Abstract
The spatial resolution of 3D imaging techniques is often balanced by the achievable field of view. Since pore size in shales spans more than two orders of magnitude, a compromise between representativeness and accuracy of the 3D reconstructed shale microstructure is needed. In this study, we characterise the effect of imaging resolution on the microstructural and mass transport characteristics of shales using micro and nano-computed tomography. 3D mass transport simulation using continuum and numerical physics respectively is also compared to highlight the significance of the Knudsen effect on the reconstructed solid surface. The result shows that porosity measured by micro-CT is 25% lower than nano-CT, resulting in an overestimated pore size distribution and underestimated pore connectivity. This leads to a higher simulated intrinsic permeability. An overestimated diffusive flux and underestimated permeability are obtained from the continuum mass transport simulation compared to the numerical ones when the molecular-wall collision is accounted, evidenced by the large deviation of the measured Knudsen tortuosity factor and permeability correction factor. This study is believed to provide new knowledge in understanding the importance of imaging resolution and gas flow physics on mass transport in porous media.
Highlights
Www.nature.com/scientificreports nanometre to micrometre)[16]
Two case studies will be presented to highlight (1) the effect of imaging resolution on describing the microstructural characteristics and mass transport properties in the direction parallel to www.nature.com/scientificreports the horizontal natural bedding of the shale gas sample; (2) the disparity of obtained mass transport parameters vertical to the natural bedding between continuum and numerical computational fluid dynamics (CFD) simulation attributed to the captured sub-micron 3D pore network
This study aims to compare the microstructural metrics and mass transport parameters as a consequence of the extra porosity which can be imaged in the nano-Computed Tomography (CT)
Summary
Www.nature.com/scientificreports nanometre to micrometre)[16]. A previous study[17] characterized the gas flow in micro and nanopores using ideal cylindrical pore model, which cannot account for the effect of complex surface roughness of the wall, the constriction and the arbitrary morphology. The wide distribution of the pore size causes two problems in the mass transport study: (1) it is not reliable to estimate the Knudsen-based diffusivity based on the averaged pore size, which could potentially over-estimate the gas flow due to the constriction effect[21,22]; (2) Viscous flow fails in smaller pore spaces as the diffusion flow mechanisms associated with pore-wall interactions become dominant[23], which leads to under-estimating the permeability This means conventional continuum physics can no longer describe the flow field in shales[24]. DSMC has been applied to study the gas flow in a variety of materials of distinct pore morphologies, such as in solid oxide fuel cells[31], cylindrical channels[32,33], random/aligned fiber orientations[30,34,35] and ablative materials[29]
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