Module 4-GR Logs Flashcards
What are the ranges for shale, shaly sand and clean sand?
Shale- 90-150
Shaly sand- 45 to 90
Clean sand-15 to 45
The clean sand line on the GR ranges between?
7 to 45 but typically ranges between 15-30
In general, what are the GR values for SS, LM, DO, AN, Shale
All low, Shale high
Vshale
GRlog-GRmin/ GRmax-GRmin
GR max- max reading on the GR log
GR Min- Min reading on the Gamma Ray log
Clay typing
Potassium and Thorium are the primary elements present in clays.
Minerology
Carbonates usually display a low gamma ray signature
Ash layer detection
Thorium is frequently found in ash layers.
How is a bed boundary always picked?
inflection point
What is the best log to determine bed thickness?
GR
What is the impact of speed on a GR log?
The sonde measures the emitted gamma rays, which is averages over a time defined by a time constant-Tc.
To increase the quality of the log, the Tc must be as large as possible (i.e.. SLOW). Although…this may cause blurriness!
What are the 2 effects caused when the GR tool is logged too fast?
- The tool response time is shifted in the direction the tool is moving, causing a lag.
- The log cannot properly respond when bed thickness is <hc.
What the equation for lag (hc)?
hc=v*tc
v=logging speed (ft/sec)
tc=the time constant (sec)
What is the size of a tool in GR detector
5-10cm diameter
What is the approximate depth of investigation of the GR?
1 foot
What % of the GR signal comes from within 1 foot?
75%, This applies horizontally and vertically
What are the 3 radioactive elements?
Potassium, Thorium, Uranium
The GR tool consists simply of a highly sensitive gamma ray detector in the form for a?
scintillation counter
What is compton scattering?
progressive reduction of energy of gamma particles after emission due to collision with other atoms in the rock
Natural Gamma rays measure?
number of natural gamma rays emitted by the rock surrounding the tool
Spectral Gamma ray logs measure?
able to break up the total gamma ray response into 3 components and clearly distinguish and differentiate the different clay mineral types in the formation.
What are the 5 factors that govern GR response?
- Radiation intensity
- Logging tool efficiency
- Time constant
- Logging Speed
- Borehole environment
What are the 5 factors that effect borehole quality?
- eccentricity
- Caliper
- Depth
- Temperature
- Cable tension
Which mud is a problem for GR tools and why?
Barite muds since they are good at absorbing GR.
The denser the mud, the ________ the Compton scattering
GREATER
