Changes

Data preparation is usually conducted in a prescribed order. The order may vary slightly because of personal preference and the nature of available computer programs. The order recommended here is as follows:

Data preparation is usually conducted in a prescribed order. The order may vary slightly because of personal preference and the nature of available computer programs. The order recommended here is as follows:

'''1. Move digital log data from logging field tapes or digitized logs to the log processing environment'''. If paper copies of logs were digitized, all digital log data should be plotted and compared with the original log hard copies.

;1. Move digital log data from logging field tapes or digitized logs to the log processing environment: If paper copies of logs were digitized, all digital log data should be plotted and compared with the original log hard copies.

'''2. Display all logging data'''. This way the user can become familiar with typical responses for the different lithologies and/or formations in the study area. This is an initial quality control step and is also used as input to the merging of logging runs.

;2. Display all logging data: This way the user can become familiar with typical responses for the different lithologies and/or formations in the study area. This is an initial quality control step and is also used as input to the merging of logging runs.

'''3. Collect all information related to the logs'''. Locate the headers for all logs, and identify the logging tool model used on each well and logging run. Collect all temperature data and mud properties. Determine the top and bottom of the valid logging measurements for each curve to be utilized for each logging run. The stacking of logging tools can lead to large differences (several tens of feet) in the starting depth of individual measurements.

;3. Collect all information related to the logs: Locate the headers for all logs, and identify the logging tool model used on each well and logging run. Collect all temperature data and mud properties. Determine the top and bottom of the valid logging measurements for each curve to be utilized for each logging run. The stacking of logging tools can lead to large differences (several tens of feet) in the starting depth of individual measurements.

'''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.

All depths should be referenced to what is termed a ''base log''. The base log is selected from a logging tool where strong or forceful tool positioning is not used. Free-moving tools travel through the borehole more smoothly than tools that are pushed with great force against the borehole wall, such as the [[Basic open hole tools#Density|density log]]. For this reason, strongly centralized tools are not selected as the base log. A [[Basic open hole tools#Resistivity|resistivity log]] ([[Basic open hole tools#Induction|induction]] or [[Basic open hole tools#Laterologs|laterolog]]) is usually selected as the base log. For example, if [[Basic open hole tools#Gamma ray|gamma ray]] logs are available from both the density tool and induction tool strings, it is wise to select the gamma ray from the induction tool as the base log. The gamma ray from the density curve and all curves recorded with the density are then shifted to match the induction log depths. The base curve should also be selected based upon its expected strong correlation with the curves to be depth matched.

:All depths should be referenced to what is termed a ''base log''. The base log is selected from a logging tool where strong or forceful tool positioning is not used. Free-moving tools travel through the borehole more smoothly than tools that are pushed with great force against the borehole wall, such as the [[Basic open hole tools#Density|density log]]. For this reason, strongly centralized tools are not selected as the base log. A [[Basic open hole tools#Resistivity|resistivity log]] ([[Basic open hole tools#Induction|induction]] or [[Basic open hole tools#Laterologs|laterolog]]) is usually selected as the base log. For example, if [[Basic open hole tools#Gamma ray|gamma ray]] logs are available from both the density tool and induction tool strings, it is wise to select the gamma ray from the induction tool as the base log. The gamma ray from the density curve and all curves recorded with the density are then shifted to match the induction log depths. The base curve should also be selected based upon its expected strong correlation with the curves to be depth matched.

Depth-shifting programs are commonly of two types: (a) automatic depth-shifting programs in which mathematical correlations are made among curves from different tool strings and the shifting is accomplished without user input, or (b) visual correlation programs in which the curves to be shifted are laid beside or on top of the base curve, allowing the user to instruct the program by noting correlative points on each log and calculating the depth offset. With older programs, the correlations can be made by using log prints and the shifts input to the screen or a file. Most programs allow the user to carry or cause the same shift to be performed on other curves recorded on the same tool.

:Depth-shifting programs are commonly of two types: (a) automatic depth-shifting programs in which mathematical correlations are made among curves from different tool strings and the shifting is accomplished without user input, or (b) visual correlation programs in which the curves to be shifted are laid beside or on top of the base curve, allowing the user to instruct the program by noting correlative points on each log and calculating the depth [[offset]]. With older programs, the correlations can be made by using log prints and the shifts input to the screen or a file. Most programs allow the user to carry or cause the same shift to be performed on other curves recorded on the same tool.

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 [[Conventional coring]], [[Core handling]], and [[Core description]].)

;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.

While interpolation is a necessary step in the depth matching of wireline logs, it is highly undesirable when shifting cores. Interpolation should not be done when a core segment is shifted. Also, core data should not be resampled if least squares correlations are planned for calibrating logs or developing porosity and [[permeability]] relationships. Linear resampling of permeability destroys porosity and permeability relationships and can make statistical inference incorrect when making core to core or log to core data correlations. It is recommended that in any of these correlations the logs be resampled, not the core data.

:While interpolation is a necessary step in the depth matching of wireline logs, it is highly undesirable when shifting cores. Interpolation should not be done when a core segment is shifted. Also, core data should not be resampled if least squares correlations are planned for calibrating logs or developing porosity and [[permeability]] relationships. Linear resampling of permeability destroys porosity and permeability relationships and can make statistical inference incorrect when making core to core or log to core data correlations. It is recommended that in any of these correlations the logs be resampled, not the core data.

'''7. Perform environmental corrections on logs'''. Well logs are recorded in the hostile borehole environment where borehole size, temperature, pressure, mud properties, and other environmental factors affect logging tool responses. Logging tools are calibrated to make correct measurements only when specific environmental conditions exist (that is, an 8-in. borehole diameter at standard temperature). The purpose of environmental corrections is to correct the logging measurements to these standard conditions. Environmental corrections can be large. Some logging tools work over a much broader range of environmental conditions than others (see Table 1 or service company charts for more details).

;7. Perform environmental corrections on logs: Well logs are recorded in the hostile borehole environment where borehole size, temperature, pressure, mud properties, and other environmental factors affect logging tool responses. Logging tools are calibrated to make correct measurements only when specific environmental conditions exist (that is, an 8-in. borehole diameter at standard temperature). The purpose of environmental corrections is to correct the logging measurements to these standard conditions. Environmental corrections can be large. Some logging tools work over a much broader range of environmental conditions than others (see Table 1 or service company charts for more details).

{| class = "wikitable"

{| class = "wikitable"

* [[Basic open hole tools]]

* [[Basic open hole tools]]

* [[Basic tool table]]

* [[Basic tool table]]

* [[Determination of water resistivity]]

* [[Determination of water resistivity]]

* [[Wireline formation testers]]

* [[Wireline formation testers]]

[[Category:Wireline methods]]

[[Category:Wireline methods]]