Análise de incertezas em fenômenos transitórios de hidráulica de poços utilizando o método Monte Carlo

Abstract

During an oil well drilling, a pressure gradient develops along the well. These pressures need to be controlled carefully, since the well only accepts a narrow pressure interval. The well pressures are hard to measure; therefore, they are usually estimated by numerical simulation. In order to aggregate reliability to the process, the pressure prediction methods are required to be as precise as possible. They can be made more robust and resilient by adding uncertainty considerations into the mathematical models. This way, the main objective of this work is to evaluate the uncertainties in numerical simulations of well hydraulics transient operations, by using the Monte Carlo Method along with Latin Hypercube Sampling. In order to achieve this, it is developed numerical-statistical implementations of such transient phenomena. It was developed a case-study, based on a drillpipe displacement, considering 1% of uncertainty in the well and drillpipe diameters, in the drilling fluid density, drillpipe velocity, yield stress and plastic viscosity. A sensibility analysis was conducted, in order to evaluate the influence of the parameters in the pressure uncertainty composition. The variable with the biggest influence was the external well diameter. In this case, it was obtained more than 2% of relative uncertainty. As the conclusions, the method is quite efficient for the uncertainty quantification. It is emphasized that the uncertainties in the transient step of the simulations are bigger than the ones at steady-state. Therefore, a reliability-based uncertainty analysis must consider the transient effects, otherwise, it will not be able to identify the critical uncertainty values.