Page 55 - The Geological Interpretation of Well Logs

- RESISTIVITY AND CONDUCTIVITY LOGS -

1000
5.Or

\ platy sand
* 100
$
F =65 (lower porosity
KA
clean sand 1
°
*
———____» a \ o < z ow
-

F = 26 (higher porosity?
clean sand 2
—~ a
oA,

hy 2
~~
E
Sag .
e

Factor - ° T KX \ spheres and shells $ z 1 0.04 6.4 | ™ mary 8 b%g 10%
4:4 mix
u

1.0
Formation \ AX shell (ragments Figure 6.6 The effect of changes in formation-water resistivity
FORMATION WATER RESISTIVITY
4
Apparent ns o \\ on F (formation resistivity factor). F will not change with
different water salinities in a clean formation. In a shaly
VN
formation F will constantly change (schematic).
XS ‘ \
e\, a poorly permeable limestone may have a value around
i ee
300-400. F is dimensionless (Figure 6.4).
S\\ F is an independent element in the expression of rock
SY resistivity. The F value of a particular rock reservoir will
remain constant no matter what the resistivity of the fluid
YY
filling the pores. In other words, F will not vary with
10 ‘ 1 1 1. 2 4 L tt 4 piss changes in formation-water salinity which entail overall

“28 30 40 50 60 70 80 90 100
rock resistivity changes (Figure 6.6). The F value is
Porosity, %
therefore constant between the reservoir containing oil
Figure 6.5 Graph of formation resistivity factor, F and
and the same reservoir containing water. This behaviour
porosity showing their relationship to grain shape (texture)
can be expressed mathematically:
illustrated by analyses of laboratory samples. A predictable
relationship between £ and porosity only exists for one type
of grain population m = cementation factor (re-drawn from
R=FKR,
Jackson et ai., 1978).
predictable (follow Archie’s Jaw, Section 6.7) (Jackson That is, F is the ratio of the formation resistivity to the
et al., 1978). But, when grain shape is changed, the resisvily of the fluid which it contains. Hence when
relationship changes, although still in a predictable way porosity is 100% (impossible) F is 1 (Figure 6.5). This is
(Figure 6.5). F is therefore strongly influenced by grain the basic relationship which is used in all calculations
shape, (theoretically because of changes in pore throat involving the resistivity measured by well logs (Section
geometry). However, in geological terms, grain shape is 6.7).
an element of texture, along with other factors such as
size and arrangement (sorting). Geologically then, F Resistivity (conductivity) of clays
becomes a texturally related term, an aspect which will Discussion thus far suggests that rock resistivity (or
be considered in more detail when the geological applica- conductivity} is only a function of the active part played
tions of resistivity logs are considered (Section 6.8). by a conductive formation water (resistivity R,) and the
For petrophysical purposes, it is necessary to quantify apparently passive part played by the rock skeleton (F).
the relationship between F and porosity (porosity being However, the part played by the rock skeleton is not always
measurable by other logs). Fundamental work by Gus passive. When shale is present it plays an active role in
Archie established an empirical relationship (Archie, conductivity and F is no longer constant (Figure 6.6).
1942), which has been confirmed by subsequent work Clays conduct electricity in two ways, through pore
(e.g. Figure 6.5). However, as indicated above, the water and through the clay itself. The porosity in clay, like
relationship varies with each population of grains and to that in other rocks, encloses conductive formation water.
establish a universally applicable relationship has This may be up to 80% in newly-deposited clays but
proved elusive (e.g., Winsauer and McCardell, 1953; diminishes rapidly through compaction (see ‘Compac-
Maute, 1992). Presently available formulae give only a tion, over-pressure and shale porosity’, Section 6.8 ).
good estimate (see ‘Basic equations of petrophysics’ Conductivity in the clay mass itself is more complex.
Section 6.7). Clay consists of stacked silicate layers which, in the
F is usually between 5 and 500, the higher numbers presence of water, become negatively charged. Clay may
indicating a greater effect due to the formation. Good in fact be considered to act like a salt, dissociating into an
porous sandstones will have an F value around 10, while immobile, negatively-charged framework and positive,
45