Determination of Formation Collapse Pressure of Bonan Shale Taking into Account Tri-axial Stresses

The lithology of the Shasan reservoir formations in Luojia, Block Bonan is mainly shales that are thick and widely spread. The shales are rich in organic carbon and oil and gas. Borehole instability has been encountered during horizontal drilling because of the developed fractures in those formations, the anisotropy of the formations, and the hydration of the formation rocks. Conventional models prognosing the collapse pressure of the formations cannot be used to calculate the collapse pressure. Those models that only consider the weak planes have no thought for the effect of intermediate principal stress on the strengths of rocks, and the prognoses are generally not done satisfactorily. Mineral composition and fracture distribution have been examined about the shale samples taken from Luojia, and based on the weak plane strength theory, a more accurate prognosis model, which takes into account the effects of triaxial stress, has been established for the prediction of collapse pressure. The application of the model shows that for wells penetrated the shale formations, borehole stability cannot be realized through only higher mud weight. Carbonate formations with higher original strengths and low porosity are not uncommon in this area. If managed pressure drilling (MPD) is used with bottom hole pressure slightly lower than formation pressure, the strength of the rocks can be increased, and hence the stability of the borehole wall. This, in turn, will help reduce drilling cost and increase ROP.