This page describes density logs profiles, in the order of their appearance over the years. This presentation style provides insights into tool evolution, and a specific tool’s capabilities and limitations. You will find most these tool types in your well files – here’s your chance to learn more about them.
Density logs first appeared in 1957, based on the principle of gamma
ray absorption by Compton scattering.
Early tools were called gamma - gamma density logs because they
emitted and recorded gamma rays. The log displayed counts per
second, which was transformed to density by a semi-logarithmic
transform. Modern tools have two detectors, which allows borehole
compensation to be applied.
tool can be used in air or mud filled open boreholes. A cased
hole tool is available in some areas.
UNCOMPENSATED DENSITY LOG
Single detector density tools are severely affected by hole size, mud weight, mud cake thickness, source type and strength, source detector spacing, and detector efficiency. The High-Low calibration method compensates for all these problems, but available charts do not. In the earliest versions of these tools, the source strength decayed rapidly, so count rates definitely need to be normalized on a well by well basis.
COMPENSATED DENSITY LOG (FDC)
Porosity can be derived from density and can be presented as a percent or as a decimal fraction on the log. This porosity may still contain artifacts from shale and minerals not accounted for by the logging computer, so this porosity is NOT a final answer.
A typical density logging tool is shown at the right. The tool is pressed against one side of the borehole by a back-up arm that also serves to measure a diameter of the borehole. Two detectors at fixed spacings from the source are shown. The source is well-shielded from the two detectors and only scattered gamma radiation is detected. The intensity of the scattered radiation will be dominated by the density variations along the path from source to detector.
If there is no stand-off (of mud or mudcake) between the tool face and the formation, and if the tool is properly calibrated, then the apparent density from both detectors will be the same and equal to the true formation density. If they are different, there must be mud between the tool face and the rock.
If there is some standoff, a correction
to the density from the long spaced detector can be generated
from the difference between the apparent density seen by the
far and the near detectors. The actual correction function
can be determined empirically by placing the density device
in a number of formations to measure the apparent long-spaced
and short-spaced densities for various thicknesses of mudcake
of a variety of densities. Computer modeling has augmented
these laboratory studies.
ADVERSE BOREHOLE CONDITIONS
In the mid 1970's a 90 degree offset tool was developed to reduce the chance of logging the large diameter of the borehole. It consisted of a second backup arm at 90 degrees to the original, pressured a little higher, that forced the tool skid into the smaller diameter. This led to the concept of the dual axis, or X and Y axis calipers. Later development led to a dual density tool, essentially two complete density logs on the same tool string, positioned at 90 degrees from each other, resulting in reasonable complete log coverage in stressed boreholes.
CAUTION: Do not use density data when you suspect standoff
problems. A reasonable guide would be a density correction more than
0.15 gm/cc (150 kg/m3) is highly suspect and greater than 0.20 gm/cc
is useless. If density porosity is greater than neutron porosity,
and no gas is expected in the rock, the density is probably useless
(provided the logs were run on a porosity scale appropriate for the
mineralogy). Noisy, hashy, or impossibly high density porosity
probably indicates a bad log, even when the caliper and correction
curve show no problems. The density skid is about 2 feet long so
there can be significant breakouts within that distance that the
caliper cannot see.
DUAL DENSITY LOG AND 90 DEGREE OFFSET TOOL
An alternative was called the dual density
log. There were literally two density tools coupled together, one
above the other at 90 degrees so that while one tool was facing the
bad side of the borehole, the other was facing the good side. Thus
two independent density logs were run simultaneously.
Cased hole formation density logs make accurate formation density measurements in cased wells. A chemical gamma ray source and three-detector measurement system are used to make measurements in a wide range of casing and borehole sizes. The density measurement made by the three detector system is corrected for casing and cement thickness.
The density data are used to calculate porosity and determine the lithology. The combination of density and neutron data is used to indicate the presence of gas.
■ Porosity determination
■ Lithology analysis and identification of minerals
■ Gas detection
■ Hydrocarbon density determination
■ Shaly sand interpretation
■ Rock mechanical properties calculations
■ Determination of overburden pressure
■ Synthetic seismogram for correlation with seismic
The density log can be presented in units of density, that is, grams per cc or Kilograms per cubic meter. Some log presentations portray the density data as its equivalent porosity, translated with a particular lithology assumption. Some show both density and density porosity, as in the image above.
The scales are usually called Sandstone or Limestone
scales to reflect the assumption that was made to create them.
Dolomite scales also exist on a few logs. The relationships
Because some logs do not have a density scale, you may have to translate the recorded log into density units so that it can be used, for example to calculate acoustic impedance for a seismic application.
To use data from a density log, you must correctly identify both the scale type, lithology assumption, and the two end point values. Other log curves are often present, such as the density correction, compensated neutron, gamma ray, caliper, bit size, cable tension, and photoelectric effect. You have to choose the correct curve from among those presented. Editing for bad hole and casing effects will be mandatory if the log is to be used to generate a synthetic seismogram. The density data should not be used for any purpose if the density correct is larger than 0.200 gm/cc (200 kg/m3).
CAUTION: The use of an inappropriate porosity scale on a
combination density - neutron log presentation can be EXTREMELY
misleading. For example, sandstone rock recorded on a limestone
scale will cause the density porosity to be higher than the neutron
porosity by as much as 6 to 8% (0.06 to 0.08 decimal fraction). This
is often interpreted to indicate the presence of gas, leading to
very expensive completion mistakes. The density - neutron crossover
needs to be considerably greater than 8% to indicate gas in this
situation. Similarly, a log run on dolomite scale through a
limestone rock will show up to 12% porosity crossover, just because
of the inappropriate scale, not because of gas. Use the PE curve to
determine lithology, then interpret the crossover correctly. PE near
2 = sandstone, PE near 3 = dolomite, PE near 5 = limestone.
DENSITY INTERFACE LOG and SONAR LOGS
During the initial mechanical interface test (MIT) of a gas storage cavern, the survey is run in time-lapse mode. With some water in the cavern, nitrogen is injected under pressure and held for at least 24 hours. If the water level changes or pressure drops more than 10 psi, the test has failed. Remedial action, if possible, must be undertaken before the cavern can be used to store gas. During operation of the cavern, the objective is to observe the water--gas contact depth in the cavern, along with the reservoir pressure, to monitor remaining gas volume.
The original cavern volume is determined by a
sonar log. This device maps the travel time of sound from the tool
to the cavern wall and back again. By pinging the sonar in varying
directions, a map of the distance to the walls can be made at
various depths in the cavern. The survey ends up as a 3-D image of
the cavern, which is used for routine gas volume modeling.
Formation Density Log Uncompensated Type (DL)
Formation Density Log Compensated Type (FDC)
This log was often presented on the same log display as the compensated neutron log, and more rarely in the right hand track on a dual induction log.
Litho-Density Log (LDT)
This log was often presented on
the same log display as the compensated neutron log,
Page Views ---- Since 01 Jan 2015
Copyright 2023 by Accessible Petrophysics Ltd.
CPH Logo, "CPH", "CPH Gold Member", "CPH Platinum Member", "Crain's Rules", "Meta/Log", "Computer-Ready-Math", "Petro/Fusion Scripts" are Trademarks of the Author