Extended Reach Drilling (ERD) Optimization in Layer a Shale through Geomechanical Characterization

Abstract

Significant rock strength anisotropy associated with weak bedding laminations in shale can lead to wellbore instability challenges especially when drilling Extended Reach Drilling (ERD) wells that require low angles of attack relative to the formation bedding planes. Past drilling experience with water-based muds in offshore Abu Dhabi showed a high frequency of hole-cleaning and stuck-pipe events for wells deviated above approximately 40° from vertical. This was attributed to shale instability due to the invasion of the drilling fluids into micro-fractures along bedding planes as longer exposure lengths and times increased with the angle of deviation. Recently the decision to further develop a giant field with wells drilled from artificial islands has created the need for large numbers of ERD wells that will cross the shale formations at angles in the range of 40° to 85°. Pilot holes have been successfully drilled across shales at angles up to 80° by using non-aqueous drilling fluid (NADF) with mud weights predicted with an ExxonMobil proprietary model that uses previous experience and limited log data. Nevertheless, it was considered advantageous, if not essential, to better understand the mechanisms for shale instability as a function of both the angle of inclination and azimuth of the section, since it is critical to reliable prediction of the mud weight and chemistry required to avoid well-bore instability. An extensive program of tri-axial compression testing of orientated preserved core plugs was conducted in order to quantify the degree of strength anisotropy associated with both a reservoir cap, Layer A shale and intra-reservoir shales encountered while drilling offshore Abu Dhabi. This work showed that Layer A shale's compressive strength can be reduced by approximately 70 to 75% and the intra-reservoir shares by approximately 45 to 50%, when the shear plane of failure aligns with the weak laminations, compared to loading parallel, or perpendicular to the bedding planes. The inclusion of the measured strength anisotropy functions into a wellbore stability model is shown to accurately predict the observed mud weights associated with induced wellbore breakout. The non-aqueous (NADF) mud weight required for wellbore stability was incorporated in an Integrated Hole Quality and Quantitative Risk Assessment (IHQ/QRA) study to evaluate the drillability of various ERD well designs. Actual field drilling performance with NADF has also been used to validate the model. Increased understanding of this wellbore failure mechanism has the potential to reduce drilling risk and significantly increase current extended reach drilling limits for ZADCO's long term field development plan offshore Abu Dhabi.