Determination of optimized mud weight for vertical wellbore drilling in fractures formation : A case study for SIE-04 wellbore in Persian Gulf oilfield

Wellbore stability modeling for drilling operations is primarily to create a safe mud weight (mud pressure) window. The designed mud density will be high enough to ensure borehole stability and low enough to not fracture the formation (i.e., mud losses do not occur) Therefore, the safe mud weight should be greater than the pore pressure gradient and shear failure gradient and less than the fracture gradient. In this article in order to investigate the effect of fracture in wellbore failure mechanism and determination of safe mud window, analytical and numerical approach was conducted. In order to shear failure analysis in wellbore in fractured formation, Mohr–Coulomb, Jaeger failure criterion was used. For numerical simulation the distinct element method was used. The simulation of a wellbore in Persian Gulf for fracture formation was implemented in order to understanding the effect of discontinuity in wellbore failure mechanism. The results showed, in a specific angle (β), that represents the direction of fracture orientation relative to the horizontal stress, the fractures do not have any effect on strength of the rock mass. Furthermore, in fractured formation, shear failure do not occur in the direction of minimum horizontal stress, but the orientation of discontinuity specifies the failure and slips direction in wellbore. In this article the maximum mud pressure to prevent failure in wellbore occurs in (β=45+ϕ_w/2=52.25). Also in the range of (17.5≤θ≤46.5) the fractures do not have any effect on strength of the rock mass. The practical mud pressure which was used in drilling operation in real condition in Siri oil field was (40.55 Mpa) and the mud pressure determine in analytical solution is (36.92 Mpa). The numerical result (distinct element method) shows, the overall condition of normalized yield zone, shear failure slips along the fractures and tensile failure in two different pressure (36.92 - 40.55 Mpa) are almost similar. Parametric study results showed that in higher horizontal stress ratio (σ_H/σ_h ), wellbore is more instable and in an isotopic stress regimes the shear and slips along the discontinuity was eliminated.