For many decades formation evaluation and petrophysics have relied on electric conductivity (or resistivity) to input to saturation models for water saturation. However, water salinity strongly affects this estimate and may lead to incorrect results. Electromagnetic propagation measurements have enhanced the conductivity measurement by dielectric permittivity to a complex-valued conductivity-permittivity pair. At measurement frequencies of about 1 MHz and above the permittivity becomes increasingly less sensitive to water salinity in low clay formations, and so facilitates a more reliable water saturation.
Directional drilling/well placement in both conventional reservoirs and unconventional resource plays requires an early, reliable prediction of water saturation, immune to uncertainty in water salinity. Such an early water-saturation estimate then permits drilling through and staying in the reservoir "sweet spot" and allows early quantification of potential producible hydrocarbon for near-term operating decisions in the expensive rig-on-location time frame.
Logging While Drilling tools include propagation-resistivity measurements, which operate from a few 100 kHz up to 2 MHz. The tools provide phase shift and attenuation measurements from a radio wave between a pair of receivers. Traditionally, these two independent measurements were converted to two, independent, apparent resistivities. More recently, the two measurements were also simultaneously inverted for apparent conductivity and permittivity. Permittivity is also very dispersive: it rapidly diminishes with increasing frequency because of several unrelated and competing effects. We propose to investigate utilizing dielectric dispersion at LWD frequencies for similar derivations without the complications of shallow wireline measurements and potential operations impacts.
We have implemented a recently published mechanistic model that describes wideband EM phenomena in earth formations. For practical application, we present case examples where we input petrophysical analysis results from field logs to this model and simulate logs of dielectric permittivity and conductivity at LWD frequencies, and in some cases where wireline dielectric and array induction inverted dielectric properties are available, at these tool frequencies also. This study presents utilization of this new forward-simulation and the new LWD inversion algorithm. These two complementary algorithms relate the measured conductivity and permittivity and attendant dispersion to water-filled porosity, salinity, and other key reservoir properties such as CEC, formation factor, and permeability. The results are self-consistent: the forward simulation followed by the inversion reliably reproduces the original input results and vice versa. The new algorithm offers critical and reliable low-clay-formation water-saturation estimates and other properties from LWD propagation-resistivity and porosity measurements in real and during the drilling and well-placement process.
