SPE Formation Evaluation and Logging 2 — Questions and Answers
Question 1: Which neutron log measurement is most directly affected by the presence of gas in the formation?
- Increased neutron porosity reading
- Decreased neutron porosity reading (Correct answer)
- No change in neutron porosity
- Increased bulk density reading
Correct answer: Decreased neutron porosity reading
Gas has very low hydrogen index compared to water or oil, causing the neutron log to read anomalously low porosity in gas-bearing zones.
Question 2: What is the primary purpose of a caliper log in formation evaluation?
- Measuring formation fluid salinity
- Determining borehole diameter and shape (Correct answer)
- Estimating formation permeability
- Measuring formation temperature gradient
Correct answer: Determining borehole diameter and shape
The caliper log measures the diameter of the borehole, identifying washouts, key seats, and zones of good mudcake formation critical for correcting other log measurements.
Question 3: In a density-neutron crossplot, a gas crossover occurs when:
- Neutron porosity exceeds density porosity
- Density porosity exceeds neutron porosity (Correct answer)
- Both logs read identical porosity values
- Both logs read zero porosity
Correct answer: Density porosity exceeds neutron porosity
Gas crossover occurs when density porosity reads higher than neutron porosity due to gas having low density and low hydrogen index simultaneously.
Question 4: The formation factor (F) in Archie's equation relates to which petrophysical property?
- Water saturation
- Cementation and tortuosity of pore space (Correct answer)
- Formation temperature
- Irreducible water saturation
Correct answer: Cementation and tortuosity of pore space
The formation factor F = R0/Rw reflects how the geometry, tortuosity, and cementation of pore channels affect resistivity relative to the pore fluid alone.
Question 5: Which log is most commonly used to identify fractures in a formation?
- Spontaneous Potential (SP) log
- Fullbore Formation MicroImager (FMI) (Correct answer)
- Compensated Neutron Log (CNL)
- Gamma Ray (GR) log
Correct answer: Fullbore Formation MicroImager (FMI)
The FMI (or similar microresistivity imaging logs) produces high-resolution borehole wall images that clearly reveal fracture orientations and apertures.
Question 6: What does a high spontaneous potential (SP) deflection (toward negative millivolts) typically indicate?
- A shale or tight formation
- A permeable, porous sand with fresh formation water (Correct answer)
- A zone with high oil saturation
- An overpressured formation
Correct answer: A permeable, porous sand with fresh formation water
Large negative SP deflections indicate a permeable bed where salinity contrast between mud filtrate and formation water drives a significant electrochemical potential.
Question 7: Invasion of mud filtrate into a formation primarily affects which logging measurements?
- Deep resistivity only
- Shallow and medium resistivity measurements (Correct answer)
- Neutron and density logs exclusively
- Gamma ray and SP logs only
Correct answer: Shallow and medium resistivity measurements
Mud filtrate invasion affects shallow and medium resistivity measurements most because those tools read within the invaded zone, while deep resistivity reads the virgin formation.
SPE Formation Evaluation and Logging 2
Which neutron log measurement is most directly affected by the presence of gas in the formation?