Understanding borehole collapse mechanisms during underbalanced drilling (UBD) in shale is becoming increasingly important for the petroleum industry, due in particular to the implementation of the UBD technique in operational practices. However, it is challenging to determine the proper constitutive material model to describe the UBD borehole environment sufficiently and accurately. Moreover, it is difficult to define how and where "borehole failure" occurs during simulation or even in laboratory conditions. In an attempt to minimize borehole instability problems, detailed and careful analysis of the excavation process is often performed during the planning stage. However, the accuracy of these analyses is highly dependent on the constitutive model adopted for the shale. Three important features of the constitutive model for shale are the dissipation of the pore pressure, the strength and the plasticity after yielding. This paper presents rock strength and borehole deformation results from a hollow cylinder (HC) testing program on Pierre-1 shale. Furthermore, a calibration was performed on a virtual borehole model  against HC laboratory data. The HC tests at underbalanced conditions were performed on samples drilled parallel and perpendicular to the bedding to properly quantify the effects of anisotropy and to determine possible scaling/geometry effects. Laboratory observations of material failure were compared with numerical outcome and showed promising similarities.
Fundamental borehole stability models for underbalanced drilling (UBD) wells are directly associated with the mechanical properties, formation anisotropy and heterogeneity, true pore pressure, mud weight, well trajectory and formation temperature of the rock. The mud weight (MW) and the well trajectory are controllable parameters and are considered key factors in the borehole design models, whereas rock strength and formation pore pressure (pf) are the main unmanageable parameters directly involved in the borehole stability models. In practice, rock strength is described by the collapse pressure (CP) of UBD wells. Determining the true rock strength is of prime concern in the drilling industry to obtain trustworthy borehole collapse simulation models. Moreover, accurate knowledge of the rock strength is essential for drilling optimization, rate of penetration (ROP) prediction  and also for sand production prediction models . Conventional drilling simulators provide a tool to determine the rock strength for the drilling engineer to further model and study the effect of different drilling parameters, where the overall drilling process performance can then be optimized. The rock strength is therefore a key element in borehole stability modeling as well as in the optimization of the drilling process. Several challenges appear when creating borehole stability analysis models. The main challenge is fitting an existing constitutive material model using the mechanical parameters of the rock for the borehole stability model to a reliable and realistic state. Another important consideration is to evaluate and to model the borehole stability in shale. The lack of relevant test data to describe shale properties accurately is due to the shale’s heterogeneity and its anisotropic behavior. Field observations indicate that most of the drilling problems occur when drilling in shale sections.