Analysis of rock failure in underbalanced drilling

Underbalanced drilling (UBD) is a method used to minimize formation damage and enhance the reservoir productivity. But, at the same time UBD greatly raises the risk of mechanical wellbore instability by purposefully keeping the wellbore pressure below the formation pore pressure. This thesis systematically examines the geomechanical response of a vertical wellbore under UBD conditions through numerical analysis. The primary objective of this study is to systematically compare wellbore stability predictions derived from three distinct constitutive models, which are the Linear Elastic, Mohr-Coulomb, and the Generalized Hoek-Brown criterion model. To achieve this, a two dimensional, plane strain numerical model of a vertical wellbore in a sandstone formation was developed using the finite difference software FLAC 2D (Itasca). The analysis was conducted across three different pore pressure scenarios which are dry, drained, and undrained. The findings demonstrate that the negative differential pressure characteristic of UBD generates tensile radial stresses at the wellbore wall, thereby elevating the risk of tensile failure. A significant discovery across all models is that elevating the level of underbalance intensifies both tensile and compressive stresses, markedly increasing the likelihood of fracturing and shear failure. While the elastic model quantifies these stress precursors, the elasto-plastic Mohr-Coulomb and Hoek-Brown models were used to predict the extent and geometry of rock mass failure. Furthermore, the analysis highlights the critical role of pore pressure conditions, with undrained scenarios showing more severe instability immediately after drilling due to localized pore pressure changes. This study quantifies the discrepancies between models, offering essential insights for reducing instability risks and enhancing drilling designs.