Page 319 - Petrophysics 2E

290 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

TABLE 4.19
MATRIX TRAVEL TIME

vm tma
Formation (ftW (PS/ft)
Sandstone:
Unconsolidated 17,000 or less 58.8 or more
Semiconsolidated 18,000 55.6
Consolidated 19,000 52.6
Limestone 21,000 47.6
Dolomite 23,000 43.5
Shale 6,000 to 16,000 167 to 62.5
Calcite 22,000 45.5
Anhydrite 20,000 50.0
Granite 20,000 50.0
Gypsum 19,000 52.6
Quartz 18,100 55.6
salt 15,000 66.7
Water 5,300 189.0

where the velocity of sand (P wave) in the matrix, Vma, is expressed as
follows:

vma= [ Pm 1 (4.152)
K f0.75G Oa5

where K and G are the bulk and shear moduli, respectively, and pm
is the density of matrix. Table 4.19 shows the velocity and matrix
travel time for various rock types. The presence of shale, fractures, and
gas complicate the sonic porosity measurements. In multiple-porosity
rocks, such as vuggy or fractured carbonates, the travel time is often
shorter than would be calculated for that given porosity. This is
because vugs or fractures are irregularly located and the compressional
sound wave goes through the formation with the least porosity, i.e.,
shortest travel time. The secondary porosity is generally estimated by
subtracting sonic porosity (Equation 4.150) from the neutron or density
porosity (Equation 4.157). In some cases, this may lead to erroneous
results.
Unconsolidated formations, almost always sandstones, tend to exhibit
longer travel times than consolidated formations having the same
porosity. Consequently, the Wyllie et al. correlation gives unacceptabIe
high porosities [49]. In this case, Equation 4.150 is modified to include