Comparative Study on Calculation Methods of Mud-Weight Window for Subsalt Wells: 1-D, 3-D, and Integrated 3-D Methods

Owing to the complex distribution of stress directions around a salt body, accurate prediction of mud-weight window (MWW) for subsalt wells has presented a challenge to the industry for a long time. This paper presents a comparative study on methods practically used in calculating the mud-weight window of subsalt wells. Selected methods, which include the 1-D analytical method, 3-D numerical method, and the integrated 3-D method, are compared. A subsalt well section in deepwater GoM was chosen as a validation example. Both workflows of each calculation method and resultant MWWs obtained with each method are analyzed and compared. The TVD depth of the investigated subsalt well section is from 22,000 to 30,000 ft. The salt body's thickness along the wellbore trajectory is 20,000 ft, and its width is 18,000 ft. With seismic sectional data, an anti-cline structure is constructed accordingly at its bottom surface. Mohr-Coulomb criterion is adopted in the calculation of MWW. A 1-D solution of MWW is calculated with the geostress field derived from overburden stress along the trajectory. A 3-D solution of MWW is calculated with the stress values obtained with finite-element submodeling techniques. An integrated 3-D solution of MWW is calculated with the 3-D stress field obtained from the finite-element method and an analytical tool for MWW calculation along the trajectory without using the submodeling technique. Comparisons have been made between the results obtained with the conventional 1-D method, 3-D finite-element submodeling method, and the integrated 3-D method. Solutions obtained with the conventional 1-D method have missed the abnormality of stress distribution at the salt base and have smaller values of MWW compared to the other solutions. The solution of MWW obtained with the integrated 3-D method is very close to the one obtained with the finite-element submodeling method, but at only one-tenth of the time cost. With the validation example of the subsalt well section, a comparison of workflows and the resultant MWW's accuracy has illustrated the advantages and disadvantages of typical 1-D, 3-D, and integrated 3-D method for MWW calculation. The results presented in this paper can be used as a reference deciding on the best practice for calculating the MWW of subsalt wells. This work also presents an example of best practice.