Log Analysis

Question 1

What to wireline log measure and how are they run?

Answer 1

Measure rock properties

Run down borehole on cable or attached to drilling assembly to provide real time measurement whilst drilling (MWD/LWD)

Question 2

Where are logging tool located?

Answer 2

Lowered on wireline cable attached to tubing

can be incorporated in the drill string while drilling

Question 3

Can tools take measurements through casing?

Answer 3

Yes but most need open hole

Question 4

What is done to ensure that tool have been unaffected by trip down hole?

Answer 4

Pre and post logging calibrations

Question 5

Why are repeat sections recorded?

Answer 5

Quality control, checks tools working consistently

Question 6

When are measurements taken?

Answer 6

When tool is being pulled out hole at a constant speed

Question 7

How are logging instruments protected from downhole pressures and temps?

Answer 7

Mounted in steel sondes

Question 8

How is data acquired from LWD/MWD instruments? Give an advantage of MWD/LWD

Answer 8

• Instruments mounted in drill collars
• Real time measurements transmitted to surface in pulses through mud system
• Memory data recorded in downhole instruments and retrieved at surface
• Measurements recorded at fixed time and resampled at fixed depth
• Measurements collected immediately after drilling are less affected by invasion

Question 9

What are the main electrical log? indicate if passive or active

Answer 9

Active: SP
Passive: Resistivity, Dipmeter

Question 10

What is the main sonic, nuclear magnetic resonance and formation imaging logs? Active or passive?

Answer 10

Sonic: DT - passive
Nuclear: NMR - active
Formation imagine - Formation Micro-scanner: passive

Question 11

In what order should interpretation from logs be done? Describe the steps

Answer 11

• Assess borehole size, flag poor hole conditions
• Define lithology and identify potential reservoirs (GR Log)
• Define porosity (neutron/density and sonic logs)
• Define fluid content (resistivity)

Question 12

What does an undersized and an oversized hole indicated by a calliper log suggest?

Answer 12

Undersized: mudcake build up, porous formation

Oversized: washout or caving

Question 13

How does a gamma ray work? What are the natural radioactivity sources common in rocks? What is natural gamma spectrometry?

Answer 13

• Measures natural radioactivity of rock using scintillation counter
• Clays, feldspars and glauconite (K, Zr, U, Th)
• NGS: measures total formation GR plus proportions of K, Th and U

Question 14

What does a neutron log measure? What does this tell you? Borehole influence?

Answer 14

Response of rocks to emitted neutrons

• Measures rock hydrogen index
• Measures total fluid content as hydrogen mainly in pore fluids
• Tool effected by hole size and borehole conditions

Question 15

When would adjustments be applied in neutron logs?

Answer 15

• In shaley formations as give an apparent higher porosity

- If gas present as gives low porosities due to lower density and has lower hydrogen content

Question 16

How does the RHOB measure the total matrix density and density of pore fluids?

What can calibration with rock densities give you? How can it be used?

When might a spurious porosity reading be given?

Answer 16

• Variations in total density due to variations in amount of matrix available to absorb gamma radiation thus can be porosity tool
• Calibration with known rock densities allows a calibration of this porosity tool, fluid densities used for calibration water/oil are similar, often used in porosity crossplot with neutron tool
• With gas as much less dense