Formation Evaluation and Logging Flashcards
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Read the first 7 Formation Evaluation and Logging flashcards as text
Which resistivity tool provides the deepest investigation into the formation?
Answer: Laterolog deep (LLD)
The deep laterolog (LLD) focuses current laterally to investigate several feet into the formation beyond the invaded zone, providing the deepest resistivity reading among standard laterolog devices.
What causes the 'horns' or spike anomalies seen on resistivity logs at bed boundaries?
Answer: Current focusing artifacts when tool straddles two beds of contrasting resistivity
Resistivity horns at bed boundaries result from geometric focusing effects when the tool simultaneously reads two formations with very different resistivities, creating a transient artifact.
In a tight gas sand with very low permeability, which log measurement would most clearly indicate gas presence?
Answer: Density-neutron separation (crossover)
Even in tight gas sands, gas causes the characteristic density-neutron crossover because density reads high apparent porosity while neutron reads anomalously low, regardless of permeability.
The cement bond log (CBL) evaluates:
Answer: Integrity of the cement bond between casing and formation
The CBL measures acoustic attenuation of a signal traveling down the casing; good cement bonding damps the casing signal strongly, while free pipe or poor bonding shows high amplitude.
What does a Repeat Formation Tester (RFT) or Modular Formation Dynamics Tester (MDT) measure directly?
Answer: Formation fluid pressure and mobility (permeability) from pretest drawdown
The RFT/MDT measures static formation pressure and, from the pretest drawdown and buildup, estimates formation fluid mobility (kh/μ), enabling pressure gradient analysis and fluid contact identification.
Which environmental correction is most critical for accurate density log interpretation in large-diameter or washed-out boreholes?
Answer: Mudcake thickness correction (Δρmc)
Mudcake on the borehole wall between the density tool pad and formation significantly affects the density reading; the spine-and-ribs plot uses the short-spaced detector to correct for mudcake thickness and density.
The Pickett plot is a graphical technique used to:
Answer: Simultaneously determine Rw and cementation exponent m from water-bearing zones
The Pickett plot graphs log(Rt) vs. log(porosity) for a suite of formations; water-bearing points align on a trend line whose slope gives m and intercept gives Rw/a.