Flowmeters - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 68

The generally accepted practice is to calibrate the continuous flowmeter in the
well itself rather than trying to estimate the effects of viscosity and friction. The
technique calls for several passes to be made in the well at different cable speeds
both against and with the flow direction. A plot may then be prepared from which
the analyst can deduce the fluid speed at each point in the well. Figure 5.11 illus-
trates the technique.

Fig. 5.11 Calibration of continuous flowmeter. Courtesy Schlumberger

The tool velocity (from the cable speed and direction recording in Track 1) is
plotted against rps for a single depth in the well for several passes of the tool. In this
example, four passes were made going down (see Quadrant I) and three coming up
(Quadrant III). The procedure is to extend a line through the “down” points to inter-
sect the zero rps line and extend another line through the “up” points to intersect the
same line. Midway between these two intersections is marked the point O'. The true
fluid velocity is then read off from zero tool velocity line to the point O'.
A graph of this sort is prepared for each point in the well for which a flow rate is
required. (Normally the service company engineer will make these calibration plots.
However, the reader should know how to do it.)
Note that flow rate is still unknown. The tool has measured the fluid speed and
direction. It remains to convert the fluid speed to a volumetric flow rate by multiply-
ing by the cross-sectional area of the pipe. Conventionally, flowmeter results are
quoted as fluid speeds in feet per minute. A useful conversion is:

=´ ´ (

)

2

BD ft / 14 . / min casingIDininches .

Before making the conversion, remember that flowmeters are run centralized in
the pipe and therefore record the speed of the fluid in the center of the pipe.