Page 11 - The Geological Interpretation of Well Logs

1

INTRODUCTION

1.1 Well logs — a definition electrical, and modern methods of data transmission do
not necessarily need a wire line so the name above is
The continuous recording of a geophysical parameter
recommended. This book therefore concerns wireline
along a borehole produces.a geophysical welt log. The
geophysical well logs.
value of the measurement is plotted continuously against
In France, where well logging was first invented by
depth in the well (Figure 1.1). For example, the resistivity
Conrad Schlumberger and Henri Doll, the original name
log is a continuous plot of a formation’s resistivity from
was ‘Carottage Electrique’ (electrical coring) as opposed
the bottom of the well to the top and may represent over 4
to mechanical coring. Today the name diagraphies
kilometres (2% miles) of readings.
différées (literally, “deferred diagrams’) is applied to
The most appropriate name for this continuous depth-
distinguish wireline geophysical well logs, which are
related record is a wireline geophysical well log,
made after drilling, from the drill logs (diagraphies
conveniently shortened to well log or log. It has often
immédiates, i.e. immediate diagrams) made during the
been called an “electrical log’ because historically the first
drilling. In English no such distinction is made — the
logs were electrical measurements of electrical proper-
word ‘log’ is universally used.
ties. However, the measurements are no longer simply
1.2 Well logs - the necessity
value
Oo Ss 10 15 Many different modern geophysical well logs exist. They
136 »- ohm msm
are records of sophisticated geophysical measurements
along a borehole. These may be measurements of sponta-
140
neous phenomena, such as natural radioactivity (the
gamma ray log), which requires a tool consisting simply
150 of a very sensitive radiation detector; or they may be
induced, as with the formation velocity log (sonic log), in
V<N] the (ime taken for the sound to reach a receiver at a set
160 which a too] emits sound into the formation and measures
m 170 distance along the too] (Table 1.1).
Geophysical well logging is necessary because
well geological sampling during drifling (‘cuttings sampling’)
180 leaves a very imprecise record of the formations encoun-
in tered. Entire formation samples can be brought to the
\Y LA
depth 190 surface by mechanical coring, but this is both slow and
expensive. The results of coring, of course, are unequivo-
PTA between ‘cuttings’ and ‘cores’, and with experience,
200 cal. Logging is precise, but equivocal, in that it needs
interpretation to bring a log to the level of geological or
210 petrophysical experience. However, logs fill the gap
calibration and computers, they can almost replace cores,

240 crop reality into the subsurface.
as they certainly contain enough information to put out-
220

230
1.3 Wireline logs -— the making

drilling tools are no Jonger in the hale. ‘Open-hole’ logs,
v Wireline geophysical well logs are recorded when the
(open-hole indicates that the formation forms the wall]
of a well, as opposed to ‘cased-hole, in which a tube
Figure 1.1 A well log. Representation of the first ‘log’ made
of metal casing lines the well), the subject of this
at Pechelbronn, Alsace, France, in 1927 by H. Doll. (From
Allaud and Martin, 1976). book, are recorded immediately after drilling. MWD