Changes

===Gamma ray tool===

===Gamma ray tool===

[[file:basic-cased-hole-tools_fig1.png|300px|thumb|{{figure number|1}}Typical presentation of a pulsed neutron log. © Schlumberger, 1986.]]

[[file:basic-cased-hole-tools_fig1.png|300px|thumb|{{figure number|1}}Typical presentation of a pulsed neutron log. &copy; Schlumberger.<ref name=Schlumberger1986>Schlumberger Well Services, 1986, Schlumberger Production Services Catalog: Houston, TX.</ref>]]

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]].)

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]].)

* [[Quick-look lithology from logs]]

* [[Quick-look lithology from logs]]

* [[Borehole imaging devices]]

* [[Borehole imaging devices]]

==External links==

==External links==