Establishment of pore pressure equilibrium in shale is a time dependent process which is governed by shale's intrinsic properties. The extremely low permeability of shale is a key parameter involved in the consolidation process and restriction of fluid flow (undrained mechanism). Underbalanced drilling in shale usually increases the risk of borehole instability due to yielding or failure of the rock adjacent to the borehole. In addition, shale has both a ductile and brittle nature. Material plasticity is strongly influenced by the consolidation and material dilation process. The distribution of pore pressure is taking a crucial role in the consolidation process, and has to be clarified for undrained mechanism. Therefore, the aim of this work is to quantify pore pressure distribution and material yielding when the borehole reaches underbalanced conditions. The M-C elasto-plastic numerical material model was applied to accomplish the study goal. The simulation results indicated that with decreasing mud pressure increased yielding in the dilating region took place. Time delayed pore pressure distribution is influenced by material dilation. Pore pressure distribution behaved anomalously after yielding. Here the dilating angle seemed to be the critical parameter. The outcome of this study will help to understand the physics of material yielding and the impact of consolidation on time delayed borehole instability.
Drilling with a bottomhole pressure less than the formation pore pressure (Underbalanced Drilling, UBD) usually increase the risk of borehole instability due to yielding or failure of the rock adjacent to the borehole. However, if the borehole would overcome the initial failure risk, the risk of instability is reduced if wellbore pressure and pore pressure are equilibrated. But due to the extremely low permeability of shale, the pore fluid cannot flow freely [7-11]. Both drained and undrained fluid flow mechanisms and their impact on evolving strain are demanding to be clarified. Underbalanced drilled wells in shale usually bring up mechanical borehole collapse challenges [1-3, 10]. Fig.1 presents several hypothesis of near wellbore stress pattern for UBD candidates. The critical region is indicated by a blue shaded area where the shear and radial tensile failure are the resultant mechanisms to cause borehole instability. Shear failure of the borehole wall will take place when the stress concentration around the borehole exceeds the compressive strength of the rock, whereas sufficient negative effective radial stress promotes radial tensile failure [7,10, 11,12]. Both of these failure mechanisms are associated with pore pressure consolidation, material dilation, redistributed borehole stresses and rock strength. The evaluation criterion of these failure mechanisms are complex and very often diagnosis does not agree with field operational practices. But, the accuracy of predictions of material failure state under undrained circumstances around the wellbore are essential in order to address wellbore instability problems. Using an adequate constitutive model for shale is vital in obtaining better predictions of the stress changes and rock deformation. However, an improved understanding of the behavior of shale during UBD will enable the main features to be included and facilitate more rational predictions.
