X-ray micro-computed tomography imaging for coal characterization

An Australian bituminous coal is imaged at high resolution of 16.1 μm with (wet) and without (dry) X-ray attenuating fluids present in the pore space using a large-field three-dimensional microfocus helical X-ray computed tomography (micro-CT) instrument. Scanning Electron Microscope (SEM) is conducted on slices of the specimen to visualize coal micro-features up to resolution of about 15 nm. Two- and three-dimensional image registration techniques are used to precisely overlay micro-CT tomograms of the core plug in dry and wet conditions and SEM images to yield detailed threedimensional visualizations of the geometry and topology of the fracture systems in coal. SEM images are also used to produce a calibration curve based on the relationship between the micro-CT intensity values and the true apertures of fractures within coal. This eliminates the need for two sets of imaging. Advanced filtering algorithms are applied to segment the micro-CT image into four distinct phases: resolved fractures, sub-resolution pores and fractures, macerals, and minerals. The application of micro-CT in determination of relative age relationships between adjacent geological features is presented. The distribution of resolved aperture size within the coal sample is investigated and the variation of permeability and porosity in several sub-samples of the coal is plotted. The analysis suggests that coal permeability is independent of porosity and is likely affected by other petrophysical properties such as lithotype. To include the effects of mineral phase on coal properties, we remove the segmented mineral phase and merge it to the resolved fracture phase. This analysis affirms that minerals are deposited in highly connected regions

Configurational diffusion transport of water and oil in dual continuum shales

Understanding fluid flow in shale rocks is critical for the recovery of unconventional energy resources. Despite the extensive research conducted on water and oil flow in shales, significant uncertainties and discrepancies remain in reported experimental data. The most noted being that while oil spreads more than water on shale surfaces in an inviscid medium, its uptake by shale pores is much less than water during capillary flow. This leads to misjudgement of wettability and the underlying physical phenomena. In this study, therefore, we performed a combined experimental and digital rock investigation on an organic-rich shale including contact angle and spontaneous imbibition, X-ray and neutron computed tomography, and small angle X-ray scattering tests to study the potential physical processes. We also used non-equilibrium thermodynamics to theoretically derive constitutive equations to support our experimental observations. The results of this study indicate that the pre-existing fractures (first continuum) imbibe more oil than water consistent with contact angle measurements. The overall imbibition is, however, higher for water than oil due to greater water diffusion into the shale matrix (second continuum). It is shown that more water uptake into shale is controlled by pore size and accessibility in addition to capillary or osmotic forces i.e. configurational diffusion of water versus oil molecules. While the inorganic pores seem more oil-wet in an inviscid medium, they easily allow passage of water molecules compared to oil due to the incredibly small size of water molecules that can pass through such micro-pores. Contrarily, these strongly oil-wet pores possessing strong capillarity are restricted to imbibe oil simply due to its large molecular size and physical inaccessibility to the micro-pores. These results provide new insights into the previously unexplained discrepancy regarding water and oil uptake capacity of shales

FracDetect: A novel algorithm for 3D fracture detection in digital fractured rocks

Fractures have a governing effect on the physical properties of fractured rocks, such as permeability. Accurate representation of 3D fractures is, therefore, required for precise analysis of digital fractured rocks. However, conventional segmentation methods fail to detect and label the fractures with aperture sizes near or below the resolution of 3D micro-computed tomographic (micro-CT) images, which are visible in the greyscale images, and where greyscale intensity convolution between different phases exists. In addition, conventional methods are highly subjective to user interpretation. Herein, a novel algorithm for the automatic detection of fractures from greyscale 3D micro-CT images is proposed. The algorithm involves a low-level early vision stage, which identifies potential fractures, followed by a high-level interpretative stage, which enforces planar continuity to reject false positives and more reliably extract planar fractures from digital rock images. A manually segmented fractured shale sample was used as the groundtruth, with which the efficacy of the algorithm in 3D fracture detection was validated. Following this, the proposed and conventional methods were applied to detect fractures in digital fractured coal and shale samples. Based on these analyses, the impact of fracture detection accuracy on the analysis of fractured rocks' physical properties was inferred

Representative elementary volume of rock using X-ray microcomputed tomography: A new statistical approach

© 2020 Taufiq Rahman et al. Rock heterogeneity is a key parameter influencing a range of rock properties such as fluid flow and geomechanical characteristics. The previously proposed statistical techniques were able to rank heterogeneity on a qualitative level to different extents; however, they need to select a threshold value for determination of representative elementary volumes (REV), which in turn makes the obtained REV subjective. In this study, an X-ray microcomputed tomography (μCT) technique was used to obtain images from different porous media. A new statistical technique was then used to compute REV, as a measure of heterogeneity, without the necessity of defining a threshold. The performance of the method was compared with other methods. It was shown that the calculated sum of the relative errors of the proposed method was lowest compared to the other statistical techniques for all tested porous media. The proposed method can be applied to different types of rocks for more accurate estimation of a REV

Society of Petroleum Engineers - SPE Asia Pacific Oil and Gas Conference and Exhibition 2018, APOGCE 2018

Copyright 2018, Society of Petroleum Engineers Despite decades of numerical, analytical and experimental researches, sand production remains a significant operational challenge in petroleum industry. Amongst all techniques, analytical solutions have gained more popularity in industry applications because the numerical analysis is time consuming; computationally demanding and solutions are unstable in many instances. Analytical solutions on the other hand are yet to evolve to represent the rock behaviour more accurately. We therefore developed a new set of closed-form solutions for poro-elastoplasticity with strain softening behaviour to predict stress-strain distributions around the borehole. A set of hollow cylinder experiments was then conducted under different compression scenarios and 3D X-Ray Computed Tomography was performed to analyse the internal structural damage. The results of the proposed analytical solutions were compared with the experimental results and good agreement between the model prediction and experimental data was observed. The model performance was then tested by analysing the onset of sand production in a well drilled in Bohai Bay in Northeast of China. Acoustic and density log along with core data were used to provide the input parameters for the proposed analytical model in order to predict the potential sanding in this well. The proposed solution predicted the development of a significant plastic zone thus confirming sand production observed by today sanding issue in this well

A review of experimental and numerical modeling of digital coalbed methane: Imaging, segmentation, fracture modeling and permeability prediction

Coalbed methane (CBM) is a form of natural gas that is extracted from coalbeds. Characterization of CBMs is very challenging mostly due to the very complex fracture system leading to ambiguous fluid and petrophysical properties. Among several important factors that control the performance of CBMs, permeability is the most crucial one, which summarizes the global fracture system, intensity, connectivity, and production ratio. As such, accurate characterization of CBMs is coupled with fracture delineation and permeability description, which resulted in the development of a wide range of methods. In this paper, all the necessary steps from imaging, segmentation, and modeling of the fractures to various methods of permeability evaluation are reviewed. This paper presents a critical review of all of the existing relevant and significant techniques and compares their performances with special reference to permeability prediction. Several practical and simplified computational methods for calculating permeability are thus reviewed and compared. Finally, this review paper summarizes the current challenges and possible future research

Digital rock analysis for accurate prediction of fractured media permeability

© 2016 Elsevier B.V. Determining the permeability of fractured rocks depends significantly on fracture geometry and topology. To date, no practical approach has been proposed to produce realistic binary images of fractured rocks from micro-computed tomography (micro-CT) images for fluid flow simulation. In this paper, a novel method is developed for generating a binary image that contains the true characteristics of fractured rocks for accurate computation of permeability. The method employs high-resolution scanning electron microscope (SEM) data for calibration of micro-CT images. Micro-CT is used to obtain 3D images of a highly fractured rock sample with a resolution of 16.5μm and SEM is applied to obtain images with nanometer resolution from polished surfaces of the sample. The SEM images are then registered to the micro-CT images to facilitate image segmentation and generate a calibration curve. The calibration curve correlates the grayscale values at the midpoint of each fracture to the true aperture size measured from SEM data. Thinned fractures of two subsets are extracted and used to obtain the gray-scale values at the midpoint of fractures. These are converted to the true aperture value using the calibration curve and subsequently grown by an adjustment algorithm to produce 3D calibrated binary images that are representative of the true fracture system. The connectivity and aperture size distribution of the subsets before and after calibration are quantified. The permeability of the subsets before and after calibration are computed using a direct numerical simulator and compared with experimental measurements. The computed permeabilities demonstrate that using non-calibrated images generates massive permeability errors whereas calibrated images produce accurate permeability results that nearly coincide with experimental measurments. The method can be applied to fractured rocks for better prediction of permeability and other petrophysical properties

Impact of dissolution of syngenetic and epigenetic minerals on coal permeability

© 2018 Elsevier B.V. Permeability of coal is a key parameter in coalbed methane recovery. Minerals are known to occlude flow paths and reduce coal permeability. Herein, pore space variation of coal due to dissolution of syngenetic and epigenetic minerals is numerically simulated. A high-resolution helical micro-computed tomography (micro-CT) scanner is used to acquire 3D images from internal structure of a coal sample that contains both syngenetic and epigenetic minerals. Two subsets are then obtained from the micro-CT image and segmented to separate syngenetic minerals, epigenetic minerals and macerals. The syngenetic and epigenetic minerals individually and together are dissolved and their impact on porosity and permeability is studied. The minerals are identified through Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS). The dissolution process is performed based on a first order kinetic reactive model. The numerical model combines lattice Boltzmann and finite volume methods. The results show that coal permeability significantly increases when a reactive solution is introduced. It is observed that the permeability increase due to change of porosity is approximately 50% greater when only epigenetic minerals are dissolved. It is demonstrated that dissolving syngenetic minerals that are contiguous to the connected flow network can enhance the permeability through increasing the available connected void spaces. Also, it is shown that the gap, which at some cases occurs due to mineral detachment from the fracture wall, has a direct impact on dissolution performance. Overall, this study improves the understanding of dissolution phenomena in different types of coal minerals

Rock Dynamics – Experiments, Theories and Applications - Proceedings of the 3rd International Conference on Rock Dynamics and Applications, ROCDYN-3 2018

© 2018 Taylor & Francis Group, London. Stress Corrosion Cracking (SCC) of cable bolts is an unresolved global issue in underground mines, which not only reduce operational efficiency at the mines but also compromise the safety of the working environments. SCC can result in catastrophic consequences without readily visible sign of damage. To understand this failure phenomenon, a new laboratory-based testing program for investigating the SCC of cable bolts is developed. A new load frame capable of inducing tensile loads on full-size cable bolt specimens is designed and constructed. To simulate the loading experienced by cable bolts in service, specimens are loaded under Periodically Increasing Strain Rate (PISR) conditions in simulated underground environments. The proposed testing program provides a platform for investigating SCC using full-size cable bolts and proved to be an appropriate and reliable technique

Mineralogically influenced stress corrosion cracking of rockbolts and cable bolts in underground mines

Stress corrosion cracking (SCC)of rockbolts and cable bolts in underground mines has been identified as a serious international problem over the past two decades. The frequent occurrence of rockbolt and cable bolt SCC in strata containing clay and groundwater suggests the presence of certain mineral types may be an influencing factor. To identify the environmental factors leading to SCC and evaluate the effects of mineralogically influenced corrosion, a laboratory testing program capable of recreating the underground bolting environments was designed and conducted. A static load was applied to specimens of rockbolts and cable bolts placed in “corrosion cells” containing groundwater, clay, coal or a combination of those materials for about 300 days. Results from these tests revealed that the mineralogical materials indirectly affected the rate of corrosion by significantly altering the local water chemistry. The mechanical load applied to the specimens was found to accelerate the rate of corrosion, and corrosion pits were observed to have formed on the steel surfaces. The generation of corrosion pits would likely result in local stress concentration and contribute to SCC. The results of this study provide further insights into the environmental factors leading to SCC and the proposed methodologies can be used for investigating this type of failure through in-situ experiments