To maintain oil production offshore Sabah, from a prolific deep-water turbidite sand reservoir, additional wellbores were planned to tap oil zones and place water injectors along the flanks. Reservoir thicknesses are variable at the flanks and complicated by multiple faults generated by an anticline thrust structure. Faults with up to 50 feet throw impart high uncertainty in navigating the wellbore within the reservoir. To aid optimization of the well bore real-time, ultra-deep resistivity (UDR) 1D inversion data proved invaluable. However, 1D inversion is limited to resistivity changes above and below the well. It is unable to define faults approaching the wellbore at an oblique angle. Post-well 3D inversion models present a representation of the 3D subsurface reservoir structure, allowing identification of lateral changes in resistivity caused by these faults.
A UDR tool was deployed to obtain real-time 1D inversions to assist in well placement operations. The well was expected to encounter multiple vertically displaced fault blocks, which were expected to be defined on the 1D inversion. Pre-drill feasibility studies were performed using nearby offset wells to reference the expected resistivity signature. Multiple geologic scenarios, from base-case, deep-case, shallow-case and other permutations were modelled to demonstrate the expected inversion results, to aid in real time decision making and to optimize the tool spacing and frequencies. Whilst the UDR 1D inversion performed admirably and was able to delineate the sand throughout the wellbore, it is limited to one plane. While drilling through the faulted blocks, the boundary appeared less distinct at the fault boundaries, an indication that the faults were not perpendicular to the well, but likely to have been intersected at an oblique angle. The UDR 1D inversion was unable to resolve the oblique fault planes relative to the well bore resulting in high misfits from the inversion algorithm.
Post well, a full 3D inversion of the UDR EM data revealed a picture of the reservoir 360° around the well bore. The resultant cylindrical volume provided a full picture of the reservoir, up to 80 feet radially from the well bore, revealing the 3D nature of the faulted structure. This provided insight into both the fault throw direction and displacement. These results correlated with near-wellbore density image logs and azimuthal ultra-deep images from the UDR tool. Slicing through or filtering different values of the inverted resistivity image revealed a deeper understanding of the reservoir sand, not previously possible with 1D inversion.
Knowledge of the lateral variation and position of the faults delineating the sand blocks will impart confidence in the future development of this reservoir sand. The UDR 3D inversion provided an indispensable aid to understanding the complex reservoir where 1D inversion only revealed vertical distribution of formations and fluid.