Fragments of a number of core plugs have been analysed using a high resolution X-ray micro-computed tomography (micro-CT) facility at resolutions down to 2 µm. The samples analysed include a range of sandstone samples and one reservoir carbonate core. We show that data over a range of porosity can be computed from a single plug. Computational results made directly on the digitized tomographic images are presented for the permeability and drainage capillary pressure and are compared to conventional laboratory measurements on the same core material. The results are in good agreement and demonstrate the potential to predict petrophysical properties from core material not suited for laboratory testing (e.g., sidewall or damaged core and drill cuttings). Pore size information, NMR response and Formation factor are also calculated on the images. Empirical correlations linking fluid permeability to Formation factor and to a number of pore size parameters based on 3D digitized images of sedimentary rock are compared.
1. INTRODUCTION
The petroleum industry is increasingly reliant on more effective reservoir characterization to reduce the risks associated with new field development, better delineate producing fields and identify new reserves. The primary tools for reservoir characterization are wellbore logging and limited core derived laboratory measurements for calibrating field logs and establishing relationships between log responses and the petrophysical properties of interest. These relationships are necessarily empirical and introduce considerable uncertainty in the interpretation of logging measurements and in the resulting reservoir description. A major source of the uncertainty is related to the present inability to effectively characterize complex rock microstructure at the pore scale. A significant reduction in the level of uncertainty requires the development of techniques to accurately characterize rock microstructure and to relate this information to measured petrophysical properties.
This paper describes the development of a capacity to characterize and predict petrophysical properties from experimental 3D images of rock microstructures. A micro-CT (µ - CT) facility for imaging, visualizing and calculating sedimentary rock properties in three dimensions (3D) is described. We image and characterise small plugs obtained from a range of cores including homogeneous sandstones, reservoir sandstone and carbonate core. Computations made directly on the digitized tomographic images are presented for permeability and drainage capillary pressure. Comparisons with conventional laboratory measurements show good agreement. A number of pore length scale parameters are derived; these include pore volume-to-surface-area ratio, channel diameters associated with mercury porosimetry measurements and length scales associated with the NMR relaxation time. The differences in length scales are discussed. Numerical predictions of Formation factor are also given. This allows one to directly compare common permeability correlations in a controlled fashion. Overall the results show the feasibility of combining digitized images with numerical calculations to predict petrophysical properties. We discuss the challenge to extend the methodology to a wider range of petrophysical properties.
2. EXPERIMENTAL METHOLOGY
This section describes the methodology of image acquisition and phase identification methods for the samples studied.
