Effect of reducing initial bubble size on production rate

The new the multiple orifice nozzle was installed at the injection point, the initial bubble sizes were reduced as discussed in the previous sections, and the distribution of air bubbles improved significantly. As a result of this, the systemic instability within two-phase flow was reduced, the vertical lifting performance was increased and gas lift system became more stable in comparison with the single sharp edge orifice. This was reported by (Hu, 2005) that the systematic flow instability can cause serious flow oscillations within the system. In most cases, these oscillations are a major cause of production losses and are harmful to operational smoothness, safety and efficiency. Figure 4.29 illustrates the performance of the multiple nozzle injection technique and single (conventional) orifice valve in increasing the outlet flow rate (oil production rate) at different injection pressures and a constant inlet flow rate 30 l/min. The flow rate is measured by a digital flow meter at the experiment outlet lines. This digital flow meter measures the volumetric flow rate not mass flow rate. As result of this, there are small differences between inflow and out flow figures. This is due to the complexity of measurement of multiphase flow and it’s behaviours within vertical test section.

Overall, the most significant feature shown in Figure 4.29 is the outlet liquid flow rate (production rate): that tends to be higher with the multiple orifice nozzle than with the single orifice nozzle. The results shows that there is an increase to 36.6 l/min when the multiple orifice nozzle injection technique was used at 0.5 bar injection pressure, compared with the single orifice nozzle of 34.8 l/min. The increment in the outlet production rate was 5.2% at low pressure. Afterwards, it rises sharply to 41.2 l/min when the MNIT was used in comparison with the SNIT, which is only 38.6 l/min at 2 bar injection pressure. The difference between both techniques at these operating conditions was 6.7%. Subsequently there was a rapid growth ending at 50.10 l/min at 5 bar injection pressure, compared with the single orifice nozzle, which is 46.6 l/min. Finally, it is concluded that the new multiple orifice injection technique increased the average lifting performance and production rate by 7.5% at different injection pressures compared with the conventional single orifice gas lift.

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Figure 4-29: Comparison between the performance of the two techniques in increasing (production rate) at a constant inlet liquid flow rate 30 l/min.

4.4 Summary

This Chapter presents the experimental results and discussions of the variables that can affect the growth of bubble sizes within upward two-phase flow and lead to flow instability in gas lift system. The two design configurations that were used Single Nozzle Injection Technique (SNIT) and Multiple Nozzle Injection Technique (MNIT) on a simulated column apparatus for gas lift optimisation were compared against each other. The summary of the findings are as following:

a) § 4.2.1 showed that when the air injection pressure was increased from 0.5 to 5 bars, the average air bubble sizes reduced from 9.75 to 5.06 mm at contant liquid phase velocity 2.4 cm/s.

b) § 4.2.2 indicated that as bubble velocity increases, the bubble sizes increases. However the velocity of bubbles reduced at 5 bar injection pressure.

c) § 4.2.3 showed that as port size increases, the bubble sizes increases and have a negative effect on the stabiltiy of the two-phase flow.

d) § 4.2.4 presented the mechanism of bubble coalescence and development at the middle of the test section when the single orifice technique was used.

e) § 4.3.1.1 showed the MNIT reduced the initial bubble sizes by 22%. 0 10 20 30 40 50 60 0 1 2 3 4 5 P ro d u ctio n f lo w r ate (l/m in )

Injection pressure (bar)

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f) § 4.3.1.2 showed that the MNIT decreased the average small bubble within two-phase flow by 16%.

g) § 4.3.1.3 indicated that the MNIT is capable in reducing large bubble sizes (Taylor bubbles) by 8%.

h) § 4.3.3 indicated that the MNIT is cable of changing the distribution of bubbles in the simulated column apparatus from the core peaking to wall peaking.

i) § 4.3.5 confirmed that the average bubbles sizes generated from the MNIT are smaller than using the SNIT, even at different length of the test section.

j) § 4.3.6 showed that the lifting performance was increased by 7.5% when the MNIT was used compared with SNIT

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The overall effect of parameters on the stability of two-phase flow in gas lifted well can also be summarised in Table 4.3.

Table 4-3: Summary of the effect of parameters on the stability of two-phase flow

Parameters Stability of Two-phase flow

Lifting

performance Production rate

Port size

The increase of port size has destabilising effect

The increase of port size decrease the lifting performance of gas lifted system

Reduce the production rate because it cause flow instability

Injection rate

At high rate stabilising effect but at low rate destabilising

Increase partially Increase partially

Injection pressure

Increase has stabilising effect because it reduces bubble sizes

Increase partially Increase partially

Distribution of gas bubbles

Core peaking distribution has destabilising effect ,however, wall peaking stabilising effect

Core peaking decreasing and wall peaking increasing

Both effects

Reducing bubble

sizes Stabilising effect Increase Increase

Single nozzle (SNIT) Destabilising effect Reducing Reducing

Multiple orifice

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5

CHAPTER 5

Two-phase Flow Modelling using Computational Fluid Dynamics

5.1 Introduction

A huge amount of oil and gas are consumed every day and the slightest enhancement in extraction efficiency will have a substantial influence on profits for companies in the oil and gas industry. Therefore, solving the flow instabilities of multiphase flow in gas lift systems is the priority for oil companies. One of the methods used to understand fluid behaviours and optimise multi-phase flow is computational fluid dynamics (CFD) (Çengel and Cimbala, 2014).

The aim of this chapter is to use ANSYS Fluent computational fluid dynamics to simulate and validate the experimental results data. The research will investigate the reasons for flow instabilities and aspects of optimising gas-lift effectively and efficiently. This will be achieved by assessing the effects of operating conditions of flow formation, bubble behaviours, pressure drop, gas void fraction and the interactions between phases. In addition, a novel technique will be introduced to stabilise gas lift systems with potential increase the total oil production rate by replacing SNIT by the MNIT to reduce the initial gas bubble sizes and improve the distribution of gas bubbles in the column. Therefore, a comprehensive three- dimensional gas-lift model was developed to simulate the gas-liquid flow consists of a 66 mm wide and 2 m high gas-lift system, and it will be presented in this chapter.