Pump Gas Theory

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Pump Gas Theory

  1. 1. Copyright by Ochiagha Victor Ananaba 2007
  2. 2. Experimental Study On The Effect Of The Internal Design On The Performance Of Down – hole Gas Separators by Ochiagha Victor Ananaba, B.Eng. Thesis Presented to the Faculty of the Graduate School of The University of Texas at Austin in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE IN ENGINEERING The University of Texas at Austin December 2007
  3. 3. Experimental Study On The Effect Of The Internal Design On The Performance Of Down – hole Gas Separators Approved by Supervising Committee: Augusto L. Podio (Supervisor) Paul Bommer
  4. 4. Dedication To God Almighty. To my loving and supporting parents Sir Emeka & Lady Nnenne Ananaba. To my siblings, Nnem, Ugochukwu, Ugwunwanyi, Ogbugo & Amah (Papa). To the woman that will be my wife.
  5. 5. Acknowledgements I specifically want to thank my supervisor Dr. Augusto Podio for his continuous support and encouragement through this research project. Under his supervision I have greatly improved my knowledge and skills in the areas of petroleum production engineering and artificial lift systems. It is an honor to have him as my supervisor and to be his friend. I wish to thank Dr. Paul Bommer for the time that he took inside his very busy schedule to read and review my thesis. I will not forget to thank our Lab. Technician and my friend Tony Bermudez whose support in maintaining and constructing my laboratory models made certain that I finished my experiments in good time with high levels of accuracy. I wish to thank Glenn Banm, Harry Linnemeyer, Ehiwario M., Acholem K., Ojifini R., Elekwachi K. and Don Sorrell who were there to help whenever I needed assistance. My special thanks go to our amiable graduate coordinator Cheryl Kruzie. I would not be in UT if not for her kind and honest counseling. Finally I would like to thank the companies that supported this research, Echometer Company, ConocoPhillips, Yates Petroleum and Chevron. The comments and suggestions from James McCoy, Lynn Rowland, John Patterson and Gabriel Diaz helped in shaping my research. I worked with Renato Bohorquez in the early days of this research and it was great. Ochiagha Victor Ananaba December 2007 v
  6. 6. ABSTRACT Experimental Study On The Effect Of The Internal Design On The Performance Of Down – hole Gas Separators Ochiagha Victor Ananaba, M.S.E. The University of Texas at Austin, 2007 Supervisor: Augusto L. Podio The re-design of the internal geometry of static down – hole gas separators directly affects the gas – liquid separation performance. This thesis describes experimental results obtained after changing the dip tube design from the conventional straight design to a helical design. Typically, a static down – hole gas separator with a conventional straight dip tube design depends on gravity to induce density difference in the flowing wellbore fluid which causes gas – liquid separation to occur. Thus, the device is known as a gravity driven down – hole gas separator. vi
  7. 7. This research compared the experimental results and visual observations from gravity driven down – hole gas separators to that of static down – hole gas separators with helical dip tube designs known as static centrifugal down – hole gas separators. The visual observations showed that not only did the driving mechanisms for gas – liquid separation inside static centrifugal down – hole separators include gravity it also incorporated other means such as induced centrifugal forces that greatly improved overall gas – liquid separation. The 6 inch/second threshold downward superficial liquid velocity generally regarded as the industry rule – of – thumb for down – hole gas separators was increased to 10 inch/second. In field units this is a 200 BPD increase in liquid production. This research also studied the effect of increasing outer diameter of gravity driven down – hole gas separators from 3inches (2.75” ID) to 4inches (3.75” ID). The results showed that liquid handling capacity increased by over 90% due to favorable flow regimes observed inside the separator. However, critical examination of gas – liquid separation performances of both 3 inch OD and 4 inch OD separators in terms of downward liquid superficial velocity reveal that gas – liquid separation results are similar. It was concluded therefore that downward superficial liquid velocity is a reliable parameter in the design of down – hole gas separators and that all gravity driven separators regardless of separator outer diameter will operate in similar fashion except at different liquid flow rates. Bubble rise experiment performed in this research project gave a range of 1 – 100 cp as region of applicability for the results discussed in this thesis. vii
  8. 8. Table of Contents Acknowledgment …………………………………………………………………v Abstract………...…………………………………………………………………vi List of Tables ........................................................................................................ xii List of Figures ...................................................................................................... xiii CHAPTER 1 1 Introduction ..............................................................................................................1 1.1 OBJECTIVE ..........................................................................................1 1.2 LITERATURE REVIEW ......................................................................3 1.2.1 PATENTED STATIC CENTRIFUGAL DOWN – HOLE GAS SEPARATORS ...........................................................................15 1.2.1.1 GAS ANCHOR - PATENT No 3128719.................................15 1.2.1.2 Continuous Flow Down – hole gas separator for Progressive Cavity Pumps - Patent No 5902378 .........................17 1.2.2 ACTIVE TYPE CENTRIFUGAL DOWN – HOLE GAS SEPARATORS ...........................................................................20 1.2.2.1 Liquid – Gas Separator Unit - Patent No 3887342 .............20 1.2.2.2 Liquid – Gas Separator Apparatus - Patent No 4481020 ...21 1.2.2.4 Apparatus for separating gas and solids from well fluids - Patent No 6382317 B1.................................................................24 CHAPTER 2 28 Experimental Facility And Procedure ....................................................................28 2.1 EXPERIMENTAL FACILITIES.........................................................28 2.2 DESCRIPTION OF EXPERIMENTAL FACILITIES .......................28 2.3 LABORATORY TEST WELL............................................................32 2.3.1 LABORATORY INSTRUMENTS ............................................34 2.3.1.1 LIQUID FLOW MEASUREMENTS ................................34 2.3.1.2 GAS FLOW MEASUREMENT ........................................35 viii
  9. 9. 2.3.2.2 PRESSURE MEASUREMENT ........................................37 2.4 EXPERIMENTAL PROCEDURE ......................................................38 2.5 SEPARATOR PERFORMANCE DISPLAY ......................................39 2.6 DOWN HOLE GAS SEPARATOR DESIGNS ..................................44 2.6.1 ECHOMETER (3X1), ECHOMETER (3X1.5), ECHOMETER (4x1), ECHOMETER (4X1.5), ECHOMETER (4X1.75) ..........44 2.6.2 PATTERSON (3X1), PATTERSON (3X1.5), PATTERSON (4x1), PATTERSON (4X1.5), PATTERSON (4X2)..................47 2.6.3 TWISTER ...................................................................................48 2.6.3.1 ECHOMETER-TWISTER ..........................................52 2.6.3.2 PATTERSON – TWISTER .........................................53 CHAPTER 3 55 Analysis Of Experimental Results .........................................................................55 3.1 EFFECT OF HELICAL DIP TUBE DESIGN ....................................55 3.1.1 PERFORMANCE RESULTS FOR THE TWISTER SEPARATOR .............................................................................56 3.1.2 PERFORMANCE RESULTS FOR ECHOMETER – TWISTER SEPARATOR .............................................................................58 3.1.3 PERFORMANCE RESULTS FOR PATTERSON – TWISTER SEPARATOR .............................................................................61 3.2 COMPARISON OF PERFORMANCES OF HELICAL DIP TUBE GAS SEPARATORS TO STRAIGHT DIP TUBE GAS SEPARATOR63 3.2.1 COMPARISON OF ECHOMETER-TWISTER AND ECHOMETER (3X1) GAS SEPARATORS ..............................63 3.2.2 COMPARISON OF PATTERSON-TWISTER AND PATTERSON (3X1) GAS SEPARATORS ...............................73 3.2.3 EFFECT OF THE NUMBER OF DIP TUBE TWISTS ON STATIC CENTRIFUGAL SEPARATORS ...............................78 3.2.4 ANALYSIS OF STATIC CENTRIFUGAL SEPARATOR DESIGNS. ...................................................................................84 3.3 EFFECT OF INTERIOR AND EXTERIOR FLOW AREAS ON SEPARATOR PERFORMANCE........................................................88 3.3.1 EFFECT OF CHANGING INTERIOR AND EXTERIOR ANNULAR AREA FOR ECHOMETER GAS SEPARATORS90 ix
  10. 10. 3.3.2 EFFECT OF CHANGING INTERIOR AND EXTERIOR ANNULAR AREA FOR PATTERSON GAS SEPARATORS103 3.4 DIP TUBE LENGTH EFFECTS ..............................................116 3.5 PERFORMANCE OF ECHOMETER (3X1) GAS SEPARATOR WITH STANDING VALVE INCLUDED BETWEEN GAS SEPARATOR AND TUBING RETURN LINE (PUMP INTAKE) .119 3.5.1 ECHOMETER (3X1) AND ECHOMETER (3X1) WITH STANDING VALVE COMPARED. .......................................123 3.5.2 ANALYSIS OF PRESSURE DROP FOR ECHOMETER (3X1) AND ECHOMETER (3X1) WITH STANDING VALVE ......125 3.6 FLOW REGIMES INSIDE THE DOWN – HOLE GAS SEPARATORS ..................................................................................126 CHAPTER 4 133 Bubble Rise Experiments .....................................................................................133 4.1 APPARATUS USED IN BUBBLE RISE EXPERIMENTS.............134 4.2 PROPERTIES OF FLUIDS USED IN THE EXPERIMENT ...........136 4.2.1 TEST FOR NEWTONIAN CHARACTERISTICS OF FLUIDS136 4.2.2 DETERMINING THE VISCOSITY OF TEST FLUIDS IN ASSOCIATION WITH WATER AT ROOM TEMPERATURE138 4.2.2.1 TEST DATA FOR GLYCERIN IN ASSOCIATION WITH WATER ....................................................................................139 4.2.2.2 TEST DATA FOR CORN SYRUP IN ASSOCIATION WITH WATER .........................................................................140 4.3 ANALYSIS OF RESULTS FROM BUBBLE RISE EXPERIMENTS141 CHAPTER 5 145 Conclusions and Recommendations ....................................................................145 5.1 CONCLUSIONS...............................................................................145 5.1.1 CONCLUSIONS FROM COMPARISONS OF GRAVITY DRIVEN SEPARATORS AND STATIC CENTRIFUGAL GAS SEPARATORS .........................................................................146 5.1.2 THE EFFECT OF INCREASING SEPARATOR OUTER DIAMETER FOR GRAVITY DRIVEN SEPARATORS .......147 5.1.3 CONCLUSIONS FROM BUBBLE RISE EXPERIMENT ....149 x
  11. 11. 5.2 GENERAL DESIGN GUIDE ...........................................................149 5.3 RECOMMENDATIONS AND FUTURE WORK ..........................150 Appendix A ..........................................................................................................152 Schematics of the Echometer Separators .............................................................152 Appendix B ..........................................................................................................155 Schematics of the Patterson Separators ...............................................................155 Appendix C ..........................................................................................................159 Original data files .................................................................................................159 Nomenclature .......................................................................................................160 Abbreviations .......................................................................................................161 References ............................................................................................................163 Vita …………………………………………………………………………….165 xi
  12. 12. List of Tables Table 2-1 - Sample Excel Spreadsheet for continuous flow test.......................................... 41 Table 2-2 – Echometer gas separators configuration ............................................................ 44 Table 2-3 – Patterson Separator Configuration ...................................................................... 46 Table 4-1 – Dimensions of bubble rise experiment apparatus ........................................... 134 Table 4-2 – Fluid Properties used in bubble rise experiment ............................................. 135 Table 4-3 – Test data for glycerin in association with water............................................... 138 Table 4-4 - – Test data for corn syrup in association with water ....................................... 139 xii
  13. 13. List of Figures Figure 1-1 - Centrifugal Separator(Kobylinski et al) ................................................................ 8 Figure 1-2 - Gas flow through centrifugal separator (Kobylinski et al) ................................ 9 Figure 1-3 - Reverse-flow separator (Kobylinski et al) .......................................................... 10 Figure 1-4 – Collar-Size down – hole gas separator (McCoy and Podio10)....................... 12 Figure 1-5- Down-hole gas separator (Patterson and Leonard11) ........................................ 14 Figure 1-6 - Jongbloed et al12 ..................................................................................................... 17 Figure 1-7 – Static Centrifugal Separator by Obrejanu Marcel13 .......................................... 19 Figure 1-8 – Invention by Bunnelle P14.................................................................................... 21 Figure 1-9 - Centrifugal Separator by Kobylnski et al ........................................................... 23 Figure 1-10 – Invention by Powers Maston15 ......................................................................... 24 Figure 1-11 – Invention by Delwin Cobb16 ............................................................................. 26 Figure 1-12 – Cross – section (3) in Figure 1-11 .................................................................... 26 Figure 2-1 – Schematic of experimental test facility .............................................................. 30 Figure 2-2 – Laboratory facility ................................................................................................. 30 Figure 2-3 – Laboratory test well .............................................................................................. 31 Figure 2-4 – Laboratory Well .................................................................................................... 32 Figure 2-5 – Turbine flow meter and valve between pump and mixer ............................... 33 Figure 2-6 - - ITT Barton floco positive displacement meter .............................................. 34 Figure 2-7 - Fisher Porter Flow Rator tube............................................................................. 35 Figure 2-8 - Thermodynamic Omega Air Flow Meter .......................................................... 36 Figure 2-9 - Sample Performance plot for Patterson (3X1) in continuous flow ............... 42 Figure 2-10 – Echometer (3 X1.5) gas separator design ....................................................... 45 Figure 2-11- Echometer entry port geometry ......................................................................... 45 Figure 2-12 – Echometer (4X1.75) gas separator design ...................................................... 45 Figure 2-13 – 4 inch OD Patterson Separator Design .......................................................... 47 Figure 2-14 – 3 inch OD Patterson Separator Design .......................................................... 47 Figure 2-15 – Twister Separator (Bohorquez) ........................................................................ 50 Figure 2-16 – Twister Connection ............................................................................................ 50 xiii
  14. 14. Figure 2-17 – Diagrammatic of the forces acting in a static centrifugal separator ............ 51 Figure 2-18 – Echometer - Twister .......................................................................................... 52 Figure 2-19 – Patterson - Twister ............................................................................................. 53 Figure 3-1- Twister results in field units .................................................................................. 56 Figure 3-2 - Twister result in terms of superficial velocities ................................................. 57 Figure 3-3 – Echometer - Twister result in terms of superficial velocities ......................... 58 Figure 3-4 – Echometer - Twister results in field units ......................................................... 59 Figure 3-5 - Patterson - Twister result in terms of superficial velocities ............................ 60 Figure 3-6 - Patterson - Twister results in field units............................................................. 61 Figure 3-7 – Comparison of Echometer – Twister and Echometer (3X1) results in terms of superficial velocity .................................................................................................................. 64 Figure 3-8- Comparison of Echometer – Twister and Echometer (3X1) results in Field Units .............................................................................................................................................. 65 Figure 3-9 - Pressure Drop between the entry ports and pump intake for Echometer – Twister and Echometer (3X1); Casing Pressure (Pc) = 10 – 13psi ...................................... 66 Figure 3-10 – Pressure measurements during the tests ......................................................... 67 Figure 3-11 – Pressure drop for Echometer-Twister and Echometer (3X1) at constant gas rates; Pc = 10 – 13 psi .......................................................................................................... 68 Figure 3-12- Pressure drop for Echometer-Twister and Echometer (3X1) at constant liquid rates; Pc = 10 – 13 psi ...................................................................................................... 71 Figure 3-13 - Comparison of Patterson – Twister and Patterson (3X1) results in terms of superficial velocity ....................................................................................................................... 72 Figure 3-14 - Comparison of Echometer – Twister and Echometer (3X1) results in Field Units .............................................................................................................................................. 73 Figure 3-15 - Pressure Drop between the entry ports and pump intake for Patterson – Twister and Patterson (3X1) separators; Casing Pressure (Pc) = 10 – 13psi ...................... 74 Figure 3-16 – Pressure drop for Patterson-Twister and Patterson (3X1) at constant gas rates; Pc = 10 – 13 psi ................................................................................................................. 75 xiv
  15. 15. Figure 3-17 - Pressure drop for Patterson-Twister and Patterson (3X1) at constant liquid rates; Pc = 10 – 13 psi ................................................................................................................. 76 Figure 3-18 – Patterson-Twister (2 twits) ................................................................................. 77 Figure 3-19 - Patterson – Twister (2 twists) results in terms of superficial velocities ....... 78 Figure 3-20 - Patterson – Twister (2 twists) gas separator results in field units ................ 79 Figure 3-21 - Comparison of Patterson – Twister (4 twists) and Patterson – Twister (2 twists) results in superficial velocity terms ................................................................................ 80 Figure 3-22 - Comparison of Patterson – Twister (4 twists) and Patterson – Twister (2 twists) results in Field Units ........................................................................................................ 81 Figure 3-23 - Pressure drop between the entry ports and pump intake for Patterson – Twister 2 twists and 4 twists gas separators; Casing Pressure (Pc) = 10 -13 psi.................... 82 Figure 3-24 – Comparison of results for all static centrifugal separators in terms of superficial velocities .................................................................................................................... 84 Figure 3-25 - Comparison of results for all static centrifugal separators in field units ..... 85 Figure 3-26 - Pump Liquid Fraction for Static Centrifugal Separators at 10 in/sec ......... 86 Figure 3-27 - Echometer (4X1) and Echometer (3X1) results compared in field units ... 90 Figure 3-28 - Echometer (4X1.5) and Echometer (3X1.5) results compared in field units ........................................................................................................................................................ 91 Figure 3-29 - Echometer (4X1.75) results in field units ........................................................ 93 Figure 3-30 – Comparison of results of all Echometer gas separators in terms of superficial velocity ....................................................................................................................... 94 Figure 3-31 - Pump Liquid Fraction for Echometer Separators at 6 in/sec ...................... 96 Figure 3-32 - Pump Liquid Fraction for Echometer Separators at 10in/sec ..................... 97 Figure 3-33 – Comparison of all Echometer 4 inch OD separator results in field units . 98 Figure 3-34 - Pressure drop between the entry ports and pump intake for Echometer 4 inch OD gas separators for 2 phase gas liquid flow; Casing Pressure (Pc) = 10 -13 psi. 100 Figure 3-35 - Pressure drop between the entry ports and pump intake all tested Echometer gas separators; Casing Pressure (Pc) = 10 -13 psi ............................................ 101 Figure 3-36 - Patterson (4X1) and Patterson (3X1) results compared in field units ....... 103 xv
  16. 16. Figure 3-37- Patterson (4X1.5) and Patterson (3X1.5) results compared in field units .. 104 Figure 3-38 – Patterson (4X1.75) results in field units ........................................................ 105 Figure 3-39 - Patterson (4X2) results in field units .............................................................. 106 Figure 3-40 - Comparison of results for all Patterson 3 inch OD and 4 inch OD separators in superficial velocity terms .................................................................................. 107 Figure 3-41 - Pump Liquid Fraction for Patterson Separators at 6 in/sec ....................... 109 Figure 3-42 - Pump Liquid Fraction for Patterson Separators between 8 – 9 in/sec ..... 110 Figure 3-43 - Comparison of all Patterson 4 inch OD separator results in field units ... 112 Figure 3-44 – Pressure Drop between the entry ports and pump intake for Patterson 4 inch OD gas separators; Casing Pressure (Pc) = 10 – 13psi ............................................... 113 Figure 3-45 - Pressure drop between the entry ports and pump intake for Patterson 4 inch OD gas separators at varying gas and liquid rates; Casing Pressure (Pc) = 10 -13psi ...................................................................................................................................................... 114 Figure 3-46 – Comparison of results for Echometer (4X1.75) with 5½′dip tube and Echometer (4X1.75) with 2¾′ dip tube in superficial velocity terms ................................................................. 116 Figure 3-47 - Comparison of results for Echometer (4X1.75) with 5½′dip tube and Echometer (4X1.75) with 2¾′ dip tube in field units ...................................................................................... 117 Figure 3-48 – Standing Valve Assembly ................................................................................ 119 Figure 3-49 – SV joint – Gas Separator Connection ........................................................... 119 Figure 3-50 – Echometer (3X1) with SV result in terms of superficial velocities ........... 120 Figure 3-51 - Echometer (3X1) with SV result in field units .............................................. 121 Figure 3-52 – Comparison of Echometer (3X1) with and without Standing Valve in terms of superficial velocities ............................................................................................................. 122 Figure 3-53 - Comparison of Echometer (3X1) with and without Standing Valve in field units ............................................................................................................................................. 123 Figure 3-54- Pressure drop between the entry ports and pump intake for Echometer (3X1) with and without Standing Valve .................................................................................... 124 Figure 3-55 - Pressure drop between the entry ports and pump intake for Echometer (3X1) with and without Standing Valve at varying gas and liquid rates; Pc = 10 -13 psi ... 125 xvi
  17. 17. Figure 3-56 –Flow Regimes observed in the gas separator annular area (courtesy Renato Bohorquez7)................................................................................................. 126 Figure 3-57 – Flow regime map for the annular space of 3 inch OD gravity driven gas separators7 127 Figure 3-58 – Flow regime map for the annular space of 4 inch OD gravity driven gas separators .. 128 Figure 3-59 – Flow regime map for the Twister separator annlus7 ................................... 130 Figure 3-60 – Flow regime for Patterson – Twister and Echometer – Twister static centrifugal separators ................................................................................................................ 131 Figure 4-1 – Schematic of Laboratory Constructed Apparatus for testing bubble rise velocity ......... 134 Figure 4-4 – Glycerin Rheology test ....................................................................................... 136 Figure 4-5 – Glycol Rheology test .......................................................................................... 136 Figure 4-6 – Corn Syrup Rheology test.................................................................................. 137 Figure 4-7 – Viscosity plot for Glycerin in association with water at room temperature ...................................................................................................................................................... 138 Figure 4-8 – Viscosity plot for Corn Syrup in association with water at room temperature ...................................................................................................................................................... 139 Figure 4-9 – Combined viscosity plots for glycerin and corn syrup in association with water at room temperature ...................................................................................................... 140 Figure 4-10 – Examples of bubble diameter sizes measured……………………….142 Figure 4-11 – Mean bubble rise velocities in stationary liquid in an annulus ................... 143 xvii
  18. 18. Chapter 1 Introduction 1.1 OBJECTIVE Most wells producing from mature reservoirs use artificial lift methods for oil and gas production. Common artificial lift methods include beam pumping, progressive cavity pumping and electric submersible pumping. All the mentioned artificial lift systems exhibit a common problem: Gas Interference The presence of free gas in beam pumps (sucker rod pumps) prevents the traveling valve from opening at the appropriate time interval during the downstroke. This is caused by the high compressibility of gas in the pump barrel. The traveling valve may eventually open when the gas inside the barrel has been compressed enough to overcome the fluid load on the plunger. In such a case fluid pound occurs. In extreme cases the peak pressure of the trapped gas on the downstroke is insufficient to overcome the hydrostatic head of the traveling valve; then the pressure is not reduced enough on the upstroke to allow the standing valve to open and admit new fluid. Both valves are essential stuck at a closed position and the pump refuses to pump. This extreme case is known as gas locking. In progressive cavity pumps (PCP) the produced liquid lubricates the rotor and the stator so as to reduce the heat caused by friction. The presence of free gas in the 1
  19. 19. produced fluid reduces the lubricating function of the produced fluid so that the rotor and stator are in direct contact. Temperature increase due to the direct contact causes damage to the pump. In other cases gas in the produced fluid in PCP may change the chemical composition of the elastomer in the stator of the pump which further complicates the problem. Electric submersible pumps (ESP) are typically used to handle high liquid flow rates. Significant volumes of gas entering the pump especially at low intake pressures degrade the pump performance, and dramatically reduce the head produced by the ESP. This may prevent the pumped liquid from reaching the surface. The ESP is composed of a down – hole motor which is connected to a seal – section which in turn is connected to a centrifugal pump. It is imperative that the motor be cooled by the produced fluid passing the outer casing. In the event that large quantities of gas pass the motor, the heat transfer from the motor to the produced fluid will be drastically reduced, potentially causing motor damage by overheating. In all cases - beam pumps, PCP and ESP the pump volumetric efficiency is reduced by the presence of gas. To combat the problem of reduced volumetric efficiency and system damage down – hole gas separators are used in conjunction with down – hole pumps. The sole purpose of down – hole gas separators* is to prevent gas from entering into down – hole pumps, or to at least reduce the quantity of gas entering into the pump to permissible ranges where the pump efficiency is still acceptable. Unfortunately many gas separator designs have not yielded the desired efficiency. The widely used ‘poorboy’ gas separator which depends on gravity segregation to separate gas from liquids has become synonymous with inefficiency. * Down –hole gas separators will mean the same thing as gas separators throughout this thesis 2
  20. 20. A thorough literature review on the subject of gas separator design was done to study previous designs and relevant applications. Sources of information included published technical papers, patents and thesis reports by Lisguiski, Guzman and Bohorquez. The scope of the present work emphasized the effect of the internal geometry and induction of centrifugal forces on gas separator performance. 1.2 LITERATURE REVIEW Schome 1 in February 1953 reported a field test of a down-hole gas separator in a well in Utah. The pump volumetric efficiency obtained before the installation the down- hole gas separator ranged between 26 and 48%. Schome1 reported that the efficiency was increased to 70%; resulting in an increased production of 50BPD after the new gas separator was installed. The author went on to describe some bottom-hole separator (as it was then referred) designs and their mode of operation. All the separator designs described in his paper depended on gravity segregation as the controlling mechanism for efficient performance and were 30 – 40 ft long with 1inch suction tubes (dip tubes). Schome1 noted that operators often faced retrieval problems when the separators were plugged with formation debris. He attributed that to inconsistent installation techniques and gas separator designs. Clegg2 did a thorough review of the different types of gas anchors (down-hole gas separators) and the principles that govern most of their operation. He pointed out that the desire of several gas separator inventors was to achieve a downward mixture 3
  21. 21. velocity of 0.5ft/sec (6in/sec) inside the separator – dip tube annular area. A downward mixture velocity of 0.5ft/sec is generally accepted as being below the rising (slip) velocity of gas in low viscosity fluids. Clegg2 and McCoy3 et al described the reasons for the inefficiency of the commonest down-hole gas separator design the ‘poorboy’ separator. The reasons for the inefficiency of the Poorboy gas separator according to the author included the high downward liquid velocity inside the Poorboy separator and size of its dip tube ID which the author considered as too small in diameter. The small ID dip tube often causes excessive pressure drop inside the separator. The Shell (Schmit Jongbloed) gas anchor formula: 100 gas anchor efficiency = (1 + C × Pwf × Vsl0.5 ) 0.66 1 Pwf= intake pressure at the anchor; Vsl=downward superficial velocity of liquids; C = gas anchor constant (usually ≈0.2 based on laboratory data) described by Clegg1 showed that the performance of any given size and type of gas separator is largely dependent on the intake pressure at the anchor and the downward superficial velocity of the fluids in the anchor. An examination on the formula done by the author reveled that at zero pressure and zero velocity the anchor/gas separator efficiency is 100% and that at high velocities (greater than 0.5 ft/sec) inside the gas separator the separation efficiency is poor. Pressures above 400psig also resulted in low efficiencies. The author however cautioned that actual experiences indicate that separation may be significantly greater than what the formula predicts. The uncertainty in the equation emerged from the use of the constant ‘C’ which represented other important variables such as viscosity, gas bubble size and dispersion. Laboratory results that were not published indicated a constant of 0.2. The author warned that the 4
  22. 22. determination of accurate values of ‘C’ is difficult for actual field conditions. Clegg2 strongly encouraged using a Natural gas anchor (installing the pump below the lowest perforation) whenever it is feasible as is gives the greatest down-pass area for the liquid thereby reducing the downward liquid velocity. The work by Campbell and Brimhall4 largely focused on developing an industry standard for determining the down-hole gas separator area; the dip tube area and the dip tube length to be used for different liquid and gas flow rates. The objective of their computer program was to aid in the design of a gas separator system and to evaluate the pressure drops within the system and thus the system efficiency. The major parameters which they noted were pivotal to gas separator design included the gas bubble velocity, diameter of the mud anchor (down-hole gas separator), length of dip tube and the pressure drops associated with the system. They agreed that the 0.5 ft/sec downward liquid velocity inside the separator was a valid rule of thumb for low viscosity fluids. The design procedure began with using Stokes Law (see Equation 2) to determine the terminal rising velocity that a given gas bubble will achieve in a liquid for a given gas bubble radius, liquid viscosity and density difference between the two phases. 2 g ( ρ1 − ρ 2 ) Rb2 U= 9μ 2 Where U = terminal velocity, ft/sec; g = 32.17 ft/sec2; ρL = liquid density; Ib/ft3 ρg = gas density, Ib/ft3; μ = liquid viscosity; Ibm/(ft-sec); Rb = bubble radius (ft) The second step used the calculated terminal velocity to calculate the area of the gas separator (also called Mud Anchor or MA) using equation 3. 5
  23. 23. 0.00935 × QL AMA = U × EV STB Where AMA = area of mud anchor, in2, QL = liquid rate, ; EV = Pump efficiency D 3 These calculated values are inputted into the computer program explained in their paper to generate relationships between (1) pressure effects on gas bubble velocity over constant viscosity and temperature (2) gas bubble velocity and diameter of the mud anchor over different liquid flow rates (3) dip tube diameter and pressure drop in the gas anchor over different liquid flow rates (4) pressure drop and dip tube length as a function of liquid rate. The results showed that gas bubbles travelled faster in smaller OD mud anchors larger dip tube diameters yielded the smallest pressure drop and longer dip tubes had the largest pressure drops. Experimental results from Lisugurski5, Guzman6 and Bohorquez7 however dispute the orders of magnitude of the results from Campbell and Brimhall4. Field results9 based on Lisugurski’s 5 thesis have shown that a 6ft long gas separator can operate efficiently at rates which would require longer gas separator lengths if Campbell 4 and Brimhall’s results were practiced to the letter. Bohorquez7 in his work however concluded that gas bubbles especially during the up - stroke of a sucker rod pumping system coalesce more readily and rise faster in smaller separator annular areas compared to larger annular areas. Kobylinski et al 8 described the design, development and laboratory testing of a new rotary gas separator, Figure 1-1 and Figure 1-2. The rotary gas separator is an active- 6
  24. 24. type centrifugal separator. Laboratory and field comparison were conducted between the Centrifugal separator and the passive-type Reverse-flow separator, Figure 1-3. Laboratory tests were done using water and air as test fluids in continuous flow condition. The Reverse-flow separator uses the gravity separation mechanism for gas – liquid separation. The Centrifugal separator achieved separation of gas and liquid by the use of cyclone and vortex technology. The characteristics of this method identified by the authors were that the separated liquid is concentrated in the vicinity of the wall of the separator while the gas phase concentrates at the center of the system. The authors stated that dimensioning of the separator should be based on the equation of the trajectory of the gas bubbles; they added that a general equation that would cover the turbulence arising in the process is not available. Kobylinski et al8 believe that since both bubble dimensions and proportionality constant between gas and liquid velocities are unknown from Stokes’s law for laminar flow(1), reliance on experimental work for design optimization remains the only alternative. A detailed discussion on bubble dynamics is analyzed in the paper. The results from the field tests8 complemented the results from the laboratory and led to a 95% average improvement in fluid production when results from the active- type centrifugal separators were compared to the passive-type reverse-flow separators in tested wells. The dimensions of both the centrifugal and reverse-flow separators were not given. 7
  25. 25. Figure 1-1 Centrifugal Separator Kobylinski et al 8
  26. 26. Figure 1-2 - Gas flow through centrifugal separator (Kobylinski et al) 9
  27. 27. Figure 1-3 - Reverse-flow separator (Kobylinski et al) McCoy and Podio10 gave a detailed description of the Collar – Size gas separator, Figure 1-4. They emphasized a maximum pressure loss of ½ PSI for friction loss in the dip tube. The authors also highlighted the need to allow for sufficient space in the gas 10
  28. 28. separator annular area. According to the authors sufficient flow area should exist so that the gas flow rate around the ports in the gas separator will allow liquid to flow or ‘fall’ into the gas separator annulus. The authors noted the necessity to balance the area available for flow in the wellbore and that inside the gas separator. Decreasing the casing annulus will result in increased upward gas velocity which when above 10 ft/second will suspend some of the liquid and allow mist flow to occur. Another consequence of casing annulus reduction and increase of gas velocity will be the prevention of liquid from flowing into the gas separator annulus. The authors stressed the need for the use of large ports. Large ports allow liquid from the casing to fall by gravity force into the gas separator because the pressures inside and outside the large ports are the same. Kobylinski et al8 in their paper also recommended that for a gas separator to operate efficiently, it must ingest the two – phase mixture with minimal pressure drop. This is necessary to prevent additional gas breakout inside the separator. The Collar – Size separator10 had a total port area which was approximately four times the area inside the gas separator. The gas separator length received special treatment by McCoy and Podio10 they suggested that the dip tube length extend at least 18 inches below the gas separator inlet perforations (separator ports). They based their calculation on a gas rise velocity of 6in/sec (0.5ft/sec) and an average pumping speed of 10 strokes per minute which translates to a pumping cycle time of 6 seconds. The authors also looked at eccentricity of the separator. Earlier studies noted by the authors showed that liquid concentrates where tubing is placed against the casing wall and thus advised that gas separator’s outer diameter should contact the casing wall, see Figure 1-4. In wells with some deviation McCoy and Podio10 advised that the separator should be allowed to rest on the low side of the casing since gas tends to flow up on the 11
  29. 29. high side of the casing annulus by installing any tubing anchors at a distance of 60 to 90 feet shallower than the pump intake. Figure 1-4 – Collar-Size down – hole gas separator (McCoy and Podio10) Patterson and Leonard11 ran some field tests in coal-bed methane wells in Wyoming with some changes in the down – hole pump setting depth interval and for an increase in gas separator OD. The authors noted that while the modifications were not fully understood or tested with significant number of installations the improvements observed warranted some discussion. The tests were conducted in two wells and are fully described in the paper. Patterson and Leonard11designed different gas separators used in the field tests in a bid to achieve greater pump efficiency. The gas separators used in the tests had smaller slot width and included vent holes and a baffle to facilitate the evolution of gas - Figure 12
  30. 30. 1-5 . The concept according to the authors assumes that a smaller slot width will reduce the amount of gas entering the gas separator and the vent holes will allow the gas that enters to vent back to the casing. The slot sizes ranged from 0.3” wide by 6” long for the 3.5” OD gas separator (2 in number) to a 3/16” wide by 10” long for the 5.5” OD gas separator (8 in number). The 5.5” OD gas separators also had three ½ “diameter holes in the swedge (see Figure 1.1-5). Both separator designs had the same dip tube OD but different dip tube lengths – 2 inches difference. The 3.5” OD gas separator was 24 feet long whereas the 5” OD gas separator was 26 feet long. The test well , 43-26, had a 3.5” OD 8’ long gas separator installed with a Progressive Cavity Pump (PCP) at 1446’ ft after a ‘bucket’ test had been conducted. After some months a 5.5”OD gas separator was attached to the PCP in test well 43-26. Although the well contained coal particles which got into the gas separator and starved the pump intake some useful evaluations on the effect of increase in gas separator cross sectional area were made from test well 43-26. Due to gas separator design changes the inlet area of the 5.5” OD gas separator design increased four times compared to the 3.5” OD gas separator design. The 5.5” OD gas separator annular area (gas separator annular area = gas separator ID – dip tube OD) increased by approximately 3 times over the 3.5” OD gas separator. The field results11 showed that no gas was produced through the tubing when the 5.5” OD separator was run with the PCP in test well 43-26. Patterson and Leonard11 infer that the differences in inlet area and cross – sectional area available for flow could have had an impact on gas separation and would appear that some combination of these differences has a grater influence on gas separation than only increasing the cross – sectional area. In another well test where a 4” OD gas separator with some modifications to the entry slot area and separator length was compared to a 3.5 OD gas separator 13
  31. 31. efficiency in the same well. The authors observed that whereas the 3.5” OD gas separator produced gas through the tubing the 4” OD gas separator did not. The authors observed that the increase in annular area must have contributed to pump efficiency improvement. They however speculated that the increase in length of the 4” OD separator or the baffle design of the gas separator might also have aided to the improvement. The authors suggested that more field tests be done and visual modeling experiments be evaluated with different geometries and configurations to better understand the reason(s) behind the improvements. Patterson and Leonard11 made other related conclusions in the paper which dealt with; downward liquid velocity, essence of vent holes, position of the inlet of the gas separator relative to the perforations and the age old theory that placing the intake of the pump below the perforated interval creates an effective natural gas anchor (gas separator). Figure 1-5- Down-hole gas separator (Patterson and Leonard11) 14
  32. 32. Guzman6 experimentally determined that placing the gas separator inlet at about 3feet below the lowest perforation results in natural separation that yields total gas – liquid separation. A gas separator is not needed in such cases as long as the annular liquid downward velocity is less than 6 inches per second. Guzman6 also suggested that the ports area should be equal the gas separator annular area so that the superficial liquid velocity does not control the flow regime inside the separator. The use of vent holes in the design of Patterson and Leonard11 in the experiments conducted by in continuous flow Guzman6 showed that the vent holes do not improve gas separation. The author suggested the use of single row slots instead of multiple rows. He however noted that for the decentralized wells the results might be difficult to predict due to well eccentricity10. Several centrifugal gas separators have been patented over the years. Most of the patented arts require the invention have moving parts whereas some do not. The next sections will initially describe the parts and mode of operation of patented static centrifugal separators and finally do same for ‘active’ patented centrifugal separators 1.2.1 PATENTED STATIC CENTRIFUGAL DOWN – HOLE GAS SEPARATORS 1.2.1.1 GAS ANCHOR - PATENT No 3128719 The invention (Figure 1-6) by Jongbloed et al12 in 1964 relates to a gas anchor consisting of a cylindrical housing sealed at the bottom, at least one sheet metal helix accommodated in the housing, and a tube (14), one side of which communicates with the space underneath the sheet metal helix. According to the invention, a discharge 15
  33. 33. conduit is centrally positioned in the housing, the conduit is provided with openings, preferably near the side of the sheet metal helix facing the bottom of the housing (19), the gas discharge conduit (17) and the sheet, communicates with the outside of the housing through the opening (21) that is above the supply openings (20). In a reported experimental arrangement in which the outer diameter of the helical channels was 7.5 cm (gas anchor ID = 3 inches), mixtures of varying gas/oil ratios were supplied to a gas anchor according to the invention. The quantity of oil passing the separator was from 1 – 1.5 cu. meters per hour (151 BPD – 226 BPD). When fluid mixtures (dispersions) having gas – oil ratios of between 5 and 20 were supplied, the gas/oil ratio of the mixture flowing through conduit (14) was less than 0.01. This invention has no moving parts. 16
  34. 34. Figure 1-6 - Jongbloed et al12 1.2.1.2 Continuous Flow Down – hole gas separator for Progressive Cavity Pumps - Patent No 5902378 This apparatus invented by Obrejanu Marcel13 in 1999 is a gas separator which can be attached to the suction of a down – hole pump to remove gas from the liquid being pumped prior to the liquid entering the pump inlet. The separator has an elongate 17
  35. 35. housing having an annular chamber with guides which direct the liquid gas mixture to flow in an annular path from the inlet to the outlet end. During this flow centrifugal forces act to displace the gas content to the central region from which it is removed via a separate central gas outlet so that liquid delivered to the pump inlet is greatly reduced in its gas content. In operation, the separator is attached in a coaxial fashion via sub (14) to the lower end of a progressive cavity pump. In a gaseous environment the liquid will contain dissolved gases and will enter the chamber (15) under formation pressure through the inlet ports (16). When the pump is operated the reduction in pressure as a result of the pump suction will cause some of the dissolved gas to come out of solution. The gas liquid mixture is drawn upwardly within the tubular housing (12) and upon encountering the helical flights (20) is guided thereby to move in a helical path. The centrifugal forces created in the liquid as a result of the helical flow act to reduce the gas content of the peripherally outer region of the flow and increase the gas content of the central region of the flow. The angular momentum created in the liquid flow by the flights (20) is maintained as the liquid moves upwardly into the expansion chamber (23). In this chamber the cross sectional area of the flow passage is expanded as a result of the termination of the flights (20), the tapering and termination of the spindle (17), and the outwards flare of the inner wall of the tubular housing (12), the combined effects of these resulting in a marked reduction in pressure of the liquid flow thus enhancing the gas separation effect. The centrifugal force in the rotating liquid is effective to confine the separated gas to the axial region of the chamber which rises above the rounded top end (22) of the spindle. The separated gas flow through the axial exit passage (26) to the exterior of the sub (14) where they can be released into the well bore, or if so desired delivered to the surface through a separate conduit. 18
  36. 36. This separator has no moving parts within the separating chamber. To force the liquid into the chamber the separator depends on both hydrostatic head and the pressure drawdown created by the action of the PCP mounted above. Another interesting feature about the invention is that multiple separation chambers could be attached just below (18) for a two stage separation process before the liquid enters into the pump. This separator design is currently been manufactured in commercial quantity in Canada. Figure 1-7 – Static Centrifugal Separator by Obrejanu Marcel13 19
  37. 37. 1.2.2 ACTIVE TYPE CENTRIFUGAL DOWN – HOLE GAS SEPARATORS 1.2.2.1 Liquid – Gas Separator Unit - Patent No 3887342 The unit was invented by Bunnelle P14 in 1975. The inventor claims that the unit when tested in the laboratory could handle high liquid flow capacity of about 82 gallons per minute (2730 BPD) at zero discharge pressure (pressure head generated by the unit) and that this rate is slightly reduced to 70 GPM when a 26 ft3/min (37440 CFD) gas is introduced into the unit. The test fluids where air and water. The separator unit operates as follows. As motor shaft (58) revolves at a constant rate impeller shaft (26) and impeller (16) likewise revolve. The impeller draws a liquid – gas mixture through intake openings (32a), into the chamber (12a). As this liquid – gas mixture moves upwardly within chamber 12a, the revolving impeller vanes (20) impart a compound motion to it. Impeller vane segments (20b) impel the mixture primarily upwardly through chamber (12a), while vane segments (20a) primarily impart circular motion to the upwardly moving mixture, thereby centrifuging the liquid component of the mixture of the mixture outwardly away from the impeller hub (18) and causing undissolved gas present in the liquid to move inwardly toward the hub (18). The separated liquid flows up through discharge channels (100) into discharge element chamber (72a). The separated gas forms a liquid – free gas column around hub, the column of gas moves upwardly into discharge element chamber 86a from where the gas flows into inlets (96b) and through gas conducting channels (96) to discharge outlets (96a) in nearly vertical, upward directions. The channel outlets discharge discrete high velocity gas streams of separated gas that are substantially upwardly directed to promote 20
  38. 38. upward movement of the discharged gas within a wellbore where the separated unit is situated. Figure 1-8 – Invention by Bunnelle P14 1.2.2.2 Liquid – Gas Separator Apparatus - Patent No 4481020 This centrifugal liquid-gas separator is same as described in earlier section (see Figure 1-1). Here the mode of operation is briefly summarized with an explanatory pictorial shown in Figure 1-9. In operation the pump, separator apparatus (Figure 1-9) and motor are submerged down – hole within a liquid – gas well fluid mixture. The liquid - gas enters the intake ports (54) of the intake head (18) through a perforated or slotted member 21
  39. 39. (100) which assists in filtering debris from the fluid mixture. From the intake ports, the fluid mixture enters the inducer (48) which pressurizes the fluid mixture and supplies it to the centrifugal separator (50) via transition region (52). The transition region, which is designed to provide a uniform rate of change through in flow direction and velocity to the fluid mixture, conveys the fluid mixture smoothly to the centrifugal separator while minimizing pressure loss. At the outlet end of the transition region, the tangential velocity of the fluid approaches angular velocity of the centrifugal separator vanes and the axial velocity of the fluid approaches the flow through velocity of the apparatus. Liquid – gas separation occurs at the inlet of the centrifugal separator region and continues throughout its length. The liquid section is supplied to the pump through (86), while the separated gas is vented via gas vents (90) into the space between the well casing and the separator. The rotary motion to the inducer and extending vanes are supplied by an attached motor (20) 22
  40. 40. Figure 1-9 - Centrifugal Separator by Kobylnski et al 1.2.2.3 Recirculating Gas Separator for Electrical Submersible Pumps - Patent No 4981175 This invention by Powers Maston15 in 1991(Figure 1-10) is a modification of Lee et al (Figure 1-9). The inventor claims that by including a recirculating means (56) for recirculating a portion of the discharged liquid from the discharge outlet (50) back to the separator chamber (46) the gas – oil – ratio in the separator becomes substantially lower than a gas – to – liquid ratio of well fluid entering the well fluid intakes (48). The recirculating means includes all of the following: 1. Extraction chamber (60) 23
  41. 41. 2. Liquid injection chamber (62) 3. Conduit (64) A drive motor will be needed to extend an upward motion to drive all the separator mechanisms. Figure 1-10 – Invention by Powers Maston15 1.2.2.4 Apparatus for separating gas and solids from well fluids - Patent No 6382317 B1 The apparatus invented by Delwin E. Cobb16 Figure 1-11, is designed to separate gas and solids from well fluids in a wellbore. 24
  42. 42. The gas and solids are removed from the well fluids in two separate steps by two separate spirals, one spiral for the gas (66) and a separate spiral for the solids (70). An upper gas spiral is positioned below the openings (60) in the outer tubular housing (44) and a separate lower spiral spaced axially from the upper gas spiral is provided for the solids. The spirals are positioned in the annulus between the outer tubular housing and the inner flow tube (46). The spirals provide a helical flow and are spaced axially from each other at a distance. The gas accumulates in the swirl chamber (80) between the spirals and is librated from the liquid. The gas normally exists as large bubbles through an inner gas annulus (72). The liquid flows downwardly in a helical path to the solids spiral. The solids, such as sand are separated from the well fluid by the solids spiral and fall by gravity into the mud anchor or other suitable collection area. The liquid then flows upwardly in the flow tube (46) to be pumped for flow to a surface location. This invention makes use of induced centrifugal motion and gravity to separate gas and solids from wellbore fluids. The invention has no moving parts. 25
  43. 43. Figure 1-11 – Invention by Delwin Cobb16 Figure 1-12 – Cross – section (3) in Figure 1-11 26
  44. 44. The research reported in this thesis extended the experiments of Kobylinski et al8 and the theory behind the inventions of Jongbloed et al12 and Obrejanu13 in the sense of using gravity, agitation and centrifugal forces as physical mechanisms to obtain improved gas – liquid separation. The difference between this research and Kobylinski et al8 is in the use of different experimental procedure, experimental facilities and most importantly that the centrifugal down – hole gas separator must have no moving parts; it must be static similar to the inventions of Jongbloed et al12 and Obrejanu13. This research also investigated experimentally the points raised by Patterson and Leonard11 in terms of the effect of the increase in the gas separator annular area and improvement in pump efficiency. Visual observation as Patterson and Leonard11 suggested was used to capture the separation mechanism(s) in the production engineering laboratory at University of Texas at Austin as described in the next chapter. 27
  45. 45. Chapter 2 Experimental Facility and Procedure This chapter fully describes the facilities, equipment and procedure used in acquiring laboratory data used throughout this research. The down – hole gas separators used for the purposes of this experimental study are described in detail. 2.1 EXPERIMENTAL FACILITIES The separator designs were installed in a laboratory well model and tested over a range of 120 – 900 BPD of water and air rates between 13 – 115 MSCFD. The input into the experimental test system was water and air at pre - determined rates Qg and Qw; the output from the system included the pressures at the entry ports, tubing pressure and the gas flow-rate through the dip tube of the separator. The inputs and outputs are combined in a mathematical model to calculate the pump liquid fraction (pump efficiency) of the separator relative to particular input values. This chapter describes the facilities at The University of Texas Production Laboratory and the procedure used to input and acquire data. 2.2 DESCRIPTION OF EXPERIMENTAL FACILITIES Figure 2-1 and Figure 2-2 show schematic and overview pictures of the production laboratory facility used for testing down-hole gas separator designs. The test facility is a closed loop system with manually controlled valves for fluid flow control. Water was pumped in a loop into and out of a 3 - phase separator into the ‘well’ (Figure 28
  46. 46. 2-3) Air was supplied to the system by a compressed air line. Water and air meet at the mixer before entering into the well. The hoses lead the mixture from the manifold through the casing perforations into the well. Water is returned to the 3 – phase separator through a return a line. Air that passes through the dip tube is carried with the water into the 3 – phase separator and the rest rises up the casing. 29
  47. 47. Figure 2-1 – Schematic of experimental test facility 30
  48. 48. Figure 2-2 – Laboratory facility Figure 2-3 – Laboratory test well 31
  49. 49. 2.3 LABORATORY TEST WELL The bottom parts of the well are made of clear acrylic pipe to allow observation of the gas and liquid phases inside the separator. Figure 2-3 shows close – up pictures of the laboratory well. Figure 2-4 shows the full laboratory well picture, notice that the down – hole gas separator is placed below the down – hole pump. All the laboratory tests were conducted with the gas separator situated in such position. The down-hole gas separator components are positioned in the laboratory well as they would in a real well. The mud anchor is the outer barrel of the separator. The mud anchor entry ports or inlets allow water and some of the air to flow into the separator. The dip tube is the small diameter tube inside the separator. The water flows down in the separator annular area to the dip tube suction. Then the water flows up through the dip tube to the tubing intake shown in Figure 2-4. The bottom part of the casing has an ID of 6 inches. The upper part is PVC pipe that extends to the rooftop of the Petroleum Engineering building at the University of Texas, approximately 80ft. as seen in Figure 2-4. The bottom section of the casing has several perforations, 31/64 inch in diameter, distributed at different positions. This way it is possible to vary the relative location of the down-hole separator entry ports with respect to the perforations. 32
  50. 50. Tubing Pressure gauge (P3) Ports Pressure gauge (P2) Casing Pressure gauge Location of the (P1) separator Figure 2-4 – Laboratory Well 33
  51. 51. 2.3.1 LABORATORY INSTRUMENTS The instruments used in the conducting all the tests used for this research are shown below. The functions that they performed are also explained. 2.3.1.1 LIQUID FLOW MEASUREMENTS Figure 2-5 is a photo of the Daniel MRT97 turbine flow meter, used to measure the water flow rate, installed in the liquid loop before the mixer. The water flow was controlled by the valve in the same picture. Figure 2-5 – Turbine flow meter and valve between pump and mixer The ITT Barton Floco positive displacement meter (ITT Barton, model 308K) was used only for reference. It is installed between the turbine flow meter and the mixer. 34
  52. 52. Figure 2-6 - - ITT Barton floco positive displacement meter 2.3.1.2 GAS FLOW MEASUREMENT The air flow into the mixer was controlled with the Fisher Porter flowrator tube and the valve shown in Figure 2-7. The flowrator tube displays the airflow as a percentage of the maximum flow rate, 16416 CFD. The percentages used in the tests were 10, 20, 30, 60 and 90. The pressure in the compressed air line was measured by a pressure transducer to convert the actual air flow rate to standard conditions using the ideal gas law since working pressure of less than 100psi and laboratory temperature, allowed assuming a Z factor in the vicinity of 1.0. 35
  53. 53. Figure 2-7 - Fisher Porter Flow Rator tube Figure 2-8 shows the Omega FMA-A2313 thermodynamic mass flow meter installed at the top outlet of the three - phase separator. This instrument gave the most important reading in the tests, the amount of air that enters the ‘pump’. The units on the display are in standard liters per minute with accuracy of ±1%. 36
  54. 54. Figure 2-8 - Thermodynamic Omega Air Flow Meter 2.3.2.2 PRESSURE MEASUREMENT The casing pressure (see Figure 2-4) was used as a control variable for the entire system. This pressure was set between 10 psig – 13 psig for all experiments. These pressure values correspond to the maximum liquid volume in the casing that can be managed at the highest gas flow rates without overflowing at the top of the well model. This value was measured using an analog pressure gauge (Ashcroft, model Q-9047). An analog pressure gauge (Ashcroft, model Q-9047) determined the annular casing pressure at the entry of the top ports of the down-hole gas separator –Figure 2-4, P2. This pressure is measured within a foot from the ports and this value is used as a reference value to determine the pressure drop inside the down-hole gas separator. The discharge pressure of the separator is considered to be equivalent to the pump intake pressure – see Figure 2-4, P3. This pressure is equivalent to the pump intake pressure. This pressure was measured using a pressure/ vacuum gauge, calibrated 37
  55. 55. in psig for positive values of pressure and in inches of mercury for vacuum. One of the applications this pressure is to determine the pressure drop that occurs between the separator in-take (P2) and the tubing pressure/discharge pressure (P3). 2.4 EXPERIMENTAL PROCEDURE Use Figure 2-1 to follow the step - by - step procedure shown next. Before beginning 1. Make sure that there is sufficient water in the separator using the level control. Add water if necessary 2. Make sure that the desired ports in the manifold are open to inject flow from the desired position relative to the down-hole gas separator entry ports Starting the flow of fluids in the loop and setting the system in steady state 3. Turn on the pump 4. Use valve G to regulate the water flow rate. The gallons per minute read by the turbine flow meter should approximately result in the desired BPD. Valve G and the turbine flow meter are shown in Figure 2-5. 5. Gradually open the air flow to the desired percentage. It is usually set at 0% for the first experiment. There are three valves involved in the airflow. First open valve D to let air in from the compressed air line. Then set the desired percentage with valve E (seen in Figure 2-7). 38
  56. 56. 6. Finally open valve F to let air mix with the water. Valve F should be opened carefully. Otherwise the sudden injection of air can cause the water to come out the top of the well. 7. By closing valve A, let water accumulate in the well until a desired bottom – hole pressure is obtained. The pressure gauge labeled BHP in Figure 2-1 indicates the bottom – hole pressure. All the continuous flow tests were run between 10 – 13 psi. Once the desired hydrostatic head is obtained, regulate the flow out of the well to match the flow entering the well using valve A. This way, the BHP and liquid level inside the three phase separator are kept constant. This control is done throughout the test. 2.5 SEPARATOR PERFORMANCE DISPLAY The performance plots are displayed as three – Dimensional graphs, Figure 2-9. The plots are presented both in terms of oil field units and in terms of superficial velocities. In terms of oil filed units the x – axis represents the input liquid flow rate in BPD entering into the well through the perforations; the y – axis is the gas flow rate in MSCFD entering the laboratory well; the z – axis is the gas rate through the separator in MSCFD. This represents the gas that would enter the pump in a real well having the down – hole gas separator installed immediately below the pump intake. In terms of superficial velocity the x – axis is labeled the superficial liquid velocity inside the separator in inch/second. The y – axis represents the superficial gas 39
  57. 57. velocity inside the casing annulus in inch/second and the z – axis is the gas rate through the separator in MSCFD. The height of each dot (and/or the vertical bar) on the 3 – D performance plot corresponds to the gas rate through the separator for a given liquid and gas rate either in terms of oil filed units or in terms of superficial velocity. The data used in plotting the performance plots are managed with an Excel spreadsheet, see a sample data set in Table 2-1. The inputs for the spreadsheet include the following: 1. The actual start and end time for each conducted test 2. The casing, ports and tubing pressures (P1, P2 and P3) psi 3. The Floco meter reading (sec/0.1 bbl) 4. The input gas meter pressure (psi) 5. The measured gas rate through the separator (SLM). The spreadsheet calculates the following; 1. The liquid input rate (BPD) 2. The gas rate (MSCFD) 3. The superficial velocities for both liquid and gas (inch/second) inside the casing and inside the separator. 4. Gas rate through the separator (MSCFD) 40
  58. 58. 5. The pump liquid fraction The spreadsheet provides all the information needed to accurately study the performance of each separator design. All the laboratory tests conducted were run under continuous flow condition. In this type of test the valve H is completely open so that there is constant liquid and gas rate throughout the system. 41
  59. 59. Table 2-1 - Sample Excel Spreadsheet for continuous flow test 42
  60. 60. Figure 2-9 - Sample Performance plot for Patterson (3X1) in continuous flow 43
  61. 61. 2.6 DOWN HOLE GAS SEPARATOR DESIGNS Seven down – hole gas separator designs were tested. Two of the four gravity driven separators were originally constructed in 2004 and 2005 and were used with minor modifications. The other two were constructed in 2007. The three static centrifugal separators were constructed between summer 2006 and summer 2007. 2.6.1 ECHOMETER (3X1), ECHOMETER (3X1.5), ECHOMETER (4x1), ECHOMETER (4X1.5), ECHOMETER (4X1.75) The naming procedure is given as: (separator name) (separator OD x dip tube OD). Continuous flow tests2 were run on these separators between spring 2006 and summer 2007 for the purposes of: a. Comparing the performance of a gravity driven separator to that of a centrifugal separator b. Studying the effect of increasing the separator annular area c. Studying the pressure drop inside the separator d. Verifying the effect of port geometry Figure 2-10 and Figure 2-12 are pictures of the Echometer (3X1) and Echometer (4X1) design respectively. There are two sets of four slots. Each slot is 4 inches long and 2 inches wide. The first set is located 11 inches below the separator thread and the second set 2All the continuous flow tests were conducted with fluid entering from below the separator through the bottom four perforations located adjacent the separator – see Figure 2-4 44
  62. 62. is 16 inches below the thread. There is a distance of 24 inches between the lower slots and the dip tube suction and 44 inches between the upper slots and the dip tube suction. Both the 3inch and 4inch OD Echometer separators have a wall thickness of 0.125 inch so that the IDs are 2.75 inch and 3.75 inch respectively. The dip tube wall thickness is also 0.125 inch, making the ID of the dip tube a quarter of an inch less than the dip tube OD. The 4 inch OD Echometer separators have the same design configurations as the 3 inch OD Echometer separators, except for larger diameter. A summary of the Echometer separator design configurations is shown in Table 2-2 below. Table 2-2 – Echometer gas separators configuration Area of Area of Size of slots Separator Number of Total area of separator casing – (WXL) type slots slots (in2) dip tube separator (inch) annulus (in2) annulus (in2) Echometer (3X1) 4 2X4 32 5.15 21.20 Echometer (3X1.5) 4 2X4 32 4.17 21.20 Echometer (4X1) 4 2X4 32 10.26 15.70 Echometer (4X1.5) 4 2X4 32 9.28 15.70 Echometer (4X1.75) 4 2X4 32 8.64 15.70 45
  63. 63. Figure 2-10 – Echometer (3 X1.5) gas separator design Figure 2-11- Echometer entry port geometry Figure 2-12 – Echometer (4X1.75) gas separator design 46
  64. 64. 2.6.2 PATTERSON (3X1), PATTERSON (3X1.5), PATTERSON (4x1), PATTERSON (4X1.5), PATTERSON (4X2) The naming principle is same as the Echometer designs. Continuous flow tests were run concurrently for both designs between spring 2006 and fall 2007. The purpose of the tests is same as listed for Echometer design. The Patterson design has 16 thin and long entry slots. The slots are 1/8 inch wide and 8 inch long. There are 0.5 in diameter vent holes. Table 2-3 is a summary of the Patterson separator configuration. Table 2-3 – Patterson Separator Configuration Area of Area of Size of Number Total area separator casing – Separator Number slots of ½ inch of slots dip tube separator type of slots (WXL) holes (in2) annulus annulus (inch) (in2) (in2) Patterson 16 4 1/8 X 8 16 5.15 21.20 (3X1) Patterson 16 4 1/8 X 8 16 4.17 21.20 (3X1.5) Patterson 16 4 1/8 X 8 16 10.26 15.70 (4X1) Patterson 16 4 1/8 X 8 16 9.28 15.70 (4X1.5) Patterson 16 4 1/8 X 8 16 8.64 15.70 (4X1.75) Patterson 16 4 1/8 X 8 16 7.90 15.70 (4X2) 47
  65. 65. Figure 2-13 – 4 inch OD Patterson Separator Design Figure 2-14 – 3 inch OD Patterson Separator Design 2.6.3 TWISTER The separator named The Twister is the first in the series of static centrifugal separators constructed since summer 2006. The initial results of the performance of the Twister as reported by Bohorquez7 pointed to the need for more inquests into the 48
  66. 66. performance of static centrifugal separators based on the Echometer and Patterson separator entry port designs. The twister design uses a wire reinforced PVC hose used as a dip tube. The hose has a 1.028 in OD and a 0.75 in. ID∗. The reinforced PVC is spiraled four full turns inside the gas separator, Figure 2-15. The hose is twirled inside the gas separator and a plate is used to secure the hose in place. The straight dip tubes (for example, Echometer (3X1)) used for gravity separator designs are directly connected to the well’s tubing. But for the centrifugal design the connection to the tubing is different. Figure 2-15 is a picture of the twister connection. The arrows show the flow path for the gas through the gas vents and the liquid through the spiral tube connection. In operation the gas – liquid mixture enters through the three circular entry ports of the Twister separator. The helical dip tube induces a centrifugal motion on the mixture entering through the ports. Gas is evolved and a coalescing zone is formed. The length of the coalescing zone depends on the gas and liquid flow rates. The liquid mass is forced to the inner walls of the separator by centrifugal forces and the gas mass accumulates at the center (coalescing zone). While the gas rises to the gas vents at the separator connection to escape into the casing annulus the liquid flows down towards the dip tube suction and thereafter into the pump by gravity forces. Figure 2-17 shows a diagrammatic of the forces acting on the mixture as soon as it enters into the separator annulus. Laboratory observations show that the helical dip tube induces the centrifugal motion by virtue of its design; a bubble coalescing zone is formed in the center of the separator; the bubbles coalesce and become bigger bubbles in the coalescing zone and thus rise faster; the liquid momentum is reduced by the helical nature of the dip tube. This greatly ∗ The full construction detail for the Twister is covered in the thesis report by Bohorquez, 2006. 49
  67. 67. improved the gas pathway through the core of the separator eliminating the need for a gas venting tube. The operation of the twister is similar to the invention by Jongbloed et al12 since both are static type separators. The basic difference between the two is that the twister does not have a gas discharge conduit instead it has inclined gas vent holes at the connection head (Figure 2-16). The Twister design also has similarity with the invention by Obrejanu13. Apart from both separator designs been static by construction both separator designs depend on the helical nature of the separator internal design to induce centrifugal motion and thus centrifugal forces on the fluid flow inside the separator. The centrifugal forces induced become the driving mechanism for gas – liquid separation. The main differences between the two are the entry port placement and gravity effects. The Twister has three entry port circles at the top of the separator; the separator design by Obrejanu13 has the entry port at the bottom part of the separator. The Twister depends to a significant extent on gravity for gas – liquid separation as well as on hydrostatic head to flow the mixture through the separator. The gas separator by Obrejanu13 depends on both hydrostatic head and pressure drawdown created by the action of the PCP to operate efficiently. Whilst the advantages of having the entry ports at the bottom part of the gas separator is founded by density difference; in the case of fines production the PCP rotor will erode at a faster pace causing pre – mature pump damage. 50
  68. 68. Figure 2-15 – Twister Separator (Bohorquez) Gas Vent Spiral Tube holes Connection Figure 2-16 – Twister Connection 51
  69. 69. coalescing zone Dip tube acts as baffles which reduce Induced liquid centrifugal momentum motion Improved gas path way Figure 2-17 – Diagrammatic of the forces acting in a static centrifugal separator 2.6.3.1 ECHOMETER TWISTER The Echometer – Twister is a static centrifugal separator. The design is very similar to the Twister as per the dip tube design which is helical. The major difference between the two separator designs is the entry port geometry. While the Echometer design has four 4X2 slots the Twister has 3 circular entry ports and 4 half inch vent holes. The main objective of constructing the Echometer Twister is to study the effect of centrifugal forces on separation performance. A head – to – head comparison is made between Echometer (3X1) and Echometer Twister to understand the controlling mechanisms. 52
  70. 70. Figure 2-18 is the picture of the laboratory constructed Echometer – Twister separator. Figure 2-18 – Echometer - Twister 2.6.3.2 PATTERSON – TWISTER The Patterson – Twister is the third in the series of static centrifugal separators constructed. This separator design (Figure 2-19) has the same entry port geometry has the Patterson (3X1). Like the Echometer – Twister it was constructed to comparatively study the effect of centrifugal forces on the previously constructed Patterson (3X1) separator design. The Patterson – Twister also has a dip tube with four full turns/twists with pitch length of 12 – 14 inches lying at an angle of 45o on the inner walls of the separator. 53
  71. 71. Figure 2-19 – Patterson - Twister 54
  72. 72. Chapter 3 Analysis Of Experimental Results 3.1 EFFECT OF HELICAL DIP TUBE DESIGN These experiments studied the effect of changing the dip tube design from the conventional straight shape to a helical form. The designs experimentally tested include the following; The Echometer-Twister, Patterson-Twister (2 twists and 4 twists) and the Twister, which was previously tested by Bohorquez7. The following parameters were examined during the experimental tests of the separators: • Liquid input rate: up to 600 BPD • Gas flow rate: 0 MSCFD to 115 MSCFD • Superficial liquid velocity in the separator: up to 14 in/sec • Superficial gas velocity in the casing: up to 70 in/sec • Casing pressure: between 10 and 13 psi 55
  73. 73. The performance plots for each of the static centrifugal separators are presented in the following sections and a comparative analysis is presented thereafter so that the effects of the change in dip tube design are effectively captured. 3.1.1 PERFORMANCE RESULTS FOR THE TWISTER SEPARATOR Figure 3-1 and Figure 3-2 show the performance plots for the Twister separator in continuous flow in both field units and in terms of superficial velocity. Notice that the Twister achieved a zero gas flow rate through the separator up to a downward superficial liquid velocity (Vsl) of 10 in/sec for all gas rates tested. This liquid rate is equivalent to 430 BPD. The area highlighted in red represents the optimum performance for the Twister separator. Approximately no gas entered into the dip tube suction in this area for these conditions of liquid and gas flow. 56
  74. 74. Separator Type: Twister OD Dip Tube = 1″; Number of Slots = 3; Number of Twists = 4; Casing Pressure = 10 – 13 psi Dimension of Slots =2 1” holes & 1 1.5” hole; Position of the Separator = Above the Perforations Figure 3-1- Twister results in field units 57
  75. 75. Separator Type: Twister OD Dip Tube = 1″; Number of Slots = 3; Number of Twists = 4; Casing Pressure = 10 – 13 psi Dimension of Slots =2 1” holes & 1 1.5” hole; Position of the Separator = Above the Perforations Figure 3-2 - Twister result in terms of superficial velocities 3.1.2 PERFORMANCE RESULTS FOR ECHOMETER – TWISTER SEPARATOR In Figure 3-3 and Figure 3-4 the performance results for Echometer–Twister separator are shown. 58
  76. 76. Separator Type: Echometer - Twister OD Dip Tube = 1″; Number of Slots = 4; Number of Twists = 4; Casing Pressure = 10 – 13 psi Dimension of Slots =2” X 4”; Position of the Separator = Above the Perforations Figure 3-3 – Echometer - Twister result in terms of superficial velocities 59
  77. 77. Separator Type: Echometer - Twister OD Dip Tube = 1″; Number of Slots = 4; Number of Twists = 4; Casing Pressure = 10 – 13 psi Dimension of Slots =2” X 4”; Position of the Separator = Above the Perforations Figure 3-4 – Echometer - Twister results in field units The region highlighted in red in both plots depicts areas where it was observed that no gas entered into the dip tube suction of the Echometer–Twister separator. Notice that in Figure 3-3 the no gas zone was established at a downward Vsl 10 inch/sec. 60
  78. 78. 3.1.3 PERFORMANCE RESULTS FOR PATTERSON – TWISTER SEPARATOR Both Figure 3-5 and Figure 3-6 show the results for the Patterson–Twister Separator in continuous flow in terms of superficial velocities and in field units. Separator Type: Patterson - Twister OD Dip Tube = 1″; Number of Slots = 16; Number of Twists = 4; Casing Pressure = 10 – 13 psi Dimension of Slots =1/8” X 8”; Position of the Separator = Above the Perforations Figure 3-5 - Patterson - Twister result in terms of superficial velocities 61
  79. 79. Separator Type: Patterson - Twister OD Dip Tube = 1″; Number of Slots = 16; Number of Twists = 4; Casing Pressure = 10 – 13 psi Dimension of Slots =1/8” X 8”; Position of the Separator = Above the Perforations Figure 3-6 - Patterson - Twister results in field units In the following sections a comparison is made between the separation performances of Echometer–Twister and Echometer (3X1) and then Patterson– Twister and Patterson (3X1) in terms of both superficial velocity and field units. 62
  80. 80. 3.2 COMPARISON OF PERFORMANCES OF HELICAL DIP TUBE GAS SEPARATORS TO STRAIGHT DIP TUBE GAS SEPARATOR This section focuses on comparing the performances of gravity driven separators to static centrifugal separators in laboratory continuous flow experiments. For critical analysis, comparisons are made between gas separators that have the same entry port geometry and exact dip tube outer and inner diameters. Section 3.2 is organized as follows: • Section 3.2.1 compares the performance of Echometer-Twister and Echometer (3X1) gas separators • Section 3.2.2 compares the performance of Patterson-Twister and Patterson (3X1) gas separators • Section 3.2.3 studies how the number of twists inside a static centrifugal separator affects performance. The performance of Patterson-Twister gas separator with 4 twists is compared to Patterson-Twister gas separator with 2 twists. • Section 3.2.4 compares all the results for static centrifugal separators. 3.2.1 COMPARISON OF ECHOMETER-TWISTER AND ECHOMETER (3x1) GAS SEPARATORS The performance of the Echometer-Twister and Echometer (3X1) separators are compared in terms of superficial velocities in Figure 3-7 in and in filed units in Figure 3-8. The test points in the performance plots are red for the Echometer-Twister and black for Echometer (3X1). As shown in the superficial velocity performance plot, Echometer (3X1) 63
  81. 81. separates all the air entering the test well for downward superficial liquid velocities below 7 in/sec, and the Echometer-Twister for downward superficial liquid velocities below 9.5 in/sec. The plots in Figure 3-7 shows that the 6 inch/sec rule of thumb threshold for downward superficial liquid velocity (Vsl) inside the separator for gravity driven separators (green highlighted region) is surpassed. The limiting downward Vsl for optimum performance inside the Echometer – Twister gas separator design is 9.5 inch/sec; a 58% increase over the conventional rule of thumb. In Figure 3-8, 9.5inch/sec optimum downward Vsl for Echometer–Twister gas separator design is equivalent to 480 BPD of water. This represents a 200BPD increase over the optimum operational area for the Echometer (3X1) gas separator design. The 7in/sec optimum downward liquid Vsl for the Echometer (3X1) design marginally exceeds the rule of the thumb. This is an additional 40 BPD gas free liquid production. 64
  82. 82. Separator: Echometer- Twister & Echometer (3X1) Number of Slots = 4; Dimension of Slots = 4”X 2”; Position of Separator = Above Perforations: Test Type = Continuous Flow; Pc = 10psi Echometer Twister Echometer (3X1) Figure 3-7 – Comparison of Echometer – Twister and Echometer (3X1) results in terms of superficial velocity 65
  83. 83. Separator Type: Echometer – Twister; Echometer (3X1) OD Dip Tube = 1″; Number of Slots = 4; Casing Pressure = 10 – 13 psi Dimension of Slots =4” X 2”; Position of the Separator = Above the Perforations Echometer Twister Echometer (3X1) Figure 3-8- Comparison of Echometer – Twister and Echometer (3X1) results in Field Units The design of gas separators is a trade off between optimizing the separator annular area and the pressure drop in the separator. Pressure drop analysis is performed on both gas separator designs to determine the pressure difference between the entry ports, P1 and the 66

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