Probabilistic Analysis of Loss of Primary Safety Barrier During Drilling Operations

Abstract

The present study aims to propose a methodology to treat probabilistically the prediction of well bottom pressures during drilling, circulation and tripping operations. The pressure generated by the fluid is the primary safety barrier and its failure can immediately initiate gains and losses events in the well, with a direct impact on the safety of the process. Two models were considered in the study: a two-phase flow model commonly used to predict hole cleaning and downhole pressures while drilling and a pressure propagation single flow transient model (which considers fluid compressibility and gelation) to account for peaks in tripping and pump restarts. A Monte Carlo method coupled with a latin hypercube strategy was implemented to propagate the uncertainties in the input variables to the resulting pressure. The probabilistic approach aims to consider the uncertainties in the input variables (weight, rheology, penetration rate, drillstring velocity and acceleration, pump flow, particle diameter, well trajectory, etc.). Monte Carlo Simulation was conducted to generate the pressure distribution curves in each of the operations: drilling, circulation, tripping and pump restarts. Probabilistic pressures are fit into probabilistic operational windows allowing the quantification of the risk of losing the barrier. The paper also details the critical aspects associated with typical pre-salt well drilling project conditions in offshore Brazil where narrow operational widows are a reality. The analysis directly impacts the choice of drilling strategy (conventional or MPD) and, consequently, rig selection. Innovative strategy to enable risk assessment strategies in the analysis of safety barriers. Pilot to a major integrated approach which will include other barriers such as cement, rock and equipment.