Fundamentals of production logging

Ideally we would like to measure radial inflow rates using a cheap and accurate sensor. Unfortunately no such sensor exists. Alternatively we could measure the axial flow rate in a well at a depths above and below the zone of interest and compute the difference and hence the inflow rate. Unfortunately there is not any practical measurement of axial flow rate beyond some special applications of oxygen activation logs. However it is possible to measure an axial velocity and combine this with an assumed or measured internal diameter to arrive at an axial flow rate. This last approach is most commonly used. Common velocity sensors include;

1.     Turbine/Spinner flowmeters.

2.     Markers/Tracers such as oxygen activation logs or radioactive iodine tracer logs.

3.     Heated anenometry

Turbines or Spinners are assumed to rotate at a speed proportional to the average fluid velocity passing through the swept area of the blades with an offset for friction/imperfections. This becomes a simple gain and offset transformation from the rotational speed of the spinner. Unfortunately the gain and offset are not constants but are a complicated function of fluid density, fluid viscosity, spinner pitch, pipe diameter, fluid velocity, etc. This means that the spinner is typically calibrated downhole by recording the spinner speed at a series of different logging speeds (usually 30, 60,90 ft/min or 10, 20, 30 m/min) and plotting the resultant average spinner speed versus the corresponding average logging speed to determine the slope (gain) and threshold (offset). The calibrated spinner velocity then needs to be converted to an average pipe velocity.The correction coefficient determined by the velocity profile across the pipe cross section can vary from 0.5 for an infinitely small spinner in laminar flow to 1.0 for a spinner that sweeps the entire pipe area. N.B. The prefix "full bore" when applied to a spinner is a marketing name. Full bore spinners rarely cover more than half the pipe cross section. If the cross sectional area of the spinner at the depth of the spinner blades occupies a significant fraction of the pipe area then the pipe area should be reduced before multiplying it by the spinner velocity and the correction coefficient.