Pore-scale analyses of heterogeneity and representative elementary volume for unconventional shale rocks using statistical tools

Lateef T Akanji,James O Adeleye

doi:10.1007/s13202-017-0377-4

Abstract

A statistical technique for the pore-scale analyses of heterogeneity and representative elemental volume (REV) in unconventional shale rocks is hereby presented. First, core samples were obtained from shale formations. The images were scanned using microcomputed tomography (micro-CT) machine at 6.7 μm resolution with voxels of 990 × 990 × 1000. These were then processed, digitised, thresholded, segmented and features captured using numerical algorithms. This allows the segmentation of each sample into four distinct morphological entities consisting of pores, organic matter, shale grains and minerals. In order to analyse the degree of heterogeneity, Eagle Ford parallel sample was further cropped into 96 subsamples. Descriptive statistical approach was then used to evaluate the existence of heterogeneity within the subsamples. Furthermore, the Eagle Ford parallel and perpendicular samples were analysed for volumetric entities representative of the petrophysical variable, porosity, using corner point cropping technique. The results of porosity REV for Eagle ford parallel and perpendicular indicated sample representation at 300 μm voxel edge. Both pore volume distribution and descriptive statistical analyses suggested that a wide variation of heterogeneity exists at this scale of investigation. Furthermore, this experiment allows for adequate extraction of necessary information and structural parameters for pore-scale modelling and simulation. Additional studies focusing on re-evaluation at higher resolution are recommended.

Highlights

Pore geometry, tortuosity, grains size and shape are properties that are important to describe and characterise fluid flow in shale rock
We developed a workflow for the characterisation of heterogeneity of unconventional shale rock samples
In order to further evaluate the degree of accuracy of the two numerical algorithms, pore volume computation using simulation-based oriented bounding box (OBB) and simulation-based voxel method (VOX) was compared with analytically calculated method (ANA)

Summary

Introduction

Tortuosity, grains size and shape are properties that are important to describe and characterise fluid flow in shale rock. The images were processed, digitised, thresholded, segmented and each feature captured using marching cube algorithm This allows the segmentation of each sample into four distinct entities, consisting of pores, organic matter, shale grains and minerals. REV is assumed to be obtained when the computed variable (in this case porosity) plotted against increasing sample size (Fig. 2d) does not change significantly at a plateau value (e.g. Figure 1) This is useful when determining what sample to be used in modelling and simulating fluid flow through them. In order to estimate physical properties and identify pore space as well as other components of each of the Eagle Ford parallel shale rock samples, manual greyscale thresholding was used to segment the images and to distinguish all its entities. Kurtosis could be classified as platykurtic, 1.E+03 mesokurtic and leptokurtic when the value is less than 3, equal to 3 and greater than 3, respectively. 1.E+02

Results and discussion

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Conclusion