Experiments for Gas-Liquid Flow in a vertical Annulus

Abstract

Shifting from coal and oil towards sustainable energy sources is currently an urgent and global challenge. Compared to coal and oil, natural gas is a cleaner fossil fuel and may serve as a transition fuel. To meet future demands for gas there is a desire to optimize the production from existing reservoirs with their production facilities. In case of wet natural gas wells this gas production can be hampered by the presence of liquids, namely water and condensate, thus creating a multiphase flow in the production tubing. The maximum gas well capacity is limited by the ever reducing upward gas flow velocity due to the subsurface reservoir pressure that is reducing over time. The reduced gas velocity results in the inability to transport the liquid upward from the downhole location to the surface. The accumulated liquids will then block the well bore (so-called liquification), making further gas production impossible. One solution it to decrease the cross sectional flow area by inserting an inner pipe in the existing tubing, thus creating an annulus. This results in a higher gas velocity. The multiphase flow characteristics in the annulus tubing are not yet fully understood, such as the pressure drop, the liquid hold-up and the different flow regimes. Therefore in the present study lab experiments were carried out to further investigate those characteristics to enable optimisation of predictive flow modeling. The main focus is on the behaviour of the liquid film along both pipe walls. In this study a 12 m mid-scale vertical annulus with a 124 mm diameter outer pipe and a 100 mm diameter inner pipe was designed and constructed. The experiments were conducted at atmospheric conditions using an air-water mixture as the working fluids. Multiple combinations of gas and liquid throughputs were studied. The liquid at the inlet could either be injected on the inner pipe, the outer pipe or equally divided on both pipes. The position of the inner pipe with respect to the outer pipe was adjustable to create different eccentricities. To measure the local film heights flush mounted conductance sensors were designed, built and installed in the half circumference of both pipes. The efficacy of the sensors was thoroughly studied and the sensors were subjected to multiple test cases. The concentric annulus experiments showed that the pressure drop does not depend on the method of liquid injection. However, the liquid hold-up fraction at lower liquid throughputs was observed to be lower for the single wall liquid injection as compared to injection on both pipes. The redistribution of the liquid film for the single liquid injection depends on the liquid throughput. Larger liquid throughputs gave thicker liquid films containing a wavier gas-liquid interface from which droplets are atomized and subsequently migrated to the other pipe wall. Furthermore, it was found that the critical superficial gas velocity at which liquid loading occurs is 14 m/s, which is neither dependent on the method of liquid injection nor on the liquid throughput. The designed sensors were able to provide a good indication of the film heights. Single phase experiments, using air as the working fluid, showed that the pressure drop is reduced by 30$\%$ in the fully eccentric case in comparison to the concentric case. The eccentric annulus configuration with air-water flow showed that the liquid films along both pipe walls may merge. This so-called liquid bridging mainly occurs at higher eccentricities and causes flow reversal in the narrow gap. This results in a much higher liquid hold-up and pressure drop. Eccentricity induces redistribution of the liquid film from the inner pipe to the outer pipe at higher superficial gas velocities. It was found that eccentricity promotes an unequal film height along the circumference of each pipe: the film height decreases when going from the narrow gap towards the wide gap.