Changes

'''4. Merge logging runs''' (if required). When merging different logging runs in a single well, the first problem is that the depths may be different (for equivalent positions in the wellbore) from run to run. Depth shifts between runs should be made when necessary. Run overlaps should be noted because they allow the user to compare common-point depth measurements from different logging runs. This comparison of two logging measurements at the same depth aids in quality control.

'''4. Merge logging runs''' (if required). When merging different logging runs in a single well, the first problem is that the depths may be different (for equivalent positions in the wellbore) from run to run. Depth shifts between runs should be made when necessary. Run overlaps should be noted because they allow the user to compare common-point depth measurements from different logging runs. This comparison of two logging measurements at the same depth aids in quality control.

'''5. Edit logs to eliminate invalid data'''. Erroneous data can be recorded in casing at the top of the logging run and/or recorded with the tools setting on bottom before pick up. These data should be deleted (replaced with a missing data flag) on the edited copy of the logging curve. Logging data that are invalid because of environmental conditions (such as hole washouts or gas in the drilling mud) should also be deleted. This will result in data gaps, but these are preferable to erroneous data. If the data gaps occur within potential reservoirs, a replacement value of all reservoir parameters (such as [[porosity]] and water saturation) should be estimated from other logging measurements if at all possible. Logging data can also be incorrect due to incorrectly calibrated or malfunctioning logging tools (see <ref name=pt04r6>Lang, W. H., Jr., 1980, [[Porosity]] log calibration: The Log Analyst, v. 21, n. 2.</ref><ref name=pt04r7>Neinast, G. S., Knox, C. C., 1973, Normalization of well log data: SPWLA 14th Annual Logging Symposium Transactions Paper I.</ref><ref name=pt04r8>Patchett, J. G., Coalson, E. B., 1979, The determination of porosity in sandstones and shaly sandstones, Part 1— quality control: SPWLA 20th Annual Logging Symposium Transactions Paper QQ.</ref>.

'''5. Edit logs to eliminate invalid data'''. Erroneous data can be recorded in casing at the top of the logging run and/or recorded with the tools setting on bottom before pick up. These data should be deleted (replaced with a missing data flag) on the edited copy of the logging curve. Logging data that are invalid because of environmental conditions (such as hole washouts or gas in the drilling mud) should also be deleted. This will result in data gaps, but these are preferable to erroneous data. If the data gaps occur within potential reservoirs, a replacement value of all reservoir parameters (such as [[porosity]] and water saturation) should be estimated from other logging measurements if at all possible. Logging data can also be incorrect due to incorrectly calibrated or malfunctioning logging tools (see <ref name=pt04r6>Lang, W. H., Jr., 1980, [[Porosity]] log calibration: The Log Analyst, v. 21, n. 2.</ref><ref name=pt04r7>Neinast, G. S., Knox, C. C., 1973, Normalization of well log data: SPWLA 14th Annual Logging Symposium Transactions Paper I.</ref><ref name=pt04r8>Patchett, J. G., Coalson, E. B., 1979, The determination of porosity in sandstones and shaly sandstones, Part 1— quality control: SPWLA 20th Annual Logging Symposium Transactions Paper QQ.</ref>).

'''6a. Depth shift all log curves not recorded with the base curve or log'''. When logging tools are run in sequence, differences always occur in depth from tool to tool and from run to run. Even when the logging tools are run in a single string there are potential depth differences due to differential cable stretch. Stretch can be pronounced when the logging tool string sticks or temporally hangs up in the hole. All logging measurements must be adjusted to a common depth reference before data processing can continue. A depth shift of [[length::3 ft]] can destroy an otherwise good correlation among logging measurements or between well logs and cores.

'''6a. Depth shift all log curves not recorded with the base curve or log'''. When logging tools are run in sequence, differences always occur in depth from tool to tool and from run to run. Even when the logging tools are run in a single string there are potential depth differences due to differential cable stretch. Stretch can be pronounced when the logging tool string sticks or temporally hangs up in the hole. All logging measurements must be adjusted to a common depth reference before data processing can continue. A depth shift of [[length::3 ft]] can destroy an otherwise good correlation among logging measurements or between well logs and cores.

The depth-shifting operation necessarily stretches or shrinks the curve being shifted, thus it should be kept in mind that data are both created and lost in the process. For this reason, subsequent depth shifts (corrections) should start with the original raw logs, not with a previously depth-shifted copy.

The depth-shifting operation necessarily stretches or shrinks the curve being shifted, thus it should be kept in mind that data are both created and lost in the process. For this reason, subsequent depth shifts (corrections) should start with the original raw logs, not with a previously depth-shifted copy.

'''6b. Depth shift core data to logs'''. The depth correlation of log data to core data is frequently characterized by numerous abrupt changes in the amount to be shifted. Every trip with the core barrel is potentially a change in the relative core or log depth, even if continuous cores are taken. Zones flagged as lost core zones often are not where they were interpreted to be. Because of this, automatic depth shift procedures generally do not work when shifting core data to well log data. An overlay procedure is recommended where the core is segmented by core run and again where missing data occurs within a core. The core is then usually shifted by segments. Segments can separate or overlap. Separation is caused by incomplete or poor core recovery, and overlap can be caused by poor core-handling procedures. Review of the field [[core description]] can help clarify some of these problems. (For more information on cores, see the chapters on “[[Conventional coring]]” and “[[Core handling]]” in Part 3 and the chapter on “[[Core description]]” in Part 5.)

'''6b. Depth shift core data to logs'''. The depth correlation of log data to core data is frequently characterized by numerous abrupt changes in the amount to be shifted. Every trip with the core barrel is potentially a change in the relative core or log depth, even if continuous cores are taken. Zones flagged as lost core zones often are not where they were interpreted to be. Because of this, automatic depth shift procedures generally do not work when shifting core data to well log data. An overlay procedure is recommended where the core is segmented by core run and again where missing data occurs within a core. The core is then usually shifted by segments. Segments can separate or overlap. Separation is caused by incomplete or poor core recovery, and overlap can be caused by poor core-handling procedures. Review of the field [[core description]] can help clarify some of these problems. (For more information on cores, see [[Conventional coring]], [[Core handling]]”, and [[Core description]].)

A core gamma ray can be a valuable aid in establishing depth correlations between core and logs. Boyle's law core porosity and core grain density can be used to construct a core bulk density curve to correlate with log bulk density to determine the amount of depth shifts required. Core bulk density usually correlates well with the density log because lithologic variations are eliminated, resulting in two similar curves being correlated.

A core gamma ray can be a valuable aid in establishing depth correlations between core and logs. Boyle's law core porosity and core grain density can be used to construct a core bulk density curve to correlate with log bulk density to determine the amount of depth shifts required. Core bulk density usually correlates well with the density log because lithologic variations are eliminated, resulting in two similar curves being correlated.

[[file:preprocessing-of-logging-data_fig1.png|thumb|{{figure number|1}}Determination of true resistivity (''R''_t) from dual induction.]]

[[file:preprocessing-of-logging-data_fig1.png|thumb|{{figure number|1}}Determination of true resistivity (''R''_t) from dual induction.]]

The logging service company chart books contain correction charts for most logging tools and instructions for their use. Users of well log data should become familiar with these so that they can either apply the corrections or recognize the environmental conditions where corrections are not significant. Table 1 gives a summary of environmental corrections for common logging tools emphasizing the conditions for which corrections can be large. Table 1 also contains a partial list of recent service company charts used for typical corrections<ref name=pt04r19>Welex, 1985, Welex log interpretation charts: Houston, TX.</ref><ref name=pt04r4>Dyos, C. J., 1987, Inversion of induction log data by the method of maximum entropy: SPWLA 28th Annual Logging Symposium Transactions, Paper T.</ref>a, b; <ref name=pt04r20>Western Atlas International, 1985, Atlas log interpretation charts: Houston, TX.</ref>. Older tools may require the use of older chart books. Slim hole models available for some tools may require their own specific charts. The user must become familiar with log headings to identify tool models so that the correct chart can be selected.

The logging service company chart books contain correction charts for most logging tools and instructions for their use. Users of well log data should become familiar with these so that they can either apply the corrections or recognize the environmental conditions where corrections are not significant. Table 1 gives a summary of environmental corrections for common logging tools emphasizing the conditions for which corrections can be large. Table 1 also contains a partial list of recent service company charts used for typical corrections.<ref name=pt04r19>Welex, 1985, Welex log interpretation charts: Houston, TX.</ref><ref name=pt04r4>Dyos, C. J., 1987, Inversion of induction log data by the method of maximum entropy: SPWLA 28th Annual Logging Symposium Transactions, Paper T.</ref>a, b; <ref name=pt04r20>Western Atlas International, 1985, Atlas log interpretation charts: Houston, TX.</ref> Older tools may require the use of older chart books. Slim hole models available for some tools may require their own specific charts. The user must become familiar with log headings to identify tool models so that the correct chart can be selected.

Environmental corrections are often performed serially. For example, [[:file:preprocessing-of-logging-data_fig1.png|Figure 1]] is a flowchart illustrating the steps used to determine true resistivity from a dual induction log. It should be noted that shoulder bed effect corrections are difficult and require complex algorithms<ref name=pt04r4 />. Also, it is possible that not performing all corrections (e.g., invasion correction without borehole corrections) may be worse than making no corrections at all.

Environmental corrections are often performed serially. For example, [[:file:preprocessing-of-logging-data_fig1.png|Figure 1]] is a flowchart illustrating the steps used to determine true resistivity from a dual induction log. It should be noted that shoulder bed effect corrections are difficult and require complex algorithms.<ref name=pt04r4 /> Also, it is possible that not performing all corrections (e.g., invasion correction without borehole corrections) may be worse than making no corrections at all.

The SP curve presents special environmental problems that are not addressed by charts. Since the SP responds in part to the mud filtrate resistivity, no two SP curves will be recorded under exactly the same environment (Bateman, 1985). (For more information on SP logs, see the chapters on “[[Basic open hole tools]]” and “Determination of Water Resistivity” in Part 4.)

The SP curve presents special environmental problems that are not addressed by charts. Since the SP responds in part to the mud filtrate resistivity, no two SP curves will be recorded under exactly the same environment.<ref name=Bateman_1985>Bateman, R. M., 1985, Open-hole log analysis and formation evaluation: Boston, MA, IHRDC, p. 15-189.</ref> (For more information on SP logs, see [[Basic open hole tools]] and [[Determination of water resistivity]].)

==See also==

==See also==