An experimental investigation on methane hydrate morphologies and pore habits in sandy sediment using synchrotron X-ray computed tomography

Abstract

As pore-scale morphologies and spatial distribution (pore habits) of natural gas hydrates in marine sediments considerably affect their physical/mechanical properties, they have extensively been investigated by X-ray computed tomography (XRCT) and especially synchrotron X-Ray computed tomography (SXRCT). While both image spatial and scan temporal resolutions are being improved over time, it is still challenging to distinguish water from methane hydrate in an image due to their low absorption contrast. In this study, methane hydrate formation and growth in wet sand were observed at submicron/micron scale using SXRCT. Saline water (Potassium iodide -KI) was used in order to improve the image contrast. Evolutions of methane hydrate morphologies and distribution at both the pore and sample scales were observed. Results are discussed based on various mechanisms related to material behaviors and experimental conditions, e.g. suction variation during methane hydrate formation, local temperature gradient in the sample, and particularly the interaction of X-rays with the sample. Methane hydrate formation is interpreted as a dynamic process, favoring the Ostwald ripening. Furthermore, morphologies and pore habits of methane hydrates under excess-gas and excess-water conditions are discussed. Some recommendations are finally given for further studies on methane hydrates-bearing sediments via XRCT or SXRCT.

Mechanical behaviors of hydrate-bearing sediment with different cementation spatial distributions at microscales

“…(2016) . This is because although water can migrate among the sand particles ( Le et al., 2020 ; Nikitin et al., 2020 ), it is still physically impossible to float at the pore space in a gas excess environment. Figure 1 E shows the hydrate spatial distribution influence on the radii distribution of the pore space.…”

“…Xenon gas is used to generate xenon hydrate (XeH) instead of methane gas to enhance the phase contrast in the CT images. Moreover, it must be noted that the physical characteristics between the XeH and NGH are also quite close ( Chaouachi et al., 2015 ; Ning et al., 2012 ), as are the pore-scale occurrences between XeH and NGH following the similar formation procedure ( Lei et al., 2019 ; Le et al., 2020 ). Furthermore, the substitutability of XeH for NGH for some pore-habit evolution studies has been widely accepted ( Chaouachi et al., 2015 ; Yang et al., 2016 ).…”

“…Nevertheless, the vast majority of studies did not consider the hydrate cementation spatial distribution influence on the mechanical behavior of hydrate-bearing sediments due to the lack of an effective nondestructive technique. Moreover, in nature, the hydrate cementation pore habit was reported to be heterogeneous affected by the effective stress, pore-size-dependent capillary pressure, and hydrocarbon accumulation history, which has been assessed directly by X-ray computed tomography (CT) (Lei et al, 2019;Le et al, 2020) or magnetic resonance imaging (MRI) (Ji et al, 2020;Song et al, 2015;Zhang et al, 2020b) and indirectly by comparing the measured seismic velocities and those calculated via elasticity models (Dvorkin et al, 2000). Luo et al (2015) studied the effect of a heterogeneous distribution of the hydrates on the wave velocity of the sediment and found that the compressional wave velocity increases with the hydrate saturation, while the heterogeneous distribution of the hydrates can have a remarkable influence on the results, especially in the low-saturation condition.…”

The Change in Geomechanical Properties of Gas Saturated Methane Hydrate‐Bearing Sand Resulting From Water Saturation

“…The proposed growth mechanism would lead to hydrate growth stiffening up particle contacts from the outset, which is supported by the rapid increase in G max as hydrate formation proceeds (Figure 4) as observed in our tests as well as others (Clayton et al, 2005;Sultaniya et al, 2018) where the excess-gas method is used. using X-ray CT imaging suggest that in a gas-saturated environment hydrate growth occurs on the mineral surfaces and grows into the pore space, while Le et al (2020) showed that both mineral coating hydrate and hydrate at grain contacts can exist locally. Given the small sampling size used in CT studies, it is highly probable that at the specimen scale extensive grain contacts "cemented" by hydrate exists that leads to the large stiffening effect.…”

Combining Optical Microscopy and X-ray Computed Tomography Reveals Novel Morphologies and Growth Processes of Methane Hydrate in Sand Pores

“…Even though it is helpful to build geophysical and reservoir models, this categorization of hydrate distribution within the pore space lacks direct experimental confirmation. The very scarce high-resolution imaging studies by means of synchrotron X-ray computed tomography (XRCT) in fact point to more complex features of gas hydrate formation and growth in sediment pores [16][17][18][19][20][21].…”