BASIC LOG TYPES
Below is a list of the main types of logs that may be run, and why they
are run.
1.2.1 Logging While Drilling (LWD)
Traditionally, petrophysicists were concerned only with wireline
logging, that is, the data acquired by running tools on a cable from a winch
after the hole had been drilled. However, advances in drilling/logging
technology have allowed the acquisition of log data via tools placed in
the actual drilling assembly. These tools may transmit data to the surface
on a real-time basis or store the data in a downhole memory from which
it may be downloaded when the assembly is brought back to the surface.
LWD tools present a complication for drilling, as well as additional
expense. However, their use may be justified when:
• Real-time information is required for operational reasons, such as
steering a well (e.g., a horizontal trajectory) in a particular formation
or picking of formation tops, coring points, and/or casing setting depths
• Acquiring data prior to the hole washing out or invasion occurring
• Safeguarding information if there is a risk of losing the hole
• The trajectory is such as to make wireline acquisition difficult (e.g., in
horizontal wells)
LWD data may be stored downhole in the tools memory and retrieved
when the tool is brought to the surface and/or transmitted as pulses in the
mud column in real time while drilling. In a typical operation, both modes
will be used, with the memory data superseding the pulsed data once the
tool is retrieved. However, factors that might limit the ability to fully use
both sets of data are:
• Drilling mode: Data may be pulsed only if the drillstring is having mud
pumped through it.
• Battery life: Depending on the tools in the string, tools may work in
memory mode only between 40 and 90 hours.
• Memory size: Most LWD tools have a memory size limited to a few
megabytes. Once the memory is full, the data will start to be overwritten.
Depending on how many parameters are being recorded, the
memory may become full within 20–120 hours.
• Tool failure: It is not uncommon for a fault to develop in the tool
such that the pulse data and/or memory data are not transmissible/
recordable.
Some of the data recorded may be usable only if the toolstring is rotating
while drilling, which may not always be the case if a steerable mud
motor is being used. In these situations, the petrophysicist may need to
request drilling to reacquire data over particular intervals while in
reaming/rotating mode. This may also be required if the rate of penetration
(ROP) has been so high as to affect the accuracy of statistically based
tools (e.g., density/neutron) or the sampling interval for tools working on
a fixed time sampling increment.
Another important consideration with LWD tools is how close to the
bit they may be placed in the drilling string. While the petrophysicist will
obviously want the tools as close to the bit as possible, there may be
limitations placed by drilling, whose ability to steer the well and achieve
a high ROP is influenced by the placement of the LWD toolstring. LWD
data that may typically be acquired include the following:
• GR: natural gamma ray emission from the formation
• Density: formation density as measured by gamma ray Compton scattering
via a radioactive source and gamma ray detectors. This may also
include a photoelectric effect (Pe) measurement.
• Neutron porosity: formation porosity derived from the hydrogen index
(HI) as measured by the gamma rays emitted when injected thermal
or epithermal neutrons from a source in the string are captured in the
formation
• Sonic: the transit time of compressional sound waves in the formation
• Resistivity: the formation resistivity for multiple depths of investigation
as measured by an induction-type wave resistivity tool
Some contractors offer LWD-GR, -density, and -neutron as separate
up/down or left/right curves, separating the contributions from different
quadrants in the borehole. These data may be extremely useful in steering
horizontal wells, where it is important to determine the proximity of
neighboring formation boundaries before they are actually penetrated.
Resistivity data may also be processed to produce a borehole resistivity
image, useful for establishing the stratigraphic or sedimentary dip and/or
presence of fractures/vugs.
Other types of tool that are currently in development for LWD mode
include nuclear magnetic resonance (NMR), formation pressure, and shear
sonic.
1.2.2 Wireline Openhole Logging
Once a section of hole has been completed, the bit is pulled out of the
hole and there is an opportunity to acquire further openhole logs either
via wireline or on the drillstring before the hole is either cased or abandoned.
Wireline versions of the LWD tools described above are available,
and the following additional tools may be run:
• Gamma ray: This tool measures the strength of the natural radioactivity
present in the formation. It is particularly useful in distinguishing
sands from shales in siliciclastic environments.
• Natural gamma ray spectroscopy: This tool works on the same principal
as the gamma ray, although it separates the gamma ray counts into
Basics 5
three energy windows to determine the relative contributions arising
from (1) uranium, (2) potassium, and (3) thorium in the formation. As
described later in the book, these data may be used to determine the
relative proportions of certain minerals in the formation.
• Spontaneous potential (SP): This tool measures the potential difference
naturally occurring when mud filtrate of a certain salinity invades the
formation containing water of a different salinity. It may be used to
estimate the extent of invasion and in some cases the formation water
salinity.
• Caliper: This tool measures the geometry of the hole using either two
or four arms. It returns the diameter seen by the tool over either the
major or both the major and minor axes.
• Density: The wireline version of this tool will typically have a much
stronger source than its LWD counterpart and also include a Pe curve,
useful in complex lithology evaluation.
• Neutron porosity: The “standard” neutron most commonly run is a
thermal neutron device. However, newer-generation devices often use
epithermal neutrons (having the advantage of less salinity dependence)
and rely on minitron-type neutron generators rather than chemical sources.
• Full-waveform sonic: In addition to the basic compressional velocity
(Vp) of the formation, advanced tools may measure the shear velocity,
Stonely velocity, and various other sound modes in the borehole,
borehole/formation interface, and formation.
• Resistivity: These tools fall into two main categories: laterolog and
induction type. Laterolog tools use low-frequency currents (hence
requiring water-based mud [WBM]) to measure the potential caused by
a current source over an array of detectors. Induction-type tools use
primary coils to induce eddy currents in the formation and then a secondary
array of coils to measure the magnetic fields caused by these
currents. Since they operate at high frequencies, they can be used in
oil-based mud (OBM) systems. Tools are designed to see a range of
depths of investigation into the formation. The shallower readings have
a better vertical resolution than the deep readings.
• Microresistivity: These tools are designed to measure the formation
resistivity in the invaded zone close to the borehole wall. They operate
using low-frequency current, so are not suitable for OBM. They are
used to estimate the invaded-zone saturation and to pick up bedding
features too small to be resolved by the deeper reading tools.
• Imaging tools: These work either on an acoustic or a resistivity principle
and are designed to provide an image of the borehole wall that may
6 Well Logging and Formation Evaluation
be used for establishing the stratigraphic or sedimentary dip and/or
presence of fractures/vugs.
• Formation pressure/sampling: Unlike the above tools, which all “log”
an interval of the formation, formation-testing tools are designed to
measure the formation pressure and/or acquire formation samples at a
discrete point in the formation. When in probe mode, such tools press
a probe through the mudcake and into the wall of the formation. By
opening chambers in the tool and analyzing the fluids and pressures
while the chambers are filled, it is possible to determine the true pressure
of the formation (as distinct from the mud pressure). If only pressures
are required (pretest mode), the chambers are small and the
samples are not retained. For formation sampling, larger chambers are
used (typically 23/4 or 6 gallons), and the chambers are sealed for analysis
at the surface. For some tools, a packer arrangement is used to enable
testing of a discrete interval of the formation (as opposed to a probe
measurement), and various additional modules are available to make
measurements of the fluid being sampled downhole.
• Sidewall sampling: This is an explosive-type device that shoots a sampling
bullet into the borehole wall, which may be retrieved by a cable
linking the gun with the bullet. Typically this tool, consisting of up to
52 shots per gun, is run to acquire samples for geological analysis.
• Sidewall coring: This is an advanced version of the sidewall sampling
tool. Instead of firing a bullet into the formation, an assembly is used
to drill a sample from the borehole wall, thereby helping to preserve
the rock structure for future geological or petrophysical analyses.
• NMR: These tools measure the T1 and T2 relaxation times of the formation.
Their principles and applicability are described in Chapter 5.
• Vertical seismic profiling (VSP): This tool fires a seismic source at the
surface and measures the sound arrivals in the borehole at certain depths
using either a hydrophone or anchored three-axis geophone. The data
may be used to build a localized high-resolution seismic picture around
the borehole. If only the first arrivals are measured, the survey is typically
called a well shoot test (WST) or checkshot survey. VSPs or
WSTs may also be performed in cased hole.
1.2.3 Wireline Cased Hole Logging
When a hole has been cased and a completion string run to produce the
well, certain additional types of logging tools may be used for monitoring
purposes. These include:
• Thermal decay tool (TDT): This neutron tool works on the same principle
as the neutron porosity tool, that is, measuring gamma ray counts
when thermal neutrons are captured by the formation. However, instead
of measuring the HI, they are specifically designed to measure the
neutron capture cross-section, which principally depends on the amount
of chlorine present as formation brine. Therefore, if the formation water
salinity is accurately known, together with the porosity, Sw may be
determined. The tool is particularly useful when run in time-lapse mode
to monitor changes in saturation, since many unknowns arising from
the borehole and formation properties may be eliminated.
• Gamma ray spectroscopy tool (GST): This tool works on the same principal
as the density tool, except that by measuring the contributions
arising in various energy windows of the gamma rays arriving at the
detectors, the relative proportions of various elements may be determined.
In particular, by measuring the relative amounts of carbon and
oxygen a (salinity independent), measurement of Sw may be made.
• Production logging: This tool, which operates using a spinner, does not
measure any properties of the formation but is capable of determining
the flow contributions from various intervals in the formation.
• Cement bond log: This tool is run to evaluate the quality of the cement
bond between the casing and the formation. It may also be run in a circumferential
mode, where the quality around the borehole is imaged.
The quality of the cement bond may affect the quality of other production
logging tools, such as TDT or GST.
• Casing collar locator (CCL): This tool is run in order to identify the
positions of casing collars and perforated intervals in a well. It produces
a trace that gives a “pip” where changes occur in the thickness of the
steel.
1.2.4 Pipe-Conveyed Logging
Where the borehole deviation or dogleg severity is such that it is not
possible to run tools using conventional wireline techniques, tools are typically
run on drillpipe. In essence, this is no different from conventional
logging. However, there are a number of important considerations.
Because of the need to provide electrical contact with the toolstring, the
normal procedure is to run the toolstring in the hole to a certain depth
before pumping down a special connector (called a wet-connect) to
connect the cable to the tools. Then a side-entry sub (SES) is installed in
the drillpipe, which allows the cable to pass from the inside of the pipe
to the annulus. The toolstring is then run in farther to the deepest logging
point, and logging commences. The reason the SES is not installed when
the toolstring is at the surface is partly to save time while running in (and
allowing rotation), and also to avoid the wireline extending beyond the
last casing shoe in the annulus. If the openhole section is longer than the
cased hole section, the logging will need to be performed in more than
one stage, with the SES being retrieved and repositioned in the string.
Pipe-conveyed logging is expensive in terms of rig time and is typically
used nowadays only where it is not possible to acquire the data via LWD.
Most contractors now offer a means to convert an operation to pipeconveyed
logging if a toolstring, run into the hole on conventional wireline,
becomes stuck in the hole. This is usually termed “logging while fishing.”