Analysis of the extent of rock failure in wellbores drilled in Bossier shale

Drilling is one of the main activities of upstream sector in oil and gas industry, where the well is drilled and hydrocarbon is extracted to the surface. This activity involves many technical disciplines, which are strictly connected together from well planning until the hydrocarbon production. It is critical to have a thorough understanding of the mechanical response of the rock that will be drilled in order to avoid drilling instability problems that can result in additional cost. The analysis of wellbore stability starts with the characterization of the rock. The interpretation of triaxial tests in the framework of selected strength criteria, gives the rock properties that rule the mechanical response during drilling operations. The coupled effect of the far field stresses, pore fluid pressure and rock properties aims at identifying scenarios of wellbore instability and a proper mud pressure. The behaviour of many rocks is transversely isotropic. In these rocks, the properties change with the orientation of loading. In wellbore stability, the focus is mainly devoted to the rock strength anisotropy, because of the occurrence of failure during drilling operations. On the other hand, the analysis of the extent of rock failure is of paramount importance for the evaluation of safe drilling conditions. The thesis aims at analyzing the extent of rock failure around a wellbore drilled in transversely anisotropic rocks. To this end the study starts with the interpretation of triaxial tests carried out on Bossier shale, with the Weakness plane model and the Hoek & Brown criterion adapted to anisotropic rocks. Some difficulties were encountered during the data fitting, but they were overcome with the approach proposed by Deangeli & Omwanghe (2018) and Deangeli (2021). The results of wellbore stability analysis identify different scenarios of the extent of failure related to the variation of far field stresses, pore pressure and the inclination of the bedding planes.