Page 544 - Petrophysics

PETROPHYSICAL PROPERTIES 51 1

Figure 8.12. Frequency of occurrence of natural fractures near major faults [25J.

in naturally fractured formations, where porosity varies over short
distances.

FRACTURE INTENSITY INDEX

Tension stress causes rock failure along major faults, giving rise to
fracture porosity (@f), and fractures of decreasing width (wf) and length
(hf) and frequency of occurrence (FII) away from the fault plane, as
shown in Figure 8.12 [25]. Thus permeability is much more affected
by fracture dimensions than the matrix or total porosity. The curve fit
equations for both upthrown and downthrown blocks are as follows:

Upthrown block:

1
dLU = 4.2 x 1Op2FII2 - 2.43 x lop5 (8.16)

Downthrown block:
1
=
~LD (8.17)
9.44 x 10-3exp(~11) - 9.3 x 10-3
where: dLU = Lateral distance to the fault for upthrown block, ft
dLD = Lateral distance to the fault for downthrown block, ft
FII = fracture intensity index, fraction.

The in-situ value of the fracture intensity index is estimated from:

(8.18)