Baojiang Sun

The mechanism for the formation of slug flow in vertical gas–liquid two-phase flow

This paper studies the mechanism for the formation of a slug flow in vertical gas–liquid two-phase flow. By analyzing void fraction waves and their instability, it is proved that the formation of a slug flow regime is due to the increase of void fraction waves, which causes the conglomeration of gas bubbles and the coalescence of bubble clusters in unstable bubbly flow. Experiments and analysis show that intense turbulence can restrain the formation of Taylor bubbles. Therefore, in a large diameter vertical pipe, a Taylor bubble can form under a condition of low continuous volume flux due to the action of void fraction waves. However, the coalescence effect of void fraction waves as it affects bubbles is suppressed in high continuous volume flux, and therefore, a slug flow regime cannot be observed in the evolution of flow patterns. Under a condition of high continuous volume flux (VL=0.15 m/s) described in the paper, the flow pattern evolution is from cap bubbly flow to cap churn flow, and then gradually to churn flow with the increase of void fraction.

The self-assembly structure and the CO2-philicity of a hybrid surfactant in supercritical CO2: effects of hydrocarbon chain length

Hybrid surfactants containing both fluorocarbon (FC) and hydrocarbon (HC) chains, as effective CO2-philic surfactants, could improve the solubility of polar substances in supercritical CO2. Varying the length of the HC of hybrid surfactants is an effective way to improve the CO2-philicity. In this paper, we have investigated the effects of the HC length on the self-assembly process and the CO2-philicity of hybrid surfactants (F7Hn, n = 1, 4, 7 and 10) in water/CO2 mixtures using molecular dynamics simulations. It is found that the self-assembly time of F7Hn exhibits a maximum when the length of the HC is equal to that of the FC (F7H7). In this case, the investigation of H-bonds between the water core and CO2 phase shows that F7H7 has the strongest CO2-philicity because it has the best ability to separate water and CO2. To explain the origin of the differences in separation ability, the analysis of the structures of the reverse micelles shows that there are two competing mechanisms with a shortening HC. Firstly, the volume of F7Hn is reduced, which thus decreases the separation ability. Moreover, this also leads to the curved conformation of the FC. As a result, the separation ability is enhanced. These two mechanisms are balanced in F7H7, which has the best ability to separate water and CO2. Our simulation results demonstrate that the increased volume and the curved conformation of the hybrid surfactant tail could enhance the CO2-philicity in F7Hn surfactants. It is expected that this work will provide valuable information for the design of CO2-philic surfactants.

Influence of Altitudes and Air Humidity to the Minimum Gas Injection Rate in Air Underbalanced Drilling

Three aspects, such as the optimization of calculation model for the minimum gas injection rate, the effect of elevation on the minimum gas injection rate, the effect of air humidity on the minimum gas injection rate, have been reviewed in this paper. Considering the influence of altitude and air humidity, correction factor for gas injection rate is calculated. The value of gas injection for altitude correction factor generally increases with the increase in altitude, especially when conducting air drilling in high altitude areas of western China the correction factor of Qinghai oilfield can be calculated as high as 1.5. When the air humidity increases, the gas injection humidity correction factor also increases; especially before raining in summer, the air humidity correction factor can be close to 1.1, under the condition of relatively high air humidity and temperature. In the light of the calculation result and example analysis, it can be found that combining with this articles altitude and air humidity correction, the result of the amended minimum kinetic energy standard using Boyun Guos correlation is the closest to the actual one, which can be used for the calculation of the minimum gas injection rate in industry practice while other methods have relatively larger errors.

Calculation on the following performance of proppant in supercritical carbon dioxide

As a new type of fracturing fluid, supercritical carbon dioxide (SC-CO2) has now become the research hotspot in domestic and foreign academia. However, the relevant technique is limited by the unclear sand-carrying mechanism. In this study, the concept of following performance is introduced to investigate the sand-carrying performances of fracturing fluid; the following performance of proppant and the sand-carrying performance of fracturing fluid are characterized by the ratio of the proppant horizontal velocity to the fracturing-fluid horizontal flow velocity. Then the following performance of proppant is used as the criteria for evaluating the horizontal sand-carrying performances of SC-CO2. Based on the classic BBO equation, the expression of drag force has been modified; in combination with the auxiliary equations of drag force coefficients, a calculation model has also been established to analyze the following performance of proppant in SC-CO2, which provides the theoretical foundation for evaluating and optimizing the transportation of proppant in SC-CO2. To verify the correctness of the model, an experiment is conducted on the following performance of proppant using the self-developed equipment, and then the factors that affect the following performance of proppant in SC-CO2 are analyzed. The results show that the following perfomance of proppant is slightly affected within the common range of density, which is consistent with the previous experimental results. In addition, a comparative analysis is performed on the following performance of sands in SC-CO2, slickwater and air. The analysis results indicate that the sand-carrying performance of SC-CO2 is more affected by its high-density property but less affected by its low viscosity. Because of high-density and low-viscosity properties, SC-CO2 is far better than air but worse than or approximately equal with slickwater in terms of sand-carrying performances. Especially at a high flow velocity, the horizontal sand-carrying performance of SC-CO2 is comparable to that of slickwater.