Page 580 - Petrophysics

CHARACTERIZING NATURAL FRACTURES 547

(2) Current properties of the fracture

(a) The fracture permeability is calculated from Eq. 8.85:

k2 78.62
kf=-=-- - 41,216 mD
km 0.15

The fracture permeability at initial reservoir pressure is:

-
-
852
kfi = - = - 48,166 mD
k2
k, 0.15
(b) The fracture porosity is obtained from Eq. 8.76.
In fractured reservoirs with deformable fractures, the fracture
compressibility changes with declining pressure. The fracture
compressibility can be estimated from the following expression [ 221 :

1 - (kf/kfi)''3
ctf =
AP
1 - (41,216/48,166)'/3
Ctf = = 10.12 x psi-'
500

The compressibility ratio is:

ctf
-- - 10.12 10-5 = 24.4
Ctm 4.15 x

Thus, the fracture compressibility is more than 24 folds higher than
the matrix compressibility, or ctf = 24.4ctm. The fracture porosity
is:

- 0.0019 = 0.19%

The total porosity of this naturally fractured reservoir is:

Cpt = (Pm + @f = 0.14 + 0.0019 = 0.1419