Pore space characterisation and permeability prediction using fast network extraction and pore throat conductance calculation

State of the art X-ray micro-computed tomography (micro-CT) of reservoir rock samples and cuttings allows reconstruction of the pore network with micrometric spatial resolution that is suitable for further petrophysical and fluid flow properties evaluation. Synchrotron tomographic beam lines, compared to conventional micro-CT systems, allow better 3D representation of the pore space. The digital rock volume is used for medial axis extraction and for 3D pore space morphological characterization (i.e. connectivity, pore-body/throat size, shape factors, etc.). The medial axis is generally derived by computed intensive digital-thinning operations for skeletonization of the digital rock volume. The alternative method we describe in this work, allows to extract the 3D centered and shortest paths to follow in order to go through the pore space from any given point to another one using simple and well-established mathematical methods. Even if we do not consider explicitly fluid flow, one path can be viewed as the streamline of the fastest fluid thread for laminar flow regime that a particle follows to go through the pore space from an intrusion point to an exiting point of the digital sample. The method is very fast, processing large voxel volumes in seconds, and it is also amenable to parallelization. The conductance of the irregular cross-sections of the throats is calculated with a fast finite element method (f.e.m.) that assumes fully developed laminar flow in axial direction. The network is solved as a linear system and the absolute permeability derived. The paper reports the results of application on real oil reservoir sandstone samples and shows the potential to predict petrophysical properties from rock material (i.e. drilling cuttings or crushed side wall cores) not suitable for conventional analyses.