Geofisica

1
An Overview
of Well Logging
1.1 INTRODUCTION
The French translation of the term well logging is carottage electrique,∗ literally
´
“electrical coring,” a fairly exact description of this geophysical prospecting technique
when it was invented in 1927 [1, 2]. A less literal translation might be “a record of
characteristics of rock formations traversed by a measurement device in the wellbore.”
However, well logging means different things to different people. For a geologist, it
is primarily a mapping technique for exploring the subsurface. For a petrophysicist,
it is a means to evaluate the hydrocarbon production potential of a reservoir. For a
geophysicist, it is a source of complementary data for surface seismic analysis. For a
reservoir engineer, it may simply supplyvalues for use in a simulator.
The initial uses of well logging were for correlating similar patterns of electrical
conductivity from one well to another, sometimes over large distances. As the measuring techniques improved and multiplied, applications began to be directed to the
quantitative evaluation of hydrocarbon-bearing formations. Much of the following
text is directed toward theunderstanding of the measurement devices and interpretation techniques developed for this type of formation evaluation.
Although well logging grew from the specific need of the petroleum industry
to evaluate hydrocarbon accumulations, it is relevant to a number of other areas of
interest to earth scientists. New measurements useful for subsurface mapping have
evolved which have applications for structuralmapping, reservoir description, and
∗ The French definition is mentioned for two reasons: as an acknowledgment of the national origin of well

logging and as one of the rare cases in which Anglo-Saxon compactness is outdone by the French.

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1 AN OVERVIEW OF WELL LOGGING

sedimentological identification. The measurements can be used to identify fractures or
provide the formationmineralogy. A detailed analysis of the measurement principles
precedes the discussion of these applications. In this process, well logging is seen to
require the synthesis of a number of diverse physical sciences: physics, chemistry,
electrochemistry, geochemistry, acoustics, and geology.
The goal of this first chapter is to discuss well logging in terms of its traditional
application toformation hydrocarbon evaluation and to describe the wide variety
of physical measurements which address the relevant petrophysical parameters. We
begin with a description of the logging process, to provide an idea of the experimental
environment in which the measurements must be made.

1.2 WHAT IS LOGGING?
The birth of logging can be dated to the first recorded event [1] at Pechelbronn
onSeptember 5, 1927 where H. Doll and the Schlumberger brothers (and a few
others) made a semicontinuous resistivity measurement in that tired old field in Alsace.
The operation was performed with a rudimentary device (a sonde) consisting of a
bakelite cylinder with a couple of metallic electrodes on its exterior. Connecting
the device to the surface was a cable/wire, thus providing us with the termwireline
logging. Wireline refers to the armored cable by which the measuring devices are
lowered and retrieved from the well and, by a number of shielded insulated wires in
the interior of the cable, provide for the electrical power of the device and a means
for the transmission of data to the surface. More recently, the devices have been
encapsulated in a drill collar, and the transmissioneffected through the mud column.
This procedure is known as logging while drilling (LWD).
1.2.1

What is Wireline Logging?

The process of logging involves a number of elements, which are schematically illustrated in Fig. 1.1. Our primary interest is the measurement device, or sonde. Currently,
over fifty different types of these logging tools exist in order to meet various information needs...