CHAPTER Five: EDITING AND REPAIRING WELL LOGS

Table Of Contents
5.00 Introduction to This Chapter
5.01 Why is Editing Needed?
5.02 What is Editing?
5.03 Log Calibrations
5.04 Calibration Shifts
5.05 Depth Shifts
5.06 Skips, Noise, and Spikes
5.07 Rough, Large, and Salty Hole Effects
5.08 Rock Alteration
5.09 Correcting Log Values in Hydrocarbon Zones
5.10 Shaping Logs for Quantitative Analysis
5.11 Editing For Seismic Modeling
5.12 Use of Check-Shot Data for Seismic Editing
5.13 True Vertical Depth Editing
5.14 In Conclusion
5.15 Exercises For Chapter Five
5.16 Bibliography For Chapter Five

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Publication History: This Chapter formed Chapter Five of The Log Analysis Handbook, Pennwell 1986. Only minor corrections were made for this electronic edition Feb 2001.

CHAPTER Five: EDITING AND REPAIRING WELL LOGS

5.00 Introduction to This Chapter
This Chapter examines why editing is needed, and illustrates some examples. Most edits are aimed at reducing apparent porosity determined from logs. It is easy to overlook a prospective zone as a result of injudicious editing. Likewise, editing a zone to appear overly favourable may not be a good idea either.

It is not unethical to edit, correct, repair, or otherwise modify a log, if corrections are needed and made properly. Keep a record and include it in the final report. Some people are horrified by the concept of modifying logs arbitrarily, preferring instead to believe either the service company can never be wrong or that bad data should not be used. This attitude results in interpretation errors or wasted data.

The watchword in editing is CAUTION! Don’t overdo it, but don’t forget to do it.

Editing is essential before using the quantitative methods outlined in later Chapters. Most calculator and computer programs assume perfect data, and are incapable of resolving problems easily recognized by the analyst. Therefore data must be edited carefully to prevent propagation of errors through the system.

5.01 Why is Editing Needed?
If logs were perfect, editing would not be required. However, logs can suffer from a number of problems.

Procedures for correcting most of these faults are presented in this Chapter.

The necessity of calibrating seismic information with edited wellbore data is inherent in seismic modeling, which has become more common in recent years. Consequently, every effort must be made to ensure that log data provides the best possible presentation of in-situ physical properties. Only edited logs can allow this.

5.02 What is Editing?
Editing is a form of interpretation. Judgments, assumptions, and comparison to other logs in the area are required. When performing a visual interpretation, the edit is often an integral step, although it may not be recognized as such. Common sense will compensate for sonic cycle skips or depth discrepancies with little conscious effort. However, quantitative calculator or computer evaluation demands more systematic editing. Since computer work is only as accurate as the data input, computer aided log analysis can create zones with poor or inaccurate results. Therefore, editing should be the first step, since it is more practical to edit raw data than edit results.

If you cannot supply better data, then the bad data should not be used. Much bad data can be recognized by a good quality control review of the logs, as described in Chapter Two. Some problems will not show up until some calculations have given silly or impossible results. In this case it may or may not be easy to find the offending data.

5.03 Log Calibrations
Calibrations are an integral part of any log. If they were not run, or are now missing (or cut off the log), the analyst should use extreme care in picking log values.

Standard calibrations for relatively modern logs (1960 to present) are documented in service company manuals, which should be part of your library. You will need dated versions of these manuals as the current manuals will show only current tools. This won't help for older logs in your files.

Some calibration examples are shown in Figures 5.01 to 5.03. Logs before about 1975 (+/-) have analog calibration tails. These look like a piece of the log and are annotated with stick-on tape labels. More modern logs have digital calibration tails that look like a computer printout. You will need the service company manuals to understand what the values should be and what the tolerances are.

Older logs (pre-1960) may have fewer, different, or no calibrations shown. Calibrations for different service companies vary slightly from each other, so be sure to obtain examples from all service companies which you normally use.

Calibrations from computerized logging units are totally different from non-computer units. These calibrations show numerical data instead of recorded (analog) curves. See Figures 5.04 to 5.05 for samples.

No calibrations are presented for sonic logs on computerized trucks, unless the logging engineer displays sonic log readings at fixed travel times on the film, simulating the calibrations on non-computerized logs. The SP calibration is also not displayed on computerized trucks.

Since calibration details vary widely, it is impractical to publish all of them in a handbook of this type. It is strongly recommended, however, that you learn how to interpret and use calibration data, such as those shown in the illustrations in this section. Calibrations usually consist of low and high end points to define the log scale, and intermediate points to define linearity of scale.

Primary calibration of a log usually occurs under laboratory conditions or a test pit of known characteristics. Secondary calibration is a method for carrying primary calibrations to the service company field location by some device which simulates the laboratory readings. These are usually called shop calibrations. A third tier of calibration is a mechanism for transporting shop calibrations to the field for use at each well site.

For example, a neutron log prototype is first calibrated in a test pit with known rock type and porosity. Then it is immediately run into a secondary calibrator of standard design, one of which will be available at each major logging center around the world. In the case of a neutron log, the secondary calibration is a tank of particular dimensions filled with diesel fuel. The readings in the secondary calibrator now constitute the main source of calibration.

Periodically thereafter, each tool is placed in the secondary calibrator, adjusted to read the correct response, and the field calibrator is placed on the tool. The tool response to this calibrator is then recorded. At each logging job, the tool is readjusted to read the same value when in the field calibrator environment. The field calibrator for a neutron log is a small gamma ray source at a short distance from the neutron log detector.

This three stage process moves the primary calibration in the test pit in Houston to each well logged by the tool. Some logs require only a two stage calibration (such as induction logs) and some only require one stage (such as spontaneous potential or sonic travel time).

Calibrations performed before the log is run are called Before-Survey Calibrations, and those run after the job are called After-Survey Calibrations. Differences between Before and After calibrations need to be accounted for only if the difference is large enough to cause errors in the results of the log analysis.

5.04 Calibration Shifts
Even though the logging engineer tries to perform calibrations accurately and consistently, calibrations may be in error before the survey starts or may drift from their set values due to electronic problems. If these conditions prevail, the calibrations are said to be shifted.

Several situations can arise if calibrations are clearly shifted. Both before and after survey, calibrations may be off by the same amount. Here, the log should be rescaled or a new scale constructed to correspond to the calibrations. Most computer aided log analysis software has a “block shift” function to do this.

A drift may occur between before and after calibrations. Here the log must be rescaled at regular intervals to use up excess drift.

For example, assume a sonic log calibration showed the following:

If a 40 to 140 scale was used for the logged interval, and the log was 3000 feet long, the following scales should be used:

These values are created by linear interpolation or extrapolation as required. Any log may be rescaled using linear algebra. A computer can apply a continuous, linear or non-linear shift as described by the user, providing the proper equation is incorporated. Most computer aided log analysis software has a “user-defined equation” function to do this type of re-calibration.

All induction resistivity and most laterologs logs should first be translated into conductivity, rescaled, and then translated back to resistivity. Most errors are in the sonde error setting, which is a linear shift in conductivity, not in resistivity. Calibrations may appear to be perfect, yet the log can read high or low in comparison to other logs in the area. Checkpoints for calibration shifts are the matrix base lines in clean, non-porous limestone, dolomite, or anhydrite, shale base lines, or overall position of the log curve with respect to another log in the same or nearby well in thick shale beds with good borehole conditions.

5.05 Depth Shifts
Logs run on separate passes can easily be off-depth from each other. Choose one curve, usually the primary porosity curve (eg. density or sonic or gamma ray), and make it the reference curve. Compare all bed boundaries on each curve to the reference curve, and build a table of depth adjustments over the interval to be analyzed. Most computerized log analysis packages can stretch or squeeze log curves based on such a table, or allow interactive picking of the table from screen images.

Curves that are run on the same pass can also be off depth with each other. Some curves are memorized by the recording equipment because they are actually recorded a few feet above the lowest curve on the logging tool. The operator can set the memory distance incorrectly or the electronic memorizer can malfunction. Even computer controlled logging does not eliminate this problem.

Should the logging tool pull tight in the hole, it may slow down or even stop, while the cable keeps moving and stretching. Since the cable movement drives the recording camera, curves will be recorded off depth during this period. As a result of the memorizer, each curve will be off depth at a different location on the log.

After the tool pulls free it will move up the hole quickly while the logging cable driving the recorder will not. Therefore, the logs are again off depth, but in the opposite direction. This problem can only be adequately resolved if a curve exists which did not pull tight. Other curves can be stretched and squeezed to match the reference curve by computer programs or by approximate depth shift tables constructed by hand.

5.06 Skips, Noise, and Spikes
Cycle skips and noise are normally related to sonic logs, but can occur on any log curve. To edit, draw a smooth log curve ignoring the spikes, following an imaginary base log located beneath the noise, as shown in Figure 5.08.

FIGURE 5.08: Editing sonic log skips

At times, a smooth log curve may not exist. Therefore, it is necessary to review an offset log or another curve from the same well. Note that in Figure 5.09, the edit gives a sonic porosity of 5% - 10% instead of 25%. It indicates a major difference and is more probable for this particular zone.

FIGURE 5.09: Editing density log for bad hole condition

CAUTION: Some noise may be the result of thinly bedded porous layers, coal, or rough hole effects. Coal spikes should be identified as such, and bad hole effects discounted. Some bad hole is caused by breakout of the wellbore at natural fractures. The density log is the most strongly affected curve. The density log porosity should not be used as an indicator of reservoir volume, but the location of the fractures should be noted.

If the sonic log is edited, the integrated transit time curve must also be corrected.

5.07 Rough, Large, and Salty Hole Effects
All logs are affected to some degree by borehole size and environment. Application of environmental corrections by computer programs may reduce the need for editing. If no computer is handy, you may have to do something yourself. Several examples are cited here. Figure 5.10 shows a resistivity log which is severely affected by very conductive mud in the borehole. The dual laterolog would have been more suitable in this instance. Since only the dual induction was run, its readings must be used.

FIGURE 5.10: Dual Induction Log in Salt Mud

As a result two problems occur. Conductive mud causes the induction log to read too low due to invasion and borehole effect. This can be corrected by using special charts for invasion and borehole corrections. These charts are unique to particular tool types and can be obtained from service companies. Borehole correction charts are available from each service company and may be incorporated in computer aided log analysis programs. Since each tool from each company requires its own individual correction chart, the charts in the computer program may contain only a generic correction.

The second problem is the hashy nature of the log in low porosity zones, caused by variations in borehole effect. An edit is required to select a reasonable resistivity value and is shown on the log. The offset log, as well as the gamma ray, sonic and density neutron curves are used to provide the proper amplitude and shape. This is obviously a very serious change of log values and all calculated results should be used with extreme caution.

Modern sonic logs are normally unaffected by hole size. At times, the hole may be too large and the tool cannot find a refraction path that will deliver sound to the receivers. The log may be edited by comparison to offset wells, or reconstructed from calculations based on other logs. Older style, single transmitter, sonic logs have a spike at each change in hole size which must be edited manually.

All pad type devices, such as density, sidewall neutron, microlog, proximity, and dipmeter logs are ineffective in large holes, where the hole diameter is beyond the reach of the pad. Tools will either read the mud value or jump from high to low values due to intermittent contact with the borehole wall. Log data from these curves must not be used in this environment.

In holes ranging from bit size to approximately 30% of the pad extension, the tool response is probably correct. From 30% to 90% extension, logs may appear reasonable, but are influenced by mud between the pad and borehole wall. A similar effect may arise in thick mud cake where the hole size is smaller than the bit size.

Rough or rugose holes will also leave excessive mud between the pad and borehole wall. This effect may not seem too noticeable on most logs, since the caliper may appear smooth. On the density log, the correction curve will be abnormally high in such zones. This problem cannot be detected on most other logs. Therefore, caution is recommended. The MSFL does have an apparent mud cake thickness curve (Hmc) which will be abnormally high in such conditions.

An example of a density log in rough and large hole is shown in Figure 5.11.

FIGURE 5.11: Density Logs in Rough Hole

We know these logs are inaccurate because the apparent porosity is too high compared to other sources of information such as the sonic and offset wells. As well, the density neutron crossover infers gas, yet this is not indicated by other logs or well data. The porosity is abnormally high for a Devonian carbonate and the density correction curve suggests caution. For reservoir evaluation, the density data should be ignored. For seismic uses, an approximate density would be estimated based on lithologic description and offset well data.

5.08 Rock Alteration
Most formations are altered when the borehole is drilled. More competent formations show either an imperceptible or massive change, while softer formations often suffer significant, obvious alterations.

Shales are altered by exposure to mud filtrate, caving, eroding, absorbing water and swelling. The degree of weathering is a function of shale type and mud property, such as water loss, filtrate salinity and weight. Exposure time and other mechanical factors involved in drilling a hole are also of importance.

Sandstones are altered by relaxation, which is a function of mud weight, as well as erosion - a result of mud weight, water loss characteristics, and bit hydraulics.

Carbonates under stress, as found in overthrust zones, often implode into the borehole, leaving a large, irregular hole which is difficult to log accurately. An example is given in Figure 5.12.

FIGURE 5.12: Density Log in Rough Hole

Evaporites often dissolve and leave a large hole which cannot be logged, as in Figure 5.13.

All the above conditions can be minimized by proper attention to drilling procedures and drilling mud control.

Editing is not usually possible for rock alteration, except in rare cases where offset data provides guidance. Re-constructing (calculating from basic principles) a log using less affected data from the same hole may also be attempted. Such results are generally used only for seismic modeling.

In altered shales, a longer spaced sonic log, will often provide better data for geophysical purposes and reservoir calculations. Other sonic logs in the area can be edited using comparative data between the long and short spaced sonic logs run in selected holes.

Figure 5.14 shows the difference between frozen and thawed rock in an Arctic permafrost situation.

This log clearly illustrates a severe case of rock alteration. In shallow or recent sediments, the difference between long and short spaced sonic may be quite marked. The longer spacing is usually the better log.

Figure 5.15 is another example of rock alteration, showing a long and short spaced sonic log in a badly weathered borehole. Here, the density log has been reconstructed by calculating the log response from the long spaced sonic data and other known properties of the rock sequence.

5.09 Correcting Log Values in Hydrocarbon Zones
The values of density, neutron, and sonic velocity logs recorded in hydrocarbon bearing zones will usually be incorrect. They may deviate from the true, in-situ values due to the invasion of mud filtrate. In Figure 5.16, the neutron, density and sonic logs are severely affected by gas in the invaded zone. This invasion does not allow the log to read the in-situ gas filled rock/fluid properties needed for seismic modeling purposes, or the correct porosity required for reservoir evaluation. Log analysis methods compensate for this problem in reservoir evaluation. Therefore, an edit is unnecessary if porosity is the only result needed from the logs.

However, to generate a reasonable synthetic seismograph, the density and sonic data should be reconstructed by calculating the log response for an un-invaded zone. This is done by reversing the usual sonic and density (shale corrected) equations with the correct fluid terms to obtain the interpreted log readings.

Seismic applications of logs are covered in more detail in Chapter Twenty-One.

5.10 Shaping Logs for Quantitative Analysis
Calculators and computers will process data exactly as it is input. Therefore, it is essential to edit or shape logs at abrupt bed boundaries. Otherwise, unreasonable answers will result in the boundary area. The example in Figure 5.17 illustrates a typical coal bed with rounded corners at the bed boundary.

FIGURE 5.17: Shaping Log Curves at Bed Boundaries

An obvious lag is seen in the density and neutron data at the top of the zone. The shaded areas indicate how the log should be shaped, so that the computer analysis program will not give false shows of hydrocarbon at the top of the zone. Sonic and resistivity logs demand similar edits.

If necessary, trim the tops and bottoms of other bed boundaries, including interfaces between clean sand and shale, and limestone - dolomite - anhydrite - salt sequences. Depth adjustments should be made concurrently.

Thin beds may also require curve shaping. Figure 5.18 illustrates two thin beds on the shallow resistivity curve at the base of each clean sand. The deep resistivity curves show only a subtle hint of these beds and do not indicate their true value.

FIGURE 5.18: Thin Bed Recognition on Shallow Resistivity Log

The analyst can ignore the problem in this example, since the zones are thin and do not contribute or detract from the reservoir, but you may need to do significant work or run specialized thin-bed logs in other cases.

A second approach is to apply thin bed corrections, available from service company charts. This work is usually done by hand as it is more difficult to write suitable computer programs. Such programs scan up and down one curve searching for thin beds and apply reasonable corrections to another curve.

A third and more pragmatic approach is to shape the deep resistivity curve to match the shallow curve. Depending on the anticipated hydrocarbon content, the curve amplitude can be amended higher, lower or equal to the shallow curve. Do not try to extend this concept to match the resolution of micro resistivity logs. Some service companies can provide this service on some newer logs.

5.11 Editing For Seismic Modeling
Competent rocks, such as limestone, dolomite and well cemented sandstone can fracture and deteriorate at the wellbore due to stress release. These fractures may also penetrate some distance into the formation, changing natural in-situ properties. Since the hole is rough, and often oval instead of round, the logging tool response is presumably incorrect, as described in earlier sections.

Log editing for seismic purposes is an interpretative judgment. Consequently, you run the risk of editing out valid data. The risk is worthwhile to avoid inaccurate information. Figure 5.19 presents a severe, but plausible, simulated editing situation for a seismic modeling job. This example is taken from "Log Editing in Support of Detailed Seismic Studies", by Brian E. Ausburn, SPWLA, 1977. A brief excerpt of his paper discusses the editing process as follows:

Figure 5.19: Editing Logs for Seismic Modeling

Interval 3000 - 5520

This interval is a sand/shale sequence where most shales are washed out, yet the majority of sands remain. The combination of hole enlargement and shale alterations can result in apparent shale velocities and densities, being markedly less than that judged to be appropriate for the section.

The judgment is often based on trend curves developed from other wells in the area, where hole conditions for the equivalent stratigraphic section were superior. In this simulated example, the correction trend is substantiated by observed shale velocity values in sections where the hole has no wash out, as given in 4960-5220 and 5370-5220. It could be speculated that 4960-5220 has not washed out since it was logged shortly after drilling, and therefore, had less exposure time. Perhaps 5370-5520 has not washed out because the mud quality was improved and was more easily maintained after the casing string was set at 5220.

Note that the shale interval below the casing shoe, 5220-5370, has washed out. The mud quality was presumably poor immediately after drilling out the casing. By the time the drill bit reached 5370, the mud properties had stabilized resulting in less hole enlargement. In addition, mud circulation hydraulics, termed annular turbulence, occasionally causes washouts beneath casing shoes.

If a mechanical reason for hole washout cannot be determined, the substitution value should be considered carefully. It is possible the zone washed out because it is different from the nearby equivalent zones.

Shale alterations and washouts also affect the density log. Hole washouts, as they pertain to density log readings, can be subdivided into rugosity and enlargements. Rugosity sufficient to cause density log errors or a lack of pad contact may occur with little hole enlargement.

Interval 5520-6900

This interval is a mixed section of limestone, sandstone, shale and salt. With the exception of a few obvious hole washouts noted on the caliper in shale sections, the majority of editing is related to the salt section. It is postulated that the salt layer, 6190-6300, is completely washed out. As a result, both the sonic and density logs are recording the properties of borehole fluids. These are markedly different from salt parameters. Note that the density log has been edited from 6790-6900 even though the hole is in-gauge throughout that section. Due to a different electron/bulk density relationship for salt, from most other sedimentary rocks, the apparent log bulk density of salt is not the true bulk density. By contrast, the sonic response in the good hole salt section substantiates the validity of the sonic edits in the washed out salts.

Figure 5.20 compares synthetic seismograms that would be obtained from raw and edited log data. As predicted from the severity of edits shown on Figure 5.19, the differences in synthetic seismograms are significant.

Note that the shallow section of the synthetic from unedited data, has more character than from edited data. These apparent events represent only the observed contrasts in the erroneous information and not any real acoustic impedance contrast in the subsurface. Note also that the last major event from the unedited set occurs at approximately 2.03 seconds, while on the edited version this event occurs at approximately 1.875 seconds. This difference reflects the significance of the uphole velocity in achieving depth/time ties between seismic and wellbore data. In this case, the too slow velocity observed on the up-hole portion of the raw log could make a difference of 500 to 1,000 feet in the location of the deeper events. Raw log data generally makes a synthetic too long.

Seismic applications of logs are covered in more detail in Chapter Twenty-One.

5.12 Use of Check-Shot Data for Seismic Editing
Check-shot or seismic reference surveys (SRS) are obvious ways to either validate or correct acoustic log information for seismic purposes. This is probably the oldest method for tying seismic times to well depths. Check shot surveys using closely spaced stations can often assist in determining interval velocities, and in editing sonic logs affected by noise, skipping or hydrocarbon effects. The service has been replaced by Vertical Seismic Profiles (VSP), but SRS surveys will be found in existing well files and will be useful for years to come.

It is incorrect merely to shift logs to give the same integrated time as the SRS or VSP. The difference between SRS and sonic for an interval should be applied to specific places on the sonic by editing. Re-integrate to determine the remaining error and correct as often as necessary.

Apply a linear shift only if SRS picks are of good quality and no obvious edits are needed. Particular data points in an SRS can be incorrect. Thus judgment is required in adjusting sonic logs.

In some cases the seismic information may itself be used to edit the well logs, by successive edits and synthetic matches. Seismic applications of logs are covered in more detail in Chapter Twenty-One.

5.13 True Vertical Depth Editing
The material in this section was taken verbatim from "Application of True Vertical Depth, True Stratigraphic Thickness and True Vertical Thickness Log Displays" by R.M. Bateman and V.R. Hepp, CWLS, 1981. The mathematics for these processing steps is given in Chapter Twenty Five.

Wells which are not vertical or wells which penetrate dipping formations present special editing problems. The logs must be adjusted to compensate for the angles involved.

The True Vertical Depth (TVD), True Stratigraphic Thickness (TST), and True Vertical Thickness (TVT) plots facilitate this task. Even so, their application is not straightforward, and rules must be set to avoid pitfalls and to take full advantage of these plots.

As long as dips and deviations do not exceed a few degrees, the simple vertical-horizontal case is approximated closely enough by the actual logs. But when deviations and dips exceed about ten degrees, corrections are needed because apparent formation thicknesses measured on logs are greater than true stratigraphic thicknesses by different amounts in different wells. This adds to the difficulty of well-to-well log correlation. Also, if wells are deviated from vertical, and if formations have substantial dip, apparent thicknesses differ from the vertical thicknesses needed for reservoir volume calculation, and must be corrected.

To achieve these corrections in a convenient manner, modern data processing affords three different computed log products: The TVD, TST and TVT plots. These logs are created by altering the depths of the recorded data according to the geometry of the situation.

Two methods exist for computation of altered depths:

1. Common surface point, assuming a hole drilled from the same surface point or formation top but with a different course.

2. Common subsurface point, assuming a hole drilled either vertically, or normal to the bed dip, from some point in the actual well course, such as a formation top, or a point of formation dip change. Depths may be reset arbitrarily at the common point.

The true vertical depth plot ignores formation dip, and corrects for well deviation only. It thus represents formations as they would look in a vertically drilled well, provided the formations had zero dip. It is useful in areas of directional drilling where dip is low, for well-to-well correlations and for reservoir volume calculations. It is usually run only in the common surface point mode.

True stratigraphic thickness plots account for formation dip, and require knowledge of true well course, whether vertical or not. It displays formations as though the well had been drilled perpendicular to the beds. If a change of dip occurs, an equal and opposite change of deviation is assumed.

If only one dip is present, the plot represents the logs that would have been obtained if the well had been drilled at the same location perpendicular to that dip.

If more than one dip is present, the interpretation becomes more complicated. At each dip change, some stratigraphic column must either disappear, or thin, or thicken, or even repeat itself.

The TST always shows formations under their minimum thicknesses.

A true vertical thickness plot is closely related to the TST, and as such accounts for both well deviation and formation dip. It shows formation thickness as though the well had been drilled vertically through the dipping beds. A TVT in a vertical hole would be identical to the original log. The TVT is meant to be used for reservoir volume calculations from deviated hole logs.

If the well is vertical and the formations are horizontal, all three transformed logs would be identical to the original log, and the processing would be a waste of computer time.

If the well is deviated and the formations are horizontal, the TST and the TVT are identical to the TVD, and running the latter is sufficient.

If the well is vertical and the formations dip, the TVD and TVT plots would be wasted computer time, but the TST may be useful in well to well correlations.

If the well is drilled perpendicular to the formation dip, (as it often tends to be in hard rocks), the TST would be a waste of computer time, but the TVT is needed for reservoir calculations, and the TVD may be of use if deviation is pronounced.

All three plots perform valuable functions, but all three may be misleading if not used with the proper caution, in particular with respect to absolute depths.

The TVD is incorrect in both formation thicknesses and in absolute depths if formations have appreciable dip.

The TST is always correct in formation thicknesses. It should be run in the common subsurface point mode. If changes of dips are present, it should reset the subsurface depth at each change of dip and make independent plots through each dip zone.

The TVT may produce apparent thicknesses greater than measured thicknesses. Such thicknesses may be fictitious, when beds are truncated in their vertical extension by unconformities or faults. It should be run in independent sections for each change of dip, in the common subsurface point mode, as for the TST.

Editing of this type is seldom performed by hand, and only some computer aided log analysis systems have all three processing modes. The mathematics is simple geometry and detailed in Chapter Twenty-Five.

5.14 In Conclusion
Editing is a form of interpretation. It is advisable to read the remaining Chapters of this handbook and practice the methods, prior to attempting your first edit on a real project. When editing is approached with limited background or experience, it is unlikely that the edit will be successful. However, good edits are an integral part of any successful log analysis.

5.15 Exercises For Chapter Five
Bonus points (up to 10% of each question) will be given for originality, good grammar, and clear, concise exposition. Deductions will be made for obvious Copy and Paste.

1. Give ten reasons why editing is needed? (10 marks)

2. What are the two crucial concepts in editing? (10 marks)

3. Define what log calibrations are. (5 marks)

4. Describe primary, secondary, and tertiary calibrations. Which one is often called the shop calibration and which is the field calibration? Give an example. (10 marks)

5. Give three reasons why a log may need to be shaped and how to do it. (10 marks)

6. When and why is true vertical depth editing needed? (5 marks)

7. Describe the two methods for computation of true vertical depths. (5 marks)

8. Define true vertical depth, true stratigraphic thickness, and true vertical thickness plots, and distinguish between them. (10 marks)

9. Describe why logs may need editing prior to use by a geophysicist. How are checkshots or VSP data used to assist the editing process? (10 marks)

10. Define rock alteration and how it affects sonic and density logs. (5 marks)

11. Edit the sonic log and density porosity log displayed below. Use a wide tip pen so the edit is clearly visible. What might be the cause of the anomalies on the sonic curve? On the density curve? (20 marks)

5.16 Bibliography For Chapter Five
1. Some Human-Electrical Factors of a Quality Logging Service, L.M. Edwards, L.E. Shane, SPWLA, 1965

2. Multipit, A Method for Calibration of Logging Systems, Merle E. Crew, SPWLA, 1979

3. Log Calibrations Surface and Downhole, A.F. Bosworth, CWLS, 1973

4. The Delta Tension Curve For Better Log Quality, B.B. Cooley, SPWLA, 1974

5. Mechanics of Log Calibrations, W.C. Waller, M.E. Cram, J.E. Hall, SPWLA, 1975

6. Recommendations for Scaling Metric Well Logs in Canada, R.E. Wyman, T.J.M. Bruseker, CWLS, 1975

7. Log Quality Control, O.R. Hold, CWLS, 1975

8. Schlumberger Log Calibrations, SWSC, 1966

9. Calibration And Quality Standards, SWSC, 1974

10. Standardization of Large Volumes of Log Data, E.T. Connolly, CWLS, 1969

11. Porosity Log Calibrations, W.H. Lang, Jr., SPWLA, 1980

12. Log Editing in Support of Detailed Seismic Studies, B.B. Ausburn, SPWLA, 1977

13. Log Quality Control, R.M. Bateman, IHRDC, Boston MA, 1985

14. The Log Analyst and the Programmable Calculator Part III, Dipmeter Computation, R.M. Bateman, C.E. Konen, SPWLA, 1978

15. The Log Analyst and the Programmable Calculator Part IV TST and TVT Computation, R.M. Bateman, C.E. Konen, SPWLA, 1979

16. Simplified TVT Calculation Using Programmable Pocket Calculator, S.W. Smith, D. Keen, SPWLA, 1979

17. True Vertical Depth, True Vertical Thickness and True Stratigraphic Thickness Logs, O.R. Holt, L.G. Schoonover, P.A. Wichmann, SPWLA, 1977

18. Application of TVD, TST and TVT Log Displays, R.M. Bateman, V.R. Hepp, CWLS, 1981

ABOUT THE AUTHOR

E. R. (Ross) Crain, P.Eng. is a Consulting Petrophysicist and 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.

His textbook, "Crain's Petrophysical Handbook on CD-ROM" is widely used as a reference to practical log analysis. Mr. Crain is an Honourary Member and Past President of the Canadian Well Logging Society (CWLS), a Member of Society of Petrophysicists and Well Log Analysts (SPWLA), and a Registered Professional Engineer with Alberta Professional Engineers, Geologists and Geophysicists (APEGGA).