Drilling with a bottomhole pressure less than the formation pore pressure (Underbalanced Drilling, UBD) usually increase the risk of borehole instability due to yielding or failure of the rock adjacent to the borehole. But evaluation criterion of this failure mechanism is complex and very often diagnosis did not fit with field operational practices. It is believed that, shear and radial tensile failures with negligible chemical instability are the common mechanisms to cause mechanical instability in UBD in shales. In a mechanical borehole instability perspective, UBD causes lots of potential challenges and formation uncertainities due to facts like:

  • - Insufficient case data related to borehole instability;

  • - Complex physics of borehole instability mechanisms ;

  • - Requirement to conduct excessive experimental work which is costly and time consuming;

  • - Scarcity of real shales specimen and reluctant for coring of shales;

  • - Reliability of fitting material constitutive model to analyse shales;

  • - Dependable model to predict wellbore collapse pressure;

  • - Uncertainty to predict pore pressure and permeability of shale formation;

  • - Variable pressure regimes (i.e., shale heterogeneity);

A detailed workflow in connection within physical, experimental, analytical and numerical investigation is required to understand and to diagnose borehole stability. Numerous studies have been carried out so far on borehole stability design, but UBD and shale instability is quite new research areas where more insight is needed. This paper discuss and presents mechanical borehole instability both analytically and numerically to quantify borehole collapse risk for inclined wells. In addition produce comparable results and therefore some physical models are also presented for in depth study. Results show that the analytical and numerical models competent to predict borehole collapse risk in terms of minimum mud weight window. Since several real- life situations were evaluated, the potential applicability of the models is apparent; the results could be used as cross checks for particular situations in the field.

The generality of this study provided an overview along with standard workflow into borehole collapse assessment which may helps to obtain proper diagnosis of material failure state with respect to reduction of instability in drilling. Findings of this study can be useful for further research within the same area.

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