SPATIAL CHARACTERIZATION OF MICRO-AND NANOSCALE MICRO-CRACKS IN GAS SHALE BEFORE AND AFTER TRIAXIAL COMPRESSION TEST
LAN Hengxing1,2, CHEN Junhui1,3, WU Yuming1
1. Institute of Geographic Sciences and Natural Resources Research, CAS, Beijing 100101;
2. School of Geological Engineering and Geomatics, Chang'an University, Xi'an 710054;
3. University of Chinese Academy of Sciences, Beijing 100049
The spatial characterization of micro-cracks and their relationship with the mineral composition in the gas shale can provide scientific guidance for the shale gas exploration. However, there has been little research on this problem because of the limitation of experiment instruments. In this study, two samples from Longmaxi Shale, which had been preserved in Sichuan Basin since Silurian, are imaged using the high performance field Scanning Electron Microscope(SEM) and energy dispersive X-ray spectrometer(EDS). Triaxial compression test is conducted on one of the two samples which called compressed sample, while the other sample remains undistrubed. The SEM and EDS are ZEISS MERLIN SEM equipped with secondary electron(SE)detectors for crack investigations at a pixel size of 10 nm and Bruker QUANTAX energy dispersive X-ray spectrometer(EDS)system for automatic identification of minerals at a pixel size of 1 μm. The scanning area is 1 mm. Based on this, the micro-and nanoscale micro-cracks before and after triaxial compression test are characterized by spatial analysis methods. The relationship between micro-fractures and mineral compositions is also investigated. Results show that the spatial distribution of all micro-cracks in both disturbed and undisturbed samples follows the power law indicating the nature of self-organization and structural hierarchy of spatial distrubiton of micro-cracks in shale. However, the parameters of the distribution(D and logC) vary distinctly with the mineral types(quartz, feldspar, carbonate and clay) implicating that different minerals have different mechanisms in response to mechanical loading and thus different contributions to the shale fracturing. Some brittle minerals such as feldspar and carbonate and their interaction with clay may play vital role in shale fracturing. The transition from tensile failure to shearing failure dominates the failure patterns for all minerals under mechanical loading. However, the cracking spatial characterization and fracturing mechanism vary among different minerals. The spatial distribution of micro-cracks concerning carbonate minerals and clay mineral changes dramatically before and after triaxial compression test and more complicated crack networks could form. Multiple failure mechanisms could co-exist within single mineral due to the complex stress condition under triaxial compression test. The high uncertainty of failure process should be taken into account due to the complicated mineral composition and structure.
. SPATIAL CHARACTERIZATION OF MICRO-AND NANOSCALE MICRO-CRACKS IN GAS SHALE BEFORE AND AFTER TRIAXIAL COMPRESSION TEST[J]. Journal of Engineering Geology, 2018, 26(1): 24-35.
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