SPE - Society of Petroleum Engineers Reservoir Characterization Questions and Answers 1 — Questions and Answers
Question 1: An operator is characterizing a newly discovered clastic reservoir. Routine core analysis (RCA) has been performed on several conventional cores. Which of the following sets of properties is MOST directly determined from RCA?
- Capillary pressure and relative permeability
- Wettability and irreducible water saturation
- Porosity, permeability, and grain density (Correct answer)
- Formation resistivity factor and cementation exponent
Correct answer: Porosity, permeability, and grain density
Routine Core Analysis (RCA) directly measures fundamental rock properties like porosity (the storage capacity), permeability (the flow capacity), and grain density. Properties like capillary pressure, relative permeability, and wettability are determined through more complex and time-consuming Special Core Analysis (SCAL) procedures. Formation resistivity factor and cementation exponent are derived from electrical property measurements, which are also part of SCAL.
Question 2: In well log interpretation for reservoir characterization, a 'crossover' effect where the neutron porosity curve reads lower than the density porosity curve in a clean, liquid-filled formation is a classic indicator of what?
- A gas-bearing zone (Correct answer)
- A high-pressure zone
- A fractured carbonate
- A shale or clay-rich interval
Correct answer: A gas-bearing zone
This phenomenon, known as 'gas crossover,' occurs because the density log responds to the low density of gas, causing it to indicate higher porosity. Conversely, the neutron log responds to the low hydrogen index of gas (compared to oil or water), making it read a lower porosity. The separation, or crossover, between the two curves is a strong qualitative indicator of a gas zone.
Question 3: A reservoir engineer is building a static geological model and needs to distribute petrophysical properties like porosity between wells. Which of the following techniques is specifically designed to account for spatial correlation and directionality of reservoir properties?
- Monte Carlo simulation
- Inverse distance weighting
- Linear regression
- Geostatistics (e.g., Kriging) (Correct answer)
Correct answer: Geostatistics (e.g., Kriging)
Geostatistics, through methods like variogram analysis and Kriging, is specifically designed to model the spatial continuity and anisotropy (directionality) of reservoir properties. It honors the data at well locations while providing statistically robust estimations for inter-well regions. Inverse distance weighting is a simpler interpolation method that doesn't account for spatial structure, Monte Carlo simulation is for uncertainty analysis, and linear regression finds relationships between variables but not their spatial distribution.
Question 4: During the analysis of a 3D seismic volume, an interpreter identifies a high-amplitude anomaly that conforms to the structural closure of a potential reservoir. This feature is often referred to as a 'bright spot'. What is the most likely cause of this bright spot?
- A dense, low-porosity carbonate stringer
- The presence of low-impedance gas-charged sand (Correct answer)
- A significant increase in water saturation
- A facies change to a very dense shale
Correct answer: The presence of low-impedance gas-charged sand
A 'bright spot' is a high-amplitude seismic reflection that often indicates the presence of hydrocarbons, particularly gas. Gas in the pore space significantly lowers the acoustic impedance (the product of density and velocity) of the reservoir rock compared to the surrounding water-wet or shale formations. This large contrast in acoustic impedance creates a strong reflection, appearing as a bright spot on seismic data.
Question 5: An asset team is trying to determine the oil-water contact (OWC) in a reservoir compartment. They have pressure data from a Repeat Formation Tester (RFT) tool run in a new well. How is the OWC identified from this pressure data?
- By identifying the depth with the lowest recorded pressure
- At the point where the pressure gradient changes from the oil gradient to the water gradient (Correct answer)
- By calculating the average pressure of the entire hydrocarbon column
- At the depth corresponding to the highest observed mud filtrate invasion
Correct answer: At the point where the pressure gradient changes from the oil gradient to the water gradient
Pressure-versus-depth plots are a robust method for identifying fluid contacts. Different fluids have distinct density-driven pressure gradients. As the RFT tool moves down the wellbore, it will record a certain pressure gradient in the oil column. When it crosses into the water leg, the denser water will cause a steeper pressure gradient. The intersection of these two distinct gradient lines indicates the Free Water Level (FWL), which is directly related to the OWC.
Question 6: Which of the following data integration approaches is most critical for reducing uncertainty in a reservoir model away from well control?
- Correlating Special Core Analysis (SCAL) data with Routine Core Analysis (RCA) data
- Calibrating well log-derived porosity with core porosity
- Integrating 3D seismic attribute data with well log petrophysical properties (Correct answer)
- Analyzing historical production data from analog fields
Correct answer: Integrating 3D seismic attribute data with well log petrophysical properties
While all options are valuable, integrating 3D seismic data is the most critical for characterizing the reservoir between wells. Seismic data provides continuous 3D information about the reservoir's structure and properties. By calibrating seismic attributes (like acoustic impedance) to petrophysical properties measured at the wells (like porosity), it's possible to create a guided model that predicts property distributions throughout the entire reservoir volume, significantly reducing inter-well uncertainty.
SPE - Society of Petroleum Engineers Reservoir Characterization Questions and Answers 1
An operator is characterizing a newly discovered clastic reservoir.
Routine core analysis (RCA) has been performed on several conventional cores.
Which of the following sets of properties is MOST directly determined from RCA?