The experimental study of counter-current two-phase flows in annuli

Abstract

Counter-current two-phase gas-liquid flows in annuli occur in many industrial applications, including coal seam gas (CSG) production. In CSG wells, gas is typically produced through counter-current gas-water flow in an annulus. Yet, the models used by CSG companies were designed for conventional wells, wherein co-current upward gas-liquid flow occurs in pipes. Accurately predicting the ensuing flow regimes and counter-current flow limitation (CCFL) is important because they are intrinsically linked to flow characteristics, and dictate operating conditions. The CCFL describes the onset of co-current upward or downward flow due to high gas (gas flooding) or liquid (liquid flooding) rates, respectively. There are currently no experimental publications on counter-current gas-liquid flow regimes in annuli, prohibiting predictive models from being validated. I collated 3,947 flow regime results from 35 experimental studies of counter-current flows in pipes and co-current flows in pipes and annuli, and confirmed that flow configuration, channel geometry, and fluid properties can significantly influence flow regimes (Chapter 2). This thesis aims to experimentally characterise flow regimes and the CCFL during counter-current two-phase flows in an annulus. These findings will allow predictive models of counter-current flow regimes in annuli to be validated.To study counter-current flow regimes in an annulus (Chapter 3), I designed and built an experimental apparatus using acrylic pipes with annulus diameters of 170 mm and 70 mm (170/70 annulus). Flow regimes and the CCFL were documented for air-water at standard conditions using combinations of superficial gas velocities within 0.014-5.794 m/s and superficial liquid velocities within 0.004-0.240 m/s. Differential pressure data were recorded over 1 m and used to infer void fractions, and to quantitatively assess the flows using fast Fourier transform (FFT). High-speed video images were captured at 4,000 frames/s. An apparatus with annulus diameters of 44 mm and 19 mm was also assembled to investigate the impact of annulus size.My observations confirm the 170/70 annulus is a large annulus in which stable slug flow regime cannot develop due to Rayleigh-Taylor instability. Subsequently, the flow regimes in this system are described as homogeneous and heterogeneous instead of bubble and slug-churn regimes.Churn-annular flow was observed, but, the CCFL was reached before a fully annular regime could develop. Existing empirical correlations for the CCFL only considered gas flooding and performed poorly when assessed against my experimental results. So, I developed an empirical correlation for the CCFL that also incorporated liquid flooding. Liquid flooding was detected using a novel experimental technique developed in Chapter 5. Essentially, a water barrel beneath the annulus, used for water re-circulation, was dual-purposed as a vertical separator. The proposed correlation for CCFL ii in an annulus is * 1/2 + 1.7 * 1/2 = 0.56, where * and * are the dimensionless gas and liquid s...