Changes

'''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 density log. For this reason, strongly centralized tools are not selected as the base log. A [[Basic open hole tools#Resistivity|resistivity log]] (induction or 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.

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| Induction

| [[Basic open hole tools#Induction|Induction]]

| H:R-1E S: RCOR-4A W:6-2

| H:R-1E S: RCOR-4A W:6-2

| Mud resistivity, hole size, and tool position

| Mud resistivity, hole size, and tool position

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| Microlaterolog microspherically focused proximity microguard

| [[Basic open hole tools#Microresistivity|Microlaterolog]] microspherically focused proximity microguard

| H.R-11 S: RXO-3 W:4-1

| H.R-11 S: RXO-3 W:4-1

| Mudcake thickness, ''R''_xo zone resistivity, mudcake resistivity, and hole size

| Mudcake thickness, ''R''_xo zone resistivity, mudcake resistivity, and hole size

| Mudcake thickness is difficult to determine

| Mudcake thickness is difficult to determine

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| Compensated neutron logs

| [[Basic open hole tools#Compensated neutron|Compensated neutron logs]]

| H: POR-14C S: POR-14C W:5-13

| H: POR-14C S: POR-14C W:5-13

| Hole size, temperature, pressure, borehole, and formation salinity; mud weight, mud type, mudcake thickness, and tool position

| Hole size, temperature, pressure, borehole, and formation salinity; mud weight, mud type, mudcake thickness, and tool position

| Charts tend to overcorrect in elliptical boreholes; some or all corrections may have been applied at the time of recording.

| Charts tend to overcorrect in elliptical boreholes; some or all corrections may have been applied at the time of recording.

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| Density logs

| [[Basic open hole tools#Density|Density logs]]

| S:POR-15A W:5-11

| S:POR-15A W:5-11

| Hole size, formation density, and mud weight

| Hole size, formation density, and mud weight