Drilling in Pakistan: Predrill wellbore stability study in a highly anisotropic rock formation for complex conditions in an exploration well.
Submitted: ADIPEC, 2021
Authors: Vladimir Vasquez, Omer Mohammed
Numerous wellbore instability problems were encountered while drilling through a shale formation in Pakistan gas fields. These problems become greater as the well inclination increases. Consequently, a wellbore stability study in the exploration well X1 (ST) was conducted to address those complications. Because of a lack of published studies regarding wellbore instability problems in this area, this valuable case study should serve as an example for similar fields in that region.The paper provides a detailed description and well bore stability analysis which include drilling issues encountered in the offset wells, failure analysis and geopressure evaluation. Also, it presents a strategy to maintain mechanical stability in shales. Finally, it carried out an uncertainty analysis using Monte Carlo simulations for quantifying uncertainty in model outputs and obtaining the required mud weight to drill successfully, which form part of the strategy for designing an optimal drilling fluid program.
The drilling events and data revealed potential anisotropic strength effects caused by bedding planes and abnormal pore pressure in this shale formation. The 1D wellbore stability model showed shear failure in rock exhibiting high stress concentration, which means that the mud weight should have been higher than that used to prevent borehole collapse. FMI data were used to estimate the angle of attack estimated in the X1 (ST) well, and revealed several intervals in the critical area that were characterized by low strength. The workflow proposed makes up a complete study useful for developing well bore stability analyses which optimize decision making in drilling process design.
The technical contributions of this study is that the conventional model prediction gives one safe mud weight for specific input data while the uncertainty model prediction provides a confidence level to each calculated safe mud weight range. This paper clarifies how to apply this procedure. Features like evaluation of attack angle and integration of uncertainty and sensitivity analysis are novel components for completing well bore stability modeling in exploration areas.
