A New Focused UDAR Inversion to Highlight Finer Geological Features in Transitional-Resistivity Formations
Authors:   Halliburton, Wu Hsu-Hsiang (Mark) , Chatterjee Amitabha ,   AkerBP, Clegg Nigel ,   Halliburton, Ma Jin ,   Halliburton, Fan Yijing ,   Halliburton, Riofrio Karol ,   Halliburton, Lozinsky Clint ,   Halliburton, Bittar Michael ,   Halliburton, Duriez Alban ,   Halliburton
Published: November 2024

Ultradeep azimuthal resistivity (UDAR) tools have made a significant impact on logging-while-drilling (LWD) geosteering, geomapping, and geostopping operations. The technology explores large formation volumes, bridging the gap from the near-wellbore to seismic scale with higher-resolution formation determination surrounding the wellbore. Due to the ultradeep detection range of up to 225 ft away from the wellbore, a multilayered inversion is required to transform the UDAR measurements into a visual representation of formation geological features. However, this introduces a high-dimensional parameter space in the inversion and potentially results in high inversion uncertainty and/or inversion artifacts, posing decision-making challenges in certain formation conditions. A new UDAR-focused inversion, improving upon an existing deterministic UDAR inversion, is proposed to address such inversion uncertainty. The focused inversion attains intricate formation insights at the cost of some loss in detection range compared to the existing inversion. Various synthetic and field examples have demonstrated great success with high-fidelity formation determination from the new inversion, especially for illustrating gradational resistivity profiles. Complex geological and fluid boundaries often show gradational changes from high-resistivity hydrocarbon zones to low-resistivity shale or water zones, resulting in low-resistivity contrasts at the boundaries and contacts. UDAR measurements highlight these gradual resistivity variations as a stepped profile across the transitional boundary. To represent a boundary in this way, the number of multilayered inversion unknowns must be significantly increased to describe these types of resistivity transitions. However, the maximum number of layers available in previous inversions limits the inversion capabilities to produce an accurate model fit in these complex environments. A new focused inversion is presented to include two novel features. First, a more realistic formation model is proposed with a resistivity transition section to better fit the gradational resistivity features. In addition, the sectional model allows fewer unknowns to the inversion to describe multilayer resistivity profiles. Finally, the new inversion promotes individual measurement sensitivity to refine and invert such formation transition zones. We first observed struggling results of the existing inversion based on a multilayer and multitransition formation model example with very low-resistivity contrast boundaries in a prewell modeling analysis. The challenge led to the new focused inversion for distinguishing more layers with low-resistivity contrast, whereas the current algorithm loses some layers based on the same UDAR measurements. Further validation on a field example is displayed in Fig. 1. The new inversion results are capable of determining the high fidelity of the formation properties; on the other hand, it can lose detection range toward the edge of the inversion as compared to the current inversion algorithm. A new UDAR-focused inversion is introduced to accurately determine the high fidelity of the formation properties, especially for gradational resistivity profiles. This improves reservoir understanding and allows more accurate well placement decisions to be made.