CPG CPG Formation Evaluation & Petrophysics 2 — Questions and Answers
Question 1: What does a high spontaneous potential (SP) deflection toward the shale baseline in a sandstone interval typically indicate?
- High formation water salinity or shaly sand (Correct answer)
- Overpressured reservoir
- Low porosity limestone
- Oil-bearing zone
Correct answer: High formation water salinity or shaly sand
Reduced SP deflection in a sandstone indicates either high formation water salinity (reducing electrochemical contrast) or the presence of clay reducing permeability.
Question 2: The Klinkenberg effect in gas permeability measurements refers to which phenomenon?
- Gas slippage at pore walls causes apparent permeability to exceed liquid permeability (Correct answer)
- Gas expansion reduces core porosity measurements
- Capillary pressure affects gas migration
- Gas compressibility alters flow rates
Correct answer: Gas slippage at pore walls causes apparent permeability to exceed liquid permeability
The Klinkenberg effect describes how gas molecules slip along pore walls at low pressures, causing measured gas permeability to exceed true (liquid) permeability.
Question 3: Which method is used to estimate formation permeability from wireline logs when no core data is available?
- Timur-Coates (NMR-based) or Wyllie-Rose empirical equations (Correct answer)
- Archie's equation applied to resistivity logs
- Gamma ray cutoff method
- Spontaneous potential baseline shift
Correct answer: Timur-Coates (NMR-based) or Wyllie-Rose empirical equations
Empirical permeability transforms such as Timur-Coates (using NMR porosity and free-fluid index) or Wyllie-Rose equations relate log-derived porosity to permeability.
Question 4: In well log interpretation, what is the primary use of the dual induction log suite (ILd, ILm, SFL)?
- Determining invasion profile to distinguish flushed zone from virgin formation resistivity (Correct answer)
- Measuring formation pressure at multiple depths
- Identifying gas-water contacts
- Calculating shale volume
Correct answer: Determining invasion profile to distinguish flushed zone from virgin formation resistivity
The dual induction suite reads at three depths of investigation, allowing the interpreter to model the mud filtrate invasion profile and determine true formation resistivity (Rt).
Question 5: What is the significance of the 'irreducible water saturation' (Swirr) concept in reservoir evaluation?
- It represents the minimum water saturation that cannot be produced, indicating maximum potential hydrocarbon saturation (Correct answer)
- It marks the depth of the free water level
- It defines the capillary entry pressure
- It measures clay-bound water volume
Correct answer: It represents the minimum water saturation that cannot be produced, indicating maximum potential hydrocarbon saturation
Irreducible water saturation is the water held by capillary forces on grain surfaces and small pores that cannot be displaced, setting the upper bound for moveable hydrocarbon volume.
Question 6: Which log combination is most reliable for identifying dolomitization in a carbonate sequence?
- Neutron-density crossplot combined with the Pe log (Correct answer)
- SP log and gamma ray
- Resistivity and caliper
- Sonic and SP
Correct answer: Neutron-density crossplot combined with the Pe log
Dolomite has a characteristic Pe of ~3.1, density of ~2.87 g/cm³, and crossplot position that distinguishes it from limestone and anhydrite when Pe is combined with neutron-density.
CPG CPG Formation Evaluation & Petrophysics 2
What does a high spontaneous potential (SP) deflection toward the shale baseline in a sandstone interval typically indicate?