DENSITY LOG BASICS
The tool can be used in air or mud filled open boreholes. Experimental tools approaching commercialization are being developed for cased hole applications.
The density logging tool emits gamma rays from a chemical source at the bottom of the tool The gamma rays enter the surrounding rocks where some are absorbed. Some gamma rays survive to reach scintillation counters mounted about 18 and 24 inches above the source. The number of gamma rays arriving at the far detector is inversely proportional to the electron density of the rock, which in turn is proportional to the actual rock density. Data from the closer detector is used to correct for borehole effects.
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.
The energy of the returning gamma rays is a function of the photoelectric capture cross section of the rock, which is indicative of mineralogy. A caliper and gamma ray curve are also presented, along with the density correction curve. Note that the correction has already been applied to the recorded density data by the computer.
Early density logs had only one detector and were recorded in counts per second. Density was derived with a semi-logarithmic transform.
Borehole gravity meters measure the pull of gravity in a station by station survey. Results are translated into formation density. Depth of investigation is large compared to normal logs so anomalies some distance from the borehole may be detected.
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.
Most modern two-detector density devices use multiple energy windows to derive the density, the photoelectric factor, and the correction curve as described above. In one three-detector wireline version, the combination of multiple detectors and multiple energy windows produce on the order of a dozen counting rate measurements at each depth. Each counting rate can be described by a forward model relating the rate to the five important parameters of density logging: formation density, formation photoelectric factor, mudcake density, mudcake photoelectric factor, and the thickness of the mudcake.
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.
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.
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.
Formation Density Log Uncompensated Type (DL)
Formation Density Log Compensated Type (FDC)
Litho-Density Log (LDT)
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