Optimizing the Wellbore Trajectory of Directional Wells Considering Wellbore Stability Subjected to the Non-Independence and Uncertainty of Geomechanical Parameters

Abstract

Abstract Effectively analyzing the wellbore stability risk in directional wells plays an important role in the exploration of oil and gas resources in complex deep formations. For the smooth execution of the drilling process, wellbore stability is related to the rock strength characteristics of the formation and the stress state of the rock around the borehole, which in turn is directly affected by the wellbore trajectory inclination and azimuth. The stress state depends on the magnitude of in-situ stresses. However, the uncertainty and non-independence of geomechanical parameters greatly impact the predicting wellbore instability pressure and wellbore stability evaluation. Therefore, this paper effectively combines the Monte Carlo method with the Nataf transformation to sample and simulate the geomechanical parameters and realize the quantitative risk assessment (QRA) of wellbore instability. The parameter sensitivity characteristics of borehole collapse and fracture pressures under different wellbore trajectories and stress states are studied on this basis. The main research shows that the risk assessment results of wellbore instability based on parameter uncertainty indicate that the predicted collapse equivalent density usually increases and the fracture pressure equivalent density decreases where the reliability is greater than 50%, which leads to a significantly narrower safe mud weight window. In addition, the influence of parameter uncertainty on fracture pressure is significantly greater than that on collapse pressure. The correlation coefficient is used to constrain the reservoir geomechanical parameters, which maintains the linear characteristics between the parameters, and then significantly reduces the uncertainty range of wellbore instability pressure. The uncertainty of in-situ stress parameters makes it possible for the formation rock to experience the type of stress state that changes from conventional strike-slip faults to normal faults. However, this possibility of stress state transition obviously affects the selection of wellbore trajectory optimization. The rock mechanics parameters including elastic modulus, Poisson’s ratio, cohesion, internal friction angle, and tensile strength have a weak effect on the collapse pressure and fracture pressure of arbitrarily inclined boreholes. However, the obvious sensitivity of in-situ stresses depends on the change of wellbore trajectory in deviated/horizontal wells. As for the vertical well, the maximum and minimum horizontal in-situ stresses always are the primary sensitivity factors of borehole collapse and fracture pressures, respectively. The methodology shown in this paper provides important guidance for engineering design by calculating the probability of wellbore trajectory optimization.