The DGA Directional Gamma tool is a Tungsten-collimated version of the NGT-T tool. These types are also known as “Focused” or “Azimuthal” Gamma tools. New versions are supplied with our NextGen electronics.
It is mechanically and electrically identical to the NGT-T, except for the weight. A smaller diameter crystal is used, but with a longer length to compensate for lower sensitivity. A window is machined along the length of the Tungsten shield that surrounds the detector. Only gamma rays entering from the formation through this window can be detected and counted.
The following two figures show how it works. The DGA is shown in cross-section in a horizontal well, where the borehole is being drilled in a reservoir rock just below a shale-bed boundary. Shale rocks are typically ten times more radioactive than most reservoir rocks. The driller would like to continue in the reservoir and avoid drilling into the non-productive shale bed.
In FIG.1 the driller rotates the DGA facing upwards, and records the count-rate from the tool. Gamma-rays coming from the shale layer above the tool easily enter the detector through a window in the tungsten shield. Relatively few gamma-rays typically come from the reservoir rock below. In FIG.2, with the tool window facing down, less counts are recorded because the detector is shielded from the shale rock. In this way, the ratio between “high-side” and “low-side” counts indicates proximity to the shale bed, and the drill-bit can be steered to avoid it.
When aligned with the tool face or other physical reference, the DGA indicates the direction from which gamma radiation originates within an angle of 50 degrees. The following polar-plot shows how the relative sensitivity varies with azimuthal angle around the tool. The front/back count ratio is 2.5 minimum.
It is important to use the correct Collar/Mud corrections for the DGA – they are NOT THE SAME as for the NGT-T. 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.