Download files
Abstract
Shale gas has become the most important natural gas resource in the U.S. and is booming in many other countries. Due to the nano-pores of shale, diffusion phenomenon is an important mass transport mechanism in shale and therefore needs to be better understood for extracting the gas more effectively. Since shale pore structure is comprised of pores with sizes ranging over several orders of magnitude, comprehensive studies on shale diffusion property require characterization of diffusion at multiple length scales. In this thesis, we develop a workflow that investigates the process of liquid/liquid diffusion through shale rock. We combine scanning electron microscopy (SEM), focused ion beam-scanning electron microscopy (FIB-SEM), pore-scale simulations and high-resolution 4D X-ray microcomputed tomography (μ-CT) to investigate shale structure and diffusion properties at various length scales. Firstly, a new mathematical framework is developed, which is able to measure local effective diffusivity from μ-CT images. This framework enables micrometer scale investigation of diffusion for millimeter samples. For achieving better measurement, the errors/uncertainties that are associated with grayscale-based quantifications are investigated and practical suggestions to alleviate those issues are provided. Secondly, 4D visualization of diffusion phenomenon in shale is realized by dynamic μ-CT imaging. Fast diffusion caused by fractures is observed. Local effective diffusivity measurement shows substantial heterogeneity within the sample. Micrometer scale compositional changes in shale matrix are found to be responsible for the heterogeneity of diffusivity. Lastly, the multi-scale pore structure of shale is investigated and a conceptual pore structure model is built based on observations. Pore-level numerical simulations on segmented FIB-SEM images are upscaled to determine effective diffusivity. The porosity-diffusivity relationship provides a way for predicting diffusivity from porosity. We conclude that three factors control the diffusion process in shale, which include fractures, matrix heterogeneity and organic matter pore structure. Fractures influence the diffusion process at the core scale (millimeter) resulting in a bulk diffusivity greater than the matrix diffusivity. Matrix heterogeneity controls diffusion at the sub-core scale (micrometer) and causes diffusivity differences by more than one order of magnitude. Pore-scale structure (nanometer) within the organic matter results in diffusivity differences mostly within one order of magnitude.