Petrophysical Characterization of Metamorphic Rocks using X-ray Micro-CT data – Implications for fluid flow

Abstract

The petrophysical properties (porosity and permeability) of rocks are significantly influenced by their microstructure and fabric anisotropy, which can be evaluated using X-ray micro-computed tomography (μCT). Through study of mica schists from the Singhbhum Shear Zone (Eastern India), we demonstrate the potential to perform X-ray micro-CT studies in metamorphic rocks and discuss the associated challenges in data processing. We show that automated thresholding of greyscale values from μCT data acquisition yields abnormally high porosity/permeability values in schists when compared to the values obtained from laboratory measurements of the same samples. We develop a modus operandi where the laboratory measured porosity value from an individual schist sample is used to calibrate the greyscale threshold range (designating void space) of μCT data from the same sample. This calibration is done using PACE3D numerical simulation framework that allows a multiphase-field approach, and it is shown that (a) porosities of schist derived from analysis of μCT data (post re-thresholding) fit well with laboratory measured values of respective samples and (b) anisotropy of permeability can be computed from μCT data. Permeability computed using the μCT data (post re-thresholding) vis-à-vis laboratory measurements are comparable in 4 out of 5 schist samples analysed here, when the samples are treated as a two phase system (void spaces as one phase and solid rock mass as the second phase) in PACE3D. The aberration in one schist sample is attributed to its heterogeneous layering and microstructure that comprises alternate layers of coarse and fine grain size aggregates of phyllosilicate + quartz. Re-computation of permeability by performing three-phase simulations for the above layered sample in PACE3D framework by introducing phyllosilicate as a third phase (in addition to void spaces and other mineral phases) yields results similar to the laboratory measurements. We conclude that our approach of integrating μCT data, laboratory measurement of petrophysical properties and microstructure modelling/simulation in PACE3D multiphase-field framework helps evaluate the role of rheological variations in controlling porosity/permeability. Thus this study has a bearing on enhancing knowledge about fluid flow in metamorphic rocks with possible implications for mineralization.