457503602-5-Gas-Well-Testing-and-Analysis-pptx.pptx

Gas well testing

1. EE053-3.5-3 Gas Engineering Gas Well Testing and Analysis

2. Outline Introduction Flow-after-flow test Isochronal test Modified isochronal test Time to stabilization Radius of investigation Classifications, limitations and use of Deliverability test Slide <2> of 13

3. Introduction Deliverability testing refers to the testing of a gas well to measure its production capabilities under specific conditions of reservoir and bottomhole -flowing pressure (BHP). A common productivity indicator obtained from these tests is the absolute open flow (AOF) potential . The AOF potential of a well is defined as the rate at which the well will produce against a zero backpressure. It cannot be measured directly but may be obtained from deliverability tests. Most common types of deliverability tests: Flow-after-Flow test Isochronal test Modified isochronal test Slide <2> of 13 VIDEO

4. Deliverability equations based on theoretical methods are : 1. Pressure solution technique 2. Pressure-squared technique 3. Pseudo pressure technique 4 Theoretical Method

5. Empirical Method W here C is the flow coefficient and n is the deliverability exponent . The equation reveals that a plot of Δ ( P 2 ) = P 2 R — P 2 wf versus q sc on log-log scales should result in a straight line having a slope of 1/ n . Once a value of n has been determined from the plot, C can be calculated by using data from one of the tests that falls on the line. Then AOF is calculated. AOF is used for these tests. AOF is maximum rate at which the well could flow against the theoretical back pressure at the sand face. Slide <2> of 13

6. Slide <2> of 13

7. Flow-after-Flow Test Sometimes called gas backpressure or four-point tests, are conducted by producing the well at a series of different stabilized flow rates and measuring the stabilized BHFP at the sandface . Each different flow rate is established in succession either with or without a very short intermediate shut-in period . This type of test was developed to overcome the limitation of long testing times required to reach stabilization at each rate. The stabilized rate and pressure are recorded; the rate is then changed and the well flows until the pressure stabilizes again at the new rate. The process is repeated for a total of three, four, or five rates . Slide <2> of 13

8. Slide <2> of 13

9. A plot of typical flow-after-flow data 9

10. Stabilized Flow Test Analysis: A flow-after-flow test was performed on a gas well located in a low-pressure reservoir. Using the following test data, determine the values of n and C for the deliverability equation, AOF, and flow rate for = 175 psia . 10 Example 1 Test q sc ( mscfd ) P wf ( psia ) - 201 1 2730 196 2 3970 195 3 4440 193 4 5550 190

11. Solution 201 psia is P R (Reservoir pressure or Initial pressure) Solve for Δ P 2 : Test 1, P wf = 196 , then P R 2 - P wf 2 = (201) 2 - (196) 2 = 1985 = 1.985*10 3 psia Test 2, P wf = 195 , then P R 2 - P wf 2 = (201) 2 - (195) 2 = 2376 = 2.376*10 3 psia Test 3, P wf = 193 , then P R 2 - P wf 2 = (201) 2 - (193) 2 = 3152 = 3.152*10 3 psia Test 4, P wf = 190 , then P R 2 - P wf 2 = (201) 2 - (190) 2 = 4301 = 4.301*10 3 psia Solve for n ; Slide <2> of 13

12. From test 4 , calculate C: Therefore, the deliverability equation is: 12

13. 13

14. Exercise 1 A flow-after-flow test was performed on a gas well located in a low-pressure reservoir. Using the following test data, determine the values of n and C for the deliverability equation, AOF, and flow rate for = 465 psia. 14 Test q sc (MSCFD) P wf ( psia ) 650 1 5230 590 2 6750 575 3 7380 560 4 8650 510

15. Exercise 2 A flow-after-flow test was performed on a gas well located in a low-pressure reservoir. Using the following test data, determine the values of n and C for the deliverability equation, AOF, and flow rate for p wf = 335 psia . Slide <2> of 13 Test q sc (MSCFD) P wf ( psia ) 520 1 3640 405 2 4270 420 3 5150 395 4 6350 375

16. Exercise 3 A flow-after-flow test was performed on a gas well located in a low-pressure reservoir. Using the following test data, determine the values of n and C for the deliverability equation, AOF, and flow rate for P wf = 850 psia . Flow-after-flow Test data are shown in the following Table. Test q sc (MSCFD) P wf ( psia ) 0 1300 1 12570 1220 2 13700 1178 3 14660 1120 4 15470 1050 16

17. Isochronal Test Isochronal test is a series of a single point tests developed to estimate stabilized deliverability characteristics without actually flowing the well for the time required to achieve stabilized conditions. This test consists of producing the well at a fixed flow rates with flowing periods of equal duration .  It is conducted by alternately producing the well, then shutting in the well and allowing it to build to the average reservoir pressure. The well is then flowed at a second rate for the same length of time . The isochronal test is based on the principle that the radius of investigation established during each flow period is not a function of the flow rate but depends only on the length of time for which the well is flowed . Slide <2> of 13

18. Slide <2> of 13

19. Example 2 An isochronal test was conducted on a well located in a reservoir that had an average reservoir pressure R of 1952 psia . The well was flowed on four choke sizes, and the flow rate and flowing bottom-hole pressure were measured at 3 hr and 6 hr for each choke size. An extended test was conducted for a period of 72 hr at a rate of 6.0 MMscfd , at which time P wf was measured at 1151 psia . The slopes of both the 3-hr and 6-hr lines are apparently equal. Determine n, C, deliverability equation and AOF.   Slide <2> of 13 t = 3 hr t = 6 hr q sc ( mscfd ) P wf ( psia ) P wf ( psia ) 2600 1793 1761 3300 1757 1657 5000 1623 1510 6300 1505 1320 6000 Ext. flow, t = 72, P wf = 1151

20. Solution Use the first and last points on the 6-hr test to calculate n : Using the extended flow test to calculate C : Given the data in the following table, the deliverability equation for in mscfd is : To calculate AOF, set = 0:

21. Deliverability data plot 21

22. Inflow Performance Relationship curve 22

23. For some low-permeability wells, the time required to obtain stabilized shut-in pressures may be impractical. Objective of modified isochronal tests is to obtain the same data as in an isochronal test without using the lengthy shut-in periods required for pressure to stabilize before each flow test is run . The modified isochronal test is essentially the same as the isochronal test, except the shut-in periods separating the flow periods are equal to or longer than the flow periods .  Isochronal tests are modelled exactly; modified isochronal tests are not. However , modified isochronal tests are used widely because they save time and money and because they have proved to be excellent approximations to true isochronal tests . 23 Modified Isochronal Test

24. Modified Isochronal test 24

25. 25 Isochronal Modified Isochronal

26. 26

27. Exercise 4 A modified isochronal test was conducted on a well located in a reservoir that had an average reservoir pressure of 2050 psia. The well was flowed on four choke sizes, and the flow rate and flowing bottom-hole pressure were measured at 6 hours for each choke size. An extended test was conducted for a period of 72 hour at a rate of 9.0 MMscfd, at which time P wf was measured at 1270 psia. Using the data in the following Table, calculate the values of n and C for the deliverability equation, AOF, and flow rate for P wf = 850 psia. Generate an IPR curve for this well. Test q sc ( Mscfd ) P wf ( psia ) 1 6500 1890 2 7600 1805 3 9200 1700 4 10400 1600 Extended flow 9000 1270 t=72 hour 27

28. 28 Gas Production

29. 29 The inflow performance/deliverability is a measure of the reservoir’s ability to produce gas to well bore. IPR is used to describe the relationship between gas production and Bottom Hole Flowing Pressure (BHFP). Performance curves characterizing a gas production system are very useful tools, and is used to visualize and graphically predict: The effects of declining reservoir pressure Changes in tubular size, increasing water production To install gas compressors To forecast future production Inflow Performance Relationship (IPR)

30. 30 The equation introduced by Rawlins and Schellhardt where: Q = gas flow-rate, MMscfd P r = average reservoir pressure, psia P wf = bottom-hole flowing pressure, psia C = stabilized performance coefficient, constant n = numerical exponent, constant IPR

31. 31 Eqn . reveals that a plot of ( P 2 ) = P 2 R — P 2 wf versus q s c on log-log scales should result in a straight line having a slope of 1/n. Once a value of n has been determined from the plot, C can be calculated by using data from one of the tests that falls on the line.

32. 32

33. 33 By taking various P wf values, the corresponding q (gas flow rates) can be calculated and then plotted.

34. Pressure Transient Analysis Also known as Well Test Interpretation or Well Test Analysis. Defined as the analysis of   pressure changes over time , especially those associated with small variations in the volume of fluid. In most well tests, a limited amount of fluid is allowed to flow from the formation being tested and the pressure at the formation monitored over time. Then , the well is closed and the pressure monitored while the fluid within the formation equilibrates. Pressure transient analysis is mainly used for reservoir characterization in Exploration and Appraisal stage. During an exploration well test, hydrocarbon is produced for the first time and flared using temporary facilities (platform rig) to evaluate the potential and reserves of a field. The analysis of these pressure changes can provide information on the size and shape of the formation as well as its ability to produce fluids. Slide <2> of 13

35. Slide <2> of 13

36. Application of Pressure Transient Analysis Productivity tests are conducted to: Determine well deliverability Characterize formation damage and other sources of skin effect Identify produced fluids and determine their respective volume ratios Measure reservoir pressure and temperature Obtain representative fluid samples suitable for PVT analysis Evaluate completion efficiency Evaluate workover or stimulation treatments. Descriptive reservoir tests are conducted to: Assess reservoir extent and geometry Determine hydraulic communication between wells Characterize reservoir heterogeneities Evaluate reservoir parameters. Slide <2> of 13

37. Types of Pressure Transient Tests Single-well Tests Involve only one well in which the pressure response is measured following a rate change. A common single-well test: Pressure buildup test. Placing a bottomhole pressure (BHP) measuring device in the well, and shutting in the well. Following shut-in, the BHP builds-up as a function of time, and the rate of pressure build-up is used to estimate well and formation properties. From a pressure build-up test, average reservoir pressure and permeability in the well’s drainage area can be estimated. Multiwell tests When the flow rate is changed in one well and the pressure response is measured in one or more other wells, the test is called a multiwall test. Concept: to produce from or to inject into one well, the active well and observe the pressure response in one or more offset wells. Multiwell tests are designed to estimate both permeability and porosity in the drainage area of the wells and quantify reservoir anisotrophy . Slide <2> of 13

38. Stabilization time is defined as the time when the flowing pressure is no longer changing or is no longer changing significantly. Physically, stabilized flow can be interpreted as the time when the pressure transient is affected by a no-flow boundary, either a natural reservoir boundary or an artificial boundary created by active wells surrounding the tested well. Stabilization is more properly defined in terms of radius of investigation. Radius of investigation, r inv , means the distance that a pressure transient has moved into a formation following a rate change in a well. 38 Time to Stabilization

39. Slide <2> of 13 The pressure in the wellbore continues to decrease as flow time increases. Simultaneously, the area from which fluid is drained increases, and the p ressure transient moves further out into the reservoir.

40. From then that is, r inv = r e , and stabilization is said to have been attained. This condition is also called the pseudo-steady-state. Pressure does not become constant but the rate of pressure decline does. Time to stabilization can be determined by: Slide <2> of 13 = average reservoir pressure

41. If the time to stabilization is of the order of a few hours, a conventional backpressure (flow-after-flow) test may be conducted. Otherwise one of the isochronal tests is preferable. The isochronal test is more accurate than the modified isochronal test and should be used if the greater accuracy is required. 41

42. Radius of Investigation Defined as the point in the formation beyond which the pressure drawdown is negligible, is a measure of how far a transient has moved into a formation following any rate change in a well. Stabilized flowing conditions occur when the radius of investigation equals or exceeds the distance to the no-flow boundary of the well. As long as the radius of investigation is less than r e , stabilization has not been reached and the flow is said to be transient. Slide <2> of 13

43. Gas well tests often involve interpretation of data obtained in the transient flow regime when for r inv < r e . If r inv = r e , the flow is pseudo-steady-state. When the radius of investigation reaches the exterior boundary, r e , of a closed reservoir, the effective drainage radius is given by r d = 0.472 r e 43 Radius of investigation after t hours of flow is Radius of Investigation

44. Example 3 Solution: Time to stabilization is: The radius of investigation is: 44 = 0.032 = 0.032   = 1219 ft

45. Exercise 5 Given the following data, calculate time to stabilization and radius of investigation. k = 5.014 mD Pr = 3500 psia Φ = 0.1113 μ g = 0.02350 cp r e = 1800 ft t = 152.4 hr Slide <2> of 13

46. In designing a deliverability test, oil operators must collect and utilize all information which includes: logs drill-stem tests previous deliverability tests conducted on that well production history gas and liquid compositions temperature core samples geological studies 46 Classifications, Limitations, and use of Deliverability Test

47. Knowledge of the time required for stabilization is a very important factor in deciding the type of test to be used for determining the deliverability of a gas well. This may be known directly from previous tests, such as drill-stem or deliverability tests, conducted on the well or from the production characteristics of the well. If such information is not available, it may be assumed that the well will behave in a manner similar to neighbouring wells in the same pool, for which data are available. 47

48. Figure.4.9 Types, limitations, and uses of deliverability tests. 48

49. Modified Isochronal Test Slide <2> of 13

50. End of lesson Slide <2> of 13