Wellbore Stability Analysis Based on Statistic Analysis Applied to a Probabilistic Method

When developing a wellbore stability analysis project, it is important to have as much information about the area to be drilled as possible due to the importance of data accuracy to construct the mud weight window. Usually, in order to avoid drilling events, data of adjacent wellbores are used to deterministically estimate the fracture and collapse gradients and stablish a mud weight range that assures the wellbore stability along the desired trajectory. This paper presents a wellbore stability analysis based on a probabilistic method - First Order Second Moment (FOSM) - that considers a statistic analysis to characterize the uncertainties associated to the failure pressures calculated. First, a deterministic method is used to calculate the fracture and collapse gradients. The input parameters are assumed to have a standard deviation of 10% of the mean value. Then, the results of the first step are used as input data to the probabilistic method that will provide scenarios for different probabilities of success (P10, P50 and P90). The probabilistic method proposed is based on a statistical analysis to define the standard deviation associated to the gradients’ calculus. This paper will approach two different ways to define this standard deviation: normal variation and random variation. Once the analysis of the results is made, it is possible to conclude for the probabilistic method which scenario presents a more conservative behavior, the standard deviation of the results and it will be possible to compare the two statistical analysis described for each of the failure model.