Quantifying fracture geometry with X-ray tomography: Technique of Iterative Local Thresholding (TILT) for 3D image segmentation

Hang Deng,Jeffrey P Fitts,Catherine A Peters

doi:10.1007/s10596-016-9560-9

Abstract

This paper presents a new method—the Technique of Iterative Local Thresholding (TILT)—for processing 3D X-ray computed tomography (xCT) images for visualization and quantification of rock fractures. The TILT method includes the following advancements. First, custom masks are generated by a fracture-dilation procedure, which significantly amplifies the fracture signal on the intensity histogram used for local thresholding. Second, TILT is particularly well suited for fracture characterization in granular rocks because the multi-scale Hessian fracture (MHF) filter has been incorporated to distinguish fractures from pores in the rock matrix. Third, TILT wraps the thresholding and fracture isolation steps in an optimized iterative routine for binary segmentation, minimizing human intervention and enabling automated processing of large 3D datasets. As an illustrative example, we applied TILT to 3D xCT images of reacted and unreacted fractured limestone cores. Other segmentation methods were also applied to provide insights regarding variability in image processing. The results show that TILT significantly enhanced separability of grayscale intensities, outperformed the other methods in automation, and was successful in isolating fractures from the porous rock matrix. Because the other methods are more likely to misclassify fracture edges as void and/or have limited capacity in distinguishing fractures from pores, those methods estimated larger fracture volumes (up to 80 %), surface areas (up to 60 %), and roughness (up to a factor of 2). These differences in fracture geometry would lead to significant disparities in hydraulic permeability predictions, as determined by 2D flow simulations.

Highlights

Over the past two decades, X-ray computed tomography has become a valuable tool for 3D visualization and characterization of geological specimens [1,2,3,4,5,6,7,8,9], including fracture geometries [10,11,12,13,14,15,16,17,18,19,20,21], pore networks [22,23,24,25,26,27,28,29,30,31], crystal sizes [32, 33], and mineral phases [3, 34,35,36,37,38,39]. xCT imaging is indispensable for non-destructive observation of geometry of fractures, which is important because fractures provide preferential flow conduits and often dominate mass transfer in geological materials. xCT imaging has been used to provide valuable insights in core-flow experiments designed to investigate fractures in the context of geohydrological, geochemical, and geomechanical processes of the deep subsurface, such as CO2 geological storage [20, 30, 40] and oil and gas operations [13,14,15, 17, 41,42,43,44,45,46]
Fracture surface roughness is important in flow modeling because it leads to an effective hydraulic aperture that is smaller than the actual mechanical aperture, reducing fracture permeability [21]
We examine the extent to which differences in fracture geometry lead to differences in estimation of fracture permeability, a key hydrodynamic variable. This gives us a sense of the importance of the quantitative differences observed in the fracture geometry estimates

Summary

Introduction

Over the past two decades, X-ray computed tomography (xCT) has become a valuable tool for 3D visualization and characterization of geological specimens [1,2,3,4,5,6,7,8,9], including fracture geometries [10,11,12,13,14,15,16,17,18,19,20,21], pore networks [22,23,24,25,26,27,28,29,30,31], crystal sizes [32, 33], and mineral phases [3, 34,35,36,37,38,39]. xCT imaging is indispensable for non-destructive observation of geometry of fractures, which is important because fractures provide preferential flow conduits and often dominate mass transfer in geological materials. xCT imaging has been used to provide valuable insights in core-flow experiments designed to investigate fractures in the context of geohydrological, geochemical, and geomechanical processes of the deep subsurface, such as CO2 geological storage [20, 30, 40] and oil and gas operations [13,14,15, 17, 41,42,43,44,45,46]. Laboratory and synchrotron-based xCT instruments that have adopted two-stage magnification can achieve micron to submicron voxel sizes at source-sampledetector separations that accommodate relatively small rock cores (e.g., 4-mm-diameter cores) within vessels that allow flow, temperature, and pressure control [52, 54]. Advanced conventional xCT scanner designs, which rely solely on geometric magnification, provide the X-ray flux and source-detector spacing needed to image larger fractured rock cores (e.g., 2 cm dia.) within larger and stronger vessels [55]. The tradeoffs between voxel size, field of view, sample size, and X-ray flux [2, 32, 33] suggest that both types of instruments will continue to play important roles in core-flow research for the foreseeable future, and regardless of xCT design, larger datasets will continue to accompany advances in instrumentation

Methods

Results

Conclusion