Summary

In the last decade, imaging techniques capable of reconstructing three-dimensional (3-D) pore-scale model have played a pivotal role in the study of fluid flow through complex porous media. In this study, we present advances in the application of confocal laser scanning microscopy (CLSM) to image, reconstruct and characterize complex porous geological materials with hydrocarbon reservoir and CO2 storage potential. CLSM has a unique capability of producing 3-D thin optical sections of a material, with a wide field of view and submicron resolution in the lateral and axial planes. However, CLSM is limited in the depth (z-dimension) that can be imaged in porous materials. In this study, we introduce a ‘grind and slice’ technique to overcome this limitation. We discuss the practical and technical aspects of the confocal imaging technique with application to complex rock samples including Mt. Gambier and Ketton carbonates. We then describe the complete workflow of image processing to filtering and segmenting the raw 3-D confocal volumetric data into pores and grains. Finally, we use the resulting 3-D pore-scale binarized confocal data obtained to quantitatively determine petrophysical pore-scale properties such as total porosity, macro- and microporosity and single-phase permeability using lattice Boltzmann (LB) simulations, validated by experiments.
Lay description

In this study, a method is described to apply confocal laser scanning microscopy (CLSM) to image, reconstruct and characterize statistically the 3-D pore space of geological rock samples. Confocal Laser Scanning Microscope has a unique capability of producing very thin optical sections of a material, with submicron resolution in the lateral and axial planes. The limitation of CLSM is the restriction on acquiring depth (z-dimension) information because the observed light intensity is attenuated with depth due to absorption and scattering by the material. It is an extension of the methods currently used in digital rock imaging to build numerical rock models using various techniques, including reconstruction made from computed tomography (CT) scans (micro-CT and synchrotron-computed micro-tomography), computer-generated sphere packs and 2-D scanning electron microscope (SEM) images. The novel method disclosed here is to image the pore space to the depth which can be accessed by the conventional CLSM approach and then grind away a slightly smaller layer of the rock followed by another imaging step. This process is repeated to acquire a 3-D image of unlimited depth. It has an advantage over sequential grinding and 2-D imaging by conventional microscopy in that far fewer grinding steps are needed. It can also be used to acquire an arbitrarily wide image without the loss of resolution by stitching together multiple scans. Micro-CT cannot obtain such a wide field of view without loss of image quality in large physical specimens. The volumetric pore space image obtained in this way can be used to quantitatively predict the macroscopic petrophysical properties, including total porosity, macroporosity, and microporosity and subsample single-phase permeability using known digital rock analysis techniques.