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Why Liquid Loading Occurs, And How This
Knowledge Helps To Prevent It.
A.T. van Nimwegen1
L.M. Portela1
R.A.W.M. Henkes1,2
G.J. de Vries3
1
Delft University of Technology, The Netherlands
Shell Projects & Technology, The Netherlands
3
NAM, The Netherlands
2
Abstract
Liquid loading is a problem encountered in the production of natural gas, which occurs when
the gas velocity in the production tubing is no longer sufficient to drag the liquid, associated
with the gas, upwards. The liquid subsequently accumulates in the well, exerting a hydrostatic
pressure on the reservoir and limiting the production of gas.
When the reservoir pressure is high, and the gas velocity in the tubing is large, and there is
relatively regular annular flow, in which the liquid moves upwards in a liquid film at the wall
and in entrained droplets in the gas core. When the gas velocity becomes low, the flow
becomes a much more irregular churn flow, in which the liquid moves upwards in large,
aerated flooding waves and, between the waves, the liquid film is very irregular and moves
up- and downwards. The transition between annular flow and churn flow is closely related to
liquid loading.
The critical gas velocity, below which liquid loading occurs, is usually predicted by the Turner
criterion [1], which states that liquid loading occurs when the gas is no longer able to drag the
largest droplets in the flow upwards. The calculation of the critical velocity therefore requires
an estimate of the size of these largest droplets. However, experiments show that the droplet
size estimated by Turner is too large: such large droplets do not occur in the flow.
Using movies made with a high-speed camera, we show that it is the reversal of the liquid film
that causes the transition between annular flow and churn flow, and not the reversal of the
largest droplets. We also show the complicated morphology of the gas-liquid interface in the
churn flow regime, where many droplets and ligaments are formed. This morphology causes
the large pressure gradient that prevents production of gas in wells.
This improved understanding of why liquid loading occurs can help us in improving the
modelling of the onset of liquid loading. But, more importantly, the knowledge that the reversal
of the liquid film is the major cause of liquid loading, helps us to develop deliquification
techniques. We review two deliquification techniques that change the nature of the liquid film.
Using a hydrophobic coating on the pipe wall, the liquid film is completely removed. By
injecting surfactants the liquid film will foam, decreasing the density of the liquid film. Both
these techniques decrease the critical velocity.
The project is funded by NAM, a Dutch subsidiary of Shell and ExxonMobil. The authors
would like to thank Kees Veeken, Ewout Biezen and Ruud Trompert, from NAM, for the
valuable discussions.
[1] RG Turner and MG Hubbard. Analysis and prediction of minimum flow rate for the
continuous removal of liquids from gas wells. Journal of Petroleum, 22(11):1475–1482, 1969.