The GPT-A is a Gamma-Gamma sensor designed to measure the density of material around the lower end of the tool. It uses a Cs-137 radioactive source mounted in the lower end of the tool, and contains a sensitive scintillator/photomultiplier gamma detector to measure the amount of gamma-rays scattered and absorbed by surrounding material. The instantaneous count-rate is recorded in a micro-controller and transmitted to the surface logging computer via the CBG digital logging system. A dual-detector version is also available by special order, to reduce the influence of the borehole fluids and casing if their properties are unknown.
This tool is also known as a 4π Density tool, since it responds equally to material inside and outside the borehole in all directions. It is commonly used to log the density of gravel-packs outside of perforated casings in the producing zones, to see if they have become displaced or clogged with sand. Count-rates are quite linearly related to percentage packing-factor in new gravel-pack situations. The tool can also be used as a Cement Plug logging tool to verify the top of a cement plug placed during well abandonment operations before the cement has hardened, and to ensure that the cement is of the required density.
The double-sealed radioactive source is typically of 50 milliCurie strength and its use will require licensed personnel, certified handling and transportation equipment, none of which is supplied by CBG. We can recommend a suitable supplier and assist with log interpretation methods. Lower-strength sources may also be used but at risk of allowing interference from NORM deposits on casing or Tracer isotopes injected into the formation.
The tool does not provide any feedthrough bus wires, so it is always mounted at the lower end of the tool stack. It cannot be used in combination with a flowmeter for that reason. The GPT-A can be provided as a digital-output version compatible with all other combinable tools including the CPT-B Combo tool, or as a standalone analog pulse-output version for low-cost operations. A rubber fin centralizer is available that fits over the lower housing and provides more accurate and repeatable logs.