Resolution dependence of petrophysical parameters derived from X-ray tomography of chalk

Abstract

X-ray computed tomography data from chalk drill cuttings were taken over a series of voxel dimensions, ranging from 320 to 25 nm. From these data sets, standard petrophysical parameters (porosity, surface area, and permeability) were derived and we examined the effect of the voxel dimension (i.e., image resolution) on these properties. We found that for the higher voxel dimensions, they are severely over or underestimated, whereas for 50 and 25 nm voxel dimension, the resulting values (5%–30% porosity, 0.2–2 m2/g specific surface area, and 0.06–0.34 mD permeability) are within the expected range for this type of rock. We compared our results to macroscopic measurements and in the case of surface area, also to measurements using the Brunauer-Emmett-Teller (BET) method and found that independent of the degree of compaction, the results from tomography amount to about 30% of the BET method. Finally, we concluded that at 25 nm voxel dimension, the essential features of the nanoscopic pore network in chalk are captured but better resolution is still needed to derive surface area.

Particle Diffusion in Complex Nanoscale Pore Networks

“…Some of the primary petrophysical parameters, i.e. porosity and surface area, which were derived from 7 tomograms for these samples, have already been published elsewhere [18]. In Figure 1, 3D subvolumes of the reconstructed data for both samples are shown before and after segmentation.…”

“…Particle size is important because in some highly compacted chalk types, the pore throats connecting the individual pores can be so small (less than 100 nm [18]) that some high molecular weight oil components are on same length scale as the pores. Consequently, the dynamics of the molecules in the pores becomes size dependent, which leads to a size dependent tortuosity.…”

Rock Characterization Using Gray‐Level Co‐Occurrence Matrix: An Objective Perspective of Digital Rock Statistics

“…Bruns et al (2017) investigated the use of grayscale images to define a statistically representative elementary volume for chalk, which is a highly heterogeneous porous medium affecting estimated porosity, specific surface area, and diffusive tortuosity. While such an analysis directly using grayscale images posed a significant challenge, it removed the dependence on image resolution (Müter et al, 2014), which is crucial for image segmentation, especially for complex rock structures as seen in chalk and samples under high-stress conditions such as coal and shales. This paper aims to present an automated rock characterization technique applied directly to micro-CT images obtained after reconstruction and is based on second-order statistics.…”

Detailed analysis of the lattice Boltzmann method on unstructured grids

“…The reconstructed volume had a voxel size of 25 nm and an optical resolution about 150 nm. Details about the data collection and reconstruction can be found in Müter et al (2014). The reconstructed images were corrected for ring artefacts before segmentation by a dual filtering and Otsu thresholding procedure Müter et al (2012).…”