Three-dimensional imaging of porous media using confocal laser scanning microscopy

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

Visualising and modelling flow processes in fractured carbonate rocks with X-ray computed tomography

Naturally Fractured Reservoirs (NFR) have typically very complex geometries from the pore scale to the field scale – discontinuities can be found at each scale. This makes NFRs hard to accurately be modelled for flow simulations. Fractures are especially difficult to incorporate in the simulations. The topology of a single fracture is usually simplified to a plane or disk, and apertures are usually averaged to be implemented in the simulation models. The fracture aperture distribution of a single fracture is already very heterogeneous though. Contact areas in fractures can detain flow, whereas connected fracture regions with larger apertures can result in preferred flow paths and lead to early breakthrough. To help understanding how well current Discrete Fracture and Matrix (DFM) models are suitable to retain fracture influences on flow in carbonates, this research project combines the simulation of miscible single-phase flow through fractures in carbonates with precise fracture measurements (comprising fracture aperture distributions and 3D topologies) and the visualization of real single and two-phase flow experiments in fractured carbonate cores. The simulation approach employs a DFM model with a hybrid finite element/ finite volume (FEFV) method. The fractured core samples and the flow experiments are imaged with high-resolution X-ray computer tomography (CT), or X-ray radiography respectively. The main goals are to develop and optimize an image processing workflow from the X-ray CT fracture measurement to an according mesh generation as input for simulations, and to be able to compare simulations and flow experiment studies qualitatively to analyse how well the DFM approach is able to capture the true nature of fluid flow in fractures with real aperture distributions. To obtain most relevant comparisons, we conduct numerical simulations and flow experiments on the same fracture geometries, which have been measured before non-destructivel

Lattice Boltzmann simulations of fluid flow in continental carbonate reservoir rocks and in upscaled rock models generated with multiple-point geostatistics

Microcomputed tomography (mu CT) and Lattice Boltzmann Method (LBM) simulations were applied to continental carbonates to quantify fluid flow. Fluid flow characteristics in these complex carbonates with multiscale pore networks are unique and the applied method allows studying their heterogeneity and anisotropy. 3D pore network models were introduced to single-phase flow simulations in Palabos, a software tool for particle-based modelling of classic computational fluid dynamics. In addition, permeability simulations were also performed on rock models generated with multiple-point geostatistics (MPS). This allowed assessing the applicability of MPS in upscaling high-resolution porosity patterns into large rock models that exceed the volume limitations of the mu CT. Porosity and tortuosity control fluid flow in these porous media. Micro-and mesopores influence flow properties at larger scales in continental carbonates. Upscaling with MPS is therefore necessary to overcome volume-resolution problems of CT scanning equipment. The presented LBM-MPS workflow is applicable to other lithologies, comprising different pore types, shapes, and pore networks altogether. The lack of straightforward porosity-permeability relationships in complex carbonates highlights the necessity for a 3D approach. 3D fluid flow studies provide the best understanding of flow through porous media, which is of crucial importance in reservoir modelling