Page 138 - Petroleum Geology

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0 0.5
log fi log T”

Fig. 6-2. Two measures of tortuosity plotted against porosity (log-log). Data of Winsauer
et al., 1952, p. 266, table 11. is the tortuosity term derived from eq. 6.5. Tm is the
tortuosity measured electrically. The lines are linear regression lines.

colleagues (who, it is interesting to note, worked for the Humble Oil and
Refining Company). However, it must be clearly understood that none of
these is a precise relationship, as the scatter about the regression lines in Fig.
6-2 shows.
Thus, if the value of the Formation Factor can be measured by saturating
a representative rock sample with an electrolyte of known resistivity and
measuring its resistivity, the resistivity of that rock saturated with a fluid
of another resistivity can be calculated if the resistivity of that fluid is
known. Alternatively, knowing or estimating the Formation Factor and the
true resistivity of the brine-saturated rock, the resistivity (and hence the
salinity) of the pore fluids can be calculated or estimated. An estimate of the
Formation Factor leads to an estimate of the porosity, and vice versa, but
there are better tools for measuring porosity in situ and from these the For-
mation Resistivity Factor can be better estimated.
So far we have considered only clean porous rock saturated with an elec-
trolyte. These relationships do not hold for “dirty” sands, that is, sands with
an appreciable clay content, because wet clay contributes to the electrical con-
ductivity of a rock. The evaluation of dirty sands presents problems that are
best referred to a specialist well-log analyst, or petrophysicist.
Contamination of the formation fluid by petroleum is very much the
petroleum geologist’s business. Oil and gas are non-conductors of electricity.
Petroleum in a permeable rock with intergranular porosity does not displace
all the water that was originally present in the pores. The oil or gas occupies
the central parts of the pores in a water-wet reservoir rock (Fig. 6-3). The
electrical effect is one of reduced porosity and therefore higher resistivity,
but there are two components to this. First, the true area available for cur-
rent flow (At) is small: secondly, the tortuosity is large because the current
flow paths are confined to the peripheral parts of the pore spaces, where the
water is.
There are great theoretical difficulties with the concept of true resistivity