Changes

|+ {{table number|1}}Application and types of basic cased hole tools

|+ {{table number|1}}Application and types of basic cased hole tools

|-

|-

! Application

! Application || Type of Device

! Type of Device

|-

|-

| Correlation

| rowspan = 2 | Correlation || [[Basic open hole tools#Gamma ray|Gamma ray]]

| Gamma ray

|-

|-

| Spectral gamma ray

| Spectral gamma ray

|-

|-

| Saturation

| Saturation || Pulsed neutron

| Pulsed neutron

|-

|-

| [[Porosity]]

| rowspan = 2 | [[Porosity]] || [[Basic open hole tools#Compensated neutron|Compensated neutron]]

| Compensated neutron

|-

|-

| Pulsed neutron

| Pulsed neutron

|}

|}

|+ {{table number|2}}Resolutions and limitations of cased hole logging devices

|+ {{table number|2}}Resolutions and limitations of cased hole logging devices

|-

|-

! Tool

! Tool || Vertical Resolution || Radius of Investigation || Applications || Limitations

! Vertical Resolution

! Radius of Investigation

! Applications

! Limitations

|-

|-

| Gamma ray

| Gamma ray || [[length::2 ft]] || [[length::12 in]] || Well-to-well correlation and estimates ''V''_sh || Affected by radioactive scale near perforations

| [[length::2 ft]]

| [[length::12 in]]

| Well-to-well correlation and estimates ''V''_sh

| Affected by radioactive scale near perforations

|-

|-

| Spectral gamma ray

| Spectral gamma ray || [[length::3 ft]] || [[length::16 in]] || Well-to-well correlation and estimates ''V''_sh || —

| [[length::3 ft]]

| [[length::16 in]]

| Well-to-well correlation and estimates ''V''_sh

| —

|-

|-

| Pulsed neutron

| Pulsed neutron || [[length::2 ft]] || [[length::18 in]] || Determines water saturation and estimates porosity || Does not work in freshwater and severely underestimates porosity in the presence of gas

| [[length::2 ft]]

| [[length::18 in]]

| Determines water saturation and estimates porosity

| Does not work in freshwater and severely underestimates porosity in the presence of gas

|-

|-

| Compensated neutron

| Compensated neutron || [[length::3 ft]] || [[length::10 in]] || Estimates porosity || Severely underestimates porosity in the presence of gas

| [[length::3 ft]]

| [[length::10 in]]

| Estimates porosity

| Severely underestimates porosity in the presence of gas

|}

|}

===Gamma ray tool===

===Gamma ray tool===

The gamma ray measurement responds to naturally occurring gamma rays from the formation. These gamma rays are able to penetrate steel casing. This permits the gamma ray to be used in a cased hole for correlation with open hole logs, for the discrimination of sands and shales, and for the calculation of shale volume. Its use is essentially identical to its use in an open hole with the exception of minor environmental corrections needed for the influence of the steel casing and cement. A common problem encountered in the cased hole use of the gamma ray device is scaling of radioactive salts in casing. When produced water containing dissolved radioactive salts enters the casing, the encountered drop in pressure may cause the salts to precipitate from the waters and deposit on the casing near the perforations. These salts will normally dominate the gamma ray response near the perforations, making the gamma ray useless in those intervals for both correlation to open hole or the estimation of shale volume. If the salts are predominantly uranium, their influence can be removed through the use of the spectral gamma ray. However, if the salts are in part potassium, their influence cannot normally be corrected through the use of the spectral gamma ray. The gamma ray is usually run in combination with a collar locator to provide a depth reference for mechanical cased hole services. (For information on the open hole gamma ray tool, see [[Basic open hole tools]].)

 

The gamma ray measurement responds to naturally occurring gamma rays from the formation. These gamma rays are able to penetrate steel casing. This permits the gamma ray to be used in a cased hole for correlation with open hole logs, for the discrimination of sands and shales, and for the calculation of shale volume. Its use is essentially identical to its use in an open hole with the exception of minor environmental corrections needed for the influence of the steel casing and cement. A common problem encountered in the cased hole use of the gamma ray device is scaling of radioactive salts in casing. When produced water containing dissolved radioactive salts enters the casing, the encountered drop in pressure may cause the salts to precipitate from the waters and deposit on the casing near the perforations. These salts will normally dominate the gamma ray response near the perforations, making the gamma ray useless in those intervals for both correlation to open hole or the estimation of shale volume. If the salts are predominantly uranium, their influence can be removed through the use of the spectral gamma ray. However, if the salts are in part potassium, their influence cannot normally be corrected through the use of the spectral gamma ray. The [[Basic open hole tools#Gamma ray|gamma ray]] is usually run in combination with a collar locator to provide a depth reference for mechanical cased hole services. (For information on the open hole gamma ray tool, see [[Basic open hole tools]].)

===Compensated neutron tool===

===Compensated neutron tool===

===Pulsed neutron tool===

===Pulsed neutron tool===

Pulsed neutron devices are electronic devices that generate pulses of high energy neutrons. These high energy neutrons bombard the formation, losing energy as they collide with atoms of the rock. Eventually, the neutrons lose so much energy that they are captured (generally by chlorine that exists as part of the salt dissolved in the formation waters). When a neutron is captured, a gamma ray is emitted. The detectors in the pulsed neutron tool are designed to measure these “capture” gamma rays, thus, a “capture [[cross section]]” of the formation through casing can be determined. This formation property allows one to estimate the water saturation when porosity and formation water salinity are known. A typical pulsed neutron log is shown in [[:file:basic-cased-hole-tools_fig1.png|Figure 1]].

 

Pulsed neutron devices are electronic devices that generate pulses of high energy neutrons. These high energy neutrons bombard the formation, losing energy as they collide with atoms of the rock. Eventually, the neutrons lose so much energy that they are captured (generally by chlorine that exists as part of the salt dissolved in the formation waters). When a neutron is captured, a gamma ray is emitted. The detectors in the pulsed neutron tool are designed to measure these “capture” gamma rays, thus, a “capture cross section” of the formation through casing can be determined. This formation property allows one to estimate the water saturation when porosity and formation water salinity are known. A typical pulsed neutron log is shown in [[:file:basic-cased-hole-tools_fig1.png|Figure 1]].

The gamma rays detected by the pulsed neutron devices can also be processed in a manner similar to the compensated neutron and provide a very similar estimation of porosity. This estimation of porosity can be reasonably good when the formation water is relatively salty and the formation does not contain significant amounts of gas. This combined measurement of capture cross section and porosity from this single device allows the pulsed neutron to be used frequently as a single pass cased hole formation evaluation device. As with the compensated neutron, the presence of gas in the formation will cause the pulsed neutron to underestimate porosity significantly.

The gamma rays detected by the pulsed neutron devices can also be processed in a manner similar to the compensated neutron and provide a very similar estimation of porosity. This estimation of porosity can be reasonably good when the formation water is relatively salty and the formation does not contain significant amounts of gas. This combined measurement of capture cross section and porosity from this single device allows the pulsed neutron to be used frequently as a single pass cased hole formation evaluation device. As with the compensated neutron, the presence of gas in the formation will cause the pulsed neutron to underestimate porosity significantly.

* [[Quick-look lithology from logs]]

* [[Quick-look lithology from logs]]

* [[Borehole imaging devices]]

* [[Borehole imaging devices]]

==External links==

==External links==

[[Category:Wireline methods]]

[[Category:Wireline methods]]