Page 251 - Petrophysics 2E

224 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

1000

ape les
O
LI
%oo
b
a-
0
m
LL
-
C
0
U
m
10
0
U
ctm Varier From
1
0.001 0.01 0.1 1.0
Poroslty ( q5 1, Fraction
Figure 4.13. Effect of overburden pressure on formation factor (courtesy of Core
Laboratories).

increases from 1.99 at zero overburden pressure to 2.23 at an overburden
pressure of 5,000 psi. Nearly all of the values of m in widespread use today
were determined on unconfined core samples. Of course, resistivity
measurements determined under representative overburden pressures
are strongly recommended for improved well log interpretation.

THEORETICAL FORMULA FOR FR

Many attempts have been made to derive a general formula relating
formation resistivity, porosity, and cementation factor. If an electric
current is passed through a block of nonconducting porous rock
saturated with a conducting fluid, only a portion of the pore space
participates in the flow of electric current. Therefore, the total porosity
@ can be divided into two parts such that [9]:

@ = @ch -k @tr (4.41)
where @ch and are, respectively, the flowing porosity associated with
the channels and the porosity associated with the regions of stagnation
(traps) in a porous rock. Apparently, @ch is equivalent to the “effective
porosity” used by Chilingarian and @tr is equivalent to the irreducible fluid
saturation [ 101. Figures 4.14 and 4.15 show that the electrical current
can flow only through the channel indicated by C, whereas no current
can flow through the traps indicated by T. In Figure 4.14 the traps are of