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
Why does the density of the drilling mud effect the GR signal?
because they attenuate gamma rays. This is corrected using a borehole correction factor.
What does a Caliper log identify?
cavities and shows the accuracy of the GR measurement
What are the 6 application of a Gamma Ray log?
Lithology Shale Content Depth matching correlations recognition of radioactive mineral deposits recognition of depositional environment
What is the main application of GR in a regional well correlation?
They can be used to correlate between wells with peaks in the thorium abundance (sign of volcanic ashes deposited at the same time).
What do caliper logs measure?
Size and shape of a recently drilled hole
How do Caliper logs work?
Mechanical arms record hole size
Hydraulic systems with calibrated potentionmeters
How are Caliper logs used?
hole size used to correct other logs
Hole volume for cementing
lithological info
Stress field from hole beak-out
Most common uses of Caliper logs?
lithological assessment
indicator or porosity and permeability due to development of mud cake
Mudcake thickness
borehole volume
