Oil and Gas Well Performance

Transcription

1 Oil and Gas Well Performance Presented By: Jebraeel Gholinezhad

2 Agenda 1. Introduction 2. Fandamentals 3. Oil Well Performance 4. Gas Well Performance 5. Tubing Flow Performance 6. Artificial Lift Systems 7. Gas Lift Design 2

3 Oil Well Performance 3

4 Inflow Performance Straight-Line IPR Deviation of IPR from Straight Line Nonlinear IPRs Prediction of Future IPR Horizontal Oil Well Performance 4

5 Straight-Line IPR straight-line IPR is only applicable to undersaturated oils when: flow is single phase, i.e., there is no free gas production (and therefore there are no relative permeability effects) fluid properties are not strong functions of pressure (this condition is met when there is little or no solution gas present) 5

6 Straight-Line IPR The equation of the straight-line IPR is given by: q o = J ( P P R wf ) where P r = average pressure in reservoir drained by the well (P i or P e can also be used with only minor loss in accuracy 6

7 Straight-Line IPR For Straight Line IPR by Manipulation: q o = J ( P P ) R wf q J o = P R P wf P wf = P R q J o 7

8 Straight-Line IPR P R p wf (Psia) pressure drawdown p wf1 slope = - 1 J q omax or q =AOF q o1 0 oil rate, q o (STB/d) 8

9 Straight-Line IPR Properties of a Straight Line IPR By convention, dependent variable, q, on x-x axis When P wf = P R, drawdown = 0, and q = 0 When P wf = 0, drawdown = P R, and q = Qmax = AOF or absolute open flow Slope = -1/J 9

10 Exercise 3 A well tested at a rate of 38 STB/d at a wellbore flowing pressure measured at 585 psia. After shut-in, static wellbore pressure was 1125 psia. Installing a gas lift system is expected to raise production to 54.6 STB/d. Calculate: productivity index, J. AOF P wf at 54.6 STB/d P wf required to produce 60 STB/d Answer 10

11 When Can We e Use Straight Line IPR With Some Degree of Accuracy? Incompressible fluids whose properties do not change significantly with reductions in pressure Undersaturated black oils Water wells Straight-Line IPR A condition of steady state or pseudo steady state flow must have been reached 11

12 Straight-Line IPR When Cannot not We Assume Straight Line IPR? Situations where the reservoir acts as an infinite system (no boundary effects are felt) Common in lower permeability formations where it make take extended periods of time (years) to feel the effects of the reservoir drainage boundaries on the wellbore performance Compressible fluid systems (saturated oils, gas reservoirs, etc) 12

13 Deviation of IPR from Straight Line For flow of fluid in porous media Darcy s s law can be expressed in a general format: 13

14 Deviation of IPR from Straight Line Above the bubble point pressure p b, the relative oil permeability equals unity (k ro =1) and the term (k ro /µ ο B o ) is almost constant. As the pressure declines below p b the gas is released from the solution which can cause a large decrease in both k ro and (k ro /µ ο B o ). 14

15 Deviation of IPR from Straight Line 15

16 Deviation of IPR from Straight Line 16

17 Deviation of IPR from Straight Line 17

18 Deviation of IPR from Straight Line p wf q 18

19 Non-Linear IPRs There are several empirical methods that are designed to predict the non-linearity behavior of the IPR for solution gas drive reservoirs. Most of these methods require at least one stabilized flow test in which Q o and p wf are measured. All the methods include the following two computational steps: Using the stabilized flow test data, construct the IPR curve at the current average reservoir pressure. Predict future inflow performance relationships as to the function of average reservoir pressures. Vogel s Method (1968) Wiggins Method (1993) Standing s Method (1970) Fetkovich s Method (1973) The Klins-Clark Method (1993) 19

20 Vogel s Equation To construct the IPR, this method requires the following data: Current average reservoir pressure p r Bubble-point pressure p b Stabilized flow test data that include q o at p wf Vogel s methodology can be used to predict the IPR curve for the following two types of reservoirs: Saturated oil reservoirs p r p b Undersaturated oil reservoirs p r > p b 20

21 Vogel s Equation Saturated oil reservoirs p r p b Vogel (1968) used a computer model to generate IPRs for several hypothetical saturated-oil reservoirs that are producing under a wide range of conditions. Vogel normalized the calculated IPRs and expressed the relationships in a dimensionless form: 21

22 Vogel s Equation Undersaturated oil reservoirs p r > p b Beggs (1991) pointed out that in applying Vogel s method for undersaturated reservoirs, there are two possible outcomes to the recorded stabilized flow test data that must be considered: P r Bottomhole Pressure P wf P b P wf Case 1: P wf >P b Case 2: P wf <P b q o q ob q o Oil Rate 22

23 Vogel s Equation a. The value of the recorded stabilized p wf p b 23

24 Vogel s Equation b. The value of the recorded stabilized p wf < p b 24

25 Example Saturated oil reservoir A well tested at a rate of 200 STB/d with a P wf of 3220 pisa.. Bubble point pressure measured on surface recombined fluid samples was 3980 psia,, very close to measured P i = 4000 psia Plot the IPR using the Vogel equation. 25

26 Example First Calculate Q max from the data: (q o ) max = STB/day 26

27 Example Now calculate several rates at specified drawdowns to plot the IPR: P wf (psia) q o (STB/d)

28 Example The Plotted IPR: 28

29 Exercise 4 Undersaturated oil reservoir An oil well is producing from an undersaturated reservoir that is characterized by a bubble-point pressure of 2130 psig. The current average reservoir pressure is 3000 psig. Available flow test data shows that the well produced 250 STB/day at a stabilized p wf of 2500 psig. Construct the IPR curve using Vogel s method. Answer 29

30 Exercise 4 Undersaturated oil reservoir The previous oil well was retested and the following results obtained: p wf =1700 psig q o =630.7 STB/day Generate the IPR using the new test data. Answer 30

31 Fetkovich s Equation Where: Fetkovich (1973) suggests that the pressure function f(p) can basically fall into one of the following two regions: Undersaturated Region p r > p b Saturated Region p r p b 31

32 Fetkovich s Equation Undersaturated Region: Saturated Region: 32

33 Exercise 5 An oil well is producing from a reservoir. The reservoir pressure is 2992 psia and the bubblepoint pressure of the reservoir fluid is 1900 psia. The well was flow tested at 3900 bbls/day with a flowing bottomhole pressure of 2392 psia. Draw the IPR for the well and determine its Absolute Open Flow Potential (AOFP). Compare the results using Vogel and Fetkovich methods. Answer 33

34 Effect of Reservoir Pressure Decline Deteriorating inflow performance is the natural result of reservoir depletion. Average reservoir pressure decreases in the absence of artificial pressure maintenance or a strong natural water drive. Gas and water injection can be used to arrest the decline in IPR caused by depletion. 34

35 Effect of Reservoir Pressure Decline Additional reductions in inflow performance may result from: Damage near the wellbore related to drilling and completion operations Reduced drainage area due to infill drilling Reduced permeability due to two-phase flow, compaction or fines migration Increased viscosity due to gas liberation from reervoir oil Transient efects usually associated with low-permeability formations 35

36 Effect of Reservoir Pressure Decline Wellbore Stimulations May Change the Slope of the IPR Curve: P R J has increased post-stimulation p wf pre-stimulation q 36

37 Standings Method: Prediction of Future IPR Standing (1970) essentially extended the application of Vogel s to predict future inflow performance relationship of a well as a function of reservoir pressure. 37

38 Exercise 6 A well is producing from a saturated oil reservoir that exists at its saturation pressure of 4000 psig. The well is flowing at a stabilized rate of 600 STB/day and a pwf of 3200 psig. Material balance calculations provide the following current and future predictions for oil saturation and PVT properties. Generate the future IPR for the well at 3000 psig by using Standing s method. P r µ o,cp present Future B o,bbl/stb k ro Answer 38

39 Fetkovich s Method: Prediction of Future IPR Fetkovich assumes that the performance coefficient C is a linear function of the average reservoir pressure and, therefore, the value of C can be adjusted as: He assumes that the value of the exponent n would not change as the reservoir pressure declines. 39

40 Exercise 7 A four-point stabilized flow test was conducted on a well producing from a saturated reservoir that exists at an average pressure of 3600 psi. a. Construct a complete IPR by using Fetkovich s method. b. Construct the IPR when the reservoir pressure declines to 2000 psi. Q o,stb/day P wf,psi Answer

41 Wiggins Method: Prediction of Future IPR Wiggins (1993) used four sets of relative permeability and fluid property data as the basic input for a computer model to develop equations to predict inflow performance: The generated relationships are limited by the assumption that the reservoir initially exists at its bubble-point pressure. Wiggins proposed generalized correlations that are suitable for predicting the IPR during three-phase flow. 41

42 Prediction of Future IPR Wiggins extended the application of the above relationships to predict future performance by providing with expressions for estimating future maximum flow rates. 42

43 Horizontal Oil Well Performance The major purpose of a horizontal well is to enhance reservoir contact and thereby enhance well productivity. Horizontal wells have been used in: naturally fractured reservoirs reservoirs with water and gas coning problems low permeability reservoirs high permeability gas reservoirs IOR Other applications are mainly related to overcoming the drilling nad drilling-related cost problems. 43

44 Horizontal Oil Well Performance Horizontal wells offer the following advantages over those of vertical wells: Large volume of the reservoir can be drained by each horizontal well. Higher productions from thin pay zones. Horizontal wells minimize water and gas zoning problems. In high permeability reservoirs, where near-wellbore gas velocities are high in vertical wells, horizontal wells can be used to reduce near-wellbore velocities and turbulence. In secondary and enhanced oil recovery applications, long horizontal injection wells provide higher injectivity rates. The length of the horizontal well can provide contact with multiple fractures and greatly improve productivity. 44

45 Horizontal Oil Well Performance The actual production mechanism and reservoir flow regimes around the horizontal well are considered more complicated than those for the vertical well. Some combination of both linear and radial flow The well may behave in a manner similar to that of a well that has been extensively fractured The productivity gain from drilling 1,500-foot-long horizontal wells is two to four times that of vertical wells. 45

46 Horizontal Oil Well Performance 46

47 Productivity Index of Horizontal Wells Borisov s Method for an Isotropic Reservoir: Borisov (1984) proposed the following expression for predicting the productivity index of a horizontal well in an isotropic reservoir, i.e., k v = k h : 47

48 Productivity Index of Horizontal Wells The Giger-Reiss-Jourdan Method: For an isotropic reservoir where the vertical permeability k v equals the horizontal permeability k h, Giger et al. (1984) proposed the following expression for determining J h : To account for the reservoir anisotropy, the authors proposed the following relationships: 48

49 Productivity Index of Horizontal Wells Joshi s Method: Joshi (1991) presented the following expression for estimating the productivity index of a horizontal well in isotropic reservoirs: a is half the major axis of drainage ellipse. Joshi accounted for the influence of the reservoir anisotropy by introducing the vertical permeability kv into the above equation: 49

50 Productivity Index of Horizontal Wells The Renard-Dupuy Method: For an isotropic reservoir, Renard and Dupuy (1990) proposed the following expression: a is half the major axis of drainage ellipse. For anisotropic reservoirs, the authors proposed the following relationship: 50

51 Productivity Index of Horizontal Wells If L >> h, then the second term in the denominator of Borisov equation is negligible: J h = hk µ B o o ln ( 4r / L) eh h Similarly other equations would reduce to this equation if L >> h and also if well length is small compared to drainage radius r eh. 51

52 Horizontal Oil Well Performance Influence of reservoir height on well productivity: For a given length of a horizontal well, the incremental gain in a thin reservoir is much more than in a thick reservoir Influence of reservoir anisotropy: This can be accounted for by modifying the reservoir thickness. In this case an effective reservoir permeability is defined as: and the reservoir thickness is modified by: The higher the k v /k h ratio the higher the J h /J v. This effect is more pronounced for thicker reservoirs. 52