Page 190 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Formation Evaluation 159

Table 5-26
Matrix Travel Times
Velocity range At range At commonly At at 10%
Materlal Wsec p sem used p se& poroslty p seCm
Sandstone 18,000-1 9,500 51 .&55.5 55.0 or 51.0 69.0 or 65.0
Limestone 21,000-23,000 43.5-47.6 47.5 61.8
Dolomite 23,000-24,000 41.0-43.5 43.5 58.0
Salt 15,000 66.7 66.7
Anhydrite 20,000 50.0 50.0
Shale 7,000-1 7,000 58.0-142 -
Water 5,300 176-200 189
Steel casing 17,500 57.0 57.0
From References 200 and 215.

porosity Porosity increases travel time. Wyllie and coworkers [208] developed
an equation that relates sonic travel time to porosity:

Atlw - Attm.
@ = Atf -At, (5-1 02)

where At,og = At value read from log, p sec/ft
Atm = matrix velocity at 0% porosity, p sec/ft
Atf = 189-190 p sec/ft (or by experiment)
The Wyllie equation works well in consolidated formations with regular inter-
granular porosity ranging from 5%-20% [209]. If the sand is not consolidated
or compacted, the travel time will be too long, and a compaction correction
factor (C,) must be introduced [208]. The reciprocal of Cp is multiplied by the
porosity from the Wyllie equation:

(5-103)

The compaction correction factor (C ) can be found by dividing the sonic
porosity by the true (known) porosity. I[ can also be found by dividing the travel
time in an adjacent shale by 100:

(5-104)

where C is a correction factor, usually 1.0 [200]. In uncompacted sands, poro-
sities may be too high even after correction if the pores are filled with oil or
gas. Hilchie [ZOO] suggests that if pores are oil-filled, multiply the corrected
porosity by 0.9; if gas-filled, use 0.7 to find corrected porosity.
Raymer, Hunt, and Gardner [210] presented an improved travel-time to
porosity transform that has been adopted by some logging companies. It is based