Page 48 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -

Water resistivity (R,), quick look and catculations Quantitatively, the SP is used to calculate formation-
Qualitatively, the greater the SP deflection, the greater water resistivity using the relationship between resistivity
the salinity contrast between the mud filtrate and the and ionic activity. lonic activity is the major contributing
formation water. A rapid look at the SP over a certain factor to the electrochemical SP, as explained previously.
series of beds in a sand-shale sequence will show water- There is a direct relationship between ionic activity and
salinity changes. Generally these will be negative the resistivity of a solution, at least for the most fre-
deflections, the formation waters being more saline than quently-encountered values in logging (Gondouin e7 ai.,
the mud filtrate. Deftections to positive values however, 1957) (Figure 5.11).
occur with fresh formation waters, or at jeast those This rejationship allows a mathematical expression of
fresher than the mud filtrate (Figure 5.10). Typically, a the amplitude of the SP deflection to be expressed in
positive SP deflection is much less marked than a nega- terms of formation-water resistivity in the following way.
tive one: the positive potential difference is much smaller
(Taherian et ai., 1992). .

SP LOG
— 4+ lithology fluids

£ | Fluids

® 3 @ oil
x @ water
_
=
SPA : a
£
__-J--§ < Ry c
4600m Ring
2
S
S
5
SSP2 S.P-° - ¢ mud
+ £&
3 3

—FCL

3
bentonite
2

Ring 0-70, =
4700m 25°C

SSP3 > Ray Rig

s.pr*

J
1800m

Figure 5.10 Behaviour of the SP in a sand shale sequence with varying formation-water salinity. A zone of fresh formation water
occurs between about 1680m and 1775m. Mud filtrate resistivity is constant.
38