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Unfocused gamma ray density logging
Unfocused gamma-ray density logging is also sometimes called gravel-pack density logging. While focused density logs assess wellbore fluids, the unfocused density logs look at the fluid below the screen or in the gravel pack.
The unfocused gamma ray density logger incorporates a compacted slug of Cesium-137 near the bottom of the device. A gamma-ray detector, located approximately 20 in. above the slug, responds to incident gamma radiation. A counter determines the counts/min (intensity) of the gamma rays; this information is transmitted through the logging cable to the surface, where the count rate is plotted against depth.
From the Cesium-137 source, some of the gamma rays are transmitted to the detector through the tool body ("direct" transmission), some by the wellbore fluid between the tool and the casing ("indirect" transmission), and some through the material outside the casing (also "indirect" transmission). Not all of the gamma rays from the source reach the detector because of backscattering by the wellbore fluid, but the majority of the detector’s response is attributable to this transmission.
As in the focused detectors, a scintillation crystal is preferable, but Geiger tubes are used in many tools. Preferably there are eight tubes, but tools of lesser quality may have as few as three.
When the tool is at the surface, radiation safety procedures should be strictly followed. Needless exposure to the radiation from the tool should be avoided. Logging-company personnel should have current radiation training and certification. Because of the long half-life of Cesium-137, the legal restrictions on the use of the tool vary from state to state and country to country. If the tool is dropped in the well or becomes stuck, it must be retrieved or cemented over.
Logging-speed and shut-in times for a shut-in survey are the same as recommended for the focused tools.
An example of an unfocused gamma log appears in Fig. 1. A gradiomanometer (pressure-gradient) density log also is shown. Logs were recorded with the well flowing and shut-in.
During shut-in, both tools identify an oil/water interface at Depth 6. The gradio shows that the density of water in the screen below the interface is approximately 1.1 g/cm3. Although the gradio responds only to fluid inside the screen, 1.1 g/cm3 is also the density of the fluid in the porosity of the pack below the interface, because this porosity is water-filled during shut-in. Above the interface, the oil density in the screen is approximately 0.6 g/cm3; although the gradio surveys only within the screen, the density of the fluid in the porosity of the pack is also 0.6 g/cm3, because this porosity is oil-filled during shut-in.
Because gamma-ray count rate is inversely related to density, the count rate on the shut-in unfocused log is lowest in the water below the interface. Whether the transmission is direct or indirect, the transmission is lower and the count rate is lower when the density of the fluid in the transmission path is higher.
Above the interface, the unfocused gamma ray shows its greatest count rate in the oil in the screen and pack.
During flow, the gradio shows essentially water in the screen below Depth 1. Farther down, at Depth 5, however, the unfocused gamma ray shows a much higher count rate than during shut-in. Because the screen at this depth is water-filled, as during shut-in, the increased response implies more transmission of gamma rays through the pack than during shut-in. More transmission through the pack is attributable to a lower density fluid in the pack. Thus, oil is present in the pack on the high side of the casing at Depth 5, but it is not in the screen. From Depths 1 to 2, the gradio shows decreasing density in the screen. This means that oil enters the stagnant water in the screen between Depths 1 and 2. Consequently, the oil in the pack at Depth 5 flows up the pack and enters the screen between Depths 1 and 2.
There is an oil-jet entry in the screen at Depth 2. Correspondingly, the gradio shows a slight spike toward lower density. The unfocused gamma ray shows a spike toward higher count rate because the oil jet lowers the density around the tool, causing more transmission of gamma rays to the detector.
At Depths 6, 7, 8, and 9, the unfocused gamma ray shifts somewhat toward higher count rate. From Depths 6 to 7, the gradio, however, shows no change of the density of the fluid in the screen. The same is true for Depths 10 to 9. Consequently, the shifts toward higher count rate of the unfocused log are attributable to increased transmission of gamma rays through the pack. Each shift, then, implies an increased presence of oil in the pack, and thus, an entry of oil to the pack at each of the four depths. At Depth 4, a slight decrease of the gradio response indicates that oil from the four entries flows up the pack and enters the screen over the 10 ft immediately above this depth.
Both logs show little or no contribution from the top, short interval at 8,600 ft. The gradio shows no change of the density of the fluid in the screen, and the unfocused gamma ray also shows no change, implying that no change occurs in the pack.
At Depth 10 (8,670 ft), the unfocused gamma ray response decreases on both the flowing and shut-in logs. During shut-in, the screen and the pack are both oil-filled above and below Depth 10; thus, the decrease during shut-in cannot be explained by a change of the density of the fluid, whether in the screen or in the pack. Also, the flowing gradio response is unchanged at Depth 10; thus, the decrease during flow cannot be attributed to a change of the density of the fluid within the screen. Instead, the decrease is attributable to a change in the porosity of the pack, with the porosity decreasing at Depth 10, resulting in an increased density of the pack above Depth 10, relative to the pack density below this depth. Above Depth 10, the increased pack density results in lower transmission of gamma rays through the pack and, thus, a lower count rate. Consequently, the decrease appears on both the flowing and shut-in logs.
In this example, it is important to note that the unfocused gamma ray, by itself, cannot distinguish between entries to the screen and entries to the pack. In addition to the unfocused gamma ray, this distinction requires a gradio log, as in this case, or a focused gamma-ray log.
The comparison of the two density logs has shown that, at least in the lower parts of the hole, oil moves upward as a separate layer on the high side of the hole. The production logging application tables show that in such situations, the count rate from the unfocused density tool is linear in the fraction of water occupying the casing’s cross-sectional area. If this concept is applied to the data in Fig. 1, then the two unfocused-density trace shows a constant water holdup of 28% below Depth 6 during flow.
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