Discrete element simulation of the mechanical properties of shale with different bedding inclinations under conventional triaxial compression

Abstract

With the growth in energy demand, shale gas has attracted considerable attention as an unconventional clean and efficient energy source. In addition, the recoverable reserves of deep shale gas in China far exceed those with a depth less than 3500 m. Thus, deep shale gas is an important replacement field for shale gas production in China. Shale, as a shale gas reservoir, forms many weak surfaces in the deposition process and shows different degrees of anisotropy in the mechanical properties. Therefore, it is of great importance to use particle flow code (PFC) to explore anisotropy of shale from the perspective of micro-level for deep shale gas production in China. Based on the experimental results obtained from the shale specimens under conventional triaxial compression, PFC2D was used to simulate the triaxial mechanical properties of shale with different bedding inclinations. The effects of bedding inclination and confining pressure on the mechanical properties of shale specimens were analyzed. The following results are obtained. (1) With the increase of bedding inclination, the peak strength and cohesion of shale all display a "U"-type variation, but the trend of peak strength is different under different confining pressures and the internal friction angle varied nonlinearly with the bedding inclination increases. (2) The effects of bedding inclinations on the displacement direction and size of surrounding particles decrease with the increase of the angle between the bedding inclination and axial stress. (3) At constant bedding inclination, the number of microcracks at the final failure of the specimen increases with the increase of confining pressure. Under the same confining pressure, the number of microcracks in the final failure of the specimen first decreases and then increases with the increase in bedding inclination. (4) With increased confining pressure, the brittleness of shale with the same bedding angle decreases as a whole. Under low confining pressure, shale brittleness is larger at both ends and smaller in the middle with the increased bedding inclinations.

Keywords

• shale
• bedding inclinations
• failure mode
• brittleness index
• discrete element simulation