Reducing Bubble Sizes within Two-Phase Flow

4.3.1.1 Reducing Initial Average Bubble Sizes

Reducing the initial bubble sizes has a major impact on the stability of two-phase flow in the vertical column in the gas lift method, especially if these bubble sizes were reduced and have a better distribution across the entire pipe area. Figure 4.13 illustrates a comparison between the performances of the new multiple nozzles injection technique (MNIT) and the single orifice gas lift valves in reducing the initial average bubble sizes at different injection pressures and a constant liquid flow rate of 5 l/min. Overall, the most significant features of the line graph are: the average bubble sizes generated by the new technique (MNIT) were

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lower than the average bubble sizes produced by the SNIT at different injection pressures. This is due to smaller diameter of each nozzle in the new technique, which creates higher jet velocity at the injection point. Figure 4.13 shows there is a steep decrease in the initial average bubble sizes from 9.76 to 7.23 mm at 0.5 bar, when the new multiple nozzles injection technique was used, representing a 25% reduction in the average initial bubble sizes. Then the size declined considerably to 4.42 mm at 4 bars, which represented a 34% reduction compared with an orifice valve. Subsequently, a rapid drop in the average bubble sizes ending at 3.89mm, when injection pressure was 5 bars. Moreover, there was a 23% decline in average bubble size between the two systems. As a result of this, the average overall reduction between the two techniques was 22% at different injection pressures with the same port size dimensions of 2.24 mm.

Figure 4-13: Comparison between the new MNIT and the SNIT for gas lift optimisation at a constant flow rate 5 l/min.

4.3.1.2 Reducing Initial Average Minimum (small) Bubble Sizes

Furthermore, there was confirmation of the performance of the new multiple nozzles injection technique (MNIT) in reducing all different bubbles sizes including the small flowing bubble sizes in the vertical test section. Further investigations were carried out to ensure that the small bubble sizes were reduced likewise. Figure 4.14 demonstrates a comparison between using the new multiple nozzles injection technique and the SNIT in

0 2 4 6 8 10 12 0 1 2 3 4 5 A VG b u b b le sizes (m m )

Injection pressure (bar)

SNIT MNIT

I

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reducing small bubble sizes at different air injection pressures during the upward two-phase flow in the test section. In gerenal, the results showed that the average minumum bubble size produced from the MNIT was lower than the SNIT under the same operating conditions. There was a gradual reduction in minimum average bubbles size to 1.17 mm when the new technique was used, compared with the single orifice technique was 1.37 mm at 0.5 bar injection pressure. Thus, the reduction in average small bubble size was 14.54% at this point, and reduced to 0.99 mm at 3 bars. The sharp edge orifice valve was 1.23 mm under the same operating conditions. Moreover, the reduction in the small bubble sizes was 19% at this point between both techniques, and then there was a regular reduction ending at 0.96 mm when the injection pressure was 5 bar, compared with the single orifice, which was 1.19 mm. In addition, the average overall reduction in the small bubble sizes between both gas lift valves was 16.1% at different injection pressures. Therefore, the new technique is effective in reducing bubble sizes, including the small sizes.

Figure 4-14: Comparison between the new MNIT and the SNIT for gas lift optimisation at constant flow rate 5 l/min 0 1 2 3 4 5 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Injection Pressure (bar)

A v er ag e m in im u m b u b b le sizes (m m ) MNIT SNIT

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4.3.1.3 Reducing Initial Average Maximum (large) Bubble Sizes

In addition, large bubbles (Taylor bubbles) flowing in the middle of the pipe have a great influence on the stability of two-phase flow in the gas lifted wells. This is due to their sizes being critical and close to the collapse region of bubbles. Therefore, it is very important to reduce the sizes of large bubbles to avoid the rupture of these large bubbles and their collision with other nearby bubbles. This is one of the main causes of the flow instability phenomenon. Figure 4.15 demonstrates a comparison between the performances of the MNIT and SNIT in decreasing the average large bubble sizes flowing in the centre of the test section. The results showed that at the first injection pressures, the average large bubble sizes produced from the new multiple nozzles injection technique were higher than for the orifice valve. Because of at the low injection pressure less than 1.3 bar, the velocity of the air was slightly low through the nozzles of the MNIT to shear the air bubbles. However, after 1.3 bar injection pressure, the trend in average large bubble sizes from the single orifice became higher than for the multiple nozzles. This means, at this intersection point and operating conditions the average bubble size of air bubbles are the same sizes. Furthermore, the average large bubble sizes started to decline gradually after the injection pressure reached 1.5 bar, followed by a slight decline to 43.49 mm when the injection pressure was 2 bar when the multiple nozzles was used. In comparison, the single nozzle valve was 48.8 mm at the same pressure, and thereafter descended moderately to 42.09 mm at 3 bar. In comparison with the single nozzle orifice average bubble size was 50.64 mm, then there was a sharp drop in the average large bubble sizes ending at 38.30 mm when the new MNIT was used, at 5 bar injection pressure, which was 43.33 mm at the same injection pressure. The overall reduction in the average large bubble sizes between both techniques at different injection pressures was 8.22%. It can be concluded that Multiple Nozzle Injection Technique is capable in reducing large bubble sizes in the test section compared with Single Nozzle Injection Technique with the same geometrical dimensions and operating conditions especially at higher injection pressures.

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Figure 4-15: Comparison between the MNIT and SNIT for gas lift optimisation at a constant flow rate 5 l/min