Page 59 - The Geological Interpretation of Well Logs

- RESISTIVITY AND CONDUCTIVITY LOGS -

Table 6.5 Variations in filtrate and formation-water resistivity replace the oil and gas immediately around the borehole,
values, as read by resistivity tools (see aiso Figure 6.9). essentially replacing them through the flushed zone,
while the original saturation in hydrocarbons is only

Formation-water salinity
found in the virgin formation (Figure 6.11,/). A resistivity

profile across a hydrocarbon zone will show a flushed
Fresh Formation- Saline
zone with a moderate to low resistivity, filled with mud
formation water salinity= formation
filtrate (with resistivity depending on mud type) and the
water filtrate salinity water
virgin formation with an extremely high resistivity
Saltwater mud
because of the high saturation in hydrocarbons. Both oil
(usual offshore) R,,<R, R=R, Ri R,
and gas are infinitely resistive and show the same effect
Freshwatermud & 2 R, RR, RR,
on resistivity logs. The resistivity profile then, shows a big
Oil-based mud _— Oil filtrate contamination inctease away from the borehole, the exact reverse of a
(special cases) Only R, induction is usable water zone. (Figure 6.11,/). This increase in resistivity
deeper into the formation, away from the borehole, is
100%
expressed very distinctly on the logs. Shallow looking
residual oil
tools which read in the flushed zone show low (relatively)
resistivity values, while deep reading tools show very
high resistivities (Figure 6.12) The separation between
iveducible_, the curves from the shallow and deep tools, plotted on the
same resistivity scale, is diagnostic of hydrocarbons. It is
water
9° sometimes called the Aydrocarbon separation and is used
> caka in the ‘quick look’ technique for locating oil or gas. A
mud quick look, however, must be verified by calculation since
a % 2 curve separation can be caused by fresh water and many
Rme RESISTIVTY hydrocarbon zones do not give any obvious separation.
NY In practice, the behaviour of fluids in a drill encoun-

tered hydrocarbon zone is not simple. Theoretically there
di
is a differential rate of flushing of formation water and
DISTANCE FROM BOREHOLE
of oil or gas by the mud filtrate. This is supposed to
create a zone where there is a high volume of formation
water with only residual hydrocarbons, the so-called low
100%
RESISTIVITY

MICRO-SPHERICALLY FOCUSED LOG (MSFL)

LATEROLOG SHALLOW ILLS)

LATEROLOG OEEP {LLO)
RS
9 ESS x

mud cake
Rt QQ
ae Ran
Rxo RESISTIVTY

WET
LOW RESISTIVITY
ANNULUS oye R deep (LLd)

dl
DISTANCE FROM BOREHOLE wae & shallow (LL)
@

Figure 6.11 Schematic oil zone resistivity profiles.
1. Simple invasion model. 2. Model with low resistivity
annulus. R,, = anoulus resistivity. For symbols see Table 6.4.
20m
Oil (hydrocarbon) zone resistivity profiles
mixing, a two phase system of miscible fluids. When
All previous examples have assumed 100% water satura-
tion in the porous and permeable bed, the resistivity
variations being due to mud filtrate and formation water
30m
hydrocarbons are present, the system becomes three Figure 6.12 Strong separation of resistivity logs in a gas zone.
phase and much more complex. The mud filtrate will Porosity is around 15%.
49