seismic petrophysics
The role of petrophysics in seismic interpretation has taken a
major leap forward in the past ten years, resulting from
important advances in seismic data processing techniques,
particularly seismic inversion, attribute analysis, and
amplitude versus offset methods that showed we could estimate
reservoir properties from such data. Coupled with the recent
advances in dipole shear sonic logging, new vistas in seismic
interpretation, dubbed seismic petrophysics, have opened.
Geophysical well logs suffer from many borehole and
environmental problems that need to be repaired before being
used for calibrating seismic models or seismic interpretations.
A primary aim of the geophysicist/petrophysicist is to create a
synthetic seismic trace from EDITED log data that accurately
represents the seismic response of the subsurface. This is
accomplished by editing, repairing, or reconstructing the log
data. Using unedited logs for seismic purposes is a waste of
time and money and, in the worst case, can lead to very
expensive exploration and development mistakes.
If
the synthetic seismic trace is a good representation of the real
seismic response, then the edited logs can be used effectively
as aids to interpretation of the advanced seismic products.
Consequently, the role of the petrophysicist has also evolved;
she must now be competent in log reconstruction as well as
conventional log analysis, and must understand the petrophysical
needs and limitations of the inversion, attribute, or AVO
results. Unfortunately, logs are not perfect measures of in-situ
rock properties and seismic data is severely band-limited
compared to log data, so there are many compromises to be made.
A significant change in mindset is also needed, as most of the
log repairs (with the exception of fluid replacement) take place
in the non-reservoir intervals - intervals that are not usually
of interest to petrophysicists.
Geophysicists engaged in seismic interpretation seldom use logs
to their full advantage. This sad state is caused, of course, by
the fact that most geophysicists are not experts in log
analysis. They rely heavily on others to edit the logs and do
the analysis for them. But, many petrophysicists and log
analysts have no ides what geophysicists need from logs, or even
how to obtain the desired results. That's a particularly vicious
"Catch-22".
Education, practical solutions, appropriate software, and
practice are the keys to success. In order for geophysicists and
petrophysicists to communicate well, each must know something of
the other's specialty.
This Chapter and the next two
provide theory, practical methods, and case histories to
accomplish this goal.
Seismic Petrophysics and Seismic Modeling
Seismic petrophysics is a term used to describe the conversion
of seismic data into meaningful petrophysical or reservoir
description information, such as porosity, lithology, or fluid
content of the reservoir. Until recently, this work was
qualitative in nature, but as seismic acquisition and processing
have advanced, the results are becoming more quantitative.
Calibrating this work to well log - ground truth - can convert
the seismic attributes into useful reservoir exploration and
development tools. Since there are an infinity of possible
inversions, it is pretty important to find the one that most
closely matched the final edited logs or the computed results
from those logs.
A
seismic petrophysics study aimed at quantifying porosity is
shown below.
Seismic petrophysics study for porosity
This
example used a geo-statistical package to distribute the dense
"fuzzy" seismic attribute data between the sparse, "accurate"
well log data. The logs, or log analysis results, in turn are
calibrated to core, well test, and production data before being
used to control seismic interpretation. The use of geostatistics
to map seismic attributes onto well logs is a relatively new
phenomenon
Seismic Petrophysics and Well Log Modeling
Unfortunately, it takes a fair amount of effort to compare
seismic results to log data. The logs will usually require some
kind of editing or modeling or both. Comparison of seismic
results to log data may indicate that further processing of the
seismic is needed, and the calibration cycle is repeated, often
several iterations are needed. In other cases, it is the logs
that need further editing.
Log
modeling or editing is required because logs don’t see the same
rock and fluid mixtures that the seismic signal sees. Drilling
fluid invasion removes gas or oil near the wellbore, replacing
it with water and altering the sonic and density log response
from the reservoir's undisturbed values. Compensating for
invasion is called "fluid replacement". Fluid replacement
calculations are also used in "what-if" scenarios to see what a
gas filled reservoir might look like on seismic. Such models are
usually run post-mortem, after a lovely seismic bright spot was
drilled to find an equally lovely porous water zone. Maybe the
models should be run BEFORE drilling?
The
author and John Boyd presented a practical solution for fluid
replacement in 1979, based on the log response equation and a
"pseudo-travel time" for typical gases. Since then, at least a
dozen, more rigorous but less friendly, solutions have been
published: Castagna, Greenberg and Castagna, Aki and Richards,
Batzie and Wang, Toksoz et al among others. Most are based on
extensions of early work (late 1950's) by Biot, Gassmann, and
later, Domenico. The final tally on fluid replacement
calculations for gas effect on the sonic log is not in,
especially in shallow, unconsolidated, or underpressured
reservoirs.
Fluid replacement calculations for the density log are straight
forward, with no pitfalls if the gas or oil PVT properties are
known. How well do you know the reservoir engineer down the
hall?
Mechanical or chemical rock alteration due to drilling usually
reduces sonic velocity and density in the environment measured
by the logging tool. This effect is somewhat subtle but
pervasive or it can be catastrophic as in hole breakouts. It can
be repaired by using information from other log curves (in the
case of bad density data), or checkshot or VSP data to calibrate
the sonic log. But many common sense rules for using checkshots
are ignored because the software doesn't think like a human
petrophysicist.
Acoustic frequency differences have to be accounted for,
especially when shear velocity is measured. High frequency shear
velocity (lab measurements and sometimes sonic log data) is
faster than low frequency (seismic) data. Anderson's 1984 paper
provides useful information but is weak on specific
recommendations.
Poor
log response due to bad hole condition or faulty logs may be an
even more serious problem.
Check-shots, offset well data, other logs, and common sense are
used to correct for this.
Rough sonic log corrected where it needs it
The
log should be edited only where it needs it using common sense
rules grounded in local and regional trends. Few practitioners
have hip pockets full of sonic and density trend data applicable
to their current projects.
Again, at least a dozen authors have provided more or less
practical solutions, such as Ausburn, Faust, Smith, Fischer and
Good, Crain and Boyd, Patchett.
Calibration methods come in three flavours: good, bad, and
really ugly. Block shifting a log is really ugly. Rescaling and
delta-T minimum methods are better but still ugly. Discreet
editing where the log needs it, or more sophisticated curve
fitting techniques based on other logs, are pretty good
approaches. The ugly methods are fast and mostly useless, as
most of the false reflectivity is still there. The good methods
take more effort, but you get what you pay for.
In
other cases, no appropriate logs exist, so sonic and density
data have to be created by transforming some other available
log. Most of the methods used to repair bad hole effects will
also generate complete sonic or density logs. In the worst case,
a set of geological tops, lithology descriptions, and an offset
well log will suffice, especially if only the density log is
missing.
Some
models are made by "cut and paste", for example thickening or
thinning a reef or pinching-out a sand bar to see what happens
to the seismic signature. Splicing realistic data from one well
to another in a geologically sensible manner can create any
number of plausible models. The more models you create, the more
likely you will find one that matches your seismic.
Smoothing and filtering may also be performed on raw or edited
logs to extract only those frequencies that are likely to be
recorded in real seismic data. Cut and paste, and filtering, are
fairly obvious operations and are not dealt with further here.
A
competent petrophysicist working closely with the geophysicist
can provide the needed expertise to solve these problems and
generate useful log data. When integrated with the geologist and
reservoir engineering members of the team, very credible
interpretations will result.
Logs Used to Aid Seismic Petrophysics
The
two logs most used by geophysicists are the sonic (also called
acoustic) log) and the density log, because these two rock
properties determine the acoustic impedance and hence the
reflection coefficients of the rock layers. A synthetic
seismogram can be calculated from these data.
Raw
logs should NEVER be used for this purpose - editing and
modeling are nearly always required.
Most
other log curves are useful to the geophysicist. For example,
the neutron, density, photoelectric effect, and spectral gamma
ray (both natural and induced) can be used to determine
lithology quite accurately. This knowledge assists seismic
modeling and inversion or attribute interpretation.
Even
the lowly gamma ray log plotted on a two-way time scale on a
seismic section can be an invaluable aid to horizon picking and
interpretation, since it is one of the best shale indicators
available.
Computed log analysis results, such as shale volume, porosity,
lithology, and hydrocarbon fill are very informative when
displayed on a seismic section, shown at the right. Notice the strong reflections caused
by even thin gas zones (pink colour on the log analysis).
Log analysis results showing hydrocarbon fill
(pink)
plotted on two-way time scale with VSP data.
These properties are all derived from appropriate log analysis
techniques. They are generally called log analysis results,
petrophysical properties, or computer processed interpretations
(CPI). They often provide the "ground truth" for calibrating
attribute or inversion interpretation.
Modern sonic logs, called full wave, array, or dipole sonic
tools, record the complete sonic waveform instead of just the
travel time of the first arrival. This allows us to process each
wavetrain to determine shear wave and Stoneley wave travel time
(and hence velocity) as well as the more usual compressional
wave travel time.
Thus
shear wave synthetics can be constructed to calibrate shear wave
seismic sections. Lithology analysis and direct hydrocarbon
detection are sometimes possible from a comparison of
compressional and shear velocities. These can be verified by the
compressional and shear synthetic seismograms. A transform of
shear and compressional data, either from logs or seismic, into
Poisson's Ratio helps distinguish between hydrocarbon and
lithology variations.
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