Flowmeters - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 74

In the flowmeter mode the PNC tool is used to record a background gamma ray
log with the neutron generator switched off. The gamma rays detected come only
from the natural decay of radioactive materials in the formation surrounding the
borehole (potassium, thorium, and uranium).
The tool is then lowered to the bottom of the hole and the neutron generator
switched on. Provided that the flow speed of the activated oxygen in the tool-casing
annulus is greater the speed of the tool as it is raised in the well then the gamma rays
emitted from the activation decay will show up as the water stream catches up with
and passes the gamma ray detector on the tool. The speed at which the activated
oxygen flows, depend simply on the upward volumetric flow rate of the wellbore
fluids and the available cross-sectional area between the tool and the ID of the cas-
ing (or tubing, as the case may be). This is beyond the control of the operator con-
ducting the survey. However the logging speed can be controlled and indeed must
be in order to assure that usable data be recoded. If the tool is reeled in too fast and
the low rate of water is low the activated oxygen may never catch up with the upper
gamma ray detector and be seen.
In practice any attempt at quantification of flow rates and water flow entry points
must rely on the knowledge of the distance L between the neutron source and the
gamma ray detector. L for the “upper” detector might typically be close to 20 ft. To
the “far” may be 2 ft. The tool speed also needs to be accurately known. The dis-
placement D between the point on the log where a difference is seen between the
original background log and the flowing log needs to be referenced to the point
where the neutron source was when the oxygen (water) sample became activated.
The speed of the flow stream V is then related to the measured quantities by the
equation:

L ´ Tool Speed

V =

DL -

The main uses today for activation logging are:

• Determination of water entry into tubing
• Identification of watered-out perforations
• Detecting the upward flow of water behind casing (channeling) in some cases

Radioactive Tracers and Thermometers

Both radioactive tracer-tool surveys and temperature logs can indicate flow rates
and flow directions. However, since they each form a subject in itself they are treated
separately. Chapter 6 covers the radioactive tracer logs in detail and Chap. 7 covers
temperature logs.