Rock Porous Structure Characterization: A Critical Assessment of Various State-of-the-Art Techniques

The porous structure of geomaterials is of utmost importance for various industrial and natural processes. In this study, various conventional porous structure characterization techniques such as mercury intrusion porosimetry (MIP), nuclear magnetic resonance (NMR), micro-X-ray computed tomography (μCT) imaging, as well as gas injection have been employed to perform a systematic and critical evaluation of all such techniques for characterization of a carbonate rock sample porous structure. The porosity obtained from μCT (5 μm/voxel) (21.5%) is closer than the overall porosity obtained by MIP (17.23%) to the gas porosimetry result (23%). The 5% difference could be due to inaccessible pores to mercury, which can be accessible to nitrogen with much smaller molecules. The porosity obtained from NMR is 21.4%. It is lower than porosity values by μCT (5 μm/voxel) and by gas injection and higher than the prediction of MIP. The porosity is obtained by μCT, but the much lower resolution (27.5 μm/voxel) results in 8.19% underestimating the porosity by around 50%. Regarding permeability, the results of the NMR technique are highly dependent on the cutoff range used and very different from other techniques, whereas the permeability obtained by MIP is around 18.42 mD, close to that obtained by gas permeameter (20 mD). The μCT imaging provides the opportunity to measure pore and throat size distribution directly, to achieve open and closed porosity, the coordination number of pores and surface and volume characteristics of the porous medium, which can hardly be performed through other techniques. The resolution of images, however, fully controls the obtained pore and throat size distribution in CT analysis. The Kolmogorov–Smirnov distribution analysis reveals that the resulting pore size distribution from MIP is rather a rough estimation of the throat size distribution obtained from μCT (5 μm/voxel), while NMR prediction can provide a rather good approximation of the pore size distribution obtained from μCT (5 μm/voxel). The NMR prediction is however dependent on the choice made for the surface relaxivity coefficient, and changing it would significantly affect the resulting distribution. The results of this study provide further insight and elucidate the differences of the quantities such as porosity, permeability, and pore and throat size distribution obtained from various techniques which are essential either as an input to numerical models of flow and transport in porous media or as a building block of the theoretical models.