Changes

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

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

|-

|-

| Spectral gamma ray

| Spectral gamma ray

|-

|-

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

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

|}

|}

===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.

[[Category:Wireline methods]]

[[Category:Wireline methods]]