Enhanced 1D Prediction of the Mud Weight Window for a Subsalt Well, Deepwater GOM

摘要

Because of the complex distribution of stress directions around a salt body,accurate prediction of the mud-weight window(MWW)for subsalt wells has presented a challenge within the industry for many years.This paper presents a method for the enhanced 1D prediction of the MWW for subsalt wells in the Viosca Knoll area in deepwater Gulf of Mexico(GOM).An initial stress solution is obtained using a 3D finite-element method(FEM).The stress solution is then used as input data to the 1D prediction tool.The field-scale model used in the calculation is a cubic block with a true vertical depth(TVD)of about 9000 m,a width of 7500 m,and a length of 7500 m.Along the wellbore trajectory,the salt body is 6000 m thick and 5500 m wide.Based on seismic sectional data,an anticlinal structure is constructed accordingly at its bottom surface.The 3D porous elastoplastic FEM calculation was performed first with the field-scale model.The effective stress ratio was provided for the 1D prediction of MWW along the given wellbore trajectory.The finite-element model simulates the geometry of the salt body in detail.Comparisons are made between the results obtained with the enhanced 1D method,the conventional 1D method,and a finite-element method.Solutions obtained with the conventional 1D method missed the abnormality of stress distribution at the salt base and have smaller MWW values,as compared to the other solutions.The solution for MWW obtained with the enhanced 1D method is near the value obtained with the finite-element method,but at only 10%of the time cost.The FEM enhanced 1D method proposed has proven to be the most cost efficient method for predicting MWW for subsalt wells.This work presents an example of a best practice.