To study the borehole stability problem of oil and gas drilling and geothermal drilling in deep formations, a basic differential equation of plane strain problem for borehole stability in deep formation is proposed on the basis of the continuum mechanics of elasticity, and a BEM calculation method of stress and displacement distribution around the wellbore is derived by using the BEM theory. To test the correctness of the BEM method, a physical model and the boundary elements of borehole stability problem are established, then the stress solution around the wellbore is compared with the analytical solution. By analyzing the borehole stability problem of deep formation in a certain oilfield of Sichuan, it is shown that the stress distribution with the BEM method is in good agreement with analytical solution, and its error is less than 0.82%. The borehole stability analysis results are basically in accord with the actual situation at 2500-3500 m in the example well, the actual drilling fluid density is lower than the equivalent density of the collapse pressure, and the hole expanding rate is above 20%. Increasing the density of drilling fluid can maintain wellbore stability, thus the accuracy of the BEM method is further verified. This method provides a new means for wellbore stability analysis.

wellbore stability  /  collapse pressure  /  boundary element method  /  numerical method