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Multiscale Characterization of Organic Hosted Porosity in Gas Bearing Shales


Pore scale imaging has developed over the last 20 years from a primarily academic technique used into a crucial industrial tool. One of the largest challenges facing today's engineers when using such techniques is that of geological heterogeneity. Frequently the resolution required to resolve fundamental pore structures comes at the expense of a field of view representative of subsurface heterogeneity. This issue is particularly problematic in organic rich shale reservoirs where pore structures range can from the mm to the sub nm scale. Helium Ion Microscopy (HIM) presents an unique technique by which we can examine such systems. The lower De-Broglie wavelength of the helium ion enables for sub-nanometer resolutions, while the scanning nature of technique releases the stringent sample requirements of TEM. What is more the same sample (prepared in the same way) can be used for imaging using light, x-ray, electron and charged ion microscopy. Previously, however, HIM was limited to 2D analysis, meaning any information about pore connectivity remained out of reach. In this paper we will show the first 3D results from this technique, imaging and analyzing an organic hosted porous network from a North American reservoir shale sample. We also show how this technique can be integrated with other microscopy platforms to enable a full characterization of shale heterogeneity. First, images of the whole sample were taken with light, x-ray and SEM techniques. These were then imported into a single correlative imaging environment, enabling high resolution 3D HIM imaging to be targeted at a representative organic regions, imaging it with a spatial resolution of 1×1×5nm. The organic hosted porosity was segmented, and found to be poorly connected, with no percolating network found, despite a moderately high porosity (around 8% of the organic region). This may well be due to the dramatic difference in pore shape between the secondary organic hosted porosity (typically convex in shape) and the primary intra-granular porosity familiar from conventional systems (typically concave in shape). These results are critically important for understanding matrix flow of hydrocarbons through such systems. With no percolating pathway, the organics hosting the porosity must deform to enable flow, inducing complex stress fields on the surrounding regions. This would be an interesting area for future study, potentially using in situ mechanical testing coupled with nano-scale X-ray CT.