CFDgravitytypedownholeseparatorprojectreport重力式井下分离力器CFD模拟报告

1、Mechanical Engineering DepartmentME 4033 - 6033“Down-hole Separator CFD Project”Prepared For: CFD for Engineers Class, Fall 2015, Dr. S.A. ShiraziPerformed By: Haiwen ZhuDate: December 7, 2013Contents1. Introduction and objectives21.1 Application21.2 Design and principle21.3 Objectives42. Approach52

2、.1 Geometry design52.2 Meshing design62.3 Solution input83. Results and Discussion103.1 Flow pattern study103.2 Sensitivity study113.2.1 Multiphase model study113.2.2 Pressure study113.2.3 Inlet velocity study123.3 Geometry improvement study143.4 Critical casing annulus velocity study.154. Summery a

3、nd Conclusions17References18Appendices19Appendix A19Appendix B26Appendix C291. Introduction and objectives1.1 ApplicationGas problems can cost a company valuable time, money and resources. The presence of gas in the pumping zone causes various problems like gas lock, gas pound, and gas interference,

4、 resulting in reduced pump efficiency and pump failures. Gas is found in two forms in oil wells: (1) in solution with the well fluids, and (2) as a free gas in the formation. Gas is held in solution by pressure and is released by turbulence, heat and reduced pressure. The presence of gas can reduce

5、pump efficiency. Gas entering the well along with the oil is the factor which causes most pump problems. It may pocket around the pump or may accumulate inside the pump and “gas lock” the valves. In wells with a high fluid level, the high head of fluid exerts high pressure causing the gas to remain

6、in solution form. When this fluid flows through the pump intake, the gas breaks out due to the pressure drop. This free gas enters the pump intake chamber and causes the gas interference, leading to gas lock and gas pound. In order to overcome these problems, down-hole separator was developed, inclu

7、ding gravity-type, rotary type, impeller type, cyclone type and combined type down-hole separator. This project uses Don-Nan Gas Separator design, which diverts the gas away from entering the pump intake and thus reducing pump failures and improving the pump efficiency. 1.2 Design and principleThe g

8、as separator (figure 1.1) is an assembly of five individual components: Ported Coupling (Figure 2.1), separator section, which contains inner & outer tube and casing (Figure 2.2), and Outer tube (Figure 2.3). The inner tube is screwed into the center hole of the ported coupling. The outer tube i

9、s screwed into the ported coupling. The presence of the packer insures that all of the formation fluid from the well bore passes through the gas separator before entering the pump intake. The function of the gas separator is to create a turbulent and low pressure atmosphere which the fluid will pass

10、 through in route to the pump intake allowing the gas in its solution form to break out and escape through the tubing/casing annulus. The gas separator is designed and installed in such a way that, once the gas breaks out of the fluid, it travels upwards at a velocity faster than the downward veloci

11、ty of the liquids being drawn into the pump (assuming that the viscosity of the liquids is within the considered range). Preventing the free gas from reaching the pump intake reduces the occurrence of gas interference and gas lock and improves the pump efficiency. Figure 1.1 Separator geometryOnce t

12、he annulus space between the outer tube and the inner flow tube is filled with fluid, it passes through the four slots that are machined on the upper portion of the outer tube and falls back in to the annulus space between the casing and the outer tube, settling on the packer. This accumulated fluid

13、 on top of the packer passes into the inner flow tube through the “L” shaped hole that is drilled in the ported coupling, travels through the inner flow tube, and finally passes into the pump intake. This diversion of fluid from the outer tube slots to the tubing/casing annulus creates a major turbu

14、lence in the flow and helps break out the gas from the fluid. This free gas travels upwards through the tubing/casing annulus space, diverting most of the gas away from entering the pump intake (figure 1.2). With this kind of fluid flow, problems such as gas interference, gas lock, and gas pound can

15、 be greatly reduced, improving the efficiency and life of the pump.Figure 1.2 Typical two phase flow in Don-nan separator1.3 Objectives J.N McCoy, from university of Texas at Austin, did the experiment and conclude the critical liquid in-situ velocity is 6in/s with research conducted in low pressure

16、 condition. The objective of this project is to use ANSYS study McCoys conclusion. The geometry is based on “Don-nan separator”. The objectives includes:1) Basic two phase flow in the separator.2) Using Taitel Dukler model to set a case of critical condition and test the results.3) Change the critic

17、al condition and study the sensitive factors.4) Geometry analysis of Don-nan down-hole separator.2. Approach2.1 Geometry designThe geometry include three parts: ported coupling, separator section and a dip-tube. The geometry parameter are as follow:ported couplingpackerseven small holespump inletODI

18、DdiameterdistanceODID5.5"2.875"0.36"0.55"1.2"1.0"Table 2.1 Ported coupling geometry dataseparator sectioncasingouter tubeinner tubeseparator inletODIDODIDODIDsizenumber5.5"4.95"2.875"2.259"1.2"1.0"0.2" * 1"2Table 2.2 Separator sec

19、tion geometry datadip-tubeODID2.875"2.259"Table 2.3 Dip-tube geometry dataFigure 2.1 Ported couplingFigure 2.2a Separator sectionFigure 2.2 b Top view of separator sectionFigure 2.3 Dip-tube2.2 Meshing designThe default meshing design is as follow (figure 2.4). The default nodes are 59383,

20、 elements are 262451. Further modification is needed for the subtle part of the geometry, such as seven small holes of the ported coupling section and separator inlet of the separator section.Figure 2.4 Default meshing geometryTherefore, in order to increase the quality of the meshing, following inp

21、uts have been made (table 2.4)SizingInflationElement SizeAdvanced Size FunctionRelevance CenterSmoothingAutomatic InflationDefaultProximity and CurvatureFineHighProgram ControlledTable 2.4 Modified meshing input dataAfter the modification, the total nodes are 132758, total elements are 476534. The m

22、odified meshing shows as follow (figure 2.5)Figure 2.5 Modified meshing geometry.Right now, the nodes and elements are still not enough. Further modification could be decreasing element size in “Body Sizing” outline. When using 0.003m for element size, the total nodes are 485223 and total elements a

23、re 2210442. However, more elements need more calculating time. Since the project is going to use transient model. The calculation time for one single case will be days. Therefore, the project uses default for element size in “Body Sizing” outline. 2.3 Solution input The project is going to simulate

24、3-D two phase flow, therefore, multiphase model is needed. In the “General” outline, Transient is set for time, and Gravity is included. In the case study, VOF model and Mixture model are tested and Mixture model is chosen for the primary model of the rest cases. For both VOF model and Mixture model

25、, water will be primary phase and air will be secondary phase. Surface tension coefficients in “phase interaction” is 0.000735 N/m. For Mixture model, the gas bubble diameter is also an important input parameter. Most cases use 1e-5 as bubble diameter, and a comparison case uses 0.003m.Realizable k-

26、epsilon model was selected for viscous model, with “scalable wall functions” for the “near-wall treatment.” The material in the project is water-liquid with density 998.2 kg/m3 and viscosity 0.001003 kg/m-s, and air with density 1.225 kg/m3 and viscosity 1.7894e-5 kg/m-s.Gas and water velocity, as w

27、ell as outlet pressure are very important and complicated for this project. The critical water in-situ velocity 6 in/s in the casing annulus space comes from the experimental research of down-hole gravity-type gas separator. In order to calculate the real in-situ velocity of water and gas in the sep

28、arator, two phase flow mechanism model is going to be needed. Fortunately, my old “advanced production class” project allows me to calculate the in-situ velocity, liquid hold up, flow pattern and etc. After the calculation, the one critical superficial water inlet velocity is 1.52867 ft/s with criti

29、cal superficial gas inlet velocity is 1.2051 ft/s. Other combination of critical velocity of liquid and gas could also be possible. The gas outlet pressure was set by assuming multiphase in casing annular above separator inlet and single phase water in rest part of the separator. Then the critical g

30、as outlet pressure uses 4000 pa and water outlet pressure 0 pa. Comparison cases with different outlet pressure are also considered. The input data for different cases are as follow. (Table 2.5)Case123456MultiphaseVOFMixtureMixtureMixtureMixtureMixtureViscousk-epsilonk-epsilonk-epsilonk-epsilonk-eps

31、ilonk-epsilond(gas)/1e-5 m1e-5 m1e-5 m1e-5 m0.001 mWater inlet1.529 ft/s1.529 ft/s1.529 ft/s1.4 ft/s3 ft/s1.529 ft/sGas inlet1.208 ft/s1.208 ft/s1.208 ft/s1.208 ft/s4 ft/s1.208 ft/sGas outlet0 pa0 pa4000 pa4000 pa4000 pa4000 paWater outlet0 pa0 pa0 pa0 pa0 pa0 paTime step0.003 s0.003 s0.003 s0.003 s

32、0.003 s0.003 sNumber>2000>2000>2000>2000>2000>2000iteration303030303030Table 2.5 Solution input for different cases.3. Results and Discussion3.1 Flow pattern studyBased on multiphase flow simulator, almost all sections of the Don-nan down-hole separator are Bubble flow. Pressure co

33、ntours in ANSYS study also show the same results. Figure 3.1 show that, for three selected cases with different multiphase model (VOF and Mixture) and different boundary condition, the pressure distribution is continuous rather than intermittent. The results shows that, bubble flow is the primary fl

34、ow in the separator. Since VOF model is more suitable for the slug flow (intermittent pressure distribution), while Mixture model is more suitable for bubble flow, most cases in this project will use Mixture model. Compare pressure distribution of case-3 to case-1&2, the results shows that: when

35、 additional pressure difference between gas and water outlet was introduced, the seven-hole design of Don-nan separator will cause a huge pressure drop because of their small diameter. Case-1 (Time=7.107s)Case-2 (Time=10.633s)Case-3 (Time=13.785s)Figure 3.1 Pressure contour3.2 Sensitivity study3.2.1

36、 Multiphase model studyComparing case-1&2 in figure 3.2&3.3, VOF model could give us a more detailed gas bubble distribution. However, the velocity distribution of two cases a big error. Since the flow pattern in the separator is bubble flow, here we choose to use Mixture model. Future study

37、 could be focus on VOF model when the quality of meshing increased. 3.2.2 Pressure studyComparing case-2&3 in figure 3.2&3.3, the pressure difference between gas and water inlet simulates pump working. Without pump at water outlet, the separator will always have a good performance because of

38、 the gravity and buoyancy. With pump working, water will carry gas bubble downwards to the pump. Therefore, pump power (pressure difference between water and gas outlet) is a very important parameter in this study. 3.2.3 Inlet velocity studyComparing case-3&4 in figure 3.2&3.3, the results s

39、hows that the critical velocity of water is around 6in/s when assuming gas bubble diameter is 1e-5m. Case-3 is based on critical casing annulus velocity 6in/s, when time=13.785s, the gas still did not occupy all casing annular space. However, as time goes on, the gas will finally occupy the casing a

40、nnular. Case-4 has a slightly smaller water inlet velocity, at 13.5s, the gas occupy less casing annular space compared with case-3. Therefore, the real critical casing annular velocity should be slightly less than 6in/s when assuming gas bubble diameter is 1e-5m.Comparing case-3&5 in figure 3.2

41、&3.3, the results shows that: with higher water and gas inlet velocity, gas will occupy separator area very soon. 3.2.4 Gas bubble diameter studyComparing case-3&6 in figure 3.2&3.3, the results shows that: larger gas bubble diameter will case higher critical velocity. This is consistent

42、 with N-S equation study of gas bubble rise velocity. Larger bubble will have higher bubble rise velocity (upward), which could increase the critical water velocity (downward).Case-1 (Time=7.212s)Case-2 (Time=10.633s)Case-3 (Time=13.785s)Case-4 (Time=13.785s)Case-5 (Time=10.212s)Case-6 (Time=19.036s

43、)Figure 3.2 Air volume fraction contour3.3 Geometry improvement studyFigure 3.3 gives velocity distribution in the separator for different cases. For all cases, the velocity in the seven small holes (ported coupling section0 is very huge, compared with figure 3.1, this design will cause very high pr

44、essure drop as well as more erosion. Case 6 shows a most homogenous distribution of the velocity. Which suggests the future study could focus on bubble diameter selection.Separator outlet at outer tube in all cases shows a high velocity, which might cause erosion or blockage. Difference outlet shape

45、 and size could be considered in the future study.Case-1 (Time=7.212s)Case-2 (Time=10.633s)Case-3 (Time=13.785s)Case-4 (Time=13.785s)Case-5 (Time=10.212s)Case-6 (Time=19.036s)Figure 3.3 Velocity contour3.4 Critical casing annulus velocity study. Figure 3.4&3.5 shows similar water downward veloci

46、ty of case-3&6. The velocity is around 5in/s which is slightly smaller than the experiment study from McCoy. For case-3 the gas have a tendency to flow into the pump, which probably is because of a small gas bubble diameter. Case-6 is more similar to the real situation, 5in/s is still below the

47、critical velocity.Figure 3.6 shows that casing annulus water velocity is close to 12in/s which is much higher than the critical velocity. From figure 3.2 case 5, gas occupy the separator rapidly. This results shows that, when casing annulus water velocity is larger much larger than critical velocity

48、, the separator efficiency will absolutely decrease. Figure 3.4 Velocity distribution at z=0.2m for case 3Figure 3.5 Velocity distribution at z=0.2m for case 6Figure 3.5 Velocity distribution at z=0.2m for case 54. Summery and ConclusionsThe study of multiphase model shows that Mixture model is more

49、 suitable for down-hole separator study. However, when future study involved high GLR two phase flow, which might cause slug flow, VOF model could be considered. Case-6 shows that gas bubble diameter is very important to this CFD simulation study. Some paper suggests 0.25in. However, this diameter c

50、ould not be done because of the poor meshing. Future study could modify the separator geometry and meshing in order to have a larger bubble diameter.Case-2 to case-6 shows that casing annular water critical velocity is close to 5in/s when assuming gas bubble diameter is 1e-5, and will be larger than

51、 5in/s with larger gas bubble diameter.Case-2&3 shows that pressure different between water and gas outlet is also very important. However, since the liquid holdup of separator various a lot for different cases. It is difficult to calculate the pressure difference between two outlets. If the pro

52、ject could have the experimental data, the results will be more reliable. Since the geometry of Don-nan separator is not exactly based on the real one (the patent didnt include the detail design), some connection parts like seven small hole does not perform very well. Therefore, modification of the

53、geometry could be consider for the further study. References1. Jyothi Swaroop Samayamantula: “An Innovative Design For Down-hole Gas Separation,” Don-Nan Pump & Supply, 2009.2. J.N.Mccoy: “Evaluation and Performance of Packer-Type Down-hole Gas Separators,” SPE Production and Operation Symposium, 20133. Jim McCoy: “Packer-Type Gas Separato