Simulating stress-dependent fluid flow in a fractured core sample using real-time X-ray CT data

Abstract

Abstract. The objective of the current study is to investigate and validate stress-dependent single fluid flow in a fractured core sample using in situ X-ray computed tomography (CT) scans and a finite-volume method solving the Navier-Stokes-Brinkman equations. The permeability of the fractured sandstone sample was measured stepwise during a loading-unloading cycle (0.7 MPa to 22.1 MPa and back) to validate the numerical results. Simultaneously, the pressurized core sample was imaged with a medical X-ray CT scanner with a voxel dimension of 0.5 × 0.5 × 1.0 mm3. Fracture geometries were obtained by CT images based on the Missing Attenuation (MA) approach. Simulation results revealed both, qualitative plausibility and a quantitative approximation of the experimentally derived permeabilities. The qualitative results indicate flow channeling along several preferential flow paths with less pronounced tortuosity. Significant changes in permeability can be assigned to temporal and permanent changes within the fracture due to applied stresses. The applied fluid flow simulations also incorporate potential fracture-matrix interaction and permeability anisotropy within the matrix caused by high-porosity layers. The deviations of the quantitative results appear to be mainly caused by the low resolution affecting the accurate capturing of sub-grid scale features and the reproduction of the actual connectivity. Furthermore, the threshold value CTmat (1862.6 HU) depicting the matrix material represents the most sensitive input parameter of the simulations. Small variations of CTmat (±17.7 HU in this study) can cause enormous changes in simulated permeability by up to a factor of 2.6 ± 0.1 and, thus, has to be defined with caution. Finally, our results are also compared with other studies showing similar results. Based on these observations various recommendations to improve CT image quality, model quality, aperture calibration and validation of qualitative fluid flow are provided.

Pore-scale tomography and imaging: applications, techniques and recommended practice

“…Saenger et al (2016) present recommendations for digital carbonate rock physics performed on imaging data sets, and Kaufhold et al (2016) investigated structures in Opalinus Clay on multiple scales after mechanical testing for the long-term storage of radioactive waste. Kling et al (2016) showcase a new approach of fluid flow modeling in fractured rocks, whereas Grathoff et al (2016) modeled porosity and permeability of organic-rich Posidonia shales from FIB-SEM data sets. Kuhlenkampff et al (2016a, b) provide an introduction to the method and application of Positron Emission Tomography.…”