SPE Formation Evaluation and Logging 3 — Questions and Answers
Question 1: Which parameter is determined by the Waxman-Smits model that the standard Archie equation does not account for?
- Formation temperature effect on resistivity
- Clay conductivity contribution to measured resistivity (Correct answer)
- Mud filtrate invasion depth
- Borehole rugosity correction
Correct answer: Clay conductivity contribution to measured resistivity
The Waxman-Smits model accounts for the additional electrical conductivity contributed by clay-bound water and exchange cations on clay surfaces, which Archie's equation ignores.
Question 2: What is the main advantage of Logging While Drilling (LWD) over wireline logging?
- Lower tool cost and simpler interpretation
- Measurements taken before significant invasion and borehole deterioration (Correct answer)
- Better vertical resolution in all conditions
- Ability to measure formation pressure directly
Correct answer: Measurements taken before significant invasion and borehole deterioration
LWD acquires data immediately after bit penetration, before substantial mud filtrate invasion alters formation fluid distribution and before borehole wall degradation.
Question 3: A formation with a photoelectric factor (PE) of approximately 4.65 is most likely composed of:
- Quartz sandstone
- Limestone (calcite) (Correct answer)
- Dolomite
- Anhydrite
Correct answer: Limestone (calcite)
Calcite (limestone) has a PE value of approximately 5.08, and dolomite is about 3.14; a PE near 4.65 is characteristic of limestone or a limestone-dominated mixture.
Question 4: In nuclear magnetic resonance (NMR) logging, the T2 cutoff is used to distinguish:
- Oil from gas in the pore space
- Bound (irreducible) water from free (producible) fluids (Correct answer)
- Porosity from matrix volume
- Invaded zone from virgin formation
Correct answer: Bound (irreducible) water from free (producible) fluids
The T2 cutoff separates short T2 signals (clay-bound and capillary-bound water) from longer T2 signals representing moveable fluids like free water and hydrocarbons.
Question 5: The dual water model for shaly sands treats clay-bound water as having:
- The same salinity as formation water
- Zero salinity (effectively fresh) (Correct answer)
- The same salinity as seawater
- Higher salinity than formation water
Correct answer: Zero salinity (effectively fresh)
The dual water model treats clay-bound water as essentially fresh (zero salinity) because cations balance the charge, making it behave like distilled water electrically.
Question 6: Which log combination best differentiates dolomite from limestone in a carbonate section?
- Gamma ray and SP logs
- Neutron-density crossplot and PE curve (Correct answer)
- Deep and shallow resistivity curves
- Caliper and temperature logs
Correct answer: Neutron-density crossplot and PE curve
The neutron-density crossplot positions and PE values clearly distinguish dolomite (PE ≈ 3.14) from limestone (PE ≈ 5.08) and can resolve mixed mineralogies.
Question 7: Formation water resistivity (Rw) can be estimated from logs using the SP log equation when the mud filtrate resistivity (Rmf) is known and the formation is:
- Gas-bearing and at irreducible water saturation
- 100% water-saturated and permeable (Correct answer)
- Cemented and tight with low porosity
- Invaded to a great depth
Correct answer: 100% water-saturated and permeable
The SP-derived Rw estimate requires a clean, water-saturated permeable bed so the SP deflection reflects only the electrochemical potential driven by the Rmf/Rw salinity contrast.
SPE Formation Evaluation and Logging 3
Which parameter is determined by the Waxman-Smits model that the standard Archie equation does not account for?