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The New Role of Petrophysics in Geophysical Interpretation

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The New Role of Petrophysics in Geophysical Interpretation

    THE NEW ROLE OF PETROPHYSICS IN GEOPHYSICAL

    INTERPRETATION

    E. R. (Ross) Crain, P.Eng.

    Spectrum 2000 Mindware Ltd.

    403 845 2527 ross@spec2000.net

Published in CSEG Recorder Sept 2003

    NEW ROLES NEW INTERACTIONS NEW INTEGRATION 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. Chapters Twenty through Twenty-Five on my website www.spec20000.net

    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 in Figure 1.

     FIGURE 1: Seismic petrophysics study for porosity

This example used a geostatistical 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 and is showing great promise.

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 is

    not in yet on fluid replacement calculations for the sonic log, 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,

    as in Figure 2. Check-shots, offset well data, other

    logs, and common sense are used to correct for

    this.

Figure 2: Rough sonic log corrected where it needs

    it delta-T minimum method

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 a little bit 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, as shown in in Figure 3. Notice the

    strong reflections caused by even thin gas zones (pink colour

    on the log analysis).

Figure 3: 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.

The key to interpretation of seismic inversion, attribute analysis, and AVO is Petrophysics. Your

    friendly, neighbourhood petrophysicist can help you integrate geophysics and petrophysics for

    more effective exploration and development. Try it, you’ll like it!

ABOUT THE AUTHOR

    Mr Crain is a Professional Engineer with over 35

    years of experience in reservoir description,

    petrophysical analysis, and management. He has

    been a specialist in the integration of well log

    analysis and petrophysics with geophysical,

    geological, engineering, and simulation phases of

    oil and gas exploration and exploitation, with

    widespread Canadian and Overseas experience.

    He has an Engineering degree from McGill

    University in Montral and is a registered engineer in

    Alberta. He wrote “The Log Analysis Handbook”,

    published by Pennwell, and offers seminars,

    mentoring, or petrophysical consulting to oil

    companies, government agencies, and consulting

    service companies around the world.

    Ross is credited with the invention of the first

    desktop log analysis system (LOG/MATE) in 1976, 5

    years before IBM invented the PC. He continues to

    advise and train people on software design,

    implementation, and training. For his consulting practice, he uses his own proprietary software (META/LOG), and is familiar with most commercial

    systems.

He has won Best Paper Awards from CWLS and CSEG and has authored more than 30 technical

    papers. He is currently building an Interactive Learning Center for petrophysics on the World Wide

    Web. Mr Crain was installed as an Honourary Member of the Canadian Well Logging Society for

    his contributions to the science of well log analysis.

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