Registros geofísicos

Porosity Logs

General
• Type of porosity logs
– Sonic log – Density log – Neutron log

• None of these logs measure porosity directly • The density and neutron logs are nuclear measurements • The sonic log use acoustic measurements • A combination of these logs gives good indications for lithology and more accurate estimates of porosity

Sonic log

General
Sonic
• • A log thatmeasures interval transit time (Δt) of a compressional sound wave travelling through the formation along the axis of the borehole The acoustic pulse from a transmitter s detected at two or more receivers. The time of the first detection of the transmitted pulse at each receiver is processed to produce Δt. The Δt is the transit time of the wave front over one foot of formation and is the reciprocal ofthe velocity Interval transit time is both dependent on lithology and porosity Sonic log is usually displayed in track 2 or 3 Units: μsec/ft, μsec/m Mnemonics: DT, AC Symbol: φ

• • • • • •

General
Sonic • Interpretation goals:
– Porosity – Lithology identification (with Density and/or Neutron) – Synthetic seismograms (with Density) – Formation mechanical properties (with Density) –Detection of abnormal formation pressure – Permeability identification (from waveform) – Cement bond quality

Sonic Porosity
Formula
• From the Sonic log, a sonic derived porosity log (SPHI) may be derived:
– Wyllie Time-average
⎛ Δt − Δt matrix ⎞ ⎟ φ s = ⎜ log ⎟ ⎜ Δ t − Δt ⎝ f matrix ⎠

–

Raymer-Hunt-Gardner
5 ⎛ Δt − Δt matrix ⎞ ⎟ φ s = × ⎜ log ⎟ 8 ⎜ Δt log ⎠ ⎝

–

For unconsolidatedformations
⎛ Δt − Δt matrix ⎞ 1 Δt sh × C ⎟ φ s = ⎜ log ⎟ × Cp , with Cp = 100 ⎜ Δt − Δ t ⎝ f matrix ⎠

• • • •

This requires a formation matrix transit time to be known SPHI Units: percent, fraction Cp = Compaction factor C = constant, normally 1.0

•

Hydrocarbon effects:
– The Dt is increased due to HC therefore: • φ = φs x 0.7 (gas) • φ = φs x 0.9 (oil)

Sonic Porosity
ChartsSonic Porosity
Vma (ft/sec)
Sandstone Limestone Dolomite Anhydrite Salt Freshwater mud filtrate Saltwater mud filtrate Gas Oil Casing (iron) 18 – 19.5 21 – 23 23 20 15 5.28 5.40 1.08 4.35 17.5

Vma (m/s)
5.5 – 5.95 6.4 – 7.0 7.0 6.1 4.575 1.610 1.649 0.33 1.32 5.33

Δtma (μs/ft)
55.5 - 51 47.6 – 43.5 43.5 50 66.7 189 185 920 230 57

Δtma (μs/m)
182 – 167 156 – 143 143 164 219 620 6073018 755 187

Δtma (μs/ft) commonly used
55.5 or 51 47.5 43.5 50 67 189 185 920 230 57

Δtma (μs/m) commonly used
182 or 167 156 143 164 220 620 607 3018 755 187

Sonic
Secondary Effects
• Environmental effects:
– Enlarged borehole, formation fractures, gas in the borehole or formation, or improper centralization can produce signal attenuation resulting in ”cycle skipping” or DT spikesto higher values – Improper centralization, lack of standoff, or excessive logging speed can result in ”road noise”, or DT spikes to either higher or lower values

•

Interpretation effects:
– Lithology: porosity calculated from sonic depends on the choice of matrix transit time, which varies with lithology – Porosity calculations for uncompacted formations may yield porosity values higherthan the actual values when using the Wyllie equation. Use instead the Raymer-Hunt-Gardner equation or correct for decompaction – Porosity calculated in gass bearing zones will be slightly higher than the actual values because the traveltime in gass is higher than in water

Density Log

General
Density
• Gamma rays emitted from a chemical source (Ce137, Co60) interact with electrons of theelements in the formation. Two detectors count the number of returning gamma rays which are related to formation electron density For most earth materials, electron density is related to formation density through a constant Returning gamma rays are measured at two different energy levels
– – High energy gamma rays (Compton scattering) determine bulk density and therefore porosity Low energy...