Page 22 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -
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The phenomenon of the replacement of formation
fluids by drilling mud filtrate is called invasion. Invasion
affects porous and permeable formations in the immediate
vicinity of a borehole. It is described by ‘depth’ or
‘diameter’ of invasion, that is the distance reached by the
invading filtrate with respect to the borehole (Figure 2.7). km
In general, invaston is small in very porous and depth, typical olffiald
permeable formations, the mud cake building up rapidly gradieate

to block dynamic filtration (Table 2.1). The contrary is
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the case in poorly permeable zones, vuggy carbonates or
fractured formations, where mud cake formation is slow
and invasion may be very deep, up to several metres.

LANA
Since excessive tnvasion is the worst -situation for

logging and takes the real formation fluids too far away
6 100 200 300 400
from the borehole to be detected, chemicals are added to temperature, °C
the drilling mud to reduce water joss creating a protective Figure 2.8 Graph of geothermal gradients. The zone of typical
mud cake as quickly as possible. Products such as ligno- oilfield gradients is indicated.
sulphonates and starch are used.
and mud is circulating, the formation is cooled slightly
Table 2.1 Depth of invasion (distance from borehole wall) vs. and the mud heated. But, the mud remains undisturbed in
porosity (approximate) from Miesch and Albright, 1967). the borehole when circulation ceases and it gradually
heats up to reach, or at least approach, the temperature of

the surrounding formation. The two, however, are rarely
Hole
size (in) 174 124% 84 in equilibrium. Logging temperatures taken in the mud

are usually measured after only 5-[0 hours of mud
Ratio
immobility: equilibrium is probably approached only
Porosity Depth of invasion invasion diameter:
Go hole diameter after 5-10 days! (Temperature is considered at greater

length in Chapter 3.)
1-8 200.0cm 140.0cm 97.0cem 10
Typical borehole tools are generally designed to

withstand temperatures up to around 200°C (400°F): this
8-20 |90.0cem 62.0em 43.0cm 5
gives a guide to maxima expected during drilling.

2.4 Logging tool capabilities
20-36 | 22.5cm 15.5em 1).0cem 2

30+ 7=3.0em 2=2.0cem <= 1.7em <2

It was suggested earlier that logging tools should be able
to sense the undisturbed formation and to make a true
2.3 Temperature environment
measurement of it. As indicated, the undisturbed
of borehole logging
formation environment is forced away from the borehole
Formation temperatures by drilling, to be replaced by the invaded zone, Logging
Normal sedimentary basins show a more or less regular tools are therefore designed either to ‘by-pass’ the
increase in temperature with depth (Figure 2.8). The invaded zone to reach the undisturbed formation, or to
increase is not linear as frequently depicted; it varies deliberately measure just the invaded zone itself. That is,
according to lithology depending principally on the they are designed with various capabilities of penetration,
latter’s thermal conductivity (see Figure 3.1). However, called the ‘depth of investigation’ (see below). Inevitably,
despite the irregularities there is an overall, persistent such demands on tool design create secondary effects.
increase in temperature with depth (Figure 2.8). This Logging is comparable to photography with its close-up
increase 1s often expressed as a gradient, the geothermal lenses and long-distance lenses. Close-up logging tools
gradient (the increase in temperature with depth). The give great resolution but little depth of investigation: long-
metric values are usually °C per 100 m or °C per km. distance logging tools give great depth of investigation but
Typical gradients for sedimentary basins are between blurred resolution.
20°C per km and 35°C per km (see Chapter 3 and Table Three inter-related phenomena of logging and logging
3.2). tools are examined below; depth of investigation,
minimum bed resolution and bed boundary definition.
Temperatures in boreholes
Geometry of investigation will also be briefly considered.
Just as the geopressure regime is disturbed by drilling,
sa is the subsurface temperature. A weil drilled into a Depth of investigation
subsurface formation introduces relatively cold mud and Most geophysical fogs have an extremely shallow depth
mud filtrate into a hot formation. While drilling continues of investigation. By ‘depth of investigation’ we mean the

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