Naturally Fractured Reservoir Characterization Using Dipole Sonic Imaging, Vertical Seismic Profile Imaging, Integrated with Borehole Micro Resistivity Image

Newly acquired data from the first exploration drilling conducted in the Ras Al Khaimah Emirate in many years holds significant potential for addressing the challenge of characterizing fracture networks within thinly bedded reservoirs. The research focuses on the extensive sedimentary deposits that have accumulated within the accommodation space during the Northern Emirates foreland basin's evolutionary process. The complete vertical well profile exhibits a diverse range of lithologies at different spatial scales. Thin sandstone beds below seismic resolution are evident within the uppermost part of the stratigraphic unit. The carbonate succession is discordantly overlain by clastic unit, causing a significant acoustic impedance change in seismic imaging. The sandstone unit features distinct bedding boundaries, lamination planes, and structural zones with varying fracture frequencies and micro-faults. Since natural tectonic fractures are the key element of the flow from the reservoir, advanced fracture identification and classification studies were conducted. The borehole micro-resistivity image reveals that the subject well dissects multiple structural breaks that isolate the analyzed interval into structural zones, where such structural complexity is associated with varying natural fracture, enhanced, induced, and micro-faulting frequencies. Most of the natural fractures were classified as electrically semi-conductive but not necessarily partially open fractures, which is why integration between the acoustic products and the micro-resistivity image fracture analysis was a critical success factor in determining fracture fluid conductivity and extension away from the wellbore which correlates with its possible impact on reservoir quality and commercial potential. Wireline sonic data helped understand both fracture conductivity and fracture continuity and distribution in the formation, extending our view of structural features to tens of meters beyond the borehole wall. The reflected and scattered shear waves from dipole-source sonic data were used to image near-borehole bedding planes, fractures, and faults. The best imaging was achieved by carefully pre-processing the full waveform data to remove the direct waves, reduce noise, and building an appropriate two-dimensional velocity model adhering to the structural geometry relative to the borehole. The sonic image was analyzed together with VSPCDP section and micro-resistivity imager data, to confirm the main features, as well as the dips of the layers. With 0.5m spatial resolution, the dipole sonic imaging fills the gap between micro-resistivity imager (10 samples/inch) and vertical seismic profiling (15m). The reservoir properties are upscaled and investigated far from the borehole. Figure shows the section of dip discontinuity from micro-resistivity image interpretation; the sonic image confirms the discontinuity showing the extent of distinctly different bedding dips and reduces the uncertainty of large-scale feature dips in the VSP p-wave image. From results of this integrated study, the comprehensive reservoir model was built, including fracture classification, distribution, density, and spatial distribution.