Three-Dimensional Analysis on Stress Patterns within a Sub-Salt Formation and an Integrated Method for the Design of a Mud-Weight Window

Borehole stability and pore pressure environments downhole transition abruptly, often detrimentally in the presence of salt formations. Geomechanical regimes can vary considerably above, through, and below salts, making accurate modeling of them necessary but challenging. This paper proposes an integrated method that has been developed for predicting the Mud-Weight Window (MWW) of subsalt wells. The high efficiency of the 1D method and high accuracy of the 3D method are deliberately combined in the proposed integrated method, while the disadvantages of those methods are avoided. This objective is achieved by calculating the effective stress ratios, which are part of the input data of the 1D method, with a 3D finite-element (FE) model. The effective stress ratio brings a 3D property into the 1D solution of MWW, thus giving it a 3D property. A 3D model, where the wellbore trajectory was accurately referenced, was built to illustrate and validate the proposed method. A salt body with a thickness of 5.8 km was included. The distribution of the effective stress ratio within the model has been calculated and used in the numerical prediction of the 1D MWW. The prediction of a 3D MWW corresponding to the wellbore trajectory is then derived in the subsalt formation and merged with the numerical prediction.