Page 273 - Petrophysics 2E

246 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

quartz (sand). Dispersed clays contain more bound water because
they are subjected only to hydrostatic pore pressure rather than
overburden pressure. In core analysis, much of this bound water
is lost during the drying process, which results in an overestimation
of the porosity of the core sample.
Electrically, the waterdispersed clay mixture or slurry may be
approximated by a single electrolyte of resistivity Rt, expressed
as [23]:

(4.74)

fh is the fraction of total porosity @im occupied by the dispersed-clay
and formation-water mixture, so that the fraction of the effective pore
volume occupied by water (1 -fshd)/fim and the fraction of dispersed
(2)
clays in the mixture fshd/fh are related by the following equation:
Sw = (r)
+
-
=
1
1
fshd
(4.75)
fshd = fraction of the total porosity fim occupied by the dispersed
shale, Le.,

C~D = density log porosity.
Qs = sonic log porosity.
Oim = total or intermatrix porosity of the formation which
consists of all the space occupied by fluids and dispersed
clays.
Rshd = resistivity of the dispersed clays, e 0.4ORsh.
Fh = formation factor related to total porosity.

(4.76)

assuming the saturation exponent n is equal to 2. Combining
Equations 4.74-4.76 and solving for S, gives:

(4.77)