A Robust Joint Inversion for Improved Structural Mapping in UDAR Applications Using Multiple Measurement Sensitivities and Uncertainties

Recent advancements in deep and ultradeep azimuthal r esistivity tools (DAR and UDAR) have widened the scope of electromagnetic (EM) measurements available at various depths of investigation. These technologies have successfully enabled geosteering decisions based on a combined assessment of both azimuthal resistivity measurements. UDAR provides valuable insights into formations located farther from the wellbore, whereas DAR excels at identifying complex structures in close proximity. While UDAR and DAR can yield differing resistivity measurements due to different measurement sensitivity, resolution, and detection range within a transition zone (Li et al., 2020; Yan et al., 2020), typically spanning 15 to 30 ft from the wellbore, experienced geosteering engineers can enhance geological decisions by considering the inversion uncertainties and potential detection ranges associated with UDAR and DAR at specific formation profiles. Nonetheless, this approach involves numerous manual steps and can be time consuming, potentially leading to subjective interpretations if conducted by engineers with limited experience and knowledge in azimuthal measurements and formation geology. To reduce manual interpretation and improve consistency, we explored approaches to incorporate UDAR and DAR measurements into an automated workflow. A robust joint inversion technique incorporating both UDAR and DAR measurements has been developed to address the limitations of the traditional human-guided approach. The inversion technique accounts for the significant differences in sensitivity between UDAR and DAR measurements by employing weighted functions tailored to individual measurement types and specific inversion zones. T hese weighted functions are determined by the sensitivity of individual measurements to formation resistivity contrasts and boundaries, individual inversion outcomes, and their associated uncertainties. The joint inversion approach on the basis of different measurements at multiple steps is employed to fine-tune separate inversion zones, at the cost of longer calculation times or increased computational resources. This paper evaluates various synthetic and field examples where both DAR and UDAR measurements are acquired within the same wellbore. Initially, independent inversion results are obtained using DAR and UDAR measurements separately. Subsequently, a joint inversion is conducted utilizing a multilayer, multitransition formation model to integrate both measurement types. Results demonstrate that the proposed joint inversion approach not only enhances the accuracy of formation property characterization but also extends the detection range into deeper formation layers. These improvements are achieved consistently without the need for manual intervention.