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Ultrahigh-Resolution 3D Imaging for Quantifying the Pore Nanostructure of Shale and Predicting Gas Transport

Pore and fracture microstructure is key to understanding gas flow in shales. The experimental determination of these microstructures is dependent on the measurement technique employed and its resolution. High-resolution three-dimensional imaging techniques coupled with image analysis and direct numerical simulations have been employed to characterise the petrophysical properties of shale samples. In this work our particular focus is on using the Nano-CT and FIB-SEM techniques at the same location in a shale rock sample in order to investigate the effect of their different resolutions and fields of view on the resulting imaged nano-pore structure, as well as to determine any differences in the consequent measurements of the shale petrophysical properties. These petrophysical properties include porosity, permeability, pore volume and size distribution, pore aspect ratio, surface area to volume and pore connectivity. The reconstructed matrix, kerogen and pore space volumes from each approach showed significant scale-dependent differences in microstructure. The shale sample displayed a high kerogen content with high connectivity. Porosity from the reconstructed shale volumes was observed to be 0.43% and 0.7% for FIB-SEM and Nano-CT approaches, respectively. Pore volume, size, surface area to volume ratio and two orthogonal pore aspect ratio distributions have also been extracted from the reconstructed image data by 3D image analysis. These data show that voids within the rock are oblate at all scales. Permeabilities have been calculated from both the FIBSEM and Nano-CT images and fall in the range 2.55 nD to 9.92 nD. A simulation has also been produced based on the permeability calculation and parameters from the image analysis. The results of the simulation of the show connectivity in the x-, yand z-directions for both the FIB-SEM and Nano-CT images, with very low connectivity in the x-direction, but higher connectivity in the yand z-directions.