Numerical Analysis Of Wellbore Stability Of Extended Reach Wells In Complicated Stratum

As petroleum exploration and development expanding continuously, drilling accidents associated with borehole instability are increasing in oil drilling engineering. Mechanical factor is one of the important reasons for borehole instability in addition to the chemical factor. Therefore, the study of wellbore stability with analysis of the stress state around a borehole is of significant engineering value. The content of the thesis is mainly composed of the following three parts.Firstly, general stress field in the wellhole coordinate system was obtained from the principal in-situ stress field by coordinate transformation. The stress field around the well hole was obtained through the theory of elasticity. Then, the stress fields around inclined wellbores were programmed and numerically calculated under complicated stratum conditions such as normal fault, over fault and shift fault. The results show that the change of wellhole azimuth angle had more influence on the location than on the value of the maximum stress around the well. And the results also indicate that the change of well hole deviation angle had less influence on the position than on the value of the maximum stress around the well.Secondly, the rigid body displacement of the model was restricted by the constraint of six linear displacements at the four corners’nodes of the finite element model. The normal and tangential loads were applied to the model using surface effect element. Stress distribution around the inclined borehole in complicated stratum was numerically simulated using ANSYS. The effects of ground stress ratio, wellhole deviation angle, wellhole azimuth angle and other factors were analyzed.Thirdly, a numerical simulation on the ANSYS platform was performed for the stress field around inclined boreholes in complicated stratum using finite element model. The effects of stratum deviation angle, stratum azimuth angle, degree of anisotropy and other factors were analyzed. The study indicates that the anisotropy of the formation do affects the stress field around an inclined wellbore to some extent. The more serious of the degree of anisotropy is, the bigger the Mises stress around a wellbore will be. While the stratum azimuth angle affects the maximum Mises stress around a borehole differently, the formation deviation angle controls maximum Mises stress with the similar manner in the three faults.