs.…”

“…Although such models are readily available via in situ synchrotron radiation Xray tomography, the analysis of such data asks for complex workflows and high computational power to maintain valuable results. Here, we present a best-practice procedure complementing data from Chaouachi et al (2015) with data postprocessing, including image enhancement and segmentation as well as exemplary numerical simulations of an acoustic wave propagation in 3-D using the derived results. A combination of the tomography and 3-D modeling opens a path to a more reliable deduction of properties of gas hydrate-bearing sediments without a reliance on idealized and frequently imprecise models.…”

Squirt flow due to interfacial water films in hydrate bearing sediments

“…Image analysis has been accomplished 11 to reduce image artifacts, such as inhomogeneity in grey scale values, streaks and edge 12 enhancement by applying a systematic image enhancement workflow comprising different 13 image filter combinations in 2D and 3D (Sell et al, 2016). One of the most interesting 14 observations made was a systematic appearance of a thin interfacial water film separating the 15 quartz grains from the GH phase (Chaouachi et al, 2015). This fluid interface was observed in 16 samples where GH was formed in quartz sand samples directly from the juvenile state not 17 involving GH dissociation, as well as where GH was formed from gas-enriched water.…”