Multi-field coupling analysis of wellbore stability in naturally fractured formation

ABSTRACT: Shale reservoirs with natural fractures are prone to wellbore instability during drilling. In this paper, a thermal-hydraulic-mechanical-chemical coupling wellbore stability model for fractured formation is established, which takes into account the interference of reservoir in-situ stress difference, reservoir temperature difference, fluid pressure difference and solute concentration difference. The coupling model is solved numerically by finite element method and verified by classical analytical solution. The law of wellbore instability of fractured formation under different parameters is revealed. The results show that the stress field, pressure field, temperature field and chemical field of formation are obviously uneven due to the development of natural fractures. The natural fracture tip is prone to stress concentration, which greatly changes the formation stress state and increases the risk of wellbore instability. The increase of horizontal in-situ stress difference will increase the area of tensile failure and collapse failure, and the increase of solute concentration difference is conducive to reducing the failure risk. Increasing drilling fluid temperature has little effect on collapse failure, but can significantly reduce the risk of tensile failure. The research results can provide theoretical guidance for drilling process design in fractured formation.