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Upscaling of Pore Network and Permeability from Micron to Millimeter Scale in Organic-Pore Dominated Mudstones


Pore-scale imaging and permeability modeling provides an important pathway to understand mudstone pore structure and network, which is critical for efficient shale gas and oil production. The technique for 3D nm-scale pore characterization, FIB/SEM, however, can only address a very small volume, usually a few microns, of rock matrix. Given the known heterogeneities of mudstones, data from small volume FIB/SEM studies are not adequate for representative characterization of pore structure and pore-permeability relationships. We propose an improved strategy for upscaling pore networks and permeability in organic-pore dominated mudstones from the μm to mm scale. The Barnett shale, in which as much as 90% of the pore volume is developed in organic pores, was sampled and milled using a FIB/SEM as an example. 3D image analysis was then conducted to characterize the pores, including porosity, pore size, geometric tortuosity, and pore connectivity. The resulting porosity and pore size distribution are in good agreement with previous findings based on SEM image analysis. Permeability, which was calculated from pore-scale modeling, displays micro Darcy values for local pore clusters. Kerogen particle size, connectivity, and distribution were also analyzed using FIB/SEM images. These data show that large kerogen particles, with sizes > 0.5 μm, are more commonly connected than smaller particles, which are more likely to be isolated. Therefore, a 3D connected-kerogen-particle network (in mm-scale) can be defined with micro-CT analysis (at a resolution of 0.5 μm). The kerogen network is the fundamental framework of the connected organic pore network, since organic pores can be connected only if the surrounding organic matter (kerogen) is connected. The integration of nm-scale pore FIB/SEM analysis and mm-scale CT analysis can produce an organic pore network model that can form the basis for understanding and characterizing pore structure and permeability. This upscaling strategy has the potential to further the understanding of pore network and pore-permeability relation of mudstones, facilitating more efficient shale oil and gas exploration and recovery.