Page 551 - Petrophysics

5 18 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

approximated by a single pore space system, and consequently the
methods developed in Chapter 3 for clastic rocks can be used in
carbonate formations. In the case where nl is approximately equal to
n2, and since it is impossible to determine nl and n2, an alternative to
the above approach is to take the geometric mean of the two capillary
systems, i.e.:

(8.30)

Using an average value of rcl and rc2, and an average value of 41 and 42,
Equation 8.30 becomes similar to the Kozeny equation.

POROSITY AND PERMEABILITY RELATIONSHIPS IN TYPE 1 NATURALLY
FRACTURED RESERVOIRS

As mentioned earlier in regard to type 1 reservoirs, fractures provide
all the storage capacity and permeability and the fluid flow behavior
is controlled by the fracture properties. The equation for volumetric
flow rate, combined with Darcy’s law, provides the basic approach for
estimating fracture permeability.
Consider a block of naturally fractured rock with n fractures, as shown
in Figure 8.15. Assuming the fractures are rectangular, smooth, and do
not contain any mineral, the Hagen-Poiseiulle equation gives:

nhfwf3 AP
q= ~- (8.3 1 a)
12 pL

unit Area
At

Figure 8.15. Unit model used in calculation of fracture permeability in type 1
naturally fractured reservoirs.