Evaluation of Rock Failure Models for The Determination of a Safe Mud Weight Window During Drilling Operations in The Niger Delta

Abstract

Abstract Unstable boreholes due to tensile & shear failure results in serious drilling problems. This increases the non-productive time spent in the field and the cost of reviving failed wells in the oil and gas industry in order to mitigate or prevent borehole failure, different rock strength criteria models have been developed to predict the optimum required mud weight but unfortunately, not all models fit for all formation considering the different drilling conditions, hence these models need to be evaluated to check their validity and accuracy for drilling stable wells in the Niger Delta. To achieve this purpose, collapse and fracture pressure that defines the lower and upper boundary of a mud weight window are calculated using derived formula equations from three failure criteria models; Hoek-Brown failure criterion, Mohr-Coulomb failure criterion and Mogi-Coulomb criterion failure criterion. This is achieved using Microsoft Excel and the simulation is done on a plot to show the different mud window by each model for comparison. The input parameters relevant for this study are pore pressure, in-situ stress data and rock mechanical properties which were acquired from a high-pressured zone in an xxx-onshore oilfield of the Niger Delta. A plot of the predicted optimum mud weight vs the actual mud weight against depth is compared to check the accuracy of each model to conclude on the failure criterion that is best fit for the formation in this field. The results showed that Hoek-Brown failure criterion gave an unrealistic outcome, when used to drill will lead to borehole breakout. Mohr-Coulomb and 3D Mogi-Coulomb criteria gave similar results which can be used but Mogi-Coulomb is recommended for drilling stabilized vertical wells due to the consideration of intermediate principal stress effect on rock strength over the conservative Mohr-Coulomb.