Understanding Uncertainty in 1D and 3D Ultra-Deep Resistivity Inversions for Improved Geosteering and Geomapping in Complex Carbonate Reservoirs, Onshore Abu Dhabi
Authors: Fares Wael , Singh Maniesh , Bazuhair Mohamed , Dhermeshwar Thakur Parmanand , N. M. Al Baloushi Mariam , Al Arfi Saif , El Gohary Mohamed , El Abd Salem , S. Al Mesafri Ahmed , Clegg Nigel , Walmsley Arthur , Aki Ahmet 
Published: September 2022

Drilling horizontal wells in mature fields undergoing enhanced oil recovery programs requires advanced high-resolution reservoir mapping to optimise well placement. Ultra-deep electromagnetic (EM) technology provides shallow and deep 1D and 3D inversion-based mapping in real-time and recorded data. All inversion results show uncertainty in the exact position of formation/fluid boundaries and inverted resistivity values. Understanding this uncertainty and deploying multiple inversions to mitigate it is essential for attaining high confidence in the quality of results.

Multi-antenna, azimuthal EM LWD tools propagate EM fields in three dimensions with an ultra-deep depth of investigation (DOI). Robust inversion algorithms both one dimensional (1D) and three dimensional (3D) derive the position and resistivity of formations within the DOI from measurements induced by the propagated fields. This provides geologists with a clearer understanding of the surrounding geology. High confidence in these results, which are models that best represent the EM field is essential. It is vital to understand any uncertainty and where possible use independent verification. Pre-drill modelling provides understanding of the expected response in each formation. Offset data and independent LWD tools provide independent verification of results but have limited DOI's. An understanding of inversion uncertainty is essential to assess quality of the inversions and allow confident geosteering decisions to be made.

Pre-drill modeling for a candidate field onshore Abu Dhabi demonstrated the capability of resolving multiple formation layers, with a DOI of more than 90ft. Uncertainty is therefore important as other LWD tools have limited DOI's and can only be used to verify results close to the wellbore. The field trail results exceeded pre-drill expectations, clearly identifying resistivity boundaries, consistent with offset logs. While drilling, the real-time ultra-deep EM tool provided high resolution mapping for precise geosteering within thin layers and mapped a varying water slumping contact 80 ft TVD above the wellbore. A simultaneous 3D EM inversion with 120 ft distance-to-boundary window also imaged the water-front and confirmed that no lateral variation existed in its orientation, it also defined the azimuth, dip and strike of a fault. Confidence in these results was essential as the real-time information helped in timely optimizing completion design to produce oil without water cut and extend the wells production life. Understanding boundary position and resistivity value uncertainty provided confidence in the quality of the results. Post-well these results aided in updating the static model with water flood areas, reservoir tops, faults and overall reservoir structure.

The results of this experience provided optimized BHA selection and maximize the benefits of running ultra-deep EM mapping tool in mature fields for multiple purposes; deep reservoir fluid mapping, multi-layered mapping and geosteering within thinner target reservoir units. The confidence in the results allowed important and timely decisions to optimize well position and maximized the hydrocarbon-bearing reservoir contact without exiting.