Chapter 16
Well Logging: Principles, Applications and Uncertainties
Nazmul Haque Mondol
16.1 Introduction
Well logging is a means of recording the physical,
acoustic and electrical properties of the rocks
penetrated by a well. It is carried out by service
companies, which work under contract for the oil
companies. Logging has the advantage that it measures
in situ rock properties which cannot be measured in a
laboratory from either core samples or cuttings.
Logging started with simple electric logs measuring
the electrical conductivity of rocks, but it is now an
advanced and sophisticated method used routinely in
different phases of hydrocarbon exploration, field
development and monitoring. It gives us a continuous
downhole record that provides a detailed subsurface
picture of both gradual and abrupt changes in physical
properties from one bed to the next. By contrast, only
selected parts of the reservoir rocks are cored, and
samples of cuttings from the rest of the well give no
more than a general idea of the lithology.
None of the logs actually measure the parameters
that are of most interest to us, such as how much oil or
gas is in the subsurface, or how much is being produced. Such important knowledge can only be derived,
from the measured properties such as gamma radiation,
density, velocity and resistivity, using a number of
assumptions which, if true, will give reasonable
estimates of hydrocarbons. To perform a logging operation on wireline, the measuring instrument is lowered
into the borehole on the end of an insulated electrical
cable after the drilling tool is pulled up (Fig. 16.1). The
cable itself is used as the depth measuring device, so
that properties measured by the tools can be related to
particular depths in the borehole.
Well logs are usually recorded while the logging
device is being winched upward through the well. The
measurements from the instruments housed in the logging tool are recorded digitally at intervals of between 3
and 15 cm and the data is processed near the well on
land, or on the platform in the case of offshore. Recording the well log involves a number of steps, beginning
with sensing and pre-processing the measurement in the
logging tool itself, transmission of this information to
the surface over several kilometers of wireline, further
processing in the logging truck computer, data storage
on disc or magnetic tape, and finally display of the data
on film or paper. A measured log shows many
variations from top to bottom (log display in
Fig. 16.1), and each wiggle has significance. Most
logs are dependent on direct contact with the rock via
the walls of the well, and have to be run after successive
intervals of the drilling, before each stage of the steel
casing is installed in the well.
16.2 Well Logs: The Necessity
Geological sampling during drilling (cuttings)
provides a very imprecise record of the formations
encountered. Entire formation samples can be brought
to the surface by mechanical coring, but this is both
slow and very expensive. In the narrowest sense, well
logging supplements the analysis of cores, side-wall
samples and cuttings. Logs are used for a variety of
N.H. Mondol (*)
Department of Geosciences, University of Oslo, Oslo, Norway
Norwegian Geotechnical Institute (NGI), Oslo, Norway
e-mail: nazmulh@geo.uio.no
K. Bjørlykke (ed.), Petroleum Geoscience: From Sedimentary Environments to Rock Physics,
DOI 10.1007/978-3-642-34132-8_16, # Springer-Verlag Berlin Heidelberg 2015
385
Well Logging: Principles, Applications and Uncertainties
Nazmul Haque Mondol
16.1 Introduction
Well logging is a means of recording the physical,
acoustic and electrical properties of the rocks
penetrated by a well. It is carried out by service
companies, which work under contract for the oil
companies. Logging has the advantage that it measures
in situ rock properties which cannot be measured in a
laboratory from either core samples or cuttings.
Logging started with simple electric logs measuring
the electrical conductivity of rocks, but it is now an
advanced and sophisticated method used routinely in
different phases of hydrocarbon exploration, field
development and monitoring. It gives us a continuous
downhole record that provides a detailed subsurface
picture of both gradual and abrupt changes in physical
properties from one bed to the next. By contrast, only
selected parts of the reservoir rocks are cored, and
samples of cuttings from the rest of the well give no
more than a general idea of the lithology.
None of the logs actually measure the parameters
that are of most interest to us, such as how much oil or
gas is in the subsurface, or how much is being produced. Such important knowledge can only be derived,
from the measured properties such as gamma radiation,
density, velocity and resistivity, using a number of
assumptions which, if true, will give reasonable
estimates of hydrocarbons. To perform a logging operation on wireline, the measuring instrument is lowered
into the borehole on the end of an insulated electrical
cable after the drilling tool is pulled up (Fig. 16.1). The
cable itself is used as the depth measuring device, so
that properties measured by the tools can be related to
particular depths in the borehole.
Well logs are usually recorded while the logging
device is being winched upward through the well. The
measurements from the instruments housed in the logging tool are recorded digitally at intervals of between 3
and 15 cm and the data is processed near the well on
land, or on the platform in the case of offshore. Recording the well log involves a number of steps, beginning
with sensing and pre-processing the measurement in the
logging tool itself, transmission of this information to
the surface over several kilometers of wireline, further
processing in the logging truck computer, data storage
on disc or magnetic tape, and finally display of the data
on film or paper. A measured log shows many
variations from top to bottom (log display in
Fig. 16.1), and each wiggle has significance. Most
logs are dependent on direct contact with the rock via
the walls of the well, and have to be run after successive
intervals of the drilling, before each stage of the steel
casing is installed in the well.
16.2 Well Logs: The Necessity
Geological sampling during drilling (cuttings)
provides a very imprecise record of the formations
encountered. Entire formation samples can be brought
to the surface by mechanical coring, but this is both
slow and very expensive. In the narrowest sense, well
logging supplements the analysis of cores, side-wall
samples and cuttings. Logs are used for a variety of
N.H. Mondol (*)
Department of Geosciences, University of Oslo, Oslo, Norway
Norwegian Geotechnical Institute (NGI), Oslo, Norway
e-mail: nazmulh@geo.uio.no
K. Bjørlykke (ed.), Petroleum Geoscience: From Sedimentary Environments to Rock Physics,
DOI 10.1007/978-3-642-34132-8_16, # Springer-Verlag Berlin Heidelberg 2015
385