Experiments and simulations of surface cleaning for a gas–liquid two-phase jet

Tubing is widely used in oil and gas production operations and plays an extremely important role in the petroleum industry as a conduit conveying oil and gas from underground to the surface. Tubing accounts for more than 10% of the steel used in the petroleum industry, whereas the expense of tubing accounts for a large proportion of the development cost of petroleum (Shen 2018). Due to high temperature and high pressure in the downhole, dirt and rust are easily formed on the surface of the tubing, resulting in the decline of the oil well's productivity, the sticking of well string, frequent disconnections, and the shortening of pump's inspection cycle. Consequently, the cost of oil and gas production increases, resulting in a decline in the economic benefits of the oil field. Practically, the production costs of oil and gas can be greatly reduced by applying tubing repair technology. As the most basic aspect of tubing repair technology, the efficiency of the tube cleaning technology directly affects the quality of the repaired tubing (Feng et al. 2010). Therefore, in-depth studies focusing on efficient tubing cleaning technology are of particular interest to researchers and engineers.

In recent years, the high-pressure water jet cleaning technology has been widely studied and applied due to its advantages of simple process, environment-friendliness, and high cleaning efficiency (Thomas 1995; Lu & Chen 2004; Song et al. 2014; Zhao et al. 2017a, 2017b; Singh et al. 2021). The mechanism of such a cleaning has been analyzed and the cleaning device has been introduced in a previous work (Xue et al. 2008). Cleaning tests using high-pressure water jet were carried out on tube coatings and workpieces with different hardness values (Guo et al. 2014), and it was found that, when a crack existed at the surface of the coating, the cleaning speed increased linearly, while the increase of cleaning speed resulted in an increase in the surface roughness of the tube. For the high-pressure water jet technology, experimental studies have been conducted to evaluate the effectiveness of the tools for the removal of surface attachments. Under the pressure of 60 MPa, with the nozzle diameter of 1.4 mm and the dwell time of 60 s, the surface attachments could be cleaned up efficiently (Huang et al. 2014). Based upon pre-mixed abrasive water jet, the internal flow field of the mixing chamber of the nozzle was numerically simulated, while reasonable structural parameters were obtained to improve the mixing homogeneity of the water jet (Liu et al. 2014). The rust and dirt covering the body of a truck were removed using abrasive water jet technology (Zuo & Liu 2014). The movement of abrasive particles in the water jet and the wear of abrasive particles on the nozzle were simulated, the results of which were used to optimize the structure of the nozzle (Deng et al. 2021; Du et al. 2021). The influence of inlet disturbance on the performance of the self-excited oscillation cavitation nozzle was evaluated by analyzing the peak value of axial pressure fluctuations and the jet's erosion intensity (Li et al. 2016). The flow field of a new type of Helmholtz nozzle was numerically simulated using computational fluid dynamics (CFD) and the factors dominating the cavitation jet effect, including nozzle cavity height, cavity width, expansion tube angle, and pump rated pressure were analyzed (Qi et al. 2020). By taking the tandem double-chamber self-excited oscillating pulse nozzle as the research object, the influence of the nozzle's structural parameters on the internal flow field of the cavitation jet was simulated (Sun et al. 2019). Due to their distinct advantages, various water jet technologies have been applied in many fields. However, the extremely high pressure (more than 150 Mpa) of the pure water jet technology poses challenges to equipment safety and also increases the pumping cost. The impact and grinding effects of an abrasive water jet are much better than those of a pure water jet. However, in the case of the former, various problems such as serious wear of cleaning equipment and nozzle, difficulty in recycling, and utilization of abrasives are frequently encountered. The strong impact force can be generated by the cavitation effect in the cavitation jet technology, while the cavitation effect is subject to a nozzle's structure. A certain amount of jet energy will be consumed due to the occurrence of spontaneous cavitation, which consequently reduces the cleaning efficiency. Comparatively, the gas–liquid two-phase jet is a cleaning method having a higher efficiency.

The gas–liquid two-phase jet technology was proposed by Eddingfield and Albrecht from the University of Illinois in the United States (Eddingfield et al. 1979). The mixed gas reduced the frictional resistance between the water jet and the surrounding environment, which improved the effective target distance. Later, the study on the impact of the gas–liquid two-phase jet on concrete specimens showed that, compared with the pure water jet, the peak pressure of gas–liquid two-phase jet on the surface of specimens remained unchanged and its erosion performance was greatly increased (Momber 2000). An optimal gas concentration maximized the erosion, while the crack area on the specimen's surface expanded obviously. Based on this technology, the former Soviet Academy of Sciences proposed the gas–liquid pulse jet rock-breaking method by changing the structural parameters and jet medium of the pulse water jet device and obtained a 27% higher effect than the ordinary pulse jet (Wang & Zhou 1988). Teruo & Hiroshi (1977) systematically studied the fluid characteristics of gas–liquid two-phase jet under submerged conditions and applied it to high-pressure rotary jet grouting technology. Based upon CFD, the morphological characteristics of the gas–liquid two-phase jet were numerically simulated after injecting compressed air into the water jet, and the experiments were carried out to verify the simulation results (Annoni et al. 2014). The mixing of a certain amount of compressed air is not only beneficial in enhancing the stability of jet flow and maintaining the consistency of a jet's structure, but also in improving the cutting quality and enhancing the cutting capability of the jet. The ability of a gas to protect the jet was studied using simulations and experiments, which showed that, compared to the pure water jet, the protective gas can reduce the jet's frictional resistance, reduce the energy loss, and greatly improve the effective target distance of the jet under submerged conditions (Li 2008). The gas–liquid two-phase jet was applied to the hydraulic fracture-cutting technology of coal and the device for gas–liquid two-phase jet fracture-breaking was designed and developed (Lin et al. 2018). Compared with the pure water jet, the pressure threshold of the gas–liquid two-phase jet coal-cracking was reduced, whereas the depth and the diameter of the crushing crater were twice those of the pure water jet.

Previous studies have mainly focused on the applications of a gas–liquid two-phase jet in the fields of rotary jet grouting and rock-breaking, whereas less focus has been dedicated to the field of cleaning. In this study, the gas–liquid two-phase jet technology has been used to obtain an optimum cleaning efficiency. Based on the VOF (volume of fluid) model, the flow field characteristics of gas–liquid two-phase jet and the influence of the variation of jet parameters on the cleaning effect were analyzed using numerical simulations. The results were verified using experiments. The simulation results provide theoretical guidance for the application of two-phase jet cleaning technology.

Based upon a fixed Euler grid, the VOF model can achieve surface-tracking in the simulations. The phase interface of each cell can be tracked by introducing a variable called phase volume fraction. The VOF model has the advantages of easy implementation, small requirements for computational power, and high precision, which is due to the reason that it tracks the fluid volume in the grid rather than the motion of the fluid particle. Therefore, the VOF model was employed to simulate the flow field characteristics of the gas–liquid two-phase jet and the influence of jet parameters.

The governing equations of the VOF model are presented as follows (Li & Shao 2018):

• (1)
  Continuity equation is given by Equation (1):

  

  (1)

  where u is the vector of velocity, is the density of fluid, and ▽ is the Hamiltonian.
• (2)
  Momentum equation is given by Equation (2):

  

  (2)

  where u is the vector of velocity, is the density of the fluid, is the viscosity of the fluid, g is the gravitational volume force, and F is the external volume force.
• (3)

  Equation of volume fraction is given by Equation (3):

The ANSYS Fluent 18.2 was used to simulate the flow field characteristics and cleaning effects of the gas–liquid two-phase jet. Various assumptions were made for the analysis, and they are given as follows: (1) the mass transfer and heat transfer between the two phases were ignored; (2) No heat exchange occurred between the fluid and the external medium, and (3) the temperature remained constant. The boundary conditions were set as follows: velocity-inlet (to investigate the targeted velocity field of the gas–liquid two-phase flow in detail), and pressure-outlet. Additionally, the pressure at the outlet was zero.

The variation of impact effect with certain jet parameters was investigated, such as the standoff distance (the distance between the nozzle outlet and the target surface), which varies within the range of 5–20 mm. The gas concentration is defined as the gas volume fraction in the gas–liquid two-phase jet (referred to as the gas volume fraction of the inlet), and varied within the range of 4–10%. The flow rate of the liquid laid within the range of 20–50 L/min. The nozzle contraction angle (tapered angle of the converging nozzle) laid within the range of 10–140° (see Table 1).

For the gas concentration, flow rate, nozzle contraction angle and standoff distance of 8%, 50 L/min, 140°, and 10 mm, respectively, the simulated pressure nephogram of the gas–liquid two-phase jet is shown in Figure 3(d). The impact pressure (93.0 MPa) acting on the targeted surface was much higher than the inlet pressure (49.3 MPa). This can be ascribed to the fact that a micro jet with a high velocity is formed and a water hammering effect is generated when the intermittently distributed liquid columns impact on the targeted surface. As a result, the extremely instantaneous pressure becomes much greater than the stagnation pressure, which greatly improves the impact effect of the gas–liquid two-phase jet (Wang 2010).

To identify the jet parameters of a gas–liquid two-phase jet, sensitivity analysis based on numerical simulations combined with laboratory experiments was implemented. Various factors such as the standoff distance, gas concentration, flow rate, and nozzle contraction angle were used in the analysis.

The experimental results are shown in Figure 5(b). For the flow rate, gas concentration and impact time of 50 L/min, 8%, and 60 s, respectively, the dependence of the diameter and depth of the impact crater on the standoff distance are depicted in Figure 5(b). With the increase in the standoff distance, the depth of the impact crater slightly increased followed by a decrease, while the diameter of the impact crater first increased and then decreased. When the standoff distance was smaller than 10 mm, the targeted surface was located inside the jet's potential core area. The jet velocity arriving at the targeted surface was almost uniform. Therefore, the crater diameter slightly increased. As the standoff distance exceeded the value of 10 mm, the targeted surface went outside the core area, so that the impact velocity and pressure began to decrease quickly, thus decreasing the crater diameter. The crater diameter reached its maximum value when the standoff distance was 15 mm. When the standoff distance was smaller than 15 mm, the gas–liquid two-phase jet energy was slightly dispersed. Meanwhile, the overall energy was concentrated, which led to an increase in the crater diameter. However, as the standoff distance became larger than 20 mm, the gas–liquid two-phase jet energy dispersed significantly. Therefore, parts of the jet energy were not strong enough to break the cement block, which led to a decrease in the crater diameter. To sum up, combining the impact pressure with the carter depth and diameter, the optimum standoff distance was chosen to be 10 mm (five times that of the nozzle diameter).

For the flow rate, standoff distance and impact time of 50 L/min, 10 mm, and 60 s, respectively, the experimental diameter and the depth of the impact crater under different gas concentrations are depicted in Figure 6(b). With the increase in gas concentration, the diameter of the impact crater increased, while the depth of the impact crater first increased, and then, decreased. With the increase in gas concentration, the liquid phase was more dispersed by the gas phase, resulting in a gradual increase of the jet's action area on the targeted surface and the diameter of the impact crater. When the gas concentration was less than 8%, the jet had a strong clustering property. With the increase in gas concentration, the impact force generated by bubble bursting enhanced the jet's energy, resulting in an increase in the depth of the impact crater. As the gas concentration continued to increase, the liquid energy was seriously dispersed by the gas phase, which decreased the jet impact force and the impact crater depth.

For the gas concentration, standoff distance, and impact time of 8%, 10 mm, and 60 s, respectively, the experimental diameter and the depth of the impact crater under various flow rates are depicted in Figure 7(b). The results showed that, with the increase in the flow rate, the diameter and the depth of the impact crater gradually increased. This is because the larger flow rate was accompanied with a higher jet energy, resulting in a larger impact force. Therefore, if the capacity of field equipment allows, the flow rate should be as high as possible to maximize the cleaning efficiency.

Due to the complexity of experiments with different nozzle contraction angles, the influence of nozzle contraction angle on gas–liquid two-phase jet was examined using numerical simulation.

In this study, a new cleaning method of gas-mixed water jet was proposed. Experimental and numerical methods were used to evaluate the cleaning effect of the gas-mixed water jet by analyzing different parameters. Major findings can be summarized as follows:

• The impact pressure of the gas–liquid two-phase jet acting on the targeted surface fluctuated with time, and exhibited obvious pulsation characteristics. The impact pressure of the gas–liquid two-phase jet was much higher than the inlet pressure.

• With the increase of standoff distance, the impact pressure acting on the targeted surface remained almost unchanged. However, when the standoff distance exceeded the value of 10 mm, the pressure decreased rapidly. The optimal standoff distance was determined to be 10 mm. The impact pressure firstly increased and then decreased with the increase of gas concentration for the concentration range of 6–10%. The desirable impact effect can be obtained when the gas concentration was about 8%. The larger flow rate yielded a greater impact pressure together with more obvious pulsation characteristics. Moreover, the diameter and the depth of the impact crater increased. With the increase in nozzle contraction angle, the impact pressure acting on the targeted surface first increased and then decreased, while the optimal contraction angle was numerically determined to be 140°.

• The jet cleaning effect is better if primary pores or cracks exist in the targeted surface. Therefore, the cleaning speed can be increased when pores or cracks exist in the coating or rust/scale layer.

This work was financially supported by the National Natural Science Foundation of China (No. 52204022), the Natural Science Foundation of Shandong Province (No. ZR2022ME152), the National Key Research and Development Program of China (No. 2021YFE0111400), the Fundamental Research Funds for the Central Universities (No. 19CX02063A), and the Science and Technology Plan of Dongying City (No. 2021ZD49).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.