In studying wellbore instability, it is generally assumed that inputs are certain and average values are used. In practice, however, there are uncertainties that exist in each one of the input variables. To overcome this deficiency a probabilistic wellbore stability model is developed for vertical wells and optimum borehole
trajectory determination. The adopted probabilistic approach captures uncertainty in input variables through running a Monte Carlo simulation to calculate the value of drilling fluid pressure as a probability distribution.
As a result, the impact of the in situ stresses and rock strength parameters on wellbore stability analysis is captured. It has been found that the maximum horizontal stress, cohesion and friction angle of the rock formation are the most critical factors in wellbore stability analysis under all in situ stress regimes. In addition, the optimum well path is studied and again the maximum horizontal stress followed by minimum horizontal stress impacted well path the most. The suggested probabilistic approach has the ability to systematically quantify the impact of uncertainty which aids more insight in wellbore stability decisions. This approach can be used as a supplement to the current available tools in designing drilling operations.