Page 60 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -

MSFL
® OIL ZONE

LLs 100%
LLd
ohm m2/m
1.0 10 100 1000 OIL
om pA et tt) go ll 44 at)
SRO
shale Me
ae 35%

10m 4." ph $

ma: “| K=600 md

“3 ok ee ee se er we sr a a ee ee eK

““
200m $ 35% 2 TRANSITION ZONE
K= 6000 md =, Seat
30m “>
OIL
K= 700 md
40m $ 34% LL deep
Figure 6.13 Active fluid movement during logging shown by
a comparison between a main run and ‘repeat’ section. MSFL

1, the ‘repeat’ section, was run 1.5 hours before MSFL 2,
the main run. The comparison shows that hydrocarbons are re-

migrating into the flushed zone, measured by the MSFL, over

the extremely permeable section (see text for explanation).
resistivity annulus on the outer fringe of the flushed zone
(Figure 6.11,2). Doubt was always cast on the stability of
such a zone, even if it was created (Threadgold, 1971).
From the data seis now accumulating from LWD
measurements (i.¢. Figure 6.10), it is clear that there is
considerable fluid movement not only during drilling
_ VIRGIN FORMATION, INVADED | FLUSHED ZONE , |
when invasion occurs, but also when drilling ceases. The
—t,

fluid equilibrium which existed before drilling attempts
Figure 6.14 Fluid mixing in a well drilled with oil-based mud.
to re-establish itself, especially in gas filled reservoirs or
7, Oil zone. 2. Transition zone. 3. Water zone. (Modified after
those with very high permeabilities. The example (Figure
Boyeldieu et ai., 1984).
6.13} shows a highly permeable reservoir containing
oil found in an offshore well drilled with a saltwater
mud and logged by a resistivity tool combination of in a water zone while it is the oil filtrate which will be
shallow, medium and deep devices (DLL-MSFL of immiscible (Figure 6.14). Thus, in an oil zone the effects
Schlumberger). A ‘repeat’ run of the tool was made and of invasion will be difficult to identify while high resistiv-
completed 1.5 hours before the same interval was logged ities will be present close to the borehole in water zones.
during the main nun. In that £.5 hours, the re-migration of As will be discussed below, boreholes in which oil-
the hydrocarbons back towards the well was taking place. based muds are used cannot be logged with the standard
This is shown by the increasing flushed zone resistivity resistivity tools: only induction devices are effective. It is
(MSFL) over the central part of the reservoir. The two only recently, with the modern array induction tools (see
deep logs (LLd and LLs) are unchanged as is the shallow Section 6.5, Induction tools) that the invasion behaviour
reading (MSFL) in the upper and lower reservoir zones of oil-based muds can be monitored by logs.
(Figure 6.13). In this case, no doubt, the re-migration of
the hydrocarbons was helped by the huge permeability of 6.4 Resistivity tools
5000 mD in the affected reservoir. However, hydrocarbon
Standard tools
movement after drilling is nicely demonstrated.
The basic circuitry of the resistivity tools was established
Oil-based mud resistivity profiles by Conrad Schlumberger in 1927. He passed a current
Many modern wells are now drilled with oil-based mud. between two electrodes in the earth and measured the
It helps stop water loss, is a good lubricant and often potential drop between two other electrodes. Modern
reduces drilling time considerably. Clearly, the invasion tools are considerably more complex than this, especially
behaviour of an oil filtrate is quite different from a water because emitted currents are ‘focused’ by contiguous
filtrate. The oil filtrate wili mix with the hydrocarbons in guard currents (Figures 6.15, 6.16). Focused currents are
a hygrocarbon zone while the water filtrate will be immis- less prone to borehole effects and can be directed at
cible: the water filtrate will mix with the formation water required areas of the formation.
50