phase-velocity log, Oil & Gas Glossary

What Is a Phase-Velocity Log?

A phase-velocity log is an electromagnetic propagation resistivity measurement that derives formation resistivity from the phase shift of a propagating electromagnetic wave between two receiver antennas, enabling real-time formation evaluation while drilling because phase shift is less sensitive to borehole geometry distortions than amplitude attenuation and provides a stable shallow-to-medium depth of investigation resistivity measurement.

Key Takeaways

Phase shift between two receivers is converted to a resistivity value using the propagation equations for the known transmitter-receiver spacing and frequency.
Phase-velocity resistivity reads shallower in the formation than attenuation resistivity from the same tool, aiding invasion profiling.
The measurement is inherently immune to skin effect distortions that affect induction tools at high conductivity.
Phase-velocity logs are a standard LWD resistivity measurement delivered in real time via mud-pulse telemetry.
Multiple frequencies (2 MHz, 400 kHz) and spacings on the same tool provide different depths of investigation for invasion analysis.

How the Phase-Velocity Log Works

The phase-velocity log measurement uses a transmitter antenna that radiates an electromagnetic wave at frequencies of 2 MHz or 400 kHz into the formation. The electromagnetic wave propagates outward from the transmitter and induces currents in the formation; the phase and amplitude of the wave at two receiver antennas placed at different distances from the transmitter are recorded. The phase shift between the two receivers (measured in degrees) depends on the electromagnetic wave velocity in the intervening formation, which is controlled by the formation's conductivity (inverse of resistivity). A high-conductivity (low-resistivity) formation propagates the wave slowly with large phase shift per unit distance; a low-conductivity (high-resistivity) formation propagates the wave quickly with small phase shift.

The conversion from measured phase shift to resistivity uses analytical solutions to Maxwell's equations for a homogeneous medium, calibrated against known-resistivity test formations. The resulting phase-shift resistivity (Rps) represents a radial average over the volume of formation sampled by the transmitter-receiver configuration. Higher transmitter frequencies produce shallower depths of investigation; larger transmitter-to-receiver spacing at the same frequency increases depth of investigation. Tools that combine multiple frequencies and spacings simultaneously provide a suite of resistivity measurements at different depths of investigation, enabling the separation of invaded and uninvaded zone resistivities needed for oil saturation calculation.

Phase-Velocity Log Applications Across International Jurisdictions

In Canada, phase-velocity LWD logs from Montney horizontal wells are transmitted in real time to the surface data hub and displayed alongside gamma ray and other LWD measurements in the geosteering workstation. AER formation evaluation requirements for Montney tight gas wells accept LWD resistivity logs, including phase-velocity measurements, as primary formation evaluation data. The ability to adjust the lateral trajectory in real time based on phase-velocity resistivity changes enables the driller to stay in the most resistive (hydrocarbon-bearing) portions of the Montney silty laminations throughout the multi-thousand-metre horizontal section.

In the United States, phase-velocity LWD measurements are the primary real-time resistivity log in deepwater Gulf of Mexico wells where wireline access is limited by formation pressure and wellbore stability concerns in extended shoe tracks. BSEE permit requirements for deepwater wells accept LWD logs as the primary formation evaluation dataset when wireline access is impractical. In Norway, Equinor's NCS horizontal drilling programmes use phase-velocity LWD resistivity in Halten Terrace wells targeting the Åre Formation gas sands, with real-time phase-velocity resistivity guiding geosteering decisions through heterogeneous reservoir sequences. In the Middle East, Aramco's extended-reach drilling at Ghawar uses multi-frequency phase-velocity LWD resistivity to map the oil-water contact in real time during drilling of horizontal producers that must land precisely above the free water level to avoid premature water breakthrough.

Fast Facts

At 2 MHz frequency and a 25-inch transmitter-to-receiver spacing, the depth of investigation of the phase-velocity resistivity measurement is approximately 10 to 20 cm beyond the borehole wall under typical formation conditions. This shallow investigation depth means the phase-velocity log responds primarily to the invaded zone in formations with significant mud filtrate invasion. In geosteering applications, this is actually beneficial because the shallow measurement responds quickly to changes in the formation immediately surrounding the borehole, providing real-time feedback on whether the bit is in the target interval or has entered an adjacent shale layer.

Phase-Velocity Versus Attenuation Resistivity

Propagation resistivity tools simultaneously measure both phase shift and amplitude attenuation between the two receivers. Phase shift and amplitude attenuation depend differently on formation resistivity and are affected differently by borehole geometry, invasion, and shoulder beds. Phase-shift resistivity is generally more accurate in high-resistivity formations (above approximately 100 ohm-m) where phase differences are large and precisely measurable. Attenuation-derived resistivity is more sensitive in low-resistivity formations (below approximately 10 ohm-m) where the amplitude ratio between the two receivers provides better discrimination. Because they read at slightly different depths of investigation, comparing phase-shift resistivity and attenuation resistivity on the same log provides an indication of radial resistivity variation (invasion) between the wellbore and the undisturbed formation.

Tip: When interpreting a phase-velocity log run at high angle or in a horizontal well through thinly bedded formations, apply anisotropy and dip corrections before using the resistivity for water saturation calculation. Phase-velocity tools respond to the horizontal resistivity component in vertical wells, but in highly deviated or horizontal wells they increasingly respond to vertical resistivity (perpendicular to bedding). In laminated sand-shale sequences, Rv is typically 2-10 times Rh because the thin shale laminae create a more resistive path perpendicular to bedding than parallel to it. Using the raw phase-velocity reading in an Archie calculation on a laminated interval in a horizontal well will overestimate water saturation because the tool is measuring a resistivity higher than the true horizontal formation resistivity relevant to sand-layer flow capacity.

Phase-velocity log is also referenced as:

Phase-shift resistivity — the output measurement name; used in LWD tool specifications and log mnemonics (typically PS or Rps) to distinguish the phase-based resistivity from attenuation-based resistivity
Propagation resistivity log — the broader category including both phase-shift and attenuation measurements from the same electromagnetic propagation tool
2-MHz resistivity or 400-kHz resistivity — the frequency-specific designations used when differentiating among the multiple resistivity measurements from a multi-frequency LWD tool

Frequently Asked Questions

How does the phase-velocity log differ from the induction log?

Both induction and phase-velocity tools measure formation resistivity using electromagnetic induction principles, but they operate at different frequencies and use different signal detection methods. Induction tools operate at low frequencies (10-200 kHz) and measure the induced current flowing in the formation through its contribution to the signal at the receiver coils; they are optimised for low-resistivity formations in oil-based mud. Phase-velocity tools operate at higher frequencies (400 kHz to 2 MHz) and measure the propagation speed of the electromagnetic wave by comparing phases at two receivers; they perform well in both water-based and oil-based muds and are not constrained to low-resistivity applications. Phase-velocity tools are the dominant LWD resistivity technology; induction tools have been more commonly used in wireline logging in oil-based mud environments.

What is the depth of investigation of a phase-velocity log?

Depth of investigation depends on transmitter-receiver spacing and transmitter frequency. At 2 MHz with a 25-inch spacing (common configuration), the depth of investigation for phase-shift resistivity is approximately 10-20 cm. At 400 kHz with the same spacing, the depth of investigation is deeper, reaching 30-60 cm into the formation. Tools with multiple spacings (16, 22, 28, 34, and 40 inches are common) at the same frequency provide a depth array from which radial resistivity profiles can be constructed, enabling the separation of invaded zone (Ri or Rxo) from uninvaded zone (Rt) resistivities needed for accurate water saturation calculation with invasion corrections.

Why Phase-Velocity Logs Matter in Oil and Gas

The horizontal well revolution that transformed the oil and gas industry from the Barnett Shale to the Bakken to the Montney to the Ghawar Arab Formation depends entirely on the ability to drill multi-thousand-metre laterals that stay within centimetre-scale reservoir targets at depths of one to four kilometres. Phase-velocity LWD resistivity is the primary real-time formation evaluation measurement that makes this precision geosteering possible: its ability to detect the resistivity contrast between hydrocarbon-bearing reservoir and bounding shale or water-bearing zones while drilling, transmitted to surface in real time through mud-pulse telemetry, allows the drilling team to adjust trajectory before the bit exits the target zone. Without real-time phase-velocity resistivity, geosteering would be blind, horizontal wells would spend large portions of their lateral sections outside the target reservoir, and the economics of unconventional and complex carbonate horizontal drilling would be fundamentally compromised.

Phase-Velocity Log: Definition, Electromagnetic Propagation Measurement, and LWD Application