Page 250 - Petrophysics 2E

FORMATION RESISTIVZTY FACTOR 223

the core is 85 ohms. The core is 3 in. in diameter and 12 in. in length.
Calculate its porosity.

SOLUTION

The resistivity of this core is (from Equation 4.3):

R,, = r,- A = 85 n (3/212 = 50ohm-in.
L 12
or
50
& = - 1.27 ohm-m
=
39.4
The resistivity of the brine having a salinity of 34,000ppm and
temperature of 120"F, is obtained from Figure 4.4:

R, = 0.12 ohm-m

The formation resistivity factor FR is equal to (Equation 4.5):

Using the Humble formula, Equation 4.40, the porosity of the core sample
is equal to:

0.5

The cementation exponent m is affected by a large number of
factors, including: shape, sorting and packing of the particulate system,
pore configuration and size, constrictions existing in a porous system,
tortuosity, type of pore system (intergranular, intercrystalline, vuggy,
fractured), compaction due to overburden pressure, presence of clay
minerals, and reservoir temperature. The main effect of these parameters
is to modify the formation resistivity factor FR. Consequently, their
combination can produce a countless number of values of FR and m for
a given porosity. For instance, compaction due to overburden pressure
generally causes a considerable increase in resistivity, especially in poorly
cemented rocks and in low-porosity rocks. Figure 4.13 illustrates the
effect of overburden pressure on the formation resistivity factor FR on
core samples from a reef-type limestone. The cementation factor m