Influence of multi-planes of weakness on unstable zones near wellbore wall in a fractured formation

Abstract

The non-production time and consumption caused by borehole instability have elicited worldwide concern in the exploration and production of fractured shale formations because of the development of bedding planes and natural fractures. Therefore, the failure zone model is established in which the transversely isotropic stress model and multiple planes of the weakness criterion are incorporated. The collapse pressure and unusable areas under a fixed wellbore internal pressure for a well drilled in transversely isotropic formation were analyzed. Further, the coupling effects of natural fractures on wellbore instability were considered based on the superposition concept. The results indicated that the commonly used single weak plane criterion has a larger error in borehole instability analysis for fractured shale formation, and the collapse pressure increases prominently with an increasing number of natural fractures. Further, the unstable zones gradually enlarged and became deeper with increasing borehole inclination when the bedding plane was considered. However, the opposite conclusion was reached when multiple planes of weakness were incorporated into the analysis. The boreholes drilled along the up-dip direction with a higher inclination have a lower collapse pressure and catabiotic unstable zones. The borehole stability can be significantly improved by optimizing the wellbore trajectory, including the fractured shale formation, considering the influence of the weakest plane, the stability of which is the key factor ensuring the safety and efficiency of drilling.