Pore scale characterization of carbonates at multiple scales: integration of micro-CT BSEM and FIBSEM

Abstract

The understanding of petrophysical and multiphase flow properties is essential for the assessment and exploitation of hydrocarbon reserves; these properties in turn are dependent on the 3D geometric and connectivity properties of the pore space. The determination of the pore size distribution in carbonate rocks remains challenging; extreme variability in carbonate depositional environments and susceptibility to a range of post-depositional processes results in complex pore structures comprising length scales from tens of nanometers to several centimeters. To increase understanding of the role of pore structure on connectivity, conductivity, permeability and recoveries requires one to probe the pore scale structure in carbonates in a continuous range across over seven decades of length scales (from 10 nm to 10 cm) and to integrate information at these different scales. In this paper experimental techniques including micro-computed tomography, backscattered scanning electron microscopy (BSEM), and Focussed ion beam SEM (FIBSEM) are used to probe the pore scale structure in carbonates across many decades of scale. Registration techniques are then used to couple information at different length scales. First an image of a 3D plug (4 cm, 20 micron voxel size) is correlated to a sample at macroporous resolutions (8 mm diameter, 4 micron voxel size). We then focus on coupling SEM and FIBSEM data at submicron resolutions to micro-CT data at ≈3-5 micron resolution. For pixel perfect registration of SEM images, an accurate template has been developed to remove warp artefacts introduced by the SEM scanning procedure and we have successfully mapped the sub-resolution porosity and pore sizes visible in the SEM image to gray scale levels in the 3D image. FIBSEM also allows one to investigate the 3D structure in samples down to tens of nanometers. We briefly discuss how this multiscale information can be used as a method for enhanced analysis of petrophysical properties of carbonates. Show more