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Log Analysis- Eval 305- Log Analysis Fundamentals > Module 4-GR Logs > Flashcards

Module 4-GR Logs Flashcards

1
Q

What are the ranges for shale, shaly sand and clean sand?

A

Shale- 90-150
Shaly sand- 45 to 90
Clean sand-15 to 45

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2
Q

The clean sand line on the GR ranges between?

A

7 to 45 but typically ranges between 15-30

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3
Q

In general, what are the GR values for SS, LM, DO, AN, Shale

A

All low, Shale high

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4
Q

Vshale

A

GRlog-GRmin/ GRmax-GRmin

GR max- max reading on the GR log
GR Min- Min reading on the Gamma Ray log

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5
Q

Clay typing

A

Potassium and Thorium are the primary elements present in clays.

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6
Q

Minerology

A

Carbonates usually display a low gamma ray signature

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7
Q

Ash layer detection

A

Thorium is frequently found in ash layers.

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8
Q

How is a bed boundary always picked?

A

inflection point

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9
Q

What is the best log to determine bed thickness?

A

GR

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10
Q

What is the impact of speed on a GR log?

A

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!

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11
Q

What are the 2 effects caused when the GR tool is logged too fast?

A
  1. The tool response time is shifted in the direction the tool is moving, causing a lag.
  2. The log cannot properly respond when bed thickness is <hc.
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12
Q

What the equation for lag (hc)?

A

hc=v*tc
v=logging speed (ft/sec)
tc=the time constant (sec)

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13
Q

What is the size of a tool in GR detector

A

5-10cm diameter

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14
Q

What is the approximate depth of investigation of the GR?

A

1 foot

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15
Q

What % of the GR signal comes from within 1 foot?

A

75%, This applies horizontally and vertically

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16
Q

What are the 3 radioactive elements?

A

Potassium, Thorium, Uranium

17
Q

The GR tool consists simply of a highly sensitive gamma ray detector in the form for a?

A

scintillation counter

18
Q

What is compton scattering?

A

progressive reduction of energy of gamma particles after emission due to collision with other atoms in the rock

19
Q

Natural Gamma rays measure?

A

number of natural gamma rays emitted by the rock surrounding the tool

20
Q

Spectral Gamma ray logs measure?

A

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.

21
Q

What are the 5 factors that govern GR response?

A
  1. Radiation intensity
  2. Logging tool efficiency
  3. Time constant
  4. Logging Speed
  5. Borehole environment
22
Q

What are the 5 factors that effect borehole quality?

A
  1. eccentricity
  2. Caliper
  3. Depth
  4. Temperature
  5. Cable tension
23
Q

Which mud is a problem for GR tools and why?

A

Barite muds since they are good at absorbing GR.

24
Q

The denser the mud, the ________ the Compton scattering

A

GREATER

25
Q

Why does the density of the drilling mud effect the GR signal?

A

because they attenuate gamma rays. This is corrected using a borehole correction factor.

26
Q

What does a Caliper log identify?

A

cavities and shows the accuracy of the GR measurement

27
Q

What are the 6 application of a Gamma Ray log?

A 
Lithology
Shale Content
Depth matching
correlations
recognition of radioactive mineral deposits
recognition of depositional environment
28
Q

What is the main application of GR in a regional well correlation?

A

They can be used to correlate between wells with peaks in the thorium abundance (sign of volcanic ashes deposited at the same time).

29
Q

What do caliper logs measure?

A

Size and shape of a recently drilled hole

30
Q

How do Caliper logs work?

A

Mechanical arms record hole size

Hydraulic systems with calibrated potentionmeters

31
Q

How are Caliper logs used?

A

hole size used to correct other logs
Hole volume for cementing
lithological info
Stress field from hole beak-out

32
Q

Most common uses of Caliper logs?

A

lithological assessment
indicator or porosity and permeability due to development of mud cake
Mudcake thickness
borehole volume

Log Analysis- Eval 305- Log Analysis Fundamentals (9 decks)

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