Abstract
Several techniques (such as scanning electron microscopy (SEM) and gas adsorption systems) have been used to study the pore features and structures of shale reservoirs. The available methods and techniques have restricted the specific research on micropores, and the morphology, genesis, volume, and main factors controlling pore characteristics are yet to be analyzed. Currently, there is no systematic understanding of the role that these spaces play in gas storage and flow. As such, our understanding of the spatial connectivity of pores and reserves of shale reservoirs is limited. In this study, the pores of the Fuling shale gas reservoir in the Sichuan Basin were systematically observed by SEM and transmission electron microscopy. Images of pores smaller than 2 nm were captured for the first time, and their morphology and genesis were analyzed by combining these images with the rock mineralogy theory. The pore size distribution characteristics of the reservoir were analyzed by the adsorption-mercury injection method and nuclear magnetic resonance, and the main factors controlling the distribution of different pore sizes were analyzed. The results show that large numbers of micropores were distributed between the mesopores and macropores in the shale reservoir, which mainly consisted intergranular pores, intermolecular pores, interlamellar pores of clay minerals, and organic matter skeleton pores. The development of pores smaller than 1 nm was mainly controlled by the clay mineral content, and the development of pores with a size of approximately 1-2 nm was related to the contents of clay minerals and organic matter. These pores could connect the macropores and mesopores well, which is important for gas storage and flow. In this paper, the types, distribution, and main controlling factors of micropores were studied, and our understanding of the reservoir space was improved from the nanometer level to the Angstrom level, which is important for gas storage and flow process analysis.
Highlights
As a typical unconventional oil and gas resource, shale gas is widely valued worldwide
Several advancements have been made in the characterization and classification of the pore structures of shale reservoirs by using materials science research methods such as scanning electron microscopy (SEM) and gas adsorption
Owing to the limited field-of-view, the sample must be fully observed under SEM before selecting the representative areas and cutting segments for transmission electron microscopy (TEM) observation to ensure that the observations of the reservoir are more comprehensive
Summary
As a typical unconventional oil and gas resource, shale gas is widely valued worldwide. Several advancements have been made in the characterization and classification of the pore structures of shale reservoirs by using materials science research methods such as scanning electron microscopy (SEM) and gas adsorption. The super-microscopic pore spaces of the brittle minerals, clay mineral particles, and organic matter skeletons were observed, and the proportions of these spaces and their main controlling factors were analyzed by gas adsorption, mercury injection, NMR, and other technologies. The roles of these spaces in gas molecular storage and flow were analyzed, which is important for attaining an in-depth understanding of the spatial characteristics of micropores in shale reservoirs and their gas flow processes
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