Page 545 - Petrophysics

5 12 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

Equation 8.16 (R2 = 0.989) is applicable for a distance range of 250 to
5,000 ft and an FII range of 7 to 25%, and Equation 8.17 (R2 = 0.998)
is applicable for a distance range of 250 to 1,250 ft and an FII range of
7 to 25%. These correlations were developed from well data obtained
by Pirson near the Luling-Mexia fault in the Austin chalk. The primary
application of these two correlations is in the exploration stage and when
the presence of a nearby fault is known a priori from seismic data, as
they provide only an order of magnitude of the distance to the fault. It
is important to emphasize that (a) FFI is influenced by several factors,
including the number of fractures and fracture geometry, and (b) not all
natural fractures are the result of faulting.
The following equations can be used to estimate fracture width and
fracture permeability in a type 1 naturally fractured reservoir:

0.064
Wf = - - Siw)FII]'.315 (8.19a)
[(l
@t
kf = 1.5 x 107@t [(I - S,i)FII]2.63 (8.19b)

where porosity, FII, and irreducible water saturation are expressed as
fractions, and fracture width and fracture permeability in cm and mD,
respectively. The fracture porosity can be directly estimated using the
following empirical correlation [ 221 :

Of = [Rmf (- 1 - -)] a
l
(8.20)
RLLS RLLD

where the range of the coefficient CT is between 2/3 (typical for Type-1
fractured reservoir) and 3/4. Rmf, RLLS and RL~ are, respectively, the
mudfiltrate, laterolog shallow, and laterolog deep resistivities in ohm-m.
are equivalent to R,, and Rt, respectively.
RLLS and R L ~

EXAMPLE

Seismic surveys and geological studies have indicated that the well
in the previous example is located in a naturally fractured zone and in
an upthrown layer. Using the given data, calculate the FII and estimate
the distance to the nearest fault, if the resistivity of the invaded zone is
7.5 ohm-m.