The Use of 4D Geomechanical Models to Predict Wellbore Stability – Added Value and Implication

Abstract

Abstract The advances on computer processing from last decade allowed marked advances in the development of 3D and 4D Geomechanical (GM) models. The advantage of using these models is that they permit a comprehensive understanding of the geomechanical behavior of a field leading to more reliable predictions on planned wells placed on structurally complex areas. They are also capable of computing stress change due to reservoir production/injection. This case study takes advantage on 4D Geomechanics models to perform wellbore stability (WBS) analysis of several wells on an offshore field from a Brazilian East margin Basin, where challenges such as very narrow mud window, stress disturbance, depletion and risk of fault reactivation take place together. This work presents how a 4D GM model has demonstrated to be a key contributor to predict WBS and successfully guide a new drilling campaign on this complex scenario. Once created a 4D GM model, a general methodology to predict WBS consists of directly extracting interval velocity, stress tensor, pore pressure and elastic properties from the model to the new survey and calculate rock mechanical properties and mud window. In the current case study, it was verified some mismatches on predicted lithology and on pore pressure during the drilling campaign These mismatches required additional data treatment before performing the wellbore stability analysis but did not prevent using the model. Stresses and pressures were adjusted considering the new lithology and the stress path concept to calculate the stresses under the updated depletion scenario. The stress path for each wellbore trajectory was calculated by progressively checking variation on pressure and stresses from the model at different production time steps. Hardening and softening effects identified along the field contributed to define the more representative stress path to update stresses. The mud weight window of each new well was estimated by using the rock mechanical properties calculated from interval velocity and the updated lithology, pore pressure and stress field scenarios. The strategy for a successful drilling campaign also included a fault stability analysis complemented with the drilling experience review in new wells. This combination of data allowed mud weight and drilling strategy optimization. Even with too narrow mud windows, the drilling campaign showed great results, with wells successfully drilled and low nonproductive time. The use of the 4D GM model showed to be fundamental to better model field behavior and gain sensitivity to crucial parameters controlling drilling risks. It has guided optimization on drilling surveys, wellbore schematics and drilling strategy, effectively preventing non-productive time (NPT). The study also showed that even when 4D model updates are required due to unexpected variations of pressure or lithology during a drilling campaign, the available model can be directly used to successfully conduct the update process directly in 1D models.