Porosity and permeability determinations of organic rich Posidonia shales based on 3D analyses by FIB-SEM microscopy

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Abstract

Abstract. The goal of this study is to better understand the porosity and permeability in shales to improve modelling fluid and gas flow related to shale diagenesis. Two samples (WIC and HAD) were investigated, both Mid Jurassic Posidonia organic rich shales from central Germany of different maturity (WIC R0 0.53 % and HAD R0 1.45 %). The method for image collection was Focused Ion Beam (FIB) microscopy coupled with Scanning Electron Microscopy (SEM). For image and data analysis Avizo and GeoDict was used. Porosity was calculated from segmented 3D FIB based images and permeability was simulated by a Navier Stokes-Brinkman solver in the segmented images. Results show that the quantity and distribution of pore clusters and pores (> 40 nm) are similar. The largest pores are located within carbonates and clay minerals, whereas the smallest pores are within the matured organic matter. Orientation of the pores calculated as pore paths showed minor directional differences between the samples, possibly due to maturation. Both samples have no axis connectivity of pore clusters in the x, y, and z direction on the scale of 10 to 20 of micrometer, but do show connectivity on the micrometer scale. The volume of organic matter in the studied volume is representative of the TOC in the samples. Organic matter does show axis connectivity in the x, y, and z direction. With increasing maturity the porosity in organic matter increases from close to 0 to more than 5 %. These pores are small and in the large organic particles have little connection to the mineral matrix. Continuous pore size distributions are compared with Mercury Intrusion Porosimetry (MIP) data. Minor differences are caused by resolution limits of the FIB-SEM and by the development of small pores during the maturation of the organic matter. Calculations show no permeability when only considering visible pores due to the lack of axis connectivity. Adding the organics with a background permeability of 1e-22 m2 to the calculations, the total permeability increased by one to two orders of magnitude depending on the direction of flow boundary conditions. Our results compare well with experimental data from the literature suggesting that upscaling may be possible in the future.

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