Abstract:

Correct understanding of the uniaxial compressive strength of shale failure characteristics and its UCS influencing factors has an important reference value in detecting shale failure mechanism, drilling and well completion, fracturing evaluation, micro crack prediction, logging interpretation and seismic response. In this paper, based on the systematic study and comprehensive analysis of the recent research results of shale uniaxial compressive characteristics and the UCS study, reviewed the shale uniaxial failure characteristics and its UCS affecting factors. The results show that, the shale inelastic deformation mechanism in uniaxial loading curve is affected by the sliding of micro cracks, the production and extension of micro cracks, and the pore collapse. The shale uniaxial compression characteristics present tensile or tensileshear fracture. The tensile fracture is easy to occur in location that has high rigidity and degree of consolidation. The shear fracture is easy to occur in location that has low rigidity and degree of consolidation. The S shape failure criterion can accurately describe the shale compressive failure strength variation from uniaxial to three axis failure. From the point of view of geology, making an induction and summary of shale UCS affecting factors, they are composition, bedding plane angle, sorting, micro fabric, organic matter content and its distribution, porosity, moisture content, and microcracks. The affecting mechanism of these factors are analyzed in detail. Among these factors, due to the complex distribution characteristics of micro cracks, the mechanism of micro cracks on shale UCS is not well understood. So, it is recommended that quantitative research of shale micro crack parameters and study of the influence mechanism of micro cracks on shale uniaxial compressional failure from the microscope scale should be strengthened in the future. This study can provide a scientific guidance for shale oil and gas exploration and development.

Key words: shale, uniaxial compressive strength, tensile failure, shear failure, micro crack, effecting factor