Fractal dimensions of pore spaces in unconventional reservoir rocks using X-ray nano- and micro-computed tomography

Abstract

Considerable attention has been paid to the development of unconventional reservoir fields due to the availability of such resources worldwide. The heterogeneities throughout the pore space structure in a rock and their effects on the occurrence of gas and oil can be quantitatively analysed using fractal theory. This study focuses on the investigation of the fractal dimension (FD) in shale and tight gas sandstones (TGS) from Brazilian oil and gas fields using 3-D X-ray nano- and micro-computed tomography. Obtaining the broad size distribution of pore space structures in unconventional reservoir rocks using 3-D images is challenging due to the multiscale characteristics of the rocks. Thus, sample images were acquired with two voxel size/resolutions (0.064 μm and from 0.46 to 1.20 μm). Very small pores (ganglia), main pore networks and organic matter grains were observed as segmented structures within sample sets for which the FD was assessed and quantified. Box-counting (BC) and pore size distribution (PSD) approaches were applied. The average FD values ranged from 1.78 to 2.59 in shale and from 2.36 to 2.46 in TGS. Correlations of the FD with the experimentally acquired specific surface area, porosity and permeability related petrophysical properties, relevant to the quantitative modelling of gas transport in unconventional reservoirs, were established. The pore scale morphologies described by the FD from 3-D X-ray CT images help on the foundation for further research in shale/tight gas sandstones.