A Three-Dimensional Finite-Element Model in ABAQUS to Analyze Wellbore Instability and Determine Mud Weight Window

Abstract

Wellbore instability is one of the main problems of the oil industry, causing high costs in the drilling operation. Knowing about the mechanical properties of involved formations and in-situ stresses is a privilege gained by determining an appropriate mud weight window (MWW). To this aim, a three-dimensional (3D) finite-element model was simulated in ABAQUS to analyze in-situ stresses and determine the MWW in the drilling operation of wellbore-D in the Azar oilfield. The results from the 3D finite model revealed that the Azar oilfield is structurally under the impact of a complex tectonic system dominated by two reverse faults with a configuration of σH > σh > σv across the Sarvak Formation. The amount of vertical, minimum, and maximum horizontal stresses was 90.15, 90.15, and 94.66 MPa, respectively, at a depth of 4 km. Besides, the amount of pore pressure and its gradient was 46 MPa and 11.5 MPa/km, respectively. From drilling wellbore-D in the direction of the maximum horizontal stress, the lower limit of the MWW was obtained at 89 pcf. In this case, the results showed that the wellbore with a deviation angle of 10° is critical with a mud weight lower than 89 pcf. It caused the fall of the wellbore wall within the plastic zone sooner than other deviation angles. Also, in the case of drilling wellbore in the direction of minimum horizontal stress, the lower limit of the MWW was 90.3 pcf. Moreover, in the deviation angle of approximately 90°, the wellbore wall remained critical while the mud weight was below 90.3 pcf. Comparison of the results of numerical and analytical modeling showed that the modeling error falls within an acceptable value of < 4%. As a result, the wellbore with the azimuth of the maximum horizontal stress needed less mud weight and decreased the drilling costs. This particular research finding also provides insights for obtaining the lower limit of the mud weight window and determining the optimal path of the well-bore when using directional drilling technology.