The process of drilling a successful extended-reach well often involves addressing many geological challenges. Some of these challenges are due to uncertainty in the structure model along the horizontal section as well as the lateral changes in the reservoir properties, from heel to toe of the well. This uncertainty may affect the borehole tortuosity as well as reduce the reservoir contact. It is clearly observed that approaching the less-porous boundaries causes undesired borehole azimuth swings. Often, the need to drill complex or challenging ERD wells efficiently,contrasts with geological aspects of data requirement and transmittal, reactive geosteering response times and accuracy of well placement. This may often require non-unique approaches in Middle East carbonate reservoirs when considering stacked sequences of reservoirs with different properties / net pay thickness; different top- and bottom-seal lithological units, and zones with limited markers requiring stringent well placement. The objective of this paper is to illustrate that by assessing the details of reservoir geology and key operational markers relevant for best practices, drilling approaches can be customized for each reservoir or scenario: for example, defining drilling ROP windows on a per-target layer basis. A well-placement methodology and workflow were developed and integrated with the calculated mechanical specific energy (MSE). The MSE is calculated using both surface drilling parameters and downhole drilling dynamics data from the drilling downhole optimization collar. The methodology also includes an optimized bottom hole assembly (BHA) design with azimuthal density image and a point-the-bit rotary steerable system (RSS) to geosteer the well within a thin target layer. This occurs while maintaining the planned target trajectory with a minimum borehole tortuosity using real-time drilling optimization. The understanding of the target layers with the analysis of the historic offset horizontal wells resulted in the delivery of engineered solutions to mitigate the uncertainty in the geological challenges.The MSE along with the downhole drilling dynamics data from the drilling downhole optimization collar has shown good correlation with real-time logging-while-drilling data and can help in making quick real-time geosteering decisions.It was reliable in determining the change in the stratigraphic movement of the well trajectory along the horizontal section. In addition, using the drilling downhole optimization collar effectively can assist in anticipating any approach to different porosity boundaries that help to reduce or eliminate the undesired borehole azimuth swings. As a result, the downhole vibrations are also mitigated and constantly remain within the allowed limits using downhole measurements for torque at bit and weight on bit.This study has shown outstanding and promising results for the potential value added when using the MSE along with the downhole drilling dynamics data in a proactive geosteering technique for a geosteering/geostopping portfolio. The uniqueness of the integration of the MSE and real-time geosteering geological model lies in its ability to address the geological challenges, enhance the drilling process, and maximize the asset value in the reservoir field of this study.Further study is needed to include different reservoirs to validate the concept.

September 2012, ZADCO drilled the longest well to date in the U.A.E. to test the productivity and injectivity of a low permeability, carbonate reservoir in a giant Middle East oil field. The extended reach well was drilled to a total measured depth of 24,390 ft., with a horizontal length of 11,015 ft.The borehole target zone was a~3.5 ft. thick calcareous dolomite sublayer sandwiched between upper and lower~4 ft. thick limestones. The critical factors for success were: (1) LWD tools capable of differentiating calcareous dolomite from the nearby limestones; and (2) LWD sensors close to the bit as possible to enable prompt geosteering reactions to possible lateral facies changes. Challenges were: (1) the capability of the jackup rig to drill the required distance; (2) minimizing hole undulations; and (3) remain within the 3.5 ft. target zone. Concerns were raised about the use of nuclear chemical source density tools and the risk of a lost nuclear source in hole should the drill pipe become stuck. To address these concerns, the second half of the lateral section was steered without a chemical nuclear source in the downhole tool string.A LWD formation pressure test tool was run post drilling to evaluate formation mobility. The result supported well placement in the higher permeability dolomite layer which was later validated by production and injection results. This paper illustrates good geosteering practices in a challenging long horizontal well with carefully selected LWD program to achieve the target and reduce risks.

Acquisition of reservoir properties measurements, high-resolution borehole images, and continuous directional surveys has historically required the deployment of multiple discrete tools. This paper details the first-time deployment of a new, integrated, multi-sensor tool, in one of the largest oil-producing reservoirs in Abu Dhabi. Deployment of the new technology helped to achieve a new record for the longest bit run with the new system. The new measurement-and-logging-while-drilling (MLWD) platform incorporates multiple sensors with significantly reduced footprint, delivering critical, high-definition measurements closer to the bit than previous technologies with modular designs. In this case study, the existing MLWD technologies were included in the drilling assembly for measurement validation and benchmarking. The new tool provided borehole size and shape measurements, and high-resolution radius and acoustic reflection-amplitude images with an axial resolution of 0.1 inches. In addition, the tool provided vibration, weight, torque, and pressure-while-drilling measurements, to help optimize the drilling parameters, and an azimuthal gamma ray sensor for formation evaluation. The new design also includes a compact directional module to provide static surveys and high-resolution continuous inclination and azimuth while drilling, facilitating wellbore placement and safe and efficient drilling by minimizing the tortuosity. The bottomhole assembly included LWD gamma ray, propagation resistivity, formation density, and neutron porosity sensors from the legacy system, along with a point-the-bit rotary steerable system and a new-generation resistivity sensor. The new system was deployed in an 8½-in. horizontal section of 14,318 ft, reaching a total depth of 25,500 ft. MD. It was the longest run of the new MLWD system globally to date. The real-time MLWD data included gamma ray and ultrasonic borehole size. Furthermore, the high-resolution radius and reflection-amplitude images captured geological features that often control the reservoir properties such as vugs, fractures, and minor faults. The radius measurements provided details of the borehole size and shape, with a three-dimensional (3D) visualization of the wellbore helping to identify borehole enlargement and breakout. In addition, the new and legacy propagation resistivity measurements matched closely. The continuous, high-definition surveys helped the drilling team to ensure smooth hole and avoid any collision risk with nearby wells. The forward plan is to continue to deploy the new generation of tools in a wider range of conditions and locations, and to add additional next-generation sensors, including higher resolution density, neutron porosity, and deep multilayer reservoir mapping sensors.

In order to develop a giant mature carbonate field offshore Abu Dhabi, four environmental Islands were built in shallow water. Due to the large areal extension of the field, most wells are Extended Reach. This paper presents a case study of the longest well drilled in the UAE. Planning to drill such an Extended Reach well with maximum reservoir contact, starts long before execution. Not only, competent rigs, capable of delivering enough mechanical and hydraulic energy to drill such well, but also placing and navigating the lateral section in multilayered anticlinal reservoir with layer cake geometry. Significant geologic uncertainties of complicated carbonate facies heterogeneities is a major challenge. Detailed study of the reservoir behavior according to pre-existing nearby wells was carried out, as well as understanding the geologic structural setting of the area since the target zone consists of 5ft thick limestone layer that overlays a 3 ft. thick dolomitic limestone with excellent permeability. The crucial success factors were; firstly, to steer in such a thin reservoir for long distances, using LWD tools to differentiate dolomite from the nearby limestone layer. Secondly, to account for possible lateral facies change, the sensors needed to be as close as possible to the bit. Considering +28600 ft. horizontal section, remarkable challenges arose related to drilling tool capability and well path smoothness. The drilling team was concerned about using nuclear source density tools to navigate the well path and needed to avoid high dogleg to prevent any fishing jobs associated with source tools, especially in fractured areas. The well was drilled to record depth of more than 45300 ft. with a horizontal section of 27,500 ft. inside reservoir. Utilizing Rotary Steerable System in all hole sections helped to minimize hole tortuosity along the well and with the help of LWD tools the well was accurately steered within target zone boundaries. Utilizing advance mud telemetry made it possible to receive data from 45,300 ft. away from surface. Successful well placement was the main factor to achieve positive overall performance in the field. This success has opened the door to drill more wells that are more challenging. In addition, it practically proved that proper planning and execution can push the limits further and gave confidence to drill even deeper. As a result, we are now aiming to drill deeper wells up to 50,000 ft.