Identification and Projected Interception of Oblique Faults up to 85 ft Away Using Ultra-Deep Azimuthal Resistivity and Geosignals: A Case History From the Norwegian Continental Shelf
Authors: Clegg Nigel , Djefel Bronwyn , Kunnas Jari , Kuld Ingi 
Published: November 2018

Resistivity inversion canvases represent the subsurface geology as a layer cake and do not account for or show resistivity changes to the sides of the wellbore. This paper uses a Norwegian continental shelf (NCS) case history to demonstrate how ultra-deep 360° azimuthal resistivity and geosignal images can be used to identify fault planes and to evaluate the formation/fluids across the boundary up to 85 ft laterally. Using multiple images, the position and orientation of the faults can be estimated and projections made to identify if and where they will be crossed by the well. When used in conjunction with a resistivity inversion canvas, these images provide a method to assess the reservoir in three dimensions.

Lateral resistivity changes can be assessed using 360° azimuthal resistivity and geosignal images. The depth of investigation (DOI) of these images increases with a larger transmitter-to-receiver spacing and decreases in frequency. With ultra-deep tools, the configuration enables the investigation of resistivity changes at a significant distance from the well. Combining the DOI of the various images with the measured depth along the well path where a resistivity change can be identified, it is possible to triangulate its distance from the well. Projecting this information ahead enables the intersection with the well path to be estimated.

In the NCS case history, the wellbore was expected to cross faults at an oblique angle; in this mature field, which has undergone significant water injection, these faults can sometimes be barriers to fluid migration.

Resistivity data and density images highlighted multiple faults; significant resistivity changes across the faults were evident (oil to water). The azimuthal density images and deep azimuthal resistivity data have a limited DOI of a maximum of 20 ft, which restricts their usefulness in the early detection of faults. Ultra-deep azimuthal resistivity and geosignal data demonstrated the most value; this data could be used to identify faults a significant distance to the sides of the wellbore; in one case, this distance was 85 ft away. The distance to this fault was calculated from multiple azimuthal resistivity and geosignal images; plotting the position of the feature made it possible to track the fault and estimate where the well would intersect it.

Ultra-deep resistivity images enable changes in resistivity to the sides of a well to be identified and distances to features estimated. Using this information, it is possible to calculate the projected intersection of the change in resistivity with the wellbore. This information can be used to steer azimuthally to maximize production and to mitigate drilling risks. It enables the assessment of the effect of changes in the fluids on production, making it possible to modify the completion strategies accordingly and to update geological models for more accurate future production estimates and development planning.