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Experimental results have shown that resistivity index can deviate from Archie's law at low conductive phase saturation. Previous works have claimed that wettability and flow history are the two main factors causing this phenomenon. Herein, we investigate how the underlying fluid morphology influences electrical resistivity. We start from digital synchrotron X-ray microcomputed tomography images at different conductive phase saturations. We then simulate two-phase flow at different capillary numbers and wettabilities to investigate deviations in resistivity index. We discover that other than water saturation the connectivity of water quantified by Euler characteristic is an important parameter in determining electrical resistivity for intermediate to purely oil-wet conditions. Deviations in electrical resistivity are found to start at low capillary number (Ca ≥ 10 −4) for intermediate and oil-wet conditions for the full range of water saturations. We study correlations between resistivity index, saturation and Euler characteristic by using the Pearson product-moment correlation and a linear regression model. We find a strong correlation between water saturation and resistivity index for the water-wet case while for the intermediate and oil-wet cases a strong correlation between Euler characteristic and resistivity index was observed. The results are explained in terms of percolation theory and a general relationship for resistivity index is proposed for intermediate-wet to oil-wet systems whereby the percolation parameter is normalized Euler characteristic. The findings explain previously observed deviations in resistivity index measurements and allow for a means to predict Euler characteristic from laboratory core-scale experiments using the proposed percolation model. Usage: Secondary publications and information retrieval purposes.

Coal seam gas is an unconventional resource for natural gas that is becoming popular due to its environmental benefit and abundance. This paper reviews recent developments on the pore-scale characterisation of coal from coal seam gas reserviors. The development of micro-computed tomography (micro-CT) imaging has enabled for the 3D characterization of the fracture system in coals. This provides detailed insights into understanding flow in these unconventional reservoirs. A novel image calibration method in which the skeleton of the fracture system is obtained from micro-CT imaging while the fracture apertures are measured from scanning electron microscopy (SEM) is described. We also show the application of micro-CT imaging for studying diffusion processes in ultralow permeability matrices and discuss the incorporation of the data into calculations of gas production from unconventional reservoirs. The extraction of statistical information from micro-CT images to reconstruct coal cleat system are also demonstrated. This technique allows for preserving the key attributes of the cleat system while the generated fracture network is not limited in terms of size nor resolution. The developments of microfluidic methods for understanding the complex displacement mechanisms in coal seams are also described. These low-cost experimental methods can provide unique information about the displacement mechanisms occurring during gas production from coal seam reservoirs. Variation of coal contact angle with pressure is analysed and results demonstrate important wettability processes that occur in coal seams. We describe numerical methods for prediction of petrophysical properties from micro-CT images of coal and discuss the associated limitations when dealing with coal samples. The paper concludes by addressing the challenges faced when characterising coal at the micro-scale and approaches for population of coal data into reservoir simulators for relaible prediction of reservoir behaviour during gas production as well as CO2 sequestration in coalbeds.