Plastic mechanics model for wellbore stability analysis of branched wells

Key words:  drilling  branch well  wellbore stability  branch well orientation  finite element method

Abstract:

Three-dimensional elasto-plastic models based on finite element method were developed for studying wellbore stability during branch well drilling, in which a mechanical system incorporating rock formation, cement sheath, casing and branch junction was considered. In the models, cement, rock formation and casing were treated as different materials with a plastic feature. Model transformation was established for stress anisotropic formations to maintain the same geometry for various branch orientations and avoid the errors caused by differential geometry model. The influence of branch orientation on wellbore stability during branch drilling was analyzed using the elasto-plastic finite element simulation. The results indicate that, when the branch azimuth angle is less than 15°, the maximum equivalent stress of the formation and cement sheath appears on the main wellbore system, which is favorable to the stability of branch wellbores. When the branch azimuth angle is greater than 75°, the maximum stress will appear on the branch wellbore, which may cause stability problems of the branch wellbores. To reduce deformation around the opening of the casing, the branch orientation should be very close to the direction of the maximum horizontal stress. Failure of the cement sheath in the main wellbore may cause problems during branch drilling, especially when the branch azimuth angle is greater than 30°. Small branch azimuth angle should be used in terms of the distribution of equivalent stress, casing deformation and cement 's plastic deformation. Azimuth angle range of 50°- 80° should also be avoided in order to ensure the stability of the wellbore.