## 3 Results and Discussions
### Flow Regime Mapping for Horizontal Flow
#### 3.1.4 Air/non-Newtonian Two-Phase Flow  
In this air/non-Newtonian fluid experiment, a 0.1% solution of xanthan gum was used as the non-Newtonian fluid. Non-Newtonian fluids are generally superior drilling fluids compared to Newtonian fluids due to their viscosity and the holding/uplifting capacity of the cuttings. Figure 17 illustrates the impact of liquid superficial velocity on slug frequency for the air/non-Newtonian flow system. In Figure 17, the slug frequency increases with the increment of liquid non-Newtonian superficial velocity when the superficial gas velocity is held constant. Therefore, at lower superficial liquid velocity, the slug frequency increases sharply, and at higher liquid velocity, the slug frequency decreases. As the liquid velocity increases, the air requires more energy to drive the viscous fluid; nevertheless, the airflow rate is constant for each set. This explains why the number of slugs decreases as the liquid velocity increases at continuous airflow (see Figure 18). Furthermore, Figure 18 highlights the relationship between gas superficial velocity and slug frequency for air/non-Newtonian flow conditions. In Figure 18, slug frequency change is shown with superficial gas velocity for a constant liquid superficial velocity. Here, 0.1% xanthan gum solution was used as a non-Newtonian fluid where the power-law index, \(n\), is equal to 0.81 and \(k\) = 0.009344. From Figure 18 it is apparent that the slug frequency decreases as the gas velocity rises to 6.5 m/s. Because the gas flow rate increases in a constant  
Figure 17: Effect of liquid superficial velocity on slug frequency for air/non-Newtonian flow. Figure 18: Effect of gas superficial velocity on slug frequency for air/non-Newtonian flow. liquid velocity, Taylor bubble formation becomes relatively more significant; therefore, the length of the slug unit increases, and slug frequency decreases. The evaluation of the slug frequency vs. mixture velocity for air/non-Newtonian fluid flow is reported in Figure 19.  
Figure 19 represents the change of slug frequency with the mixture velocity of air-xanthan gum flow. In addition, it is seen that until 6.5 m/s, the mixture velocity slug frequency is minimal. [PERSON] & [PERSON] ([PERSON] & [PERSON], 2010) also obtained similar patterns for air/non-Newtonian flow. The same phenomena also occurred in Figure 11 for gas/Newtonian flow. However, the minimum slug frequency was around 5 m/s mixture velocity, which happened significantly earlier than the gas/non-Newtonian two-phase flow. Here, we can observe the inevitable effect of viscosity. Water viscosity at 20 \({}^{\circ}\)C room temperature is around 1 cP, and the experimental viscosity of 0.1% xanthan gum is 2.4 cP, which is slightly more viscous than water. The flow mechanism of slug flow is that the gas bubble is trapped in the water and drives the water forward almost at the same velocity as the gas velocity. Nevertheless, when the liquid becomes viscous, the gas requires more energy to drive the liquid forward. At a constant airflow rate, it is hard to achieve extra energy, so the whole process becomes slow, and the slug velocity and number of slugs decrease ([PERSON] et al., 2018). If further experiments were conducted for gas/non-Newtonian fluids, slug frequency would increase again, with an increased gas flow rate in the slug to bubbly flow transition zone as the gas/water two-phase flow results in the transition from slug to dispersed bubble flow. The turbulence of the flow structure begins to increase, the unit slug starts to break down, and the number of slugs increases at higher gas flow rates. It has also been observed that the number of dispersed bubbles increases in the slug pocket and wet film area. This indicates the beginning of the transition of the flow pattern.