Multi-scale 3D Imaging for Characterization of Microstructural Properties of Gas Shales

In recent years, gas shale has attracted renewed attention as an unconventional energy resource, with massive, fast-growing, and largely untapped reserves. Shale is a fine-grained sedimentary rock containing a high content of organic matter (kerogen) from which gas can be extracted. The identification of the pore structure and quantification of the geometry, sizes, volume, connectivity, and distribution of extremely fine-grain pores, kerogen, and minerals are all extremely significant for the characterization of gas shale reservoirs. These features determine fluid flow and ultimate hydrocarbon recovery, however, they are also highly challenging to determine accurately. X-ray micro and nano-computed tomography (μ-CT and Nano-CT) combined with 3D focused ion beam scanning electron microscopy (FIB-SEM) are used in this thesis to address this challenge and to provide more information for understanding the complex microstructures in 3D from multiple scales within shale samples. In this thesis, state-of-the-art multi-scale imaging with multi-dimensional potential was applied to the image and quantified the microstructures' properties of gas shale. Samples were first imaged with X-ray micro-and nano-tomography (μ-CT and Nano-CT), and then with Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) measurements. Results of image analysis using SEM (2D), μ-CT (3D), ultrahigh-resolution Nan-CT (3D), and FIBSEM (3D) under backscattered electron (BSE) images reveal a complex fine-grained structure at specified phases such as pores, kerogen, and minerals within samples. The results show low connectivity of pores and high connectivity of kerogen which suggests that porous gas flow through samples used in this study cannot be the main transport through the pores. This implies that the gas transport through the pores is unlikely to be important, but cannot be ignored, as it is very important and constitutes the basis for understanding permeability in the rock. However, the high connectivity of kerogen provides the potential pathways for gas flow throughout the whole sample. The combination of multiscale 3D X-ray CT techniques (micro-nano) with 3D FIB-SEM provides a powerful combination of tools for quantifying microstructural information including pore volume, size, pore aspect ratio and surface area to volume distributions, porosity, permeability in shales, and also allowing the visualization of pores, kerogen and minerals phases over a range of scales. Other methods, such as physical measurements Gas Research Institute (GRI), mercury injection (MIP), and nitrogen adsorption (N2) are also presented in this study. The combination of these data sets has allowed the examination of the microstructure of the shale in unprecedented depth across a wide range of scales (from about 20 nm to 0.5 mm). Overall, the shale samples from Bowland shale formation shows a porosity of 0.10 ± 0.01%, 0.52 ± 0.05%, and 0.94 ± 0.09% from three FIB-SEM measurements, 0.67 ± 0.009% from the nano-CT data and 0.06 ± 0.008% from one μ-CT measurement, which compare with 0.0235 ± 0.003% from nitrogen adsorption, and 0.60 ± 0.07% from MIP. The porosity was also observed to be 0.43± 0.009% and 0.7% ± 0.007% for FIB-SEM and Nano-CT methods, respectively in different shale reservoirs from a Sweden formation. The data vary due to the different scales at which each technique interrogates the rock and whether the pores are openly accessible (especially in the case of nitrogen adsorption). The measured kerogen fraction is 32.4 ± 1.45% from nano-CT compared with 34.8 ± 1.74%, 38.2 ± 1.91%, 41.4 ± 2.07%, and 44.5 ± 2.22% for three FIB-SEM and one μ-CT measurement. The pore size imaged by nano-CT ranged between 100 and 5000 nm, while the corresponding ranges were between 3 and 2000 nm for MIP analysis and between 2 and 90 nm for N2 adsorption. The distribution of pore aspect ratio and scale-invariant pore surface area to volume ratio (σ) as well as the calculated permeability shows the shale sample in this study to have a high shale gas potential. Aspect ratios indicate that most of the pores that contribute significantly to pore volume are oblate, which is confirmed by the range of σ (3−30). Oblate pores have greater potential for interacting with other pores compared to needle-shaped prolate pores as well as optimizing surface area for the gas to desorb from the kerogen into the pores. Permeability has also been calculated and values of 2.61 ± 0.42 nD were obtained from the nano-CT data, 2.65 ± 0.45 nD from MIP, 13.85 ± 3.45 nD, 4.16 ± 1.04 nD, and 150 ± 37.5 nD from three FIB-SEM measurements and 2.98 ± 0.75 nD from one μ-CT measurement, which are consistent with expectations for generic gas shales (i.e., tens of nD). The quantitative results of 2D and 3D imaging datasets across nm-μm-mm length scales provided a view of understanding the heterogeneous rock types, as well as great value to better understand, predict and model the pore structure, hydrocarbon transport, and production from gas shale reservoirs