A Mechanical Model of Borehole Stability for Weak Plane Formation Under Porous Flow

Based on influence of porous flow on weak plane model, the authors established a mechanical model of borehole stability for weak plane formation under porous flow and analyzed effect of weak plane on borehole stability under porous flow. The results indicated that porous flow decreased strength of weak plane, enlarged the affecting domains of weak plane for rock mass strength, and worsened borehole instability in weak plane formations. With porous flow increasing, water content of weak plane increases. For the weak plane with DIP < 10 degrees, the borehole is stable; for 10 degrees < DIP < 30 degrees, the borehole is likely unstable when azimuth is close to the direction of maximum stress, and the larger the dip, the worse the borehole stability; for DIP > 30 degrees, it is opposite to the situation of 10 degrees < DIP < 30 degrees. While weak plane formation is next to be saturated, the minimum drilling fluid density for borehole stability does not change with weak plane azimuth and borehole stability is the worst. The mechanical model of borehole stability for weak plane formation under porous flow is applied to a well in Tarim Basin piedmont.