GR LogThe GR log is a long established part of formation evaluation. Gamma rays in the formation are emitted mainly by radioactive isotopes of Potassium, Thorium and Uranium. These elements occur primarily in shales, and so the GR log is a good shale indicator.
Engineering Applications of the GR log from an MWDThere has been a big increase in the use of GR logs run in combination with the MWD directional tool.
Since any change in lithology must be known as quickly as possible the GR sensor should be placed as near the bit as possible, below the directional sensors. Running an MWD GR log has the added problems of rigging up a depth tracking system.
The type of sensors used to detect gamma rays must be both robust and efficient. The most robust sensor is the Geiger-Muller tube, but unfortunately it will only detect a small percentage of the rays being emitted by the formation.
A more sensitive but less rugged sensor, is the Scintillation counter. Both types are used in MWD GR tools but the scintillation counter is the more popular.
Comparisson Between MWD & WL Logs:(i) The logging speeds are very different (wireline @ 1800 ft/h MWD @ 10 -100 ft/h). The resolution of the two logs will therefore be affected.
(ii) Wellbore conditions may be different since the MWD log was made, e.g. cavings
(iii) MWD log is made through a drill collar, so the attenuation of gamma rays will be greater.
(iv) Central position of the sensors may be different, especially in high angled holes.
Directional sensors and GR sensors are well established for MWD use. More sensors are being developed and the term LWD - Logging Whilst Drilling is now used to describe these tools.
Comparison between GR and Resistivity logs from an MWD tool & from wireline logging
|Comparison of MWD and Wireline Log|