The CBG NGT-T Gamma tool has become the industry standard for Geosteering and MWD applications, with over 5000 tools currently operating worldwide. When we count all the customized variants, the number more than 6000! Since 2008 all tools have been manufactured with our NextGen electronics for industry-leading stability, reliability, and low power-drain. A version with a larger crystal giving higher count-rates (to compensate for higher attenuation in 6-1/2″ and larger collar sizes) is described on the NGT-CN page.
The CBG NGT-T was initially developed for the Steering tool industry in 1994 and was later upgraded to meet the severe environmental challenges of Measurement-While-Drilling. The design is constantly being improved to meet new drilling conditions. Older tools currently in the field can be retro-fitted with our NextGen electronics as a Special Upgrade Service! We can also supply the NGT-TX tool with the NextGen electronics in a package that is 2″ shorter.
The standard model is equipped with an MDM 15-pin male connector on the top electronics end of the tool and an MDM 15-pin female connector on the bottom. Many custom versions have been made for our clients, with different connectors and mechanical details. Standard electrical connections are Pin #1 for Ground, Pin #4 for Power and Pin #8 for Signal Output. All 15 bus wires are passed along a protected wire guide from top connector to bottom.
Electronics are fully encapsulated for additional shock protection. The crystal and photomultiplier are packaged in-house, utilizing our proprietary and unique design, for ease of repair while retaining full shock ratings. All tools include the power-down option defined in the qBus standard, by pulling the output line to ground potential. In this condition, all internal functions are disabled and current demand is less than 1 mA. Also, all CBG gamma tools incorporate floating Battery-ground connection so that they can be used with EM telemetry systems without any problems.
The NGT-T Gamma Tool can now be ordered to include the complete Tensor/GE-compatible mechanical assembly, for a plug-and-play solution.
The NGT-T can be mounted to the lower Bulkhead Retainer through a standard snubber assembly but we recommend use of our new SNB-55 snubber for 6 times greater Z-axis shock protection.
At the lower end, a connector pigtail converts the MDM 15-pin connector on the tool to a 200 deg.C, GE 4Pin/6Socket connector mounted in the lower Intermodule End. At the top, a pigtail converts the MDM 15-pin connector on the NGT-T to a 200 deg.C, GE 6Pin/4Socket connector mounted in the top Intermodule End.
A custom 24-inch long BeCu Pressure Barrel is available, resulting in a significantly shorter and lower-cost tool than was previously available to the market. A version in the original 52-inch long BeCu Pressure Barrel is also available at a higher price, for those customers who want to retain the old standard tool length.
The graph below shows a typical temperature calibration chart for the NGT-T. Notice that we carefully control the maximum heating and cooling rate of the tool, similar to the rates experienced when tripping the well. Failure to observe this procedure when testing the tool can fracture the scintillator crystal and will void the warranty.
The first graph below shows the gamma attenuation effect of various thicknesses of drill collar and barrel. The second graph shows the effect of various weights of mud. The correction factor is the number that the Gamma probe readings should be multiplied by to find the true API log value.
Both graphs can be used separately or in combination to correct log data when correlation with existing logs is required. When combining the corrections, it is important to multiply the individual correction factors to get the total correction factor.
For the collar correction, subtract the collar I.D. from the O.D. and divide by two to get the thickness. The Tensor/GE barrel is 3/16 inch BeCu. For the mud correction, subtract the collar O.D. from the bit size and divide by two, then subtract the barrel O.D. from the collar I.D. and divide by two, add both parts to get the total mud thickness.
Potassium is naturally slightly radioactive. The following chart can be used to compute the additional API recorded by the Gamma probe in Potassium Chloride muds. Use the bit size for borehole diameter and the KCl mud concentration to find the API, then divide by the correction factor from the Collar/Mud weight graphs above to get the final amount of API shift on the Gamma log.