Page 244 - Petrophysics

FORMATION RESISTIVITY FACTOR 217

where Rtsh is the true resistivity of the shale zone, obtained from any
resistivity log. If the zone is over-pressured, Rwsh is approximately equal
to the Rw of the formation. For normal-pressured zones, the formation
water resistivity at reservoir temperature is obtained from Equation 4.8,
where the water resistivity of the zone at the reference temperature of
75°F is obtained from the following statistical correlation:

Rw75 = 0.0123 -k 3,647-5(ap%) -0.955 (4.24)

The empirical constant ap is approximately equal to 7 for normally
pressured zones and 1 for over-pressured zones. The total solids
concentration in ppm, %, is obtained from Figure 4.4 or:

1.047 [ ~.562-10g(~~~'~')]
C, = a, = 10 (4.25)

The shale-properties method yields acceptable values of R, , primarily in
the U.S. Gulf Coast area.
The formation resistivity factor FR of a reservoir rock is an extremely
valuable tool in the area of formation evaluation. It depends on numerous
parameters, including:

(1) Salinity of connate water.
(2) Formation temperature.
(3) Rock porosity.
(4) Irreducible water saturation.
(5) Amount, distribution, and type of clays.
(6) Amount, distribution, and type of conductive minerals.
(7) Number and type of fractures.
(8) Layering of sand beds.

Many researchers have investigated the effect of these factors on resis-
tivity, and a large number of correlations were published. Only a few,
however, survived the test of usefulness.
CORRELATION BETWEEN FR AND POROSITY

Inasmuch as clean sedimentary rocks conduct electricity by virtue
of the salinity of water contained in their pores, it is natural that the
porosity is an important factor in controlling the flow of electric current.
As a first approximation, one would expect that the current conductance
would be no more than that represented by the fractional porosity, e.g.,
a formation with 20% connate water saturation and 80% oil saturation
would be expected to transmit no more than 20% of the current that