Wireline logging is referred to as the compiling of a detailed geophysical analysis data, which is undergone with respect to the depth of a dug well. This is combined with the estimation of other services that will be needed. In oil and gas industries, they make use of wireline logging in compiling a consistent record of the properties of a formed rock. Wireline logging is majorly carried out by the wireline companies. They employ the service of expert wireline loggers. However, there are two types of wireline logging. They are cased-hole wireline logging and opened-hole wireline logging. Cased-hole wireline logging is carried out after the well has been lined with casing or production tape, while opened-hole wireline logging is carried out by the wireline companies before the oil well is cased.
In cased-hole wireline logging, a series of measurements are taken through casing by the wireline companies, these measurements are mainly to:
Supplement measurements taken in open hole: Sometimes, the conditions of a dug well can lead to an inappropriate measurement of holes in the well. These can make it unreliable and unsafe. Therefore, it may be crucial to take other measurements through casing to supplement the previous measurements taken in open hole.
To check for any changes in formation properties: In some cases, sustained production may lead to certain changes in the saturation of the void by water or oil and gas in a well. Thus, it may be of high importance to take extra measurements to check for any changes in the formation properties of the well.
To give a depth reference: Extra measurements are taken by the wireline companies to give a depth reference points between cased-hole and opened-hole.
Also, there are few basic tools used in cased-hole wireline logging. These tools are grouped into three, they are:
- Correlation device
- Saturation device
- Porosity device
The three groups have different areas of applications. Some of the tools utilized in their applications are:
Gamma ray tool
This tool gives high response to the gamma rays that occur naturally from the element formation. They have the properties of penetrating different types of steel casing. This property makes gamma ray suitable for correlating a cased-hole with an open-hole. However, certain challenges come with the use of gamma ray tool such as the checking of radioactive salts in cased-hole logs. Often times, the gamma ray tool is used with a collar locator. This is to give a depth reference point.
Pulsed neutron tool
It is an electronic device, which produce pulses of charged energy neutrons. The charged energy neutrons hit the formation sporadically, and their energy reduces with the rate of collision with the formation. The pulsed neutron tools are designed with detectors in them to measure the captured gamma rays. The gamma rays can also be processed to give an estimated value of porosity.
Compensated neutron tool
Measurements taken with a compensated neutron tool are slightly affected by the steel casing. On the other hand, compensated neutron tool will underestimate porosity when a certain amount of gas is found in the formation.