Formation Evaluation and Logging Flashcards
7 cards from real SPE practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Formation Evaluation and Logging flashcards as text
Dipmeter and borehole image logs are used to determine formation dip and azimuth. What is the minimum number of independent pad readings needed to compute a unique dip solution?
Answer: Three non-collinear measurement points on the borehole wall
Mathematically, three non-collinear points define a unique plane, so three independent measurements are the minimum needed to solve for dip magnitude and azimuth.
In the Archie saturation exponent (n), what physical phenomenon does a value significantly greater than 2.0 typically indicate?
Answer: Wettability alteration or oil-wet conditions creating discontinuous water paths
High saturation exponents (n > 2) occur when water occupies disconnected, isolated pores in oil-wet formations, forcing current through tortuous paths and inflating apparent water saturation calculations.
What does the term 'Rxo' represent in log analysis?
Answer: Resistivity of the flushed (fully invaded) zone
Rxo is the resistivity of the flushed zone immediately adjacent to the borehole, where original formation fluids have been displaced by mud filtrate, and it's measured by shallow-reading tools like the MSFL.
Which technique is used to determine fluid contacts (oil-water contact, gas-oil contact) from RFT/MDT pressure data?
Answer: Plotting formation pressure vs. depth and finding intersection of fluid gradient lines
Fluid contacts appear on a pressure-depth plot as the intersection of straight lines with slopes proportional to fluid gradients (gas, oil, water) because each fluid exerts a distinct pressure gradient.
In cased-hole logging, pulsed neutron capture (PNC) logs measure:
Answer: Sigma (Σ), the thermal neutron capture cross-section, to evaluate water saturation
PNC logs measure the macroscopic thermal neutron capture cross-section (sigma), which is strongly influenced by chlorine in saline formation water, enabling water saturation evaluation through casing.
The 'tornado chart' in log analysis is used to correct resistivity readings for:
Answer: Invasion profile — simultaneously solving for Rt, Rxo, and invasion diameter
Tornado charts use ratios of shallow-to-deep resistivity measurements to simultaneously determine true formation resistivity (Rt), flushed zone resistivity (Rxo), and the invasion diameter.
Which log is most sensitive to the presence of uranium-bearing minerals and can cause a misleading high gamma ray reading in non-shale formations?
Answer: Standard (total) gamma ray log
The total GR log sums contributions from K, Th, and U; organic-rich carbonates and phosphatic zones can contain uranium that elevates total GR without indicating shale, making spectral GR necessary for discrimination.