Numerical Study of Consolidation Effect on Time Delayed Borehole Stability During Underbalanced Drilling in Shale

Time delayed mechanical borehole stability is mostly depending on the pore pressure consolidation process. Establishment of pore pressure equilibrium in shale is a time dependent process which is characterized by shale intrinsic properties. In shale, water movement is greatly restricted by the low permeability of shale which may cause pore pressure storage and lead to redistribution of borehole stresses. To prevent wellbore instability problems, accurate predictions of stresses and deformations around the wellbore are essential, particularly when drilling in underbalance in shale. Using an adequate constitutive model for shale is crucial in obtaining accurate predictions of the stress changes and deformations. Recent research shows that several mechanisms contribute to shale instability, including swelling, pressure diffusion, plasticity, anisotropy, capillary effects, osmosis and physcio- chemical alteration. It is clear that it may be impossible to consider all features of shales in one model. However, an improved understanding of the behaviour of shales during UBD will enable the main features to be included and facilitate more rational predictions. This study was developed numerical material model. The model is enabled to predict borehole collapse risk with the impact of time delayed consolidation effect. The generality of this study is to enhance knowledge of downhole