The last marine cycle of Dubai Lower-Cretaceous has left reefal carbonate deposits on paleo-highs from deep basement faults. Those shores face deposits at the edge of the former Bab basin and developed very good reservoir but had a prolonged emersion prior to the deposition of Nahr Umr seal and are often karstified. A successful development relied on high-angle drilling through an unstable seal and advance geosteering within a complex reservoir with a thin oil leg. A unique drilling approach in the region was designed and executed to deliver these wells safely. Oil- based mud (OBM), casing while drilling (CWD), and steering assembly were deployed to build safely in unstable shale and land at a high angle within the reservoir close to the roof in a relatively slim hydrocarbon column. The reservoir presents a succession of clinoforms. Good porosity and permeability streaks are present with tighter zones. Karsts, often linked with faults, represent a threat to drilling and well completion. The new high-definition reservoir mapping while drilling (RMWD) (6-in reservoir) was introduced to accurately steer within this reservoir. Simulations of drilling dynamics revealed the best BHA and casing designs. Over seven wells, this method was optimized by using an RSS and executing the 3D trajectory with required build rate and with no compromise on ROP. CWD was used to drill up to 12,481ft MD, which is the deepest in the world for such technology. Without it, inevitable slump down to the water leg would compromise the development of the entire field. The new RMWD technology revealed the reservoir details by mapping over 100 ft thickness of structure and fluid contacts. Subseismic Karsts and the irregular shale-carbonate contacts were precisely delineated. This allowed proactive reactions while drilling and avoided costly sidetracks. Clinoforms were resolved; transition zones and oil water contact depths were mapped. Over the course of seven wells, the information gathered allowed a redefinition of the entire field model from both a geological and reservoir perspective. As result, the original oil in place (OOIP) was recalculated with a renewed field development plan. The technical execution enabled the production of dry oil wells while steering as high as possible within the most permeable layers at a high rate, mitigating karst risk. The introduction of successful CWD eliminated the need to drill the 8^-in section. Four days were saved during the well construction process, and drilling risks associated with unstable and reactive shale were greatly reduced. The new RMWD technology in the lateral section potentially saved millions in development strategies and completion plans by mapping the complete oil column, transition, and water zones while providing safe geosteering away from karstic shales.

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).