Lecture 1 & 2 - Wireline logging

Question 1

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

Answer 1

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

Question 2

What is wireline logging used for?

4

Answer 2

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

Question 3

How is well drilling done?

Answer 3

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

Question 4

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

Answer 4

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

Question 5

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

Answer 5

• 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

Question 6

What physical properties can wireline logging measure?

7

Answer 6

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

Question 7

What is the compromise between logging speed and resolution?

Answer 7

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

Question 8

What does hole size indicate about rock type?

Answer 8

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

Question 9

What does the GR well logging tool measure?

What are the units?

Answer 9

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

Question 10

Give average GR values for:

1. Clean sands, limestones, dolomites
2. Salt and Anhydrite
3. Shale

Answer 10

1. 10-20 API
2. 0-5 API (“a few”)
3. Up to 200, average 100 API

Question 11

What is GR predominantly used for?

Answer 11

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

Question 12

How can you identify shale using resistivity logs?

Answer 12

The logs plot on top of eachother

Question 13

How can resistivity give information about rock permeability?

Answer 13

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

Question 14

What is ‘Spontaneous Potential’ a measure of?

Answer 14

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

Question 15

What are the 2 sources of Spontaneous Potential (SP)

Answer 15

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)

Question 16

What is the value of SP that we measure?

Answer 16

The sum of the membrane potential and the liquid junction potential

Question 17

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

Answer 17

Negative deflection of the SP curve

Question 18

How is shale normally indicated on an SP graph?

Answer 18

Positive deflection of the SP curve

Question 19

What is the deflection of the SP curve dependent on?

Answer 19

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»_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

Question 20

When is SP unusable?

Answer 20

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

Question 21

What is SP used for?

Answer 21

1. Identifying permeable and impermeable zones

2. Determine the HC saturation

Question 22

How do we measure resistivity?

Answer 22

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

Question 23

What does resistivity depend upon?

Answer 23

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

Question 24

What equipment is used for

1. LLS or LLD surveys
2. ILS or ILD surveys
3. MSFL

Answer 24

1. Laterolog
2. EM induction tools (for oil-based drilling fluid)
3. Rubber pad pressed against borehole wall with current & potential electrodes

Question 25

What does resistivity curves plotted apart indicate?

Answer 25

Zone is PERMEABLE

Question 26

What does overlapping resistivity curved indicate?

Answer 26

Zone IMPERMEABLE e.g. shale

Question 27

How can you identify HCs using resistivity curves?

Answer 27

High deep resisitivity compared to low shallow resistivity

Question 28

What are the indicators of permeable zone? (4)

Answer 28

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

Question 29

What are the indicators of impermeable zone? (3)

Answer 29

1. High GR 2. No mudcake 3. Overlapping resistivity curves

Question 30

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

Answer 30

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

Question 31

What is measured in a sonic log?

Answer 31

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

Question 32

What is a sonic log used for?

Answer 32

To determine formation velocity and porosity

Question 33

What is cycle skipping in sonic logs?

Answer 33

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

Question 34

How do you measure formation density?

Answer 34

- 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

Question 35

What are the variables in the formula that relates bulk density to porosity? Φ = (ρma - ρb)/(ρma - ρf)

Answer 35

``` Φ = Porosity ρma = assumed matrix density ρb = bulk density ρf = fluid density ```

Question 36

How do you measure neutron porosity?

Answer 36

- 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

Question 37

How can you identify shale? | 5

Answer 37

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

Question 38

How do you identify clean limestone?

Answer 38

- Low GR | - Neutron & density plot on top of eachother

Question 39

How to you identify clean sandstone?

Answer 39

- Low GR | - Neutron to the RIGHT of density

Question 40

How do you identify clean dolomite?

Answer 40

- Low GR | - Neutron to the left of density

Question 41

Why do we need crossplots?

Answer 41

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)

Question 42

What is the gas effect and why does it occur?

Answer 42

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

Question 43

Which logs are porosity indicators?

Answer 43

Sonic, neutron and density logs

Question 44

What is a sonic cross plot useful for?

Answer 44

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

Question 45

Sw = √(Ro/Rt) What does Sw, Ro and Rt represent?

Answer 45

``` Sw = Water saturation Ro = Resistivity of deep, water-filled zone Rt = Resistivity of deep HC filled zone ```

Question 46

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

Answer 46

``` 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) ```