11th Interpore 2019 in Valencia / Spain (May 07-09, 2019)

Permalink: www.researchgate.net/publication/333220741

Heterogeneity of geological materials poses various problems when evaluating reservoir quality and storage potential. We analysed samples of different sedimentary facies of a Rotliegend sandstone from the Flechtingen High (Northern Germany) to determine the influence of depositional environment and diagenetic history on mineralogical composition and its impact on porosity and permeability. We employed high resolution computer tomography (CT) (voxel size: 2.4 µm) and focussed ion beam – scanning electron microscopy (FIB-SEM) (voxel size: 0.01 – 0.03 µm) for direct pore space and mineral distribution analyses with focus on sub-micrometer zones like feldspar cement boundaries and diagenetically grown illite meshwork pore fillings. As shown by [1], sub-resolution porosity in CT models can drastically influence flow properties. We found that about 20 - 30 % of the segmented initial pore space is rather porous (φ < 100 %) than pure void (φ = 100 %). Thus, we utilized a Navier-Stokes-Brinkmann approach that allows to refine flow properties of observed CT models using void and porous domains. Porous domain properties were derived from Navier-Stokes simulations on FIB-SEM models. For comparison low temperature N2 adsorption and He-porosimetry data were used to characterize bulk permeability (Kbulk), total porosity (φtotal), BET surface area and pore size distributions (PSD). X-ray diffraction (XRD) Rietveld analyses gave quantitative mineralogical information about the different facies and the clay mineral pore fillings. Cross bedded layers show slightly higher Kbulk values (3.8 – 5.1 mD) compared to laminated layers (1.5 – 2.3 mD) which correlates with feldspar cement content. However, calcite and clay cement phases show variance between the samples but no correlation with permeability and porosity. This can be attributed to the pore structure of these cement/grain interfaces with pore radii below 0.05 µm as observed by FIB-SEM. Interfaces connected to feldspar cements show open pore networks with pore radii up to 0.5 µm. Such areas are particularly relevant for quantifying fluid flow as treating these spaces as pure voids, when in fact they represent semi-porous rock, will lead to an overestimation of simulated permeability values (compared to measured Kbulk) by more than one order of magnitude (40 – 50 mD).