Navigating Through a Complex and Highly Fractured Carbonate Structure: A Case Study from an Onshore Abu Dhabi Field U.A.E.

Abstract

Abstract The onshore oilfield was brought on stream in mid-2019, with the objective of reaching 5.8 million barrels of crude oil export annually. Being recently put on development, these carbonate reservoirs racked petrophysical and geological unknowns and challenges. Identifying high producing streaks and achieving risk free precision drilling are the cornerstone of the intended successful outcomes. Heterogeneities and lateral discontinuities coupled with high displacement fractures were the main obstacles that were efficiently overcome. Due to field production and reservoir model, drilling through a fault could not be avoided. Risk mitigation procedures and proper Logging While Drilling (LWD) technologies must be factored in while building the well profile. A 3D trajectory was optimized. The proper Rotary Steerable System (RSS) and bit were selected. With the target reservoir being less than 10ft thick, a high fault displacement might require an open hole side-track. Ultra-high resolution LWD electrical images in tandem with Near Bit GR (NBGR) were used while drilling to identify fractures, formation dips, and to build a True Stratigraphic Thickness (TST) model to accurately estimate fault throw. Initial seismic interpretation revealed faults close to a 100ft displacement. When a major fault was crossed in the horizontal 6-inch section, the new RSS with a higher steering force and faster response-maintained wellbore control. NBGR with a short sensor offset provided early warning, and mitigation procedures were proactively enforced. With the use of Resistivity Images, dip trends were extracted, and their attributes alongside the rest of the logs were correlated to estimate displacements. Drilling polarities were examined, and with the input of the formation dips analysis in a processing software, a TST output was generated detailing the true thicknesses of the target bed and adjacent layers. A fault throw estimation was extracted, which allowed the well to be safely geosteered back into target, avoiding unnecessary side-tracks and early termination which would compromise production targets. A post-well analysis revealed two major faults in a basin structure and cumulative displacement was calculated. This basin was undetected on the seismic cross section. The geosteering model was finalized with the true formation dips and thicknesses, faults angles and throws. Points and polygons were generated and incorporated into the field geological model which was updated with the faults’ dips and azimuths and new surfaces’ depths. Fit-for-purpose technologies along with smart processing capabilities gave way to a successful well execution in otherwise hostile drilling conditions. Proper drilling and LWD methods allowed flawless geosteering techniques, while novel interpretation workflows filled the gaps of a 3D model in a new developing field. This method optimizes delivery cost by complementing software analysis with a slimmed down, more focused, and advanced technology.