Probability of Wellbore Intercept Made Easy

Abstract

Abstract Wellbore intercept probability calculations are important not only for avoiding collisions but also for relief-well planning and planning for wellbore reentry in abandonment projects. This study reviews the current methods for calculating probability and their sensitivity to the input variables. A probability of intercept calculation is presented that is proven, robust, and can be implemented in a spreadsheet as input-wellbore positions and survey-error dimensions. Given the lack of availability of empirical wellbore collision-frequency data, it was necessary to model the wellbore-intercept scenario using a Monte Carlo simulator constructed by breaking the two wellbores into short segments. The validity of the simulator is highly sensitive to the probability distribution of the input-survey errors. The input-probability distribution of well separations in the high side and lateral directions is determined from survey-comparison studies for both systematic and random-error sources. The simulator is used to compare the accuracy and reliability of different probability integration types and sensitivity to the input data and distribution of errors. It is observed from survey comparisons that the distribution of survey inclination and azimuth errors behave as "heavy tailed" probability distributions such as Laplace or Student's t. This increases the probability of intercept at higher sigma levels than the previously assumed normal distribution. The results of this study exhibit the sensitivity of the probability result to the degree of sophistication of the probability calculation from simple one-dimensional (1D) projection to three-dimensional (3D) integration of probability density. There is a law of diminishing returns with the sensitivity of complex models being lost in the reliability of the input-error values and assumptions on probability distributions. One observation is that there is no "pedal curve" effect for high-angle well crossings. The simple point-to-point collision calculations using this vector method do not include the confidence in the relative direction of the two wellbores. Therefore, probability calculations and separation-factor calculations that engage the pedal-curve distances are overly conservative. A reliable prediction of wellbore-intercept probability is achieved using a relatively simple method. This study shows the importance of the angle of intercept to enhance interception probability for relief wells and abandonment re-entry operations.