Focused gamma ray density logging

Unlike traditional density logging, focused gamma-ray density-logging tools focus on the density of the wellbore fluid.

Focused gamma ray density logging tool

The focused gamma-ray density-logging tool incorporates a compacted slug of Cesium-137 at the bottom of an open cage. The Cesium emits gamma rays, and a lead lens focuses gamma rays in a narrow beam brllel to the axis of the cage. Because the cage is open, wellbore fluid is present inside the cage, and the fluid is in the path of the focused beam. The gamma rays have an energy level low enough that the rays are deflected by the electron cloud surrounding the nucleus of any atom. Furthermore, the amount of backscatter (or absorption) is directly related to the density of the electron cloud and, therefore, to the density of the wellbore fluid. Those gamma rays that are backscattered or absorbed do not emerge from the wellbore fluid at the top of the cage.

At the upper end of the tool cage, a gamma detector responds to the gamma rays that still remain when the beam emerges from the wellbore fluid. A counter determines the counts/min (intensity) of the gamma rays; this information is transmitted through the logging cable to the surface.

Types of gamma-ray detectors

The preferred gamma-ray detector is a scintillation crystal because of its high sensitivity and short dead time (the interval required between detection of different gamma rays). Many tools use Geiger tubes as the gamma-ray detector. A Geiger tube has a much longer dead time than a scintillation crystal. It is necessary to use multiple Geiger tubes to ensure detection of all the gamma rays. Preferably, there are eight tubes, but tools of lesser quality may have as few as three.

Count rate is inversely related to the density of the fluid in the wellbore and is recorded in g/cm 3 vs. depth. The count-rate/density relationship is calibrated first with air in the open cage, yielding a count rate corresponding to a density of 0 g/cm³ and then with tap water in the open cage, yielding a count rate corresponding to a density of 1 g/cm³. For some tools, there is an aluminum block of known density that can be placed in the open cage. There is enough nonlinearity in the relationship to justify a calibration with three known density values, but typical practice is to use only two. Often, the recording system is adjusted so that the range from 0 to 1 g/cm³ spans the right-side track of a typical log. With 20 chart divisions in this track, the recorded log is shown at a sensitivity of 0.05 g/cm³. Attempts to increase the recording sensitivity appreciably over that used for the typical display result in a trace with many irregularities because of the statistical nature of nuclear events. The irregularities can make log interpretation difficult.

The advantage of the focused tool is that it measures only the density of the wellbore fluid. If an unfocused (gravel-pack) tool is used, the gamma rays investigate not only the density of the wellbore fluid, but also that of the pipe wall as well as the material and fluids that are close to the outside wall. The gamma tool can distinguish between the phases in a two-phase mixture, but it cannot distinguish among the phases of a three-phase mixture.

Procedure

The tool should be logged downward at logging speeds between 15 and 30 ft/min in a flowing production well. A constant logging speed should be used, and the same speed should be used for all runs. It is best to log downward from a starting depth above all the perforations, and to log all the way to the deepest depth that can be surveyed. In a slugging or churning multiphase flow, the log may show variable behavior even in intervals that are not perforated. In that case, another logging run is advisable to establish the degree of repeatability. If the result is less than desirable, a stationary measurement can be time-averaged for each selected location. Usually a logging run is also made with the well shut-in. This log should be run after at least two or three hours of shut-in, to be sure that the fluid distribution is stable. If the well has been shut in overnight, a shut-in log can be recorded, but the well must flow for 2 or 3 hours before the first flowing log is recorded. If the tool is centralized, there is a tendency for the recorded density to be somewhat low in a multiphase flow relative to the average density across the wellbore cross section. This is because the lighter phase tends to rise through the center of the cross section, leaving the remainder of the cross section for the heavier phase.

Hazards

Although the health, safety, and environmental risks are generally low, radiation safety procedures should be strictly followed when calibrating or running a gamma-ray density logger. Needless exposure to the radiation from the tool when it is at the surface 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.

Pressure-gradient density log

Another common density tool is the pressure-gradient instrument. As the name implies, this device determines density from a differential; pressure measurement across a spacing of a few feet along the wellbore. These instruments are often called "gradiomanometers" or "differential manometers." Owing to their linearity, a two-point, air/water calibration is sufficient for such instruments. Furthermore, the resolution of the tools is higher than that of the gamma-ray densiometers. However, at high fluid velocities, the apparent density provided by these tools is corrupted by frictional losses in pressure and requires correction. Likewise, in wellbore intervals containing intense fluid turbulence, the apparent values are again corrupted and are uncorrectable. Finally, the apparent density must be corrected for deviation of the wellbore from the vertical.

A pressure-gradient density log is shown in Fig. 1. Near the top of the figure, about halfway across the chart, is a trace labeled "Density." This trace is scaled from 0 to 1.5 g/cm³. Below the perforations, there is a stagnant mud column in the wellbore; the mud’s density is somewhat in excess of 1.5 g/cm³.

Fig. 1 – Flowing logs from a new gas well flowing 2 MMcf/D.

Spent acid and formation water enter the wellbore at the bottom perforation, Depth 1, where a density of approximately 1.2 g/cm³ is measured. This density continues to the perforations at Depth 2. At Depth 2, spent acid, water, and a small amount of gas enter the wellbore and mix with the flow from below. After mixing, the gas contributed at Depth 2 causes a density decrease from approximately 1.2 g/cm³ to approximately 1.1 g/cm³. The density remains at that level to Depth 3.

Additional entries of spent acid and gas at Depths 3 and 4 result in further, slight reductions in density.

Major gas entries at Depths 5 and 6 reduce the density to less than 0.3 g/cm³.

Redistribution of phases occurring in the 20 ft above 9,300 ft results in a density averaging approximately 0.4 g/cm 3 below and within the tubing. The actual density of the gas/liquid mixture decreases significantly in the tubing stream relative to the mixture density in the casing just below the end of tubing. This happens because the gas holdup in the stream in the tubing increases in response to the increase in fluid velocity. This significant decrease is hidden on the density trace by the increased frictional loss associated with the increased velocity.

The four liquid entries and the two major gas entries correlate with the temperature behavior. Please refer to the discussion of the temperature logging tool for a detailed interpretation of the temperature data.

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Focused gamma ray density logging

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