Image based CFD modeling of sandstones and tight rocks

Digital Rock Physics (DRP) is an emerging technology adopted in combination with traditional core analysis and used for the investigation of pore-scale phenomena. It combines modern imaging techniques and advanced numerical simulations, in order to predict petrophysical and fluid flow properties of rocks. A typical DRP workflow consists of imaging the rock samples by using micro-computed tomography and other scanning technologies, followed by the generation of digital 3D models showing the internal structure of the porous rock. On these models fluid dynamics simulations are performed and a variety of physical properties is calculated. The DRP workflow developed in this work is based on open-source tools (OpenCV, Fiji, OpenFOAM). Starting from the acquired datasets, micro- CT images are processed in order to enhance contrast, reduce noise and segment the sample into its phases. Different denoising filters have been tested and an automatic histogram-based segmentation procedure has been developed. SEM images are properly analyzed and calibrated on the three- dimensional imaged volume, according to a multi-scale 3D approach aimed at enriching the low resolution of X-ray microtomography with the additional information provided by these high-resolution images. The fluid dynamics analysis is performed by means of a finite volume CFD approach based on the continuum assumption. The generation of the computational grid is based on a novel procedure that creates the mesh working directly on the image dataset. Single-phase flow simulations are considered, with the application of specific Maxwell boundary conditions accounting for the slippage effect occurring at the fluid-solid interface for high Knudsen numbers. Suitable fluid dynamics models have been introduced in order to reproduce the flow in sub-resolution regions. The results of the DRP analysis performed on the available rock samples are compared with the relative experimental measurements, in order to validate the approach and discuss strength points and critical issues related to the developed workflow.