The objective of this paper is to incorporate multi-weakness plane failure into a new anisotropic wellbore stability model for transversely isotropic rock media involving naturally fractured and foliated formations. The model provides a good prediction of wellbore failure, safety angles of weakness planes and estimation of mud weight for drilling optimization.

The near wellbore stresses are calculated using Lekhnitskii-Amadei (1983) solution. Intact rock matrix failure and multi-weakness plane failure are incorporated to characterize anisotropic rock strength and its failure criterion. The intact rock matrix strength is described using either Mohr-Coulomb criterion or Hoek-Brown criterion. Instead of incorporating single-plane weakness theory, a multi-weakness plane failure model is used to describe joint strength induced by multiple weakness planes. Effects of intermediate principle stress are discussed. Using the proposed model, borehole failures dominated by either weakness planes or in-situ stress induced rock matrix shearing is predicted along the borehole trajectories by coupling with elastic properties from sonic logs. A comparison is made between single weakness plane theory and the proposed model. Results from both methods can be compared and calibrated with caliper logs and borehole image logs (FMI) when available. By extension, estimation of the mud weight window is obtained from the model prediction. Some sensitive analyses of weakness planes distribution, dip angle of weakness planes and horizontal stress orientations are conducted along with mud weight estimation analysis. Data from the Western Canada Sedimentary Basin (WCSB) is used for a case study.

The proposed multi-plane weakness model generates more reliable estimations of borehole failures (as opposed to single weakness plane theory) in high naturally fractured and foliated formations. Mud weight increases with high weakness plane numbers and fracture density.

Most previous models are based on Jaeger (1929)'s single weakness plane theory, which is affected by intermediate principle stress and does not work well in naturally fractured formations. In practice, results are highly sensitive to azimuth and dip direction of fractures, well inclination and fracture density. The proposed model takes into account all these parameters and consequently provides more accurate results.

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