Fractures and their macro-mesoscopic mechanical evolution pattern in coals in the Benxi Formation, Ordos Basin

Abstract

ObjectiveThis study aims to determine the relationship between multi-scale fracture characteristics and the mechanical behavior of deep coals, thereby helping gain further insights into the mechanisms behind the macro-mesoscopic damage and failure of the coals and providing a scientific basis for the stability assessment and mechanical modeling of deep coalbed methane (CBM) reservoirs. To this end, it is necessary to examine the differences in the coals’ mechanical properties under varying scales. MethodsThis study conducted uniaxial and triaxial mechanical tests, nanoindentation experiments, and CT scanning of the No. 8 coal seam in the Benxi Formation within the Ordos Basin. Based on the differences in fracture density and modulus of elasticity across varying scales, this study developed an upscaling model through fitting. Then, it investigated the influence of confining pressure on the mechanical parameters of fractures using this model. Results and Conclusions The structurally intact coal samples exhibited a fracture network formed by the dense interactions between cleats on the end faces and inside. The end faces had a face cleat density of 6–12 per 5 cm and a butt cleat density of 9–16 per 5 cm. CT scanning-based statistics reveal an internal cleat density of approximately 10 per 5 cm. At the microscale, fracture widths ranged from 0.52 μm to 13.41 μm. The presence of fractures significantly reduced the macroscopic modulus of elasticity of the coal samples, with values obtained from triaxial tests proving about 21% lower than those from nanoindentation experiments. As confining pressure increased, fractures gradually closed, leading to nonlinear increases in both the peak strength and modulus of elasticity of the coal samples. Correspondingly, the failure mode transitioned from tensile splitting to shear failure. The fitted model indicates that normal stiffness increased nonlinearly with confining pressure, with the increasing rate gradually slowing down. This model can be used to predict the law of changes in the macroscopic modulus of elasticity of coals under varying confining pressures. The results of this study provide an experimental basis for the multi-scale refined characterization and parameter prediction of the mechanical behavior of the coal reservoirs in the Benxi Formation within the Ordos Basin.

Keywords

• coal in the benxi formation
• fracture
• confining pressure
• ct
• modulus of elasticity
• nanoindentation