Page 162 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -

drilling mud and brought to the surface. They do not washed from the well walls, higher up the hole, so-called
usually ‘float’ in the mud, but are pushed upwards by ‘cavings’. By the time they reach the surface, samples are
friction and drag. At the surface, the drilling mud is therefore considerably mixed and heterogeneous. An
passed through a Jarge metal sieve, the ‘shale shaker’, and experienced rig geologist will usually recognize cavings
the cuttings are recovered. It is from the shale shaker that and eliminate them from his count. The mixture of cut-
the geological cuttings sample is taken. tings, however, from the various lithologies is the reason
On the mud log, lithology is usually recorded in per- why percentages are recorded. All lithological boundaries
centage of cuttings types in a particular sample, say 10% have become very blurred.
sand and 90% brown shale. New samples are taken every When interpreting the cuttings logs, it is the arrival of
2 to 20 metres, depending on the rate of drilling. Fast a new lithology which is significant. During drilling from
drilling rates of perhaps | minute per metre at the top of a thick shale into a thick sandstone, when the bed is
the hole allow only one sample to be taken every 20-25 actually penetrated only a small percentage of the cut-
m drilled (i.e. | sample per 20-25 minutes). At greater tings will be sandstone. This increases, but there will be
depths, rates of 30 minutes to dull 1 metre of formation 80% sandstone only several metres lower (Figure 11.2).
are common, and a sample can be taken every 2 metres The drilling rate however will correlate with major litho-
(i.e. 1 sample per hour) or even closer. logical changes — the so-called ‘drilling break’. Gas levels
To reconstruct the mud log, the time it takes a sample are also likely to change.
to reach the surface after being cut, the dag rime, must be Clearly, the difficulties in interpreting the mud log and
calculated. A sample drilled at 3000 m will take perhaps the need for immediate, accurate information, were
| hour and 10 minutes to reach the surface. It will be behind the requirement for LWD logs, or logs made while
travelling at 43 metres per minute up the hole (about 2.6 drilling. If these are available, the techniques described
km/h), the exact rate depending on the rate at which below for lithology interpretation from logs (Section
the mud is ‘circulated’, that is pumped through the mud 11.4), can be used while drilling continues, rather than
system. The drilling rate is used as the basic curve for after drilling, as is the case with the wireline logs.
the mud jog (Figure 11.1) and is presented as ceal depth.
A metre drilled is instantly recorded, but the cuttings Some pitfalls
sample is ued to the drilling-rate log, so the lithology Certain lithologies are notorious for appearing on wire-
corresponding to the depth 3000m-3002m will only be line logs, but not in the cuttings samples. Such is the case
recovered at the surface 1 hour and 10 minutes after it has for loose sands, silts and soluble evaporites (e.g. salt).
been drilled. The mesh of the sieving ‘shale shakers’ is such that loose
The same principle of lag time is applied to gas read- grains of sand or silt, even coarse sand, pass through the
ings and to shows as to cuttings. mesh. If this is suspected, the mud may be diverted through
On most mud logs the geologist has recorded not only de-sanders, where all small grains are extracted. However,
an analysis of the percentages of the cutting types pre- de-sanding is not routine and there are many cases where
sent, but also what this means in terms of real subsurface unconsolidated sand reservoirs have been drilled and shale
lithology. For instance, if a sand-shale-coal sequence is has been recorded on the mud log! Salt is a very similar
being dniled, ajl these lithologies become ‘smeared’ case. Unless the drilling mud is salt-saturated, no cuttings
while travelling to the surface. In the cuttings sample will will be found on surface. Shale, mostly cavings, will be
be seen 50% shale, 40% sandstone and 10% coal. The recorded. An experienced rig geologist, however, will note
geologist will then make a ‘guess’ at the real lithological roud salinity changes along with drilling-rate indications
column using the drilling rate, knowing that shales will which suggest the presence of salt.
drill slowly, sandstones faster (Figure 11.1). The exact opposite exists where lithologies which
There are occasions when the drilling rate can be seem to appear on the mud log do not actually occur in
compared to a sonic log or a gamma ray log (Figure the formation. A typical cause is the use of ligno-
11.2) and a good interpretation of bed boundaries has sulphonate, a mud additive which reduces water loss. It
been made from it. However, in genera}, the drilling rate resembles lignite and has very often been interpreted as
involves too many variables, such as weight on the bit, such on mud fogs, but this interpretation is suspect if it
bit wear, pump rates and so on, for it to be an accurate implies the presence of coals in pure shale intervals: real
boundary indicator. The mud-log interpretation of coals mostly occur in zones of both sand and shale.
lithology should not be used to interpret boundaries on Despite these various anomalies, the mud log is
the well logs. essential to the lithological interpretation of wireline Logs.
It is important, therefore, to use the cuttings percent- As previously indicated, it represents the only continuous
ages in the right manner. As described previously, a rock record of real lithologies (except where cores exist: see
cutting from 3000 m takes over one hour to reach the below). However, because of the way in which the samples
surface. During this time it becomes mixed with other are collected, lithologies have become mixed and bed-
cuttings taken at shallower depths and moving more boundaries smeared. Wireline logs can be used to separate
slowly up the hole. It also becomes mixed with chips the mixed elements and define the bed boundaries.
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