Wellbore stability during underbalanced drilling

Underbalanced drilling (UBD) is one of the fastest growing drilling techniques in the petroleum industry for the past two decades.; Underbalanced drilling is preferred over conventional drilling in some cases to save cost of drilling and increase production rate by increasing rate of penetration, avoiding or minimizing formation damage, reducing lost circulation and getting earlier production. Along with its many advantages, there however are few limitations in UBD. Since the bottomhole pressure is always kept below the formation pore pressure in UBD, there is an increased risk of wellbore instability, which must be addressed at both planning and drilling phases.; The operating bottomhole pressure is limited by the lower limit (the collapse pressure) and the upper limit (the pore pressure) in UBD. It is important to estimate the collapse pressure with sufficient accuracy and to operate the drilling pressure within the allowable range.; In this report, several computational models are used to analyze the collapse pressure at various depths using the data from Wellstab plus software program database provided by Maurer Technology Inc. The computational methods include linear elastic models with isotropic stress field and poroelastic models with permeable and impermeable borehole wall.; The computed collapse pressure with linear elastic solution without considering pore pressure gave a wider safe window for mud weight than that with poroelastic solution. A sensitivity test was performed on rock strength parameters and analyzed their effects on collapse pressure.