Sub-micron Digital Image Analysis, Pore Geometries and Electrical Resistivity in Carbonate Rocks

Jan H. Norbisrath, Gregor P. Eberli, Ralf J. Weger, Klaas Verwer, Janos Urai, Guillaume Desbois, and Ben Laurich

Assessment of electrical flow properties in heterogeneous carbonate rocks requires a multiscale analysis, as pore sizes span several orders of magnitude. A new technique allows for imaging and quantification of the sub-micron pore space (BIB-SEM) in unprecedented detail. It utilizes Broad-Ion- Beam (BIB) milling to produce true 2D surfaces suitable for quantification of the nanometer pore space on SEM image mosaics.

Four samples were chosen from different depositional and diagenetic environments to compare their distinct microstructures. All samples have similar porosity (16%) for the sake of comparability. Electrical resistivity was measured on all samples in a 4-electrode setup. Pore throat size distribution was analyzed with MICP, and macropore structure was analyzed with conventional thin-section digital image analysis (DIA). To analyze the micropores, sample surfaces were milled down to nanometer-precision roughness with a BIB cross-section polisher (JEOL SM-09010). The large BIB surfaces (up to 2 square mm) are investigated at 5000x and 15000x magnification (pixel length: 58.6 nm and 18.5 nm, respectively). Resulting mosaics are composed of up to 570 images each. Integrating DIA results from BIB-SEM and thin-section DIA yields a multiscale digital image analysis (Ms-DIA).

The large, nanometer-resolution BIB-SEM image mosaics reveal the diverse micro-architectures of the different rock types, allowing for qualitative estimation of flow properties. The most interesting finding from quantitative DIA is that pore size distribution follows a power law. This implies that pore densities outside the scale of investigation can be predicted. Pore-body (DOMsize) and pore-throat size (Critical diameter Dc from MICP) show excellent correlation with electrical resistivity. Smaller pore-bodies and also smaller pore-throats result in better electrical flow. Pore-body-to-throat ratios (BTR) show unexpected results, as larger BTR values result in better flow.

AAPG Search and Discovery Article #90181©2013 AAPG/SEG Rocky Mountain Rendezvous, University of Wyoming, Laramie, Wyoming, September 27-30, 2013