The Gas Holdup tool is a unique Full-Bore fluid density measurement to determine the Gas/Liquid ratio or the Gas Holdup. The physical principle of operation uses back-scattered radiation from a low-energy isotope source into a scintillator/PMT sensor. It is able to provide equally accurate measurements in Horizontal or Vertical wells and is unaffected by the borehole orientation or the fluid-flow regime. In the special case of slug flow in inclined wells, averaging of the data over a suitable period of time results in an accurate holdup measurement. The tool was invented by the founders of CBG Corporation and patented by Halliburton. CBG has added the tool to its suite of digital Production Logging products.
The tool operates most accurately when centered and requires the well to be cased. Calibration depends on casing internal diameter so this parameter must be known for accurate interpretation. The source is located inside the pressure-housing of the tool, so it is most conveniently transported as if permanently installed and the entire tool is carried in a shielded case.
CBG does not provide radioactive sources so these should be procured by a licensed customer from a specification provided by CBG. An independent vendor (Isotope Products Laboratories, Inc.) has developed a Cobalt-57 source for use in the tool. The normal source intensity is 3 milliCurie for logging speeds up to 60 ft./min. at full rated accuracy and resolution, but it is also possible to use a 100 microCurie exempt-quantity source is situations where logging speed is not important. In those situations, logging speeds up to 10 ft./min. may be possible with some small reduction in resolution.
Certification is necessary for the full-strength source, while the exempt-quantity source does not require certification in some locations and has no special handling requirements.
A method of calibrating the tool in the field office or at the wellsite has been developed, and a custom fixture has been designed. It incorporates a piece of standard 6.35 inch casing and an accurate tool-centering device. CBG can supply this fixture or, if the customer prefers, provide drawings for customer manufacture. The single casing-diameter fixture can be used to calibrate the tool for all casing sizes, eliminating the need for a complex and error-prone method with different diameters of casing.
It is most convenient to transport the complete tool with source installed in a lead-shielded case to avoid complicated source installation in the field. CBG has designed and can supply a suitable case at reasonable cost. The case design has successfully passed both the external radiation tests and the DOT physical abuse tests performed by an outside vendor (NSSI). Documentation with a White Level-I label (the least restrictive classification) will be required. The documentation is only required when the full-strength 3 milliCurie source is in the tool.
The GHT output data consists of a simple count-rate that is transmitted as a 12-bit word 16 times each second. Surface software (running in a Warrior Logging Computer for example) takes the raw data and applies calibration factors relating to casing size, nominal liquid density, tool sensitivity, and downhole temperature. The fast sampling rate allows detailed understanding of the downhole flow regime. The software calculates Gas Holdup, or Gas/Liquid Ratio, as desired by the customer.
CBG will provide detailed algorithms for calibration and interpretation to registered customers.