WELLOG                Scintillation Gamma

Revised 8-11-2007

© 2004 - 2007 WELLOG

All Rights Reserved

A Scintillation gamma log is the best tool to provide a quick, low cost correlation of water producing zones in a water

well.

The same tool has applications in mining, CBM, oil, and environmental wells.

Scintillation gamma is the most useful log for providing formation depths, type, and thickness. It is used for identification of sand and clay formations in all types of wells.

WHERE IT WORKS:

Scintillation gamma works in both cased and open holes. This method can be used to define lithology on wells that have NO DRILLING LOG or other records.

THE SYSTEM:

A scintillation gamma tool is connected to a wireline cable.  The wireline cable is connected to a winch that is used to lower the tool into the well.  The tool is lowered to the bottom of the well and then slowly raised to the surface.  As the tool is raised, it measures the natural gamma radiation in the different formations. Depth of the measurement is also recorded. The result is recorded as a log of formation type related to depth and stored on a computer.  A computer generated final print (well log) is given to the customer.

HOW IT WORKS:

The scintillation gamma tool measures the natural gamma radiation emitted by the formations thru which it passes. Clay formations emit more radiation than clean sandstone or limestone formations.  A specialized crystal material referred to as a Sodium Iodide scintillation crystal is used.  The scintillation crystal emits tiny flashes of light when gamma rays enter. The intensity of the light is proportional to the energy of the gamma ray. Gamma rays have different energies depending on their source. Pulses of light from the scintillation crystal are detected by a photo-multiplier tube and are amplified many thousands of times. The result is an electrical voltage pulse. Depending on the amount of gamma radiation, many pulses are produced by the photo-multiplier tube. The photomultiplier tube has a certain amount of low-level noise. A circuit called a discriminator is used to establish a threshold above the noise voltage in order to not detect noise pulses and allow only those pulses that exceed the discrimination level to pass. After pulses go through pulse amplification and waveform shaping, they are counted by a count rate meter. The output of a count rate meter is a DC voltage that is proportional to the number of counts. The resulting output voltage is recorded as a gamma ray counts. Scintillation detectors are considered the most sensitive type of gamma ray detector.

WHAT IS MEASURED?

All materials are radioactive. Some are more radioactive than others. Shales and clays are more radioactive than other natural occurring materials within the earth with the exception of known radioactive materials containing uranium, thorium, or potassium. Each of these radioactive materials are found in other materials in varying amounts. Gamma rays have a specific energy that is measured in thousands of electron volts (Kev) or millions of electron volts (Mev).

WHY IS IT USEFUL?

A scintillation gamma log provides useful information about what types of formations are located in a well. It helps in the final design of a well to determine the proper location of the perforated section of casing in the well. Since water is commonly produced from formations that contain mostly sand (sandstone) or lime in the form of limestone, it is very important to know where these formations are located and how thick they are. Formations containing a high amount of clay are generally considered poor formations to produce water. If a water well is drilled into bedrock, it is possible that perforations may be inadvertently placed at or below bedrock.  If bedrock is not fractured or otherwise able to produce water in the case of a water well, then the well will be non productive.

ARE THERE OTHER USES?

The scintillation gamma tool has applications in uranium mining.  Coal exploration wells can be logged to determine the thickness of coal seams. In coal bed methane wells, (CBM wells) correlation logging can establish accurate definition of coal seams and their relation to casing joints after casing is installed. Wells having radon or radium producing formations can be logged to find potential problem areas.  Because uranium is an important mineral – uranium exploration wells are often logged to find the formations that are most favorable for production of uranium. Mineral exploration is an example of an application where formation definition is important.  Because mineral deposition is frequently associated with areas having faulting or fracturing, and veins having high silica content are distinct from surrounding wall rock formations, a scintillation gamma log may be used. WELLOG can also perform spectral gamma logging.

FOR MORE INFORMATION:

For more information contact WELLOG at info@wellog.com