Lecture 1 & 2 - Wireline logging Flashcards
In oil exploration, what is the ideal conditions to find?
- Thick layer of reservoir rocks
- High Saturation of HCs
- Reservoir rocks permeable, clean (No shale)
- Capped by shale or salt
What is wireline logging used for?
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- Determining lithology
- Determining pore-fluid type
- Determining saturation
- Calibrate seismic data
How is well drilling done?
By the rotary drilling method
Rotate drill bit attached to drill pipe (31 or 46ft)
Simultaneously apply weights to bit using the drill collar.
What is the purpose of pumping drilling fluid into the drill pipe? What is the drilling fluid made up of?
- Usually oil or water based mud
- Cools and lubricates bit
- Brings cuttings to surface
- Prevents blow-outs
What is a mudcake?
How does it form?
What are the problems of forming a mudcake?
- 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
What physical properties can wireline logging measure?
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- Hole size
- Natural Gamma Ray count
- Porosity
- Formation density
- Travel time
- Spontaneous potential
- Resistivity
What is the compromise between logging speed and resolution?
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
What does hole size indicate about rock type?
- Hole size matches bit size = strong rock
- Hole size smaller = mud cake, permeable rock
- Hole size larger = weak rock (collapse of hole)
What does the GR well logging tool measure?
What are the units?
Natural gamma rays, usually emitted by potassium-40 in rocks.
Units: API
Give average GR values for:
- Clean sands, limestones, dolomites
- Salt and Anhydrite
- Shale
- 10-20 API
- 0-5 API (“a few”)
- Up to 200, average 100 API
What is GR predominantly used for?
To tell where shale rocks are and find shale volume, Vsh
How can you identify shale using resistivity logs?
The logs plot on top of eachother
How can resistivity give information about rock permeability?
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
What is ‘Spontaneous Potential’ a measure of?
Measure of the natural potential difference between an electrode at the surface and an electrode downhole
What are the 2 sources of Spontaneous Potential (SP)
- Membrane Potential
- Due to difference between permeable & impermeable zone ( and the effect of selective passage of ions) - Liquid junction potential
- Due to invaded zone next to an uninvaded zone (transfer of ions)
What is the value of SP that we measure?
The sum of the membrane potential and the liquid junction potential
How is a permeable zone normally indicated on an SP graph?
Negative deflection of the SP curve
How is shale normally indicated on an SP graph?
Positive deflection of the SP curve
What is the deflection of the SP curve dependent on?
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
When is SP unusable?
- When drilling with oil based muds as resistivity is so high
- If upper electrode not properly grounded
What is SP used for?
- Identifying permeable and impermeable zones
2. Determine the HC saturation
How do we measure resistivity?
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
What does resistivity depend upon?
Rock type
Depth of invasion of drilling mud
Fluid type in formation
What equipment is used for
- LLS or LLD surveys
- ILS or ILD surveys
- MSFL
- Laterolog
- EM induction tools (for oil-based drilling fluid)
- Rubber pad pressed against borehole wall with current & potential electrodes
What does resistivity curves plotted apart indicate?
Zone is PERMEABLE
What does overlapping resistivity curved indicate?
Zone IMPERMEABLE e.g. shale
How can you identify HCs using resistivity curves?
High deep resisitivity compared to low shallow resistivity
What are the indicators of permeable zone? (4)
- Low GR
- Presence of mudcake
- Resistivity curve separation
- Negative SP deflection
What are the indicators of impermeable zone? (3)
- High GR
- No mudcake
- Overlapping resistivity curves
How can you tell using resistivity if invasion is deep or shallow?
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
What is measured in a sonic log?
The transit time Δt of p-waves arriving at the upper and lower recorders (average between the two)
A measure of FORMATION VELOCITY
What is a sonic log used for?
To determine formation velocity and porosity
What is cycle skipping in sonic logs?
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
How do you measure formation density?
- 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
What are the variables in the formula that relates bulk density to porosity?
Φ = (ρma - ρb)/(ρma - ρf)
Φ = Porosity ρma = assumed matrix density ρb = bulk density ρf = fluid density
How do you measure neutron porosity?
- 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
How can you identify shale?
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- High GR
- High apparent neutron porosity
- Neutron porosity to the left of density, ρb
- No resistivity curve separation
- Normally positive SP curve deflection
How do you identify clean limestone?
- Low GR
- Neutron & density plot on top of eachother
How to you identify clean sandstone?
- Low GR
- Neutron to the RIGHT of density
How do you identify clean dolomite?
- Low GR
- Neutron to the left of density
Why do we need crossplots?
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)
What is the gas effect and why does it occur?
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
Which logs are porosity indicators?
Sonic, neutron and density logs
What is a sonic cross plot useful for?
Identifying salt and anhydrite:
Salt = low density, high velocity
Anhydrite = high density, high velocity
Sw = √(Ro/Rt)
What does Sw, Ro and Rt represent?
Sw = Water saturation Ro = Resistivity of deep, water-filled zone Rt = Resistivity of deep HC filled zone
How can you measure Rw and hence Sw using the SP log?
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)
