In an oil field, drilling engineers had encountered numerous drilling troubles such as sloughing and stuck pipe problems across a shale interval overlaying the reservoir. In a preliminary study the authors carried out, the instability phenomena contradicted to the simple theory of homogeneous material. For example, hole condit ion worsened in the highly deviated well although the stress regime is strike-slip fault type.

The study aims to find out the root cause of such complex instability phenomena, to developnu merical method that can be used for mud weight optimization, and to establish the drilling guideline to mitigate the troubles

Using the 3D poroelastic and laminated fo rmation model, we found that wellbore condition can be worsened as hole inclination increase even in the strike-slip stress condition. In such case, higher mud weight cause wider failure a readue to the fluid pressure penetration into weak planes of the laminated shale formation.

The paper shows that the appropriate numerical method with three-dimensional poroe lastic model with heterogeneous the strength and permeability can predict such unconventional wellbore instability phenomena.

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