3D Look Ahead Electromagnetic Inversion in Near Vertical Wells – A Bridge Between Vertical and Horizontal Wells 3D Geology Understanding

Abstract 1D look ahead inversion for ultra deep azimuthal resistivity (UDAR) tools has been demonstrated in vertical and near vertical wells, allowing detection of resistivity ahead before a boundary is intersected, optimizing the setting of casing or coring points. Horizontal look ahead UDAR inversion is frequently requested; however, this requires 3D inversion with look ahead sensitivity to account for variability in resistivity encountered. A bridge between these technologies is look ahead 3D inversion in a near vertical well. UDAR tools acquire data while rotating and measure the full 3D electromagnetic (EM) field while drilling. Models generated match predicted data to measured data to generate a representation of the subsurface. Inversions often simplify modelling to limit the number of unknowns, the most common approach treating the subsurface as a layer cake scenario with 1D inversion. The advent of 3D inversion enables mapping fully 3D geological scenarios. This approach, combined with a tool with look ahead sensitivity in near vertical wells where geology is simple, is demonstrated and the results verified. The detection of a high resistivity salt prior to entry and detection of low resistivity inclusions within the salt was the ideal environment to test a new look ahead technology. This represents both ends of the resistivity spectrum with regards to applications for the setting of casing points prior to breaching a boundary. Initial tests were for a 1D inversion which has since been verified in other fields, and was identified as an ideal data set to test the new 3D look ahead inversion. The salt entry is a simple boundary from low to high resistivity with the vertical well perpendicular to the target boundary. 3D inversion results using the full resistivity tensor show the boundary 45ft below the UDAR transmitter 36ft ahead of the bit. The 3D inversion additionally identified low resistivity inclusions approximately 70ft ahead of the transmitter, with increased depth of detection due to the surrounding high resistivity environment. Due to simplification of the modelling the 1D look ahead inversion identifies these boundaries earlier, however the 3D inversion showed that the inclusions are not simple horizontal beds but have a more complex morphology. Entry to all boundaries was confirmed as the well intersected the target units with shallow logging while drilling (LWD) measurements. Full 3D inversion is the key to look ahead inversion in horizontal wells, demonstrating this ability in a near vertical well, is key to taking the technology beyond horizontal wells. 3D inversions can allow Geoscientists to have a mechanism to validate the assumptions used while building the conceptual geological model with sparse data and get better understanding of rock and fluids properties distribution.