Increasing Subsurface Resolution with 3D Deep Azimuthal Resistivity Inversion

Abstract Ultra-deep azimuthal resistivity (UDAR) tools are frequently deployed to map resistivity boundaries in 3D around the wellbore. The high depth of investigation allows mapping of resistivity bodies over 100ft away. However, in thinner reservoirs, deep azimuthal resistivity (DAR) tools are deployed. To date these only depict a vertical inversion plane to approximately 30ft, ignoring lateral variability. This paper presents high resolution 3D inversion of DAR data to increase reservoir understanding. Deep azimuthal Resistivity tools measure the full electromagnetic (EM) field surrounding the wellbore as they rotate. The single collar transmitter to receiver spacing is smaller than multi collar UDAR tools and the transmission frequencies are higher, resulting in a decreased depth of investigation but increased resolution. Resistivity boundary mapping in the vertical plane allows these tools to display the subsurface geology and optimise well placement, but this ignores the lateral component of the EM field. A full 3D inversion of this data bridges this gap, providing deeper reservoir understanding while having increased resolution when compared to UDAR 3D inversions. A standard one-dimensional EM inversion of the target well showed a sand channel sequence with a complex morphology with 5 to 15ft thick sand units. Detailed investigation of the azimuthal resistivity and density images show that when considering geology to the sides of the reservoir there is lateral heterogeneity as well as the variability in boundary positions displayed along the well path, but it is unclear exactly what this heterogeneity is representing. 3D EM inversion clearly shows this lateral heterogeneity to be a series of oblique delineations resulting in thinning of the reservoir. Inversion quality control is critical to validate this new technology. Independent verification of the defined structures with density image log dip picks and qualitative assessment of the azimuthal resistivity images provides additional support for the results to backup statistical assessment of misfit in the inversion. Formation and fluid boundaries can show variability in 3D driven by the depositional environment and post deposition deformation. 3D inversion of EM data for UDAR and now deep azimuthal resistivity tools allows assessment of this variability away from the wellbore, improving well optimization and geological models of the reservoir.