Feasibility study of underbalanced drilling using geomechanical parameters and finite element method

Abstract

Wellbore instability causes main concerns in the oil and gas industry due to personnel safety and overall expenditure. Several signs such as wellbore spalling, the rate of penetration reduction, pipes sticking, well unnatural wash out, wellbore collapsing, induce fracture could indicate wellbore instability during drilling. A balance between stress concentration near the wellbore and rock strength is the primary condition for a wellbore stability during drilling. Thus, instability due to the failure of wellbore rocks occurs when the intensity of the effective stress exceeds rock strength. In underbalanced drilling (UBD), the wellbore is considered unstable since no pressure or maybe not enough pressure is applied on the well from the drilling fluid, and the pore pressure is considered undesirable stress on the wellbore, so the difference between these two pressures is applied on the wellbore. Consequently, the wellbore instability issue in the UBD approach is of great importance. UBD is mainly used to reduce the damage to geological formation and the risk of drilling fluid loss while increasing the drilling rate. This study has investigated the stability of a well in Iranian Oilfield using FLAC software considering yielded regions (plastic) for different mud weights. The investigation and analysis of the FLAC output plots, especially the plasticity plot, concluded that the ideal and optimal pressure for applying UBD conditions while also ensuring wellbore stability ranges 15.2–16.8 MPa.