Problems associated with wellbore instability are one of the main causes of high costs and operational challenges in the oil and gas industry. Numerous geomechanical modeling techniques have been developed to address these issues. However, the existing models are limited due to the fact that they fail to account for the inherent variability of the rocks's mechanical and petrophysical properties, as well as the uncertainty in the values for the rock properties and other critical input parameters. This paper describes the adaptation of probabilistic techniques for performing a wellbore stability analysis in order to quantify the model uncertainty and improve the predictions made. This technique involves the use of a probabilistic approach that captures uncertainty in input variables through running a Monte-Carlo simulation to calculate the safe mud weight window as a probability distribution. A sensitivity analysis is also carried out by using once at time method (OAT) to identify the most significant input parameters. Results of the analysis illustrated that various uncertainties of input data based on measurement errors from well log tools are the main source of uncertainty in some key influential parameters used in wellbore stability models. The methodology presented here can be used as a pre-drilling design tool to predict optimal mud weight windows for a better drilling program.


Drilling is an expensive and necessary operation for petroleum and gas exploration (Asmaa et al., 2020). One of the most critical challenges affecting drilling operations is wellbore instability (Mansourizadeh et al., 2016). When drilling starts through solid rock and the drilling fluid replaces the removed rock, the equilibrium of in-situ stresses around the borehole are disturbed, which causes a stress concentration at the wall of the borehole (Dahab et al., 2020). Hence, wellbore failure is anticipated to begin there. In the case that the utilized mud pressure (mud weight) does not counterbalance (less than) the pore pressure in the permeable formation, formation fluids enter the well, and even well blowout can be expected. Thus, the pore pressure limit defines the minimum mud weight required to maintain hydraulic safety. In addition, if the pressure force from an overbalanced drilling mud column is less than the formation breakout pressure, borehole breakouts may occur due to the fact that the mud pressure is not high enough to support the rock on the borehole wall (Gholami et al., 2016). On the contrary, if the hydrostatic pressure of the drilling mud column exceeds the minimum horizontal principal stress magnitude, the tensile condition is dominant and the tensile failure may lead to reopening the natural fractures or any other conductive fissures existing around the borehole, which leads to loss of drilling fluid (Mohammed et al., 2018; Ellafi et al., 2020). Furthermore, if the hydrostatic pressure in the wellbore exceeds the breakdown pressure of the formation, tensile failure will occur in the intact rock and drilling-induced tensile fracture (DITF) will begin in the borehole wall (Abbas et al., 2019; Ellafi et al., 2019). Such wellbore instability problems would result in loss of time and occasionally equipment that may account for at least 10% of the drilling costs (Aadnoy and Ong, 2003).

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