Study on Borehole Stability of Coal Seam in Oil-Water Wells Base on Discrete Element Method

There are essential differences between coal rock and shale rock in the term of their instability mechanism due to the characteristics of well-developed joints, fragility and low strength of coal rock. The traditional method of dealing with borehole stability in shale is not suitable for handling coal beam borehole stability. The analysis of the past is lack of considering the seepage. In this paper, Maximum element displacement and joint shear displacement under different drilling fluid pressure under seepage condition is calculated and analyzed, and the results show that the maximum displacement and shear displacement changes linearly when fluid pressure is less than a certain value, but they increases by orders of magnitude when the fluid pressure is greater than the value. A reasonable drilling fluid density defending collapse of coal seam is determined by comparing the different element displacement under different drilling fluid density under seepage and non-seepage condition considering fluid structure interaction, which provides an important guide in the case of borehole stability of coal seam.