Density Log

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Exploration Technique: Density Log

Exploration Technique Information
Exploration Group: Downhole Techniques
Exploration Sub Group: Well Log Techniques
Parent Exploration Technique: Well Log Techniques
Information Provided by Technique
Lithology: provides data on the bulk density of the rock surrounding the well
Stratigraphic/Structural: Stratigraphic correlation between well bores.
Hydrological: Porosity of the formations loggesd can be calculated for the Density log andprovide an indication potential aquifers.
Cost Information
Low-End Estimate (USD): 0.4040 centUSD
4.0e-4 kUSD
4.0e-7 MUSD
4.0e-10 TUSD
/ foot
Median Estimate (USD): 0.6868 centUSD
6.8e-4 kUSD
6.8e-7 MUSD
6.8e-10 TUSD
/ foot
High-End Estimate (USD): 0.8080 centUSD
8.0e-4 kUSD
8.0e-7 MUSD
8.0e-10 TUSD
/ foot
Time Required
Low-End Estimate: 0.69 days0.00189 years
16.56 hours
0.0986 weeks
0.0227 months
/ job
Median Estimate: 0.69 days0.00189 years
16.56 hours
0.0986 weeks
0.0227 months
/ job
High-End Estimate: 1.39 days0.00381 years
33.36 hours
0.199 weeks
0.0457 months
/ job
Density Log:
Density logging is a well logging tool that can provide a continuous record of a formation's bulk density along the length of a borehole. In geology, bulk density is a function of the density of the minerals forming a rock (i.e. matrix) and the fluid enclosed in the pore spaces.
Other definitions:Wikipedia Reegle

Density logs have long been used for determing the bulk density of of the formations being logged. Today the density logging tool emits gamma rays from a source at the bottom of the tool. The gamma rays emitted by the source penetrate the rocks as the tool passes by where some are absorbed. A scintillation counter, placed 18"-24" above the source recordes the gamma rays that were not absorbed. The number gamma rays recorded by the scintillation counter is inversely proportional to the electron density of the rock (grams/cc or kg/meter cubed).

This information can be used to calculate parameters such as rock porosity and rock type. It is calculated by measuring the difference in flux between a source and a detector which are at a set spacing.

Use in Geothermal Exploration
Lithology identification, perosity

Field Procedures
The geophysical/well logging service company conducts the down hole logging operation and produces both digital and hard copy logs. The Drilling contractor trips the drill pipe and bit and conditions the well bore for logging.

Data Access and Acquisition
The data acquired is based on the average density of the rock between the sensor and the detector.
Best Practices
If the density log is taken as part of a wireline measurement, to minimize the influence of mud on the results, a pad is pressed into the borehole wall to focus the source and detector sensors into the formation. If instead the density measurement is taken during well drilling, a sleeve can be added around the sensors to exclude the mud coming between them. To further reduce the influence of mud, two or more sensors at different spacings can be used.
Potential Pitfalls
An unstable well bore (sluffing, wash outs, etc) can be of concern in any well logging operation. In extreme condition, the loss of the logging tool down hole could possibly result in the loss of the hole and would require premature abandonment or the necessity to side track to complete the well drilling operation. It is important to minimize the mud between the sensors, the source and the detector, because that can impact the accuracy of the signal. Also, changes in the borehole diameter can affect the measurement and provide inaccurate results because the distance between the source and the detector can change with depth in the well.

NEPA Analysis
Well logging is a standard operation associated with the drilling permit approval and is included in the down hole analysis of the drilling program.

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