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Investigating the Effect of Salinity on Counter-Current Two-Phase Flow Regimes in Annuli

In coal seam gas (CSG) operations gas and associated water enter the casing-tubing annular space of wells 
from multiple producing intervals, forming counter-current two-phase flow patterns within the wellbore. 
These flow patterns, or regimes, determine the pressure profile in the wellbore, influencing flow and the 
production efficiency. Salts such as sodium chloride (NaCl) are abundant in the formation water, and are 
known to inhibit the coalescence of gas bubbles within gas-liquid flows. However, the effect of salinity on 
all of the flow regimes and their transitions has not previously been clarified. 
To investigate the influence of salinity on two-phase flows we conducted experiments in an apparatus 
designed to resemble a pumped CSG well in Queensland, Australia consisting of 7-in casing and 
2¾-in tubing. Two-phase flow regimes (homogeneous and heterogeneous) and their transition zones 
(homogeneous-heterogeneous and post-heterogeneous) were induced using saline solutions of up to 
11,198±142 ppm, and associated differential pressure measurements recorded for comparison with results 
using fresh water. Formation water produced from a Queensland CSG well was also tested. Signal 
processing techniques (autocorrelation, power spectral density, Shannon entropy and permutation entropy) 
and high-speed image analysis were applied to characterize the two-phase flow regimes. 
Reduced bubble coalescence resulted in higher void fractions developing within saline water at equivalent 
flow rates to fresh water during the homogeneous regime and homogeneous to heterogeneous transition 
zone, with an increase in void fraction of up to 5-6%. These flow behaviours predominantly consist of 
small gas bubbles and so are most affected by salinity. Once the flow rates were increased and the system 
transitioned to heterogeneous and post-heterogeneous flow, the impact of turbulence became increasingly 
dominant and the effect of salinity was less significant.