Apex Spectral Technical Summary
January 1, 2014
The reason the E&P industry check shot corrects borehole p-wave velocity
measurements, is because fluid mobility causes the check shot travel times to be slower
than integrated borehole p-wave velocity travel times. Said differently in the presence of
fluid mobility, dispersion (i.e. frequency dependent velocity) causes check shots, which
are measured at much lower frequencies than borehole measurements, to measure
slower travel times.
While despite Dr. Ed White (1975) predicting though not proving material dispersion
inside the seismic bandwidth, the industry largely by default has assumed for decades
that dispersion inside the seismic bandwidth is negligible. However, real world results
prove otherwise, and in fact pronounced dispersion occurs within the seismic bandwidth,
especially at lower frequencies. This has profound implications. Frequency dependent
velocity means frequency dependent reflection coefficients, and this in turn means the
reflected wave will be altered versus the incident wave. The ADF® DHI works by
measuring the changes to the reflected wave spectra caused by dispersion, which are in
turn caused by fluid mobility.
So the ADF® DHI measures fluid mobility and common hydrocarbons like Brent, WTI and
gas have much higher fluid mobility when excited by a seismic wave than brine saturated
reservoirs due to their much lower viscosity. High fluid mobility causes a spectral
signature that is distinguishable by certain spectral characteristics. Typically these
characteristics include anomalously rapid energy decay on frequencies below Dominant
Frequency, which we measure with Alpha low (“αl”), anomalously rapid decay on
frequencies above Dominant Frequency, which we measure with Alpha high (“αh”) and a
change in Dominant Frequency “DF”. So, where such reservoirs have sufficient
thickness, porosity and hydrocarbon saturation, αl and αh will steepen and DF will locally
change.
After completion of spectral processing, an ADF® DHI fluid mobility cube is computed by
normalizing and summing αl, αh and DF values such that high values in the ADF® DHI
cube represent places that have very high fluid mobility and are therefore likely
hydrocarbons.
Naturally the AVO setting affects the nature of the change in the reflected wave spectra
versus the incident wave. By building on White (1975) and Carcione (2006), Apex has
modeled the effect of AVO class on dispersion and therefore ADF®, and found the
modeled result predictions to correctly predict real work results in AVO Class I, II and III
settings.
Because the technology is seismic based, well data isn’t required to set run parameters,
but can be especially helpful to generally interpret the results, and to support exploration
and development leads that are be identified.
Apex Spectral offers its services on a commercial basis, if you’d like to know more,
please feel free to contact us.
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