Factors influencing wellbore stability during underbalanced drilling of horizontal wells – When fluid seepage is considered

• A new collapse pressure model for UBD of horizontal wells is introduced.
• Fluid seepage make wellbore more unstable during UBD of horizontal wells.
• A more stable wellbore is obtained in a slim hole during UBD of horizontal wells.
• Effects of inclination and azimuthal angle on wellbore stability are fully studied.
• The value of θ where maximum equivalent collapse density is obtained is analyzed.

Underbalanced drilling (UBD) of horizontal wells has been one of the efficient technologies in the exploration and development of oil and gas fields, while wellbore instability poses a problem during the whole operation process, for fluid seepage induced by the flow of formation fluid into wellbore exerts additional stresses on wellbore. However, the impact of fluid seepage has usually been ignored by conventional analysis of wellbore stability during UBD. This paper, taking the effects from fluid seepage into consideration, introduces a new collapse pressure model for UBD of horizontal wells. A comparison of the new model with the conventional one reveals that maximum equivalent collapse density (MECD) reduces with the decrease of borehole radius and that the wellbore is more stable in a slim hole during UBD of horizontal wells. And with the change of the inclination angle, MECD is higher when fluid seepage is considered under a certain relative azimuthal angle, indicating a narrower mud weight window and a more unstable wellbore; while the variation trend of MECD with the inclination angle are quite different at relative azimuthal angle = 90° and 0°. With the change of the relative azimuthal angle, MECD obtained in consideration of fluid seepage is also greater when the inclination angles is fixed, and MECD in both conditions (when fluid seepage is considered and otherwise) decreases with the increase of the relative azimuthal angle; meanwhile, the value of θ where MECD is obtained is also analyzed.