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.