Application of the Concept of Probabilistic Risk Assessment for the Borehole Stability Evaluation in Horizontal Directional Drilling

Abstract

The current practice of assessing the risk of hydraulic fracture in horizontal directional drilling (HDD) is conducted by making comparisons between the minimum required annular pressure to maintain drilling fluid circulation (Pmin) and the formation limit pressure (Pmax) along the HDD bore path. Despite the potential spatial variability and heterogeneity of ground conditions along the HDD bore path, the current practice of selecting appropriate models and input parameters for estimating the Pmin and Pmax relies heavily on the engineering judgments made by individuals rather than handling the uncertainties systematically. In this paper, a probabilistic method is implemented for analyzing the likelihood of hydraulic fracture occurrence to overcome the limitations of the deterministic nature of current practice. By incorporating the spatial variability and heterogeneity of ground conditions while selecting the appropriate parameters and models, a comparison between the estimates of Pmin and Pmax could be presented as a distribution of the factor of safety. This probabilistic method is also applied to a case study, which presents an HDD constructed within a clayey stratum with its properties varying along the bore path. The probabilistic analysis demonstrated herein logically handles the uncertainty associated with determining the input geotechnical parameters, especially for the portions of the HDD bore path where information on the ground conditions is limited. By analyzing the probability of hydraulic fracture occurrence as a distribution of the factor of safety, this new method could reduce the burden and subjectivity within the engineering judgments made while designing the HDD bore path.