Reducing Structural Uncertainty and Maximizing Reservoir Contact Through Precise Mapping of 8 Minor Faults Using Advanced Deep Reservoir Mapping Solutions, A Case Study from Onshore Abu Dhabi
Authors: Fares Wael , Shehab Sami , Marzouqi Salman Al , Menhali Saeed Al , Aftab Muhammad , Bazuhair Mohamed , Sayari Saleh Al , Amin Muhammad Rizky , Manurung Vinda , Reda Ashrakat , Mubeen Muhamamd 
Published: November 2024

Onshore Abu Dhabi's reservoirs lie within the Arabian Gulf Basin. They have been affected by the collision of the Arabian and Eurasian plates, developing a complex structural deformation. Consequently, a combination of anticlinal and faulted oil traps has developed at the center of the basin. In the planning stage for a well, the available seismic information has been used as the field's basic guideline to target these oil traps. However, during drilling operations, many structural uncertainties from sub seismic faults result in the trajectory exiting the target layer. With this geological understanding, well placement techniques play a significant role in reaching certain reservoirs. A case study in this paper will elaborate the accurate mapping of formation boundaries and minor geological features by utilizing deep reservoir mapping solutions.

The production section of this target well was drilled in 6in lateral hole, targeting a single carbonate layer ~10 feet TVT thick in MRC (maximum-reservoir-contact) well profile. To support effective real-time well placement, LWD (logging-while-drilling) tools utilized were: 4-3/4″ OD Deep-Azimuthal-Resistivity with real-time 1D inversion capabilities. In addition, conventional logs (gamma ray, resistivity, density, and porosity) and density images were included. Prewell modelling was established in advance, to acquire optimum steering strategies to identify multiple minor faults. Deep-Reservoir-Mapping was confirmed to be set for specific frequencies to map 20 feet TVD above and below trajectory. Seismic-based geophysical evaluation was expecting up to 10 minor faults to be intercepted while drilling within the desired layer, in addition to major geological undulations. An advanced push the bit rotary steerable drilling system (RSS) was part of the bottom hole assembly (BHA) to steer the well precisely in the target zone and achieve any required correction to the well trajectory while drilling.

As a starting point for the 6″ hole drilling, all LWD tool responses inside the target zone were verified at the landing point according to the geologic environment showing optimum reservoir properties. Along the well trajectory, the changes in the resistivity around the wellbore were successfully mapped and promptly interpreted by real-time deep-azimuthal-resistivity 1D inversion, providing the confirmation of well position and formation dip. A total of 8 minor/major faults and the variations in formation dip from flat to 3 degrees down dip and 1.5 degree up dip were successfully mapped. The real-time deep 1D inversion aided in correcting the well position and maximized the reservoir contact in the target zone.

This case study portrays the successful practice of well placement despite the challenging structural/geological environment and seismic uncertainty. Major benefits from utilizing deep-reservoir-mapping solutions were: reducing uncertainty in the stratigraphic position of the well, optimizing geosteering decisions, and achieving effective drilling interval with a smooth profile. This success in the drilling operation has set a benchmark for onshore fields with similar geological obstacles.