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Advanced Applied Geophysics > Lecture 1 & 2 - Wireline logging > Flashcards

Lecture 1 & 2 - Wireline logging Flashcards

1
Q

In oil exploration, what is the ideal conditions to find?

A
  • Thick layer of reservoir rocks
  • High Saturation of HCs
  • Reservoir rocks permeable, clean (No shale)
  • Capped by shale or salt
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2
Q

What is wireline logging used for?

4

A
  1. Determining lithology
  2. Determining pore-fluid type
  3. Determining saturation
  4. Calibrate seismic data
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3
Q

How is well drilling done?

A

By the rotary drilling method
Rotate drill bit attached to drill pipe (31 or 46ft)
Simultaneously apply weights to bit using the drill collar.

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4
Q

What is the purpose of pumping drilling fluid into the drill pipe? What is the drilling fluid made up of?

A
  • Usually oil or water based mud
  • Cools and lubricates bit
  • Brings cuttings to surface
  • Prevents blow-outs
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5
Q

What is a mudcake?
How does it form?
What are the problems of forming a mudcake?

A
  • Mud flows into the formation if permeable as pressure in hole is greater that the lithostatic pressure (to prevent blowouts)
  • Bigger grains of mud stay on the outside of the borehole to form a mudcake
  • Must take presence of 1/3 - 3/4 inch thick mudcake into account then analyzing physical properties
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6
Q

What physical properties can wireline logging measure?

7

A
  1. Hole size
  2. Natural Gamma Ray count
  3. Porosity
  4. Formation density
  5. Travel time
  6. Spontaneous potential
  7. Resistivity
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7
Q

What is the compromise between logging speed and resolution?

A

Expensive to do. Equipment expensive to rent so want to minimise logging time so reduce costs
Between 1800 and 5400 ft/hr
Faster logging speed = lower resolution

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8
Q

What does hole size indicate about rock type?

A
  • Hole size matches bit size = strong rock
  • Hole size smaller = mud cake, permeable rock
  • Hole size larger = weak rock (collapse of hole)
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9
Q

What does the GR well logging tool measure?

What are the units?

A

Natural gamma rays, usually emitted by potassium-40 in rocks.
Units: API

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10
Q

Give average GR values for:

  1. Clean sands, limestones, dolomites
  2. Salt and Anhydrite
  3. Shale
A
  1. 10-20 API
  2. 0-5 API (“a few”)
  3. Up to 200, average 100 API
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11
Q

What is GR predominantly used for?

A

To tell where shale rocks are and find shale volume, Vsh

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12
Q

How can you identify shale using resistivity logs?

A

The logs plot on top of eachother

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13
Q

How can resistivity give information about rock permeability?

A

Try to measure resistivity at a range of depths (flushed zone, transition zone & uninvaded zone)
Can use to see how deeply mud filtrate has penetrated which is indicative of permeability

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14
Q

What is ‘Spontaneous Potential’ a measure of?

A

Measure of the natural potential difference between an electrode at the surface and an electrode downhole

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15
Q

What are the 2 sources of Spontaneous Potential (SP)

A
  1. Membrane Potential
    - Due to difference between permeable & impermeable zone ( and the effect of selective passage of ions)
  2. Liquid junction potential
    - Due to invaded zone next to an uninvaded zone (transfer of ions)
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16
Q

What is the value of SP that we measure?

A

The sum of the membrane potential and the liquid junction potential

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17
Q

How is a permeable zone normally indicated on an SP graph?

A

Negative deflection of the SP curve

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18
Q

How is shale normally indicated on an SP graph?

A

Positive deflection of the SP curve

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19
Q

What is the deflection of the SP curve dependent on?

A

The difference between the resistivity of the mud filtrate that has entered the rock formation and the resistivity of water/HCs in the formation

  • If Rmf ~ Runinvaded - no deflection
  • Rmf slightly greater than Runinvaded then small
    deflection
  • Rmf&raquo_space; Runinvaded large deflection
  • Rmf < Runinvaded deflection reversed (rare)

Normally Rmf is ‘fresher’ than Rw in formation
Rfreshwater sand = 80-120
Rsaltwater sand = 2-20

20
Q

When is SP unusable?

A
  • When drilling with oil based muds as resistivity is so high
  • If upper electrode not properly grounded
21
Q

What is SP used for?

A
  1. Identifying permeable and impermeable zones

2. Determine the HC saturation

22
Q

How do we measure resistivity?

A

By passing current into a formation. Value of emitted current is proportional to resistivity
Close to the borehole = MSFL
Intermediate depth = LLS or ILS
Deeper = LLD or ILD

23
Q

What does resistivity depend upon?

A

Rock type
Depth of invasion of drilling mud
Fluid type in formation

24
Q

What equipment is used for

  1. LLS or LLD surveys
  2. ILS or ILD surveys
  3. MSFL
A
  1. Laterolog
  2. EM induction tools (for oil-based drilling fluid)
  3. Rubber pad pressed against borehole wall with current & potential electrodes
25
Q

What does resistivity curves plotted apart indicate?

A

Zone is PERMEABLE

26
Q

What does overlapping resistivity curved indicate?

A

Zone IMPERMEABLE e.g. shale

27
Q

How can you identify HCs using resistivity curves?

A

High deep resisitivity compared to low shallow resistivity

28
Q

What are the indicators of permeable zone? (4)

A
  1. Low GR
  2. Presence of mudcake
  3. Resistivity curve separation
  4. Negative SP deflection
29
Q

What are the indicators of impermeable zone? (3)

A
  1. High GR
  2. No mudcake
  3. Overlapping resistivity curves
30
Q

How can you tell using resistivity if invasion is deep or shallow?

A

If LLS (intermediate) and LLD (deep) resistivity logs are not separated (overlapping) then invasion is shallow and permeability is low.

If they ARE separated then invasion is deep

31
Q

What is measured in a sonic log?

A

The transit time Δt of p-waves arriving at the upper and lower recorders (average between the two)

A measure of FORMATION VELOCITY

32
Q

What is a sonic log used for?

A

To determine formation velocity and porosity

33
Q

What is cycle skipping in sonic logs?

A

When the 1st arrival p-wave is too weak to e detected so get large values in Δt.

Can be due to caved in hole

34
Q

How do you measure formation density?

A
  • A radioactive source emits gamma rays
  • When the gamma rays collide with electrons in the
    formation they are scattered and lose energy
  • The proportion of gamma rays that reach the detectors is related to electron density
  • Electron density is related to bulk density
35
Q

What are the variables in the formula that relates bulk density to porosity?

Φ = (ρma - ρb)/(ρma - ρf)

A 
Φ = Porosity
ρma = assumed matrix density
ρb = bulk density
ρf = fluid density
36
Q

How do you measure neutron porosity?

A
  • Using a radioactive source emitting neutrons
  • Detect no’ neutrons arriving at the detector
  • Greatest energy loss occurs when neutrons strike a H nucleus
  • It is therefore indicative of porosity and fluid content
  • As fluids have lots of H
37
Q

How can you identify shale?

5

A
  • High GR
  • High apparent neutron porosity
  • Neutron porosity to the left of density, ρb
  • No resistivity curve separation
  • Normally positive SP curve deflection
38
Q

How do you identify clean limestone?

A
  • Low GR

- Neutron & density plot on top of eachother

39
Q

How to you identify clean sandstone?

A
  • Low GR

- Neutron to the RIGHT of density

40
Q

How do you identify clean dolomite?

A
  • Low GR

- Neutron to the left of density

41
Q

Why do we need crossplots?

A

Neutron and density plots scaled to be overlapping for clean limestone and gives its real porosity.

Must use cross-plots to determine the real porosity of sandstone and dolomite (and others)

42
Q

What is the gas effect and why does it occur?

A

Gas gives low porosity on neutron log and high porosity on density log - the two are widely separated

Neutron plots to the right of density

Because gas has less H than water and oil

43
Q

Which logs are porosity indicators?

A

Sonic, neutron and density logs

44
Q

What is a sonic cross plot useful for?

A

Identifying salt and anhydrite:
Salt = low density, high velocity
Anhydrite = high density, high velocity

45
Q

Sw = √(Ro/Rt)

What does Sw, Ro and Rt represent?

A 
Sw = Water saturation
Ro = Resistivity of deep, water-filled zone
Rt = Resistivity of deep HC filled zone
46
Q

How can you measure Rw and hence Sw using the SP log?

A 
Measure static SP (SSP)
SSP = SP value at shale line - SP value at clean line
SSP = -Klog(aw/amf)
aw = activity of water
amf = activity of mud filtrate
aw/amf ~ Rmf/Rw
Hence SSP = -Klog(Rmf/Rw)
Calculate Rw and sub into Sw = √(Ro/Rt)

Advanced Applied Geophysics (4 decks)

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