Abstract: At present, the performance test and efficiency evaluation of deaerators are performed on-site. The field test conditions are relatively limited. It is difficult to analyze the factors that affect the performance of the deaerator based on the test results. It is not accurate to use traditional theoretical analysis. To predict its outgassing performance. To this end, the CFD technology is used to study the internal flow field of the vertical vacuum deaerator, analyze the factors affecting the degassing performance, and evaluate the degassing efficiency. The results show that the inclination and area of the separation umbrella have a greater impact on the deaerator, and reasonable design of the inclination and area can not only achieve high degassing efficiency, but also reduce the volume of the deaerator; a multiphase flow mixing model is used to quantify The prediction of the degassing efficiency of the deaerator, the reasonable design can make the efficiency of the vertical vacuum deaerator reach more than 90%.
Key words vacuum degasser numerical simulation flow field degassing efficiency
Degasser is one of the key equipments in drilling fluid processing system. It is used to remove the gas from drilling fluid to stabilize the performance of drilling fluid. There are many types of degasser used in oil and gas field drilling, such as vacuum type, atmospheric type and centrifugal type. The most widely used and the best effect is the vacuum type deaerator. Among them, the vertical type vacuum deaerator is widely used at home and abroad due to its small size and small footprint. Its working process is: start the vacuum pump to generate a vacuum in the degassing tank, the drilling fluid is sucked in through the suction pipe under the action of the vacuum, and flows into the degassing tank through the umbrella piece. The gas in it is exposed to the surface of the drilling fluid under vacuum, and is pumped away by the vacuum pump through the top of the degassing tank. Drain to the system. The flow of drilling fluid in the degassing tank is relatively complicated, so that there are many factors affecting the degassing efficiency. Traditional theoretical analysis cannot accurately predict its degassing performance. Laboratory testing is also very complicated and the cost is very high. Research. At present, domestic performance testing and efficiency evaluation of deaerators are performed on-site. The field test conditions are relatively limited, and it is difficult to analyze the factors affecting the performance of the deaerator based on the test results. To this end, the research on the performance and efficiency evaluation of the vertical vacuum deaerator was cooperated. The CFD technology was used to study the internal flow field of the vertical vacuum deaerator, analyze the factors affecting the degassing performance, and evaluate the degassing efficiency. The author's research results are of great guiding significance for the design and use of vertical vacuum deaerators.
1 Model establishment
1.1 3D computing model and model discretization
The parameters of the separation umbrella determine the degassing performance of the vacuum deaerator, so the separation umbrella is the object of numerical simulation research. According to the structure of the separation umbrella, the overcurrent part is represented by the solid body, and the other parts are represented by the virtual body. The calculation model of the deaerator separation umbrella is shown in Figure 1. When simulating the flow of gas invasion drilling fluid on the separation umbrella, in order to reduce the amount of calculation, one of the umbrella pieces can be used for simulation. The model diagram is shown in Figure 2.
In three-dimensional problems, tetrahedral mesh, hexahedral mesh, pyramid mesh, and wedge mesh, or a mixture of two meshes can be used to mesh the calculation model. Because the shape of the calculation area is more complicated, I use a hybrid grid.
1.2 Boundary conditions and physical parameters of the medium
The vacuum degasser works mainly by sucking fluid by the degree of vacuum in the degassing tank. Therefore, the inlet boundary is the pressure boundary, and the outlet boundary is also the pressure boundary, and the pressure is the degree of vacuum in the degassing tank. The condition of non-slip solid wall is adopted, and the fluid flow near the solid wall is determined by the standard wall surface function. The working medium is a drilling fluid containing a certain amount of gas, the gas integral number is 18%, the bubble diameter is 0.8mm, the liquid dynamic viscosity is 0.06Pa · s, and the liquid density is 1 200kg / m 3.
2 Numerical simulation analysis of gas-liquid two-phase flow in vacuum deaerator
The fluid flow on the separation umbrella can be considered as turbulent flow, and the turbulence model adopts the RNG κ-ε model. This model is derived from strict statistical techniques.It is similar to the standard κ-ε model, but has the following improvements: The RNG model adds a condition to the ε equation, which effectively improves the accuracy.The RNG model considers turbulent vortices, and at the same time It also improves the accuracy in this respect; RNG theory provides an analytical formula for the turbulent Prandtl number, while the standard κ-ε model uses a user-provided constant; the standard κ-ε model is a high Reynolds number model, RNG theory An analytical formula considering the flow viscosity of low Reynolds number is provided to calculate the low Reynolds number effect.
The fluid in the deaerator is a liquid-gas two-phase flow, and the multi-phase flow model uses a mixed model. The mixed model allows phases to penetrate each other. The volume fraction of a control volume depends on the space occupied by the phases. The concept of slip velocity is used to allow phases to move at different speeds. The mixed model solves the continuity equation of the mixed phase, the momentum equation and energy equation of the mixed phase, the volume fraction equation of the second phase, and the algebraic expression of the relative velocity. Applications of mixed models include multi-phase flows such as low-load particle loading, bubble flow, sedimentation, and cyclonic separation. The numerical calculation uses a separate implicit solution method. The momentum equation is discretized using a second-order upwind difference format, the volume fraction calculation is discretized using the Quick format, and the SIMPLE algorithm is used to couple speed and pressure.
The flow field in the deaerator mainly includes the velocity field, the pressure field, and the distribution of the volume fraction of each phase. At the same time, the degassing efficiency can be calculated by counting the density values of the inlet and outlet. The author takes the separation umbrella flow field distribution of the deaeration tank vacuum of 0.03MPa as an example to introduce its characteristics.
(1) Pressure distribution performance: There is a significant pressure difference between the inside and outside of the slurry tube, so that the liquid flows through the slurry holes to the separation umbrella under the effect of the pressure difference. The larger the vacuum degree in the deaerator, the larger the pressure difference, and the larger the flow rate when the slurry pore size is constant.
(2) Velocity distribution performance: The velocity near the outlet of the slurry separation hole is relatively large, and the liquid on the separation umbrella sheet flows downward along the inclined separation umbrella under the effect of gravity, so the speed is very small. In the work, it is hoped that the liquid flowing out from the slurry separation hole first fills the space at the top of the separation umbrella, and the separation umbrella separation holes are multiple small holes.
(3) The distribution of gas-liquid integral numbers in the separation umbrella: In the space between the two layers of separation umbrellas, the uppermost part is gas, the middle part is gas-liquid mixture, and the bottom part is liquid with little gas content. It can be seen that under the action of the separation umbrella, most of the gas has been removed from the liquid film, and the remaining liquid film contains very little gas, thereby achieving the purpose of degassing. Further research found that the liquid flowing out of the slurry separation hole near the slurry separation pipe directly flushed to the upper back of the upper separation umbrella, so the gas-liquid interface formed in the separation umbrella space facing the slurry separation hole was not Obviously, in other areas where the slurry holes are not opposite, a good gas-liquid interface is formed, and the two phases of gas and liquid are well separated.
3 Analysis of factors affecting degassing efficiency
The statistics of outgassing efficiency can be calculated according to formula (1), that is η = ρl-ρiρ0-ρi × 100% (1)
Where ρ0 --- the original liquid density;
ρi --- inlet gas invasion density;
ρl ———— the medium density of the liquid outlet.
3.1 Influence of the angle and diameter of the deaerator separation umbrella on the degassing efficiency
The angle and diameter of the separation umbrella determine the area of the separation umbrella. Numerical simulation analysis shows that under the same area of the separation umbrella, the smaller the separation umbrella inclination angle, the more obvious the degassing effect. The author takes a smaller inclination angle (20 °) to analyze the influence of the separation umbrella area on the degassing efficiency. Fig. 3 is the relation curve between the separation umbrella area and the degassing efficiency. It can be seen from Figure 3 that as the area increases, the outgassing efficiency continues to increase. When the area of the separation umbrella is increased from 2.00m 2 to 2.50m 2, the degassing efficiency is significantly improved; when the area of the separation umbrella is increased from 2.50m 2 to 3.00m 2, the gas removal efficiency is improved, but it is not obvious. From this, it can be judged as follows: When the area of the separation umbrella is less than 2.50m 2, the bubbles cannot completely overflow from the liquid film, so the degassing efficiency is very low; the area of the separation umbrella is greater than 2.50m ...Read More »