Changes

==Basic tools==

==Basic tools==

The basic cased hole tools can be divided into three general groups: correlation, saturation, and [[porosity]]. The correlation device is used to correlate cased hole measurements with open hole measurements and to estimate shale volume. The saturation device is used to determine water saturation when porosity and water salinity are known. The porosity devices are used to estimate porosity when lithology is known. Table 1 lists the types of devices used for these purposes, and Table 2 gives the resolutions and limitations of the various devices. (For more details on tool specifications, see “[[Basic tool table]]”.)

The basic cased hole tools can be divided into three general groups: correlation, saturation, and [[porosity]]. The correlation device is used to correlate cased hole measurements with open hole measurements and to estimate shale volume. The saturation device is used to determine water saturation when porosity and water salinity are known. The porosity devices are used to estimate porosity when lithology is known. Table 1 lists the types of devices used for these purposes, and Table 2 gives the resolutions and limitations of the various devices. (For more details on tool specifications, see [[Basic tool table]].)

{| class = "wikitable"

{| class = "wikitable"

===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 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===

The compensated neutron measurement is little affected by the presence of steel casing. This permits the compensated neutron to be used in cased hole to estimate porosity when lithology is known. However, the presence of gas in the formation will cause the compensated neutron to underestimate porosity significantly. (For more information on the compensated neutron tool, see “[[Basic open hole tools]]”.)

The compensated neutron measurement is little affected by the presence of steel casing. This permits the compensated neutron to be used in cased hole to estimate porosity when lithology is known. However, the presence of gas in the formation will cause the compensated neutron to underestimate porosity significantly. (For more information on the compensated neutron tool, see [[Basic open hole tools]].)

===Pulsed neutron tool===

===Pulsed neutron tool===

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

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

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]].