After fast-tracking exploration success into an early production opportunity, Dubai Petroleum Establishment (DPE) recognized directional-casing-(and-liner)-while-drilling (DCWD) technology as enablers to optimizing the full field development of a thin oil rim within an interbedded steeply dipping reef build-up, just below the Nahr Umr Shale. The field development strategy was to land the well as close to the roof of the reservoir as possible and target the attic oil. This required entering the reservoir at higher inclinations and hence drilling the Nahr Umr Shale (cap rock for the reservoir) at more than 80° inclination. The regionally well-known borehole stability issues related to drilling the Nahr Umr Formation at high inclinations (and especially at azimuths that compound issues associated with in-situ stresses) presented a significant challenge for this field development.
The objective of this paper is to share an innovative and successful solution which was implemented to mitigate or eliminate the potential issues associated with drilling operations through problematic and unstable formations at high angles (almost horizontal).
An extensive engineering process was conducted involving different teams across the operator and the CWD service provider. The process included bottom hole assembly (BHA) drilling dynamics modelling using 4D time-based calculations and the typical drilling engineering calculations (torque and drag, hydraulics, alternating stress, etc.) to enhance the drilling performance during the drilling campaign. As an example, in one of the wells, the team drilled an interval in excess of 1200-ft, reaching the new world-record depth of 12,481-ft_MD, whilst meeting the operator's expectations in terms of rate of penetration (ROP), steerability of the directional BHA (achieving dogleg severity values higher than 4°/100ft), and flawless execution.
The paper shows the methodology followed by the drilling team to evaluate the feasibility of the application, the challenges and lessons learnt, and the innovative solutions (telemetry, mud system, logging while drilling [LWD], motor design, underreamers, etc.) which were put in place to optimize the process over the course of the first six jobs in this new field.
The DCWD technology enabled the well construction requirement of drilling an unstable shale on a high inclination trajectory (from 60° to 88° inclination), minimizing the stuck-pipe risk and typical unplanned time (NPT) associated with hard reaming, tight spots, and stuck pipe, and eliminating the additional 8 ½-in. hole requirement.
The application allowed the operator to enhance the wellbore construction process and eliminated the requirement for an additional short 8.5-in. hole section, which then had to be cased off with a 7-in. liner to isolate the problematic formations. This represents a revolutionary change compared with the way that this challenge has been historically tackled by the operators in the Middle East (including limiting the inclination to a maximum 35°-40° or using oil-based mud [OBM] fluids with extremely high density that increases the chances of losses in the above formations, among others).
The potential time savings due to the elimination of an additional interval to cover the problematic shale are estimated in excess of four days. In addition to the time savings, the operator has significantly reduced the associated cost for the BHA lost in hole (LIH) while drilling through the problematic shale without limiting the maximum inclination before landing in the target reservoir. This also creates a positive impact on the directional requirements for the horizontal-producing section (which can now be drilled in either 8 ½-in. and or 6-in. to address reservoir mapping and production conformance requirements).
