Aplicando un enfoque colectivo al estudio del afecto.

The distribution of air bubbles has a major impact on the lifting performance and the stability of two-phase flow in the gas lift method in the oil industry. Therefore, the path of moving air

0 50 100 150 200 250 0 1 2 3 4 5 Nu m b er o f b u b b les

Injection pressure (bar)

95

bubbles should be changed from the middle of the vertical pipe to across the entire pipe area. This will reduce the possibility of the coalescence process among air bubbles, avoid the creation of large bubbles in the test section, and thus delay the development of two-phase flow regimes. This agrees with distribution of air void fraction reported by (Szalinski et al., 2010).

Figure 4.19 demonstrates a comparison between the performances of the new Multiple Nozzles Injection Technique and the Single Nozzle Injection Technique in distributing initial bubbles at 0.5 bar injection pressure.

Figure 4.19 shows that there is a considerable increase in smaller bubble frequency of bubbles sizes between 6.5 and 7 mm when the MNIT is used, compared with the SNIT, which had a frequency of 11 with larger bubbles between 9 to 9.5 mm at 0.5 bar injection pressure. This is because MNIT creates a large number of smaller bubbles via its small orifices.

In addition to that, when these small bubbles enter the test column they spread across the pipe area and even close to the pipe wall. This is perceived to be due to the high bubble velocities’ penetration in the liquid phase across the pipe area at the injection point and the smaller orifice sizes in the new technique. This type of bubble distribution is called wall peaking, as typified in Figure 4.20. This type of distribution increases the liquid phase lifting performance. On the contrary, the path of bubbles which exit port-size of the single orifice valve moves directly to the centre of pipe and then they rapidly coalesce with neighbouring bubbles. This is called core peaking and it is one of main reasons for the development and instability of the two-phase flow in a vertical pipe, as this type of bubble distribution gives a high possibility for bubbles to coalesce and develop in the middle of the vertical column.

96

Figure 4-19: Comparison between the distribution of air bubbles using the MNIT and SNIT at 0.5 bar; with S.D =1.01

Figure 4-20: Changing bubble distribution from core peaking to wall peaking using the MNIT

0 2 4 6 8 10 12 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 10.5 11 11.5 12 Nu m b er o f Fre q u en cy Bubble size (mm) MNIT SNIT

97

Moreover, when both techniques were investigated at higher injection pressures, it was found that as the injection pressure increases, the distribution of bubbles improved significantly, especially when the new multiple nozzle injection technique was used, due to the increase in the number of smaller bubbles occupying the entire area of the pipe, including in the vicinity of the pipe wall. There was sufficient distance between the upward flowing bubbles to minimise the collision and coalescence processes between neighbouring bubbles when the superficial velocity increased and with the development of the flow patterns (bubbly, slug, churn and annular). Therefore, this two-phase flow development can be controlled by reducing the initial air bubble sizes and changing the distribution of bubbles in the column. Furthermore, this has the potential to increase the total oil production from the gas lift method in the oil industry because the performance is improved, as shown in Figure 4.16.

Figure 4.21 demonstrates the comparison between the bubble distributions of the new multiple nozzle injection technique and a single nozzle injection technique at 5 bar injection pressure. As can be seen in Figure 4.21 there was a sharp increase in the frequency of bubbles with sizes between 3 to 3.5 mm when comparing the two techniques. There was peak frequency of bubble sizes between 5.25 to 5.5 mm with SNIT. Figure 4.21 also indicates that the bubble distribution shifted to smaller bubble sizes with better distribution when MNIT was used. In addition, the air injection rates were the same for both cases. Finally, could be concluded that the distribution of initial small bubble sizes has a major effect on the stability of two-phase flow behaviours and increases the performance of the gas lifting process. Thus, the growth rate of bubble sizes generated by the new technique along the vertical pipe makes it impossible for the bubbles to reach the size of the large bubbles produced from the SNIT, even if the flow is developed. This is because the type of bubble distribution from the new technique is maintained wall peaking. This means that the possibility of smaller bubbles coalescing and developing is lower and is due to these smaller bubbles travelling along the pipe with almost the same size and mixture velocity. In comparison, the bubbles produced from the SNIT have a greater opportunity to grow and coalesce with other bubbles, creating large bubble sizes then collapsing when they reach their critical size. This is one of the main reasons for the pressure fluctuations and two-phase flow instability along the test section.

98

Figure 4-21: Comparison between the bubble distribution of the two techniques at 5 bar injection pressure