Page 21 - The Geological Interpretation of Well Logs

~- THE LOGGING ENVIRONMENT -

mud
rock density, ofem?
2.00 2.40 2.80
i lL

Quaternary
+

Tertiary
average lithostatic
gradiant =2.4g/ em?
Cretaceous

us and “| ‘ - mud
ermosoie | =| fT emit
Jurassic jormation . - | —_ ~

~
Triassic “4 ov “/ !
.
Figure 2.5 Schematic representation of dynamic filtration as a

mud filtrate invasion

Permian
build-up.
0 500 1000 bit enters a porous formation. Note the progressive mud-cake
2
lithostatic pressure, kg/om
eukP dept —_ —
ion
7
Figure 2.4 True rock density profile and average lithostatic hn ot inva sid
gradient from a North German well. (Redrawn from
Meyer-Giirr, 1976). oy
—15,000 psi). Most oilfield Jogging tools are designed to
withstand pressures up to a maximum of 1050-1750 ——— Wore ays
kg/cm? (15,000—20,000 psi), significantly above the high-
increase
est pressure usually encountered.
Invasion-drilling pressures rate of invasion
Under ideal conditions, the pressure exerted by the
column of drilling mud will be such that when a porous
and permeable formation is encountered, as the drill mud cake tnicknesS
enters the formation, mud will be forced into it (Figure
2.5). The porous rock will then begin to act as a filter, 4 10 100 1000 10 000
separating the mud into its siquid and solid constituents. time (min after penetration)
The mud filtrate (the water used to mix the mud) will flow Figure 2.6 Graphic representation (schematic) of invasion
into the formation, while the solids (the mud) will form a and mud-cake build-up as a porous formation is penetrated
(modified from Dewan, 1983).
deposit around the borehole wall once the bit has passed.
In the hole just drilled, the solid deposit around the bore- INVADED ZONE
hole wall, the mud cake, will gradually build up to form a original drilling mud
formation tluids NMirate
skin over the porous interval.
Initially, as the bit enters the porous formation there is
complete disequilibrium and dynamic filtration takes
place (Figure 2.6). That is, below and around the bit
there is a continuous flow of filtrate into the formation,
provided of course that the mud pressure is sufficient.
Gradually, as the mud cake builds up, it creates a barrier
and the movement of fluids diminishes, unti] finally the
mud cake becomes impermeable and filtration practically
ceases (Figure 2.6). A cross-section through the borehole
at this stage would show mud in the hole, mud cake on
the borehole wall and then the porous formation
—~——_——
depth of invasion
now filled almost entirely by mud filtrate. The original
diamaier of invasion
formation fluids have been pushed away from the hole
(Figure 2.7). This is usually the situation when the open- Figure 2.7 Invasion: simple representation of the effect of
hole well togs are run. drilling on fluids in a porous and permeable formation.