Evaluation of Rock Integrity during Well Drilling Activities using Reliability Analysis

Abstract The main risk during the construction of oil wells is the possibility of uncontrolled hydrocarbon flow occurring from the well to the environment, commonly known as a blowout. After the Macondo accident, in the Gulf of Mexico, in 2010, there has been, in recent years, a significant increase in researches related to both well integrity and operational safety. A possible approach is to seek tools that can predict disastrous scenarios and support the decision making process. In this context, this work evaluates the reliability of the weakest rock in an open hole section, which is the formation immediately below the last settled casing shoe. Reliability, in this case, refers to the likelihood that the rock does not fracture during a possible kick or during drilling fluid circulation without a kick. The formulation of this problem involves addressing the inherent uncertainties, which may cause the failure of the “rock” barrier element. Modeling was performed by assigning probabilistic distributions to the parameters carrying uncertainties and by proposing two solutions. If all the uncertainties can be modeled using the normal distribution, it is possible to calculate the reliability analytically; meanwhile, if the variables are modeled using more than one type of distribution, an analytical solution may be hard to achieve and Monte Carlo simulations are preferred to obtain a numerical solution. The numerical solution does not require mean values and standard deviations of the output variables and becomes particularly useful when these characteristics are unknown. Therefore, this methodology evaluates the competence of the rock as a barrier element, enhancing operational safety and maintaining well integrity. The calculated reliability values can be used to assist in real-time drilling monitoring, from a rock integrity standpoint. The results of this methodology include the rock reliability as a function of the well depth, based on the current drilling phase. This includes the two aforementioned scenarios, with and without a kick.