Page 81 - Basic Well Log Analysis for Geologist

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CHAPTER

POROSITY LOGS

Sonic Log
The sonic log is a porosity log that measures interval Atna = interval transit time of the matrix (Table 6)
transit time (At) of a compressional sound wave traveling Atyg = interval transit time of formation
through one foot of formation. The sonic log device consists Ate i] interval transit time of the fluid in the well bore
of one or more sound transmitters, and two or more (fresh mud = 189; salt mud = 185)
receivers. Modern sonic logs are borehole compensated
The Wyllie et al (1958) formula for calculating sonic
devices (BHC*). These devices greatly reduce the spurious
porosity can be used to determine porosity in consolidated
effects of borehole size variations (Kobesh and Blizard.
sandstones and carbonates with intergranular porosity
1959), as well as errors due to tilt of the sonic tool
(grainstones) or intercrystalline porosity (sucrosic
(Schlumberger, 1972).
dolomites). However, when sonic porosities of carbonates
Interval transit time (At) in microseconds per foot is the
with vuggy or fracture porosity are calculated by the Wyllie
reciprocal of the velocity of a compressional sound wave in
formula, porosity values will be too low. This will happen
feet per second. Interval transit time (At) is recorded in
because the sonic log only records matrix porosity rather
tracks #2 and #3 (example Fig. 26). A sonic derived
than vuggy or fracture secondary porosity. The percentage
porosity curve is sometimes recorded in tracks #2 and #3,
of vuggy or fracture secondary porosity can be calculated by
along with the At curve (Fig. 26). Track #1 normally
subtracting sonic porosity from total porosity. Total porosity
contains a caliper log and a gamma ray log or an SP log
values are obtained from one of the nuclear logs (i.e.
(Fig. 26).
density or neutron). The percentage of secondary porosity,
The interval transit time (At) is dependent upon both
called SPI or secondary porosity index, can be a useful
lithology and porosity. Therefore, a formation’s matrix
mapping parameter in carbonate exploration.
velocity (Table 6) must be known to derive sonic porosity
Where a sonic log is used to determine porosity in
either by chart (Fig. 27) or by the following formula (Wyllie
unconsolidated sands, an empirical compaction factor or Cp
etal, 1958):
should be added to the Wyllie et al (1958) equation:
Ati z Ata
Table 6. Sonic Velocities and Interval Transit Times for Psonic = Aty _ Atona x 1/Cp
Different Matricies. These constants are used in the Sonic
Porosity Formula (after Schlumberger, 1972). Where:
®xonic = SOnic derived porosity
Atna
Atma = interval transit time of the matrix (Table 6)
(psec: ft)
Atj,y = interval transit time of formation
Vina Alina commonly
At, = interval transit time of the fluid in the well bore
(ft/sec) (usec/tt) used
(fresh mud = 189; salt mud = 185)
Sandstone 18.000 to 19.500 55.5 to S1.0 55.5 to $1.0 Cp = compaction factor
Limestone 21,000 to 23,000 47.6 to 43.5 47.6
The compaction factor is obtained from the following
Where: = compaction factor "100
23.000 to 26.000
Dolomite
43.5 1o 38.5
43.5
formula:
50.0
Anhydrite
50.0
20,000
— Aty x C
66.7
Salt
15.000
67.0
CP =
Casing
17.500
(Iron)
57.0
57.0
Cp
Aty, = interval transit time for adjacent shale
C
= aconstant which is normally 1.0 (Hilchie. 1978).
Alog = Ata
Psonic =
Ate — Ata
The interval transit time (At) of a formation is increased
Where:
due to the presence of hydrocarbons (i.e. hydrocarbon
66 Psonic = SONic derived porosity effect). If the effect of hydrocarbons is not corrected, the