钻井液驱替下古龙页岩力学特性及弱化规律OA北大核心

In the process of shale oil drilling,due to the redistribution of ground stress around the well-bore and the intensification of hydration,wellbore instability becomes an urgent problem to be solved.The microphysical and macroscopic mechanical properties of the Gulong shale oil layer are studied.The results show that the average pore diameter of the bedding seam is 160.94 μm,and the average pore di-ameter of the sandy strip is 1.50 μm.The shale has significant anisotropic permeability,in which the permeability parallel to the bedding direction is about twice that of the vertical bedding direction.The upper and lower limit angles of shale bedding plane failure are defined,and a chart showing the distribu-tion of shale strength with the dip angle of weak plane under different confining pressures is made.The failure model considering shale oil bedding plane is established and verified with the experimental re-sults.The average error is about 8.25％.In the vertical bedding direction,the triaxial compressive strength of rock after displacement by water-based drilling fluid,alcohol-based drilling fluid and oil-based drilling fluid decreased by 20.31％,17.67％ and 13.69％,respectively.In the horizontal bedding direction,the rock triaxial compressive strength of water-based drilling fluid,alcohol-based drilling fluid and oil-based drilling fluid after displacement decreased by 36.15％,31.78％ and 22.44％,respectively,and the failure criterion of shale after weakening was modified.Based on the field data,this paper sum-marizes the current progress in the study of wellbore instability in Gulong shale,and determines the safe drilling fluid density window of Qingshankou Formation:1.71-1.73 g/cm3.The drilling fluid with this parameter can form an effective mechanical support to the wellbore,help to effectively prevent forma-tion spalling,and provide a clear adjustment direction for the optimal ratio of main and auxiliary emulsi-fier.The research results have certain guiding significance for the safe drilling of Gulong shale oil drill-ing project.