Page 318 - Petrophysics 2E

FORMATION EVALUATION 289

The Rwa technique gives excellent results when drilling fluid (mud)
invasion is not deep, as in the high-porosity formation, or when
low-water-loss mud is used. This technique, however, can yield
pessimistic results when formation is shaly and FR is determined from
the density log,

Porosity
Three logging porosity tools have been developed to determine
porosity, namely, the sonic-acoustic log, the formation density log, and
the neutron porosity log. In addition to porosity, these logs are affected
by other parameters, such as lithology, nature of the pore fluids, and
shaliness. Combinations of these logs are used to determine lithology
and porosity as well as the fracture porosity.
The sonic log measures the interval transit time, At, i.e., the shortest
time required for a compressional sound wave to travel through one foot
of formation parallel to the wellbore. The speed of sound in the formation
depends on the nature of minerals making up the rock, porosity, pore
space fluids, temperature, pressure, and rock texture. Inasmuch as for
any given lithology the zone of investigation of the sonic tool is essentially
in the invaded zone containing mud filtrate, the speed of sound, i.e., the
interval transit time, is primarily a function of porosity.
The velocity of sound in the formation depends on the density and
elastic properties of the medium, such as bulk and shear moduli of
elasticity. It is faster in a hard substance than in a liquid. Hence, if one
considers a rock composed of only solid and liquid, the following ratio
of the transit times can be used to obtain porosity 1491 :

(4.150)

where: t = total transit time, ps/ft.
tma = matrix travel time, p~/ft.
tfl = fluid travel time, ps/ft.

Equation 4.150 is commonly used for determining the approximate
value of porosity of clean consolidated sandstones as well as that of
carbonate formations with intergranular porosity. The fluid travel time is
approximately 190 ps/ft, whereas the matrix travel time can be obtained
from the following equation:

(4.1 5 1)