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Mapping Complex Geological Surface Morphology During Landing Operations Using 3-D Inversion of Ultra-Deep Electromagnetic LWD Data


 Ultra-deep azimuthal electromagnetic (EM) logging-while-drilling (LWD) tools are frequently used during landing operations for early detection of the reservoir top. This enables alterations to the well plan before the reservoir is penetrated. To date, this approach has relied on one-dimensional (1-D) inversions that accounts only for changes in resistivity above or below the wellbore. When geology is complex, resulting in lateral changes in resistivity, 3-D inversion of EM data is required to provide increased reservoir understanding.
 This paper presents a case study from offshore Brazil, targeting a turbidite deposit. A complex reservoir surface was expected, as defined by seismic data for the area. Although top structure rugosity and lateral position uncertainty had been incorporated into the prognosis, the impact of surface topography on inversion results while landing was not anticipated. During real-time operations, 1-D EM inversion was used along with correlation of shallow LWD data to map the reservoir top. It was clear the geology was more complicated than depicted by the 2-D geological model constructed from the 1-D inversion and that lateral changes in surface morphology may be occurring. Post well a 3-D inversion of the EM data revealed the 3-D geological structure.
 During the initial approach, the 1-D inversion indicated that relief of the reservoir top was more exaggerated than expected; the well intersected a sharp peak prior to approaching the target zone. The misfit on the 1-D inversion indicated there was potential for lateral variation in resistivity, influencing the 1-D results; lateral changes can produce artefacts that obscure the subsurface structure. This was confirmed after drilling with analysis of ultra-deep azimuthal resistivity images, indicating significant changes in resistivity to the left and right of the borehole. A 3-D EM inversion was run to depict these complex subsurface geometries. The 1-D inversion results were better understood post-drill with the 3-D inversion results, which show a high point in the reservoir top to the side of the wellbore that was drilled past, but not penetrated by, the well. This high-resistivity zone had a negative effect on the 1-D inversion results and made delineation of the reservoir top difficult.
 Understanding lateral variations in formation and fluid boundaries can improve well placement and reservoir understanding. This knowledge can impact landing scenarios and well placement within the reservoir. Three-dimensional inversion of ultra-deep azimuthal EM LWD data in real time will provide a clearer picture of the position of resistivity changes while drilling. This will enable decisions to be made that affect the azimuthal position of a well, as well as its vertical position during drilling, thereby facilitating optimal well placement, even in complex geological environments or for infill wells requiring precise well placement.