Effect of Reynolds number on the development of bubble sizes

It is essential to consider the bubbles’ movements, velocities and their Reynolds number in this case to investigate whether Reynolds number is one of the variables causing bubbles to grow. Figure 4.7 demonstrates the relationship between the Reynolds number of bubbles and bubble sizes at a constant liquid phase velocity of 2.4 cm/s and at different injection pressures. In general, the increasing in Reynold number has a great impact on bubble size. The results showed that as Reynold number increases, the bubble sizes rise, and this agrees with the results obtained by Liu et al., 2016, and Guet, 2004. It should also be noted that this effect was observed for all injection pressures. The maximum bubble size measured at 3 and 4 bar. In addition, it was found that the velocity of bubbles was strongly dependent on bubble sizes and the large bubbles had higher velocities compared with smaller bubbles and these results matched with observations by Acuña and Finch, 2010 that found that large bubbles are faster than bubbles with diameter less than 2.5 mm. This was very clear from the velocity profiles and data. However, there was a slight a reduction in Reynold number of bubbles when injection pressure increased to 5 bar due to reduction in bubble sizes and their velocities at higher pressure at these operating conditions. Therefore, if the bubble sizes were reduced, then the Reynolds number of bubbles would decrease and then the development of two-phase flow would be optimised and postponed, and would not reach the higher Reynolds number where bubbles start to collapse and cause oscillations and turbulence within vertical flow.

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Figure 4-7 The relationship between the gas phase Reynolds number and bubble size at constant liquid velocity 2.4 cm/s.

Furthermore, the measurements of bubble velocities during the upward flow gave very interesting and clear evidence that there were fluctuations within the two-phase flow due to rupture and collapse of bubbles. This happens when bubbles reach critical sizes at specific velocity. Therefore, it is necessary to reduce the average flowing bubble sizes to minimise the development of two-phase flow. During the oil production process, any restrictions to flow within production tubing in the gas lift method are not recommended, because there are other operations undertaken during the lifting process such as wireline operation Therefore, there should be a new technique to change the initial fluid behaviours during the lifting process and prevent bubbles reaching these critical sizes, especially when bubbles collapse and collide with neighbouring flowing bubbles. These regions are known as the transitional flow regions, where the flow behaviour is critical and depending on interfacial forces between phases. There is another type of velocity that must be considered in two-phase flow in the gas lift method. It is known as the liquid phase velocity and in the oil industry it is referred to as the reservoir response (PI) (Asheim, 1988). It is the ability of fluid to flow though reservoir rocks to the wellbore when gas is injected through gas lift valves to the production tubing. Figure 4.8 illustrates the effect of liquid-phase velocity (water) on average bubble size. The results showed that the average bubble size remained stable at 10 mm at low liquid phase velocity

0 5 10 15 20 25 30 35 40 45 50 0 500 1000 1500 2000 2500 3000 3500 E q u iv alen t b u b b le sizes (m m ) Reynolds number

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from 2.4 cm/s to 4.8 cm/s, then it rose gradually ending at 14.02 mm when the liquid phase velocity increased from 4.8 to 14.6 cm, as shown in Figure 4.9. This confirms that the effect productivity index of the reservoir (PI) should be considered in any gas lift flow instability criteria, because as the liquid-phase velocity increases, so to do the bubble sizes. This is supporting the unified criteria for continuous gas lift instabilities by Alhanati et al, 1993 which was neglected by Asheim,1988. Thus, the flow oscillations within the two-phase flow increase. Therefore, average bubble size within the two-phase flow must be reduced to reduce these fluctuations and to achieve better stability.

Figure 4-8: The effect of liquid phase velocity on average bubble size

Figure 4-9: The effect of liquid phase velocity on average bubble size at 1m length

0 2 4 6 8 10 12 14 16 0 2 4 6 8 10 12 14 A VG b u b b le size (m m )

Velocity of liquid phase (cm/s)

I

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The large bubbles (Taylor bubbles) flowing in the centre of the pipe have the highest velocities. This is due to their large size and buoyancy as highlighted by Acuña and Finch, 2010 and shown in Figure 2.12. The results showed that the maximum bubble size increased gradually from 34 mm to 37 mm when the liquid phase velocity was increased from 2.4 cm/s to 14.6 cm/s when the injection pressure was 0.5 bar, as shown in Figure 4.10. This is evidence that the velocity of the liquid phase has a slight effect in contributing to large bubble sizes. As the flow develops downstream these bubbles collapse along the test section. These fluctuations could be reduced if the bubble sizes were reduced slightly.

Figure 4-10: The influence of liquid phase velocity on maximum bubble size