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.|
|Low-End Estimate (USD):|| 0.4040 centUSD |
4.0e-10 TUSD / foot
|Median Estimate (USD):|| 0.6868 centUSD |
6.8e-10 TUSD / foot
|High-End Estimate (USD):|| 0.8080 centUSD |
8.0e-10 TUSD / foot
|Low-End Estimate:|| 0.69 days0.00189 years |
0.0227 months / job
|Median Estimate:|| 0.69 days0.00189 years |
0.0227 months / job
|High-End Estimate:|| 1.39 days0.00381 years |
0.0457 months / job
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.
Lithology identification, perosity
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.
The data acquired is based on the average density of the rock between the sensor and the detector.
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.
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.
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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