This incident illustrates the importance of adjusting field compressor operation to maintain a minimum velocity in the production tubing. The velocity must be sufficient to entrain water, which migrates into the well, up into the high pressure separator. Based on a limited amount of data taken in gas field operation and a more substantial data base developed in the process industry, the following rule of thumb is suggested:
This equation for entrainment velocity is in the form of Stokes Law for settling of particles in a fluid. The coefficient of 1.2 will vary with gas viscosity, depth of the producing formation and the presence of surfactants in the well liquids. The reader should develop a suitable coefficient from his own experiences. Correlations developed by other workers in this field suggest that the minimum velocity to “unload” a well is greater than that shown above. Note that adding soap sticks to a well reduces the DL term in the above equation by over 50% and thus effectively lowers the entrainment
velocity.
