Investigating Dielectric Property from LWD Propagation Measurements a Modeling Study

Abstract Formation dielectric property that is highly affected by formation fluid saturation can be an additional physical parameter for geo-steering in low resistivity pay (LRP) formations. Unlike wireline dielectric tools, LWD propagation tools operate at much lower frequencies where formation dielectric constant estimation is challenging. Thus, the sensitivity of dielectric properties for different tool's operational parameters should be investigated, as the main objective of this study. In this paper, finite-element numerical modeling was validated and used to calculate the electromagnetic (EM) fields in a cylindrical borehole and homogenous isotropic formation. The EM propagation is governed by a set of antennas including a transmitter and two receivers geometrically modeled as in current LWD tools. During the sensitivity analysis, different operational parameters such as tool diameter, frequency, antenna spacing, borehole size, mud and formation properties were varied. Signal attenuation and phase shift across the two receivers were utilized in this study. The sensitivity of attenuation and phase shift were assessed against varying formation dielectric properties for each combination of operational parameters. The accuracy of the numerical modeling results was validated against an established magnetic dipole solution. For a standard tool configuration and operational frequencies, i.e., 2 MHz, the sensitivity of formation dielectric constant on simulated attenuation and phase shift increases rapidly as formation resistivity is higher than 100 ohmm and dielectric constant is higher than 20. Also, the higher the frequency the better the sensitivity to the formation dielectric property. When the frequency is higher than 10 MHz, the measurement sensitivity to the formation dielectric constant is dramatically improved and thus makes it applicable for well placement in the low resistivity LRP formations. Results also indicate that the shorter Tx spacing may help the dielectric constant sensitivity but mainly improve the signal-to-noise ratio. Meanwhile, the Rx spacing does not influence the dielectric constant sensitivity significantly. The findings of this study further confirm our recent study (Al-Ofi et al., 2024) and open a path for advanced geosteering in complex drilling conditions where some bottom-hole assembly may be limited and restricted such as in a slim hole under balanced coil tubing drilling, or when other LWD sensors cannot perform effectively.