SPE - Society of Petroleum Engineers Formation Evaluation and Logging Questions and Answers 1 — Questions and Answers
Question 1: A well is drilled with fresh mud (Rmf > Rw) into a sequence of formations. The Spontaneous Potential (SP) log shows a strong negative deflection against a thick, permeable sandstone formation. What is the most likely interpretation of this response?
- The formation is likely a clean, permeable sandstone containing saline formation water. (Correct answer)
- The formation is a low-permeability shale.
- The formation contains hydrocarbons, causing the negative deflection.
- The mud filtrate and formation water have nearly identical salinities.
Correct answer: The formation is likely a clean, permeable sandstone containing saline formation water.
The SP log measures the natural potential difference between the borehole fluid and the formation water. A negative deflection occurs when the formation water is significantly more saline (less resistive) than the mud filtrate. This potential is generated at the boundary between the permeable bed and the adjacent shales, indicating a clean, permeable formation.
Question 2: While analyzing a suite of logs from a carbonate reservoir, you observe a zone where the neutron porosity log reads significantly higher than the density porosity log, both on a limestone matrix scale. The gamma ray log shows a very low reading in this interval. What is the most probable lithology?
- Sandstone
- Limestone
- Dolomite (Correct answer)
- Anhydrite
Correct answer: Dolomite
In a clean (low gamma ray) formation, when both neutron and density logs are scaled for a limestone matrix, a crossover where neutron porosity is higher than density porosity is characteristic of dolomite. This is because dolomite's grain density (approx. 2.87 g/cc) is higher than calcite's (2.71 g/cc), making the density log read a lower porosity. Conversely, the neutron log sees a higher apparent porosity in dolomite compared to limestone.
Question 3: Archie's Law is fundamental for calculating water saturation (Sw). Which of the following parameters is NOT a direct component of the basic Archie equation (Sw^n = (a * Rw) / (Φ^m * Rt))?
- Formation Temperature (Correct answer)
- Formation Water Resistivity (Rw)
- Porosity (Φ)
- True Formation Resistivity (Rt)
Correct answer: Formation Temperature
The basic Archie's equation relates water saturation (Sw) to porosity (Φ), formation water resistivity (Rw), and the true resistivity of the formation (Rt), using the tortuosity factor (a), porosity exponent (m), and saturation exponent (n). While formation temperature is crucial for correctly determining the value of Rw, it is not an explicit variable within the Archie formula itself.
Question 4: A log analyst is evaluating a potential pay zone and needs to determine the lithology with high confidence, especially in a zone suspected to contain gas. Which logging measurement is least affected by the presence of gas and provides a strong indication of mineralogy?
- Neutron Porosity
- Density Porosity
- Sonic (Acoustic) Porosity
- Photoelectric Factor (Pe) (Correct answer)
Correct answer: Photoelectric Factor (Pe)
The Photoelectric Factor (Pe) is a measurement of the absorption of low-energy gamma rays and is primarily dependent on the average atomic number of the formation matrix. This makes it an excellent lithology indicator. Unlike density and neutron logs, which are significantly affected by the low density and low hydrogen index of gas (the 'gas effect'), the Pe measurement is relatively insensitive to the type of fluid in the pore space.
Question 5: In a well drilled with water-based mud, a resistivity log suite shows three curves: a shallow (SFL), a medium (ILM), and a deep (ILD) reading. In a permeable, hydrocarbon-bearing zone, what is the typical separation pattern observed among these curves?
- ILD > ILM > SFL
- SFL > ILM > ILD
- ILD = ILM = SFL
- SFL < ILM < ILD (Correct answer)
Correct answer: SFL < ILM < ILD
In a hydrocarbon-bearing zone drilled with water-based mud, the mud filtrate invades the near-wellbore area, displacing some of the resistive hydrocarbons. The shallow reading tool (SFL) sees this invaded zone with conductive mud filtrate, resulting in the lowest resistivity. The medium (ILM) and deep (ILD) tools investigate progressively further from the borehole, seeing more of the native, resistive hydrocarbon-saturated formation. Therefore, the resistivity increases with the depth of investigation: SFL < ILM < ILD.
Question 6: The bulk density (ρb) measured by a density log is a function of the matrix density (ρma), fluid density (ρf), and porosity (Φ). If a clean sandstone formation (ρma = 2.65 g/cm³) is completely saturated with fresh water (ρf = 1.00 g/cm³) and the density log reads a bulk density of 2.32 g/cm³, what is the calculated porosity?
- 15%
- 25%
- 20% (Correct answer)
- 33%
Correct answer: 20%
Porosity can be calculated from the density log using the formula: Φ = (ρma - ρb) / (ρma - ρf). Plugging in the given values: Φ = (2.65 - 2.32) / (2.65 - 1.00) = 0.33 / 1.65 = 0.20. Therefore, the porosity is 20%.
