Continuous Flow Gas Lift Design By Jack R. Blann 2/9/2008 1

Transcription

1 Continuous Flow Gas Lift Design By Jack R. Blann 1

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3 Petroleum Engineering Handbook Gas Lift appears as Chapter 12 in: Volume IV Production Operations Engineering Edited by Joe Dunn Clegg Chapter 12 was Co-authored by: Herald W. Winkler, and Jack R. Blann 3

4 Two Types of Continuous Flow Gas Lift Design Design for Normal Wells. Winkler Design for High gas Injection rates. 4

5 Iranian Gas Lift Well In Agha Jhari Field Produced: Natural Flow = 37,000 B/D Gas Lift = 43,000 B/D Had 26 mandrels and valves to ft. 5

6 Libyan Gas Lift Well 6

7 Production Manifold in Libya 7

8 Libyan Gas Lift Tests 8

9 Continuous Flow Installation Design Based on Constant Decrease in Operating Injection-Gas Pressure for Each Succeeding Lower Gas Lift Valve All Gas Lift Valves have same port size. There is a constant decrease in the operating injection pressure for each succeeding lower valve. Port size that allows the injection-gas throughput for unloading and operating the well. 9

10 This installation design method is recommended for gas lift valves with small production-pressure pressure factors. When the ratio of the port area to the bellows area is low, the decrease in the injection gas pressure between gas lift valves, based on additional tubing effect pressure for the top valve, is not excessive. 10

11 Injection-Pressure Operated Gas Lift Valve Production Pressure factor = Ratio of Area of Port to Area of Bellows 11

12 Gas Lift Valve Specifications 12

13 The effect of bellows-assembly load rate on the performance of the valves is not considered in the installation design calculations. Safety factors included in these calculations should allow sufficient increase in the operating injection gas pressure, which is necessary to provide valve stem travel for adequate injection-gas passage through each successively lower unloading valve without interference from upper valves. 13

14 Selection of a constant injection- gas pressure decrease, or drop, in the surface operating injection pressure for each succeeding lower gas lift valve should not be arbitrary, as proposed in some design methods. The pressure decrease should be based on the gas lift valve specification to minimize the possibility of upper valves remaining open while lifting from a lower valve. 14

15 The additional tubing-effect pressure for the top gas lift valve is a logical choice for this decrease in the operating injection- gas pressure between valves. 15

16 Closing or reopening of an injection- pressure-operated gas lift valve is partially controlled by the production pressure effect, which is equal to the production pressure factor for the valve multiplied by the difference in flowing production pressure at the top valve depth. 16

17 The flowing pressure at an unloading- valve depth changes from the transfer pressure, ( (P ptfd ) min, To a higher flowing production pressure after the next lower valve becomes the operating pressure 17

18 The additional tubing-effect pressure is the difference between ( (P ptfd ) min, and the maximum flowing-production pressure at the unlading valve depth, (P ptfd ) max, after the point of gas injection has transferred to the next lower valve. 18

19 As the unloading gas lift valve depths increase, the distance between valves and the difference between ( (P ptfd ) min, and (P ptfd ) max decrease. Although the additional tubing-effect pressure decreases for lower valves, the injection-gas requirement for unloading increases with depth. 19

20 An increased stem travel, or stroke, is usually needed for the lower valves to generate the larger equivalent port area necessary for the higher injection gas requirements with the lower pressure differentials that occur across these deeper valves. 20

21 A constant decrease in the operating injection-gas pressure equal to the additional tubing effect pressure for the top valve allows a a greater increase in the injection gas above initial opening pressure for lower gas lift valves. 21

22 API Gas Lift Design 22

23 Gradient Curve 23

24 Gradient Curve 24

25 When the injection-gas pressure significantly exceeds the flowing- production pressure, an arbitrary increase in injection-gas gas-pressure, P io can be added to the initial production- pressure effect for the top valve for calculating the spacing and the initial opening pressures of the unloading gas lift valves 25

26 The total decrease in the injection-gas pressure is distributed equally between each successively lower gas lift valve rather than having a sizeable pressure drop across the operating gas lift valve or orifice-check check valve This reduces the possibility of multipoint injection through the upper unloading valves by ensuring that the valves remain closed after the point of gas injection has transferred to the next lower valve, 26

27 Determination of Valve Depths Because the final injection gas pressure is not known until the installation is designed, a pressure difference of 100 to 200 psi between the unloading P iod and P pfd traverses is assumed for locating the deepest valve depth. 27

28 Determination of Valve Depths The assumption of P iod -P pfd = 100 to 200 psi should ensure calculation of the operating valve depth. The static bottomhole pressure, P wsd and temperature, T wsd are usually referenced to the same depth, which is the lower end of the production conduit, D d. 28

29 Determination of Valve Depths Calculate the maximum unloading GLR based on the maximum injection gas rate available for unloading and the maximum daily design total fluid rate. 29

30 Well Information Tubing size = 2 7/8 in. OD Tubing Length, У = 6000 ft. Max. Valve Depth, D v(max) = 5970 ft. Static bottomhole pressure at D d, P wsd = 1800 psig at 6000ft. Daily production rate = 800 STB/D Water Cut = 50% Formation GOR = 500 scf/stb Oil Gravity = 35 o API Gas Gravity У g = 0.65 Produced-water specific gravity, У w = 1.08 Bottomhole temperature, T wsd =170 o F at 6000ft. Design unloading wellhead temperature, T whf = 100 o F. Load-fluid pressure gradient, g ls = 0.46 psi/ft. U-Tubing wellhead pressure, P whu = 100 psig. Flowing wellhead pressure, P whf = 100 psig. 30

31 Well Information Static fluid level = 0 ft. Surface kickoff injection-gas pressure, P ko = 1000 psig. Surface operating injection-gas pressure, P io = 1000 psig. Maximum unloading injection-gas rate, q giu = 800 Mscf/D. Operating daily injection-gas rate, q gi = 500 Mscf/D. Wellhead injection-gas temperature, T gio = 100 o F Assigned valve-spacing pressure differential at valve depth, P sd = 50 psi. Test rack valve temperature, T vo = 60 o F. Assigned minimum decrease in surface operating injection-gas pressure between valves, P io = 20 psi. Minimum distance between valves, D bv(min) = 150 ft. Gas lift valves : 1.5 inch O.D. nitrogen charged with A b = 0.77 in 2, and sharp edged seat. 31

32 Step 1: Calculate Maximum Unloading GLR R giu =q giu / q lt Where q giu = maximum unloading injection rate, Mscf/D, q lt = total liquid daily production rate, B/D, and R giu = maximum unloading GLR, scf/stb 32

33 Calculate Maximum Unloading GLR R giu =q giu / q lt = 800,000 scf/d / 800 STB/D R giu = 1000 scf/stb 33

34 Step 2: Determine flowing Production pressure, P at pfd D d 34

35 Gradient Curve 35

36 Step 3:Calculate the unloading-flowing flowing- pressure-at at-depth gradient, g pfa above point of gas injection, g pf. g pfa = / 6000 g pfa = psi/ ft. 36

37 Step 4 Calculate the operating injection gas pressure at the lower end of the production conduit. where: P io = injection-gas pressure at surface, psia P iod = injection-gas pressure at depth, psia e = Napierian logarithm base = γ g = gas specific gravity (air = 1.0), dimensionless. D = true vertical depth of gas column, ft. bar T = average gas column temperature, R bar z = compressibility factor based on gas column average pressure P and temperature, T, dimensionless. 37

38 Step 4 P iod API Design Technique = 1154 psig at 6000 ft. = ( ) / 6000 =.0257psi/ft g gio = (1154 Since P iod - P pfd = = 254 psi Exceeds 200 psi, the maximum valve depth of 5970 ft can be attained. 38

39 Step 5: Calculate the unloading gas lift valve temperature at depth gradient, g Tvu. Bottomhole temperature, T wsd =170 o F at 6000ft. Design unloading wellhead temperature, whf = 100 o F at surface. T whf g Tvu = ( ) / 6000 = o F/ft. 39

40 Step 6 D v1 = depth of top valve, ft P ko = surface kick-off or average field injection-gas pressure (optional), psig P whu = surface wellhead U-tubing U (unloading) pressure, psig PsD = assigned spacing pressure differential at valve depth, psi where g ls = static-load (kill) fluid pressure gradient, psi/ft = injection-gas pressure at depth gradient, psi/ft g gio Step 6: D v1 D v1 v1 = ( ) / ( ) v1 = 1957 ft. 40

41 Step 7 Calculate the minimum flowing-production pressure, (P pfd1 )min,, the injection-gas pressure, P iod1, and the unloading gas-lift valve temperature, T vud1, at the top valve depth by multiplying the appropriate gradient by the valve depth, D v1, and adding to the appropriate surface values (where n = 1 for top valve): 41

42 Step 7. Calculate the minimum flowing-production pressure, ( (P pfd 1)min, injection-gas pressure, PioD1,, and the unloading flowing temperature, T vud1 at D v1 of 1,957 ft as described on previous slide: (P pfd1 )min = (1,957) = 361 psig P iod1 = 1, (1,957) = 1,050 psig T vud1 = (1,957) = 123 F 42

43 API Design Technique Step 8 Calculate the depth of the second gas-lift valve, D v2, where n = 2 on the basis of the assigned minimum decrease in surface injection- gas pressure, p io, for spacing the gas-lift valves and the P iod -traverse. A valve spacing differential of around 20 to 30 psi will usually be sufficient for most 1.5-in. OD gas-lift valves. However, 1-in. 1 OD valves with large ports may require a higher, p io. This can be checked by calculating the additional production-pressure pressure effect, P pe1, after the valve depths are calculated for the assigned p io. 43

44 Step 8 API Design Technique (P pfd(n-1) 1)) min +g ls (D bv )=[P iod(n-1) Solve For D bv : D bv = P iod(n-1) [(n-1) 1) P io ]- (P pfd(n and D v(n) = D v(n-1) + D bv 1)-(n-1) 1) P io ]- P sd pfd(n-1) 1)) min sd +g gio (D bv ) min - P sd / ( (g ls ls g gio The decrease in surface injection-gas pressure for calculating D v2 is P io, and for D v3 is 2 ( P( io ), and for D v4 is 3 ( Pi( o ), and this procedure continues for each succeeding lower valve. 44

45 Step 8 API Design Technique Solve For D bv : D bv = P iod(n-1) [(n-1) 1) P io ]- (P pfd(n pfd(n-1) 1)) min min - P sd / ( (g ls ls g gio D bv = / ( ) = 1472 ft. and Dv(2) = D v(n-1) + D bv D v(2) = = 3429 ft. Repeat steps 7 and 8 for each valve location until reaching maximum depth 45

46 The calculated valve spacing for the sixth valve, D v6, would exceed the maximum valve depth, D v(max), of 5,970 ft. Because an orifice-check check valve will be placed in the bottom wireline-retrievable retrievable valve mandrel, no test-rack valve setting information is required. This completes the valve spacing calculations. 46

47 Graphical Representation 47

48 Determination of Gas-Lift Valve Port Size and Calculation of Test-Rack Opening Pressures. The gas-lift valves port ID and test-rack opening pressure calculations follow: Step 1. Determine the port size required for the gas-lift unloading valves and the operating orifice-check check valve orifice ID. The upstream injection-gas pressure, P 1, is based on P od5 of the last unloading valve corrected to the orifice-check check valve depth of 5970 ft: P 1 = 1, (5,970 5,762) = 1,073 psig at 5,970 ft 48

49 Determination of Gas-Lift Valve Port Size and Calculation of Test-Rack Opening Pressures. Step 1 (Continued) The downstream flowing-production pressure, P 2, is equal to the minimum flowing-production pressure at 5,970 ft. P 2 = (5,970) = 896 psig at 5,970 ft P ov = 1, = 177 psi across the orifice- check valve 49

50 Determination of Gas-Lift Valve Port Size and Calculation of Test-Rack Opening Pressures. Step 1 (Continued) From the Thornhill / Craver correlation, the required equivalent orifice size is near 14/64-in.; therefore, the next largest gas-lift valve port ID is 1/4-in. This size is sufficient for all of the upper unloading valves because they have a higher injection-gas operating pressure and a greater differential pressure between Pi od and pfd )min. (P pfd An equivalent orifice size of 12/64-in. to 13/64-in. is required to pass the operating injection-gas rate of 500 Mscf/D. 50

51 Determination of Gas-Lift Valve Port Size and Calculation of Test-Rack Opening Pressures. Step2 Record the valve specifications for a l.5-in. OD gas-lift valve having a 1/4-in. ID port with a sharp-edged seat where A b = 0.77 sq. in. from table: (A p/ A b ) = 0.064, (1 A p /A b ) = 0.936, and F p =

52 Determination of Gas-Lift Valve Port Size and Calculation of Test-Rack Opening Pressures. Step 3 Calculate P od1 P od1 = P iod1 where P iod1 = injection-gas pressure at valve depth, psig P od1 = injection-gas initial gas-lift valve opening pressure at valve depth, psig P od1 = 1,050 psig at 1,957 ft 52

53 Step4 API Design Technique Calculate the test-rack set opening pressure of the first valve (n = 1), P vo1, using Eqs and or 12.46: P bvd(n) = P od(n) (1-A p /A b )+ P pfd(n)min +(A p /A b ) P = C vo(n) T(n) (P bvd(n) ) / (1- A p /A b ) P vo(n) = 1050 (0.936)+361 (0.064) = 1006 psig. 53

54 Step 5 P = C vo(n) T(n) (P bvd(n) ) / (1- A p /A b ) for C T1 = (Calculated using T vud1 = 123 o F): vo1 = (1006)/0.936 = 942 psig at 60 o F P vo1 Step 6 Calculate the injection-gas initial opening pressure of the second gas-lift valve at depth (n = 2), P od(n) = P iod(n) (n-1) P io od(2) = = 1068 psig at 3429 ft. P od(2) 54

55 Step 7 Calculate the maximum flowing-production pressure opposite top unloading valve immediately after the point of gas injection has transferred to the second (lower) valve, (P pfd1 ) max. ( (P pfd1 ) max is shown graphically in Figure and can be calculated using the following equation: (P pfd1 ) max = P wf + Dv1 [(P od2 P whf )/D v2 ] (P pfd1 ) max = (( )/3429) (P pfd1 ) max = 652 psig at 1957 ft. 55

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57 Step 8 Determine if the assumed decrease in surface injection- gas pressure, P io, is sufficient for the required gas-lift valve port size by calculating the additional production- pressure effect, P pe1, at the top valve: Ppe1 = F p [(P D1D1)max - P D1D1)min ] = (652 = 361 = 20 psi. P pe1 If P pe1 is less than or equal to the assumed P io proceed with the design. If P pe1 is greater than the assumed P io, then set P io = P pe1 and redo the spacing design. This is a conservative approach and many operators use actual operating experience to determine which P io to use. 57

58 Repeat Steps 6, 4 and 5 for remaining gas-lift valves: An orifice-check check valve is recommended for the sixth valve at 5,962 ft. The orifice ID should be 1/4-in. to pass sufficient gas to gas lift the well. A tabulation form for these calculations is given in table. 58

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61 High Rate Continuous-Flow Installation Design ( Winkler) The application of the injection-gas rate throughput performance for injection-pressure pressure-operated gas-lift valves is illustrated in the high daily liquid rate continuous-flow installation design. The importance of valve performance data for high daily injection-gas rates is shown, and its unimportance for low injection-gas rate installation designs is illustrated. Valve performance data is of no value in selection of the top two unloading gas-lift valves in this installation. For these two upper valves, an assumed reasonable decrease in the surface injection-gas pressure of 20 psi for each valve ensures unloading the well and these upper valves remaining closed while lifting from a lower valve. When the required daily injection-gas rate increases for lifting from the third and fourth gas-lift valves, valve performance information becomes very important. A pressure vs. depth plot for this continuous-flow installation is shown in the Figure 61

62 High Rate Continuous-Flow Installation Design ( Winkler) 62

63 High Rate Continuous-Flow Installation Design (Winkler) Although the flowing-production transfer pressure traverse method for locating the depths of the valves may require an additional valve, or valves, in some installations, this design method has several advantages in wells requiring a high daily injection-gas rate for unloading. Because the injection-gas requirement to uncover the next lower valve is reduced, smaller valve ports can be used and the increase in the injection-gas pressure to stroke the valve stem is less. The unloading operations are faster because of the lesser difference in injection-gas requirement between unloading valves. 63

64 High Rate Continuous-Flow Installation Design ( Winkler) The surface origin and final downhole termination pressures for the flowing-production transfer pressure traverse are arbitrary. The 20% in this example for locating the surface transfer pressure traverse is widely used. The unloading injection-gas requirements for uncovering each lower valve increases as the percentage decreases and decreases as the percentages increase. The flowing-production transfer pressure at datum depth should be at least 100 to 200 psi less than the available design operating injection-gas pressure at the same depth 64

65 Simplified Mathematical Gas-Lift Valve Performance Model Because performance equations for specific gas-lift valves are not available from gas-lift valve manufacturers, a simplified gas-lift valve performance computer model was used to illustrate the calculations in this paper. The model is based on static force balance equations and several simplifying assumptions. This computer model describes qualitatively the injection-gas rate throughput of unbalanced, single-element element gas-lift valves using the Thornhill-Craver equation. 65

66 High Rate Design Data Tubing size = 4-1/24 1/2-in. OD (ID = in.) and Length = 6,000 ft Casing size = 8-5/88 5/8-in. OD, 44 lb/ft (ID = in.) Datum depth for bottomhole pressures and temperature, D d = 6,000 ft Bottomhole temperature at D d, T wsd = 170 of Shut-in (static) bottomhole pressure at D d, P wsd = 2,000 psig Maximum depth for bottom valve, D v(max ) = 5,900 ft Productivity index (gross liquid), PI = 6.3 BPD/psi Oil gravity = 35 o API (γ o = 0.850) Gas specific gravity (air = 1.0), γ g = 0.65 Water specific gravity, γ w = 1.08 Water fraction, f w = 0.50 (50%) Formation gas/oil ratio, R go = 400 scf/stb Formation gas/liquid ratio, R glf = 200 scf/stb Assigned minimum daily unloading production rate, q lu = 1,000 BPD Design total (oil + water) daily production rate, q lt = 5,000 BPD Wellhead U-tubing U unloading pressure, P whu = 100 psig Surface flowing wellhead pressure, P whf = 100 psig Static load (kill) fluid pressure gradient, g ls = psi/ft 66

67 High Rate Design Data (Continued) Surface operating injection-gas pressure, P io = 1,400 psig (at wellsite) Assigned daily injection-gas rate, q gi = 2,000 Mscf/day Unloading wellhead temperature, Twhu = 120 o F (basis for calculation of Pvo) Wellhead injection-gas temperature, Tgio = 120 o F Surface kickoff injection-gas pressure, Pko = 1,400 psig (at wellsite) Minimum assigned surface injection-gas pressure decrease between valves, P io = 20 psi (represents minimum surface injection-gas pressure increase for stroking gas-lift valve) Valve spacing design line percent factor at surface = 20% ( (f pt = 0.20) Minimum transfer-production pressure difference ( (P iod P ptd ) at D d, P ptd = 200 psi Valve spacing pressure differential at valve depth, P sd = 50 psi Minimum distance between valves D bv(min) = 400 ft Gas-lift valve test-rack setting temperature, T vo = 60 of Gas-lift valves: 1.5-in. OD wireline-retrievable, retrievable, unbalanced, single-element, element, nitrogen-charged bellows with A b = 0.77 in.2, B lr = 600 psi/in., and square sharp-edged seat. 67

68 Valve Performance First Valve 68

69 Valve Performance First Valve 69

70 Winkler High Rate Gas Lift Design 70

71 Continuous Flow Gas Lift Design As Presented in 2007 SPE PRODUCTION ENGINEERING HANDBOOK 71