Determination of Physical Properties of Tight Porous Media Using Digital Core Physics/Analysis

ABSTRACT

In this study we use pore-scale SEM images of tight porous media, and generate threholded binary images which are then used to reconstruct three-dimensional (3-D) pore structures. Computational physics is then employed in order to calculate the physical properties of the porous media, such as the porosity, permeability, electrical resistivity and formation factor, and the NMR spectra. This involves generating an unstructured 3-D grid in the pore space in which we solve the corresponding governing equations. The original thresholded images have high resolution which allows a more precise masking of the pore spaces. However, there is a high computational cost associated with the high resolution images. Reducing the resolution will reduce the mask precision in the pore areas and this will lead to a different numerical solution. Therefore, we much optimize between the computational cost and accuracy when performing the numerical simulation. The threshold values for the samples at hand were selected by trying to match porosity and permeability of a neighboring plug. The choice of threshold value has a profound effect on the porous medium properties. For the fluid flow calculations, we solve the velocity and pressure in the pore space by solving the NavierStokes equations and using the results to obtain the absolute permeability. The electrical resistivity is obtained by solving for the current density through Ohm’s law, and for the NMR study we solve for the equations of molecular diffusion. This approach can be described as a version of digital core analysis (DCA) or digital core physics (DCP).

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