Workflow for creating a PVTi project, tuning EoS and generating output for compositional and blackoil simulation (Based on PVTi Tutorials 2-4) This exercise describes how to specify fluid properties in PVTi. It covers the basic functionality of PVTi; The PVT report for this fluid contains details of three experiments: a Constant Composition Expansion experiment, a Differential Liberation experiment, and a Bubble Point experiment. The later steps describe how the experimental results may be used to fit an equation of state to the experimental behavior, and how this fitted equation of state can be used to generate PVT tables for use in reservoir simulations. Creating a fluid system This part of the exercise shows how to set up basic fluid properties in PVTi and how to display the phase envelope for the defined fluid.

1. Start PVTi. 2. Select PVTi: File | New... 3. Enter BLACK.PVI as the project name in the file selection window. The Fundamentals panel opens so that basic project information can be entered. 4. Enter CO2, N2, C1 and C6 into the Components column. 5. Click Apply. 6. Click Yes so that PVTi fills in the library component names. 7. Enter the mole fractions from figure and the details for the C7+

component into the Fundamentals panel and click OK.

Note The components for which no mole weight or specific gravity has been specified are automatically set to use the PVTi component properties library. The component properties can be examined by selecting PVTi: Edit | Fluid Model | Components.... This panel can also be used to add additional components, the select alternative characterization methods and to manually defined component properties

Selecting an equation of state Three-parameter Soave-Redlich-Kwong equation of is fitted to the results of experiments carried out on the defined fluid. The Lohrenz-Bray-Clark correlations is used for viscosity.

8. PVTi: Edit | Fluid Model | Equation Of State... This opens the Equation of State and Viscosity Correlation panel. 9. Select the 3-parameter Soave-Redlich-Kwong equation of state (SRK3). 10. Click on OK. 11. Click on OK to change the parameters to SRK3 defaults.

Program options

12. PVTi: Utilities | Program | Options... This opens the Program Options panel. 13. Set the Separator GOR calculation to use Liquid at Stock Tank

Conditions. 14. Set the Temperature-dependence for volume shifts to be calculated by

Polynomial correlations. 15. Set Treatment of Volume Shifts to Independent and click on OK.

View fluid attributes

Now that a fluid has been defined, there are two plots available to review the fluid we have entered. First is the fingerprint plot of mole fraction versus molecular weight; the second is a phase plot. 16. Right-click on ZI in the project tree-view and select Fingerprint Plot from

the popup menu. The plot should look like Figure 5.1.

17. PVTi: View | Samples | Phase Plot... 18. Request Sample ZI, 5 quality lines. 19. Click on OK. 20. The plot should look like Figure ?. 21.

Saving the SYSTEM section for future use

22. PVTi: File | Save (Concise)... 23. Call the file FLUID_DEF.PVI. The complete project can be saved using PVTi: File | Save... This, effectively, saves a history of the project.

Simulating experiments This step describes how experimental observations can be entered into PVTi and how the experiments can then be simulated from an existing fluid definition. Setting units

24. Utilities | Units... 25. Set the Unit Type to Field 26. Set the Temperature Unit Type to Fahrenhei 27. Set Mole Fraction or Percentage to Percentage 28. Set Absolute or Gauge Pressure to Gauge. 29. Click on OK.

Defining experiments for simulation nsion experiment are

xperiments... ssure Depletion | Constant Composition

32. nt Entry window now shows three folders: General,

33. folder. table and select Pressure from the drop-

35. select Relative Vol. from the drop-down list.

36. can then be further

37.w shows two folders. The Components folder has

39. re (220 F).

In this section data from a constant composition expabrought into PVTi.

30. PVTi: Edit | E31. Experiment Entry: Add | Pre

Expansion... The ExperimeObservations and Components. These folders are used to define the experiment entry form. Select the Observations

34. Click in the top left cell of the down list in that cell. In the second column

The ability to tailor the table means that data entry accelerated by importing observations from a text file or the clipboard. Click on Next.

38. The table nodisappeared as there were no component observations selected; the General folder now shows an entry field to select fluid type and another to enter the temperature of the experiment. In the General folder, enter the temperatu

40. Select the Observations folder. 41. The Observations folder now shows a two-column table with the columns

selected previously. Table is provided in the file CCE_TABLE.TXT 42. Right-click in the table and select Table Import | From file...

43. Select CCE_TABLE.TXT and click on Open. 44. In the Text Import Wizard turn on Ignore Records and set the number of

records to ignore to 1 (since we want to ignore the column headings). The view of the table should no longer contain the first row.

45. Click on OK. Note The error message “Cannot delete rows from this table” appears This is because the table has a fixed length and the file we are importing from has fewer rows than the table. This message can be safely ignored. 46. Click on OK to remove the message “Cannot delete rows from this table”. 47. Click on Next to create the experiment.

The data tree now shows the created experiment (CCE1). The asterisk (*) next to the experiment’s name means that it is active. CCE1 has one observation node, for the relative volume measurements. 48. Click Close to shut the panel.

Plotting simulation results

49. Click on the Relative Vol. observation in the Data Tree and drop it over the Main Plot Window. The Main Plot Window should now look like Figure 5.3.

Differential liberation experiment

50. PVTi: Edit | Experiments... 51. Experiment Entry: Add | Pressure depletion | Differential Liberation... 52. In the Observations folder, set the table headings to match those in Table

Pressure, Oil Rel. Vol., Gas-Oil ratio, Vapor Z-factor, Liquid Density, Gas Gravity, Gas FVF.

53. Click on Next 54. Enter 220 F as the temperature in the General folder.

55. Import the file DL_TABLE.TXT into the table in the Observations folder, remembering to ignore the first line, which contains column headings.

56. Click on Next to create the experiment.

The Experiment Entry panel now shows that there are 2 experiments defined.

Defining the bubble point experiment Finally, there is a bubble point experiment at 220o F to be added.

57. Experiment Entry: Add | Single Point | Bubble Point... 58. In the Observations folder set the first column heading to Sat. Pressure

and the second to Liquid Density

59. Click on Next 60. Enter the temperature, 220o F in the General folder. 61. Select the Observations folder.

62. Enter the saturation pressure as 2516.7 psig and the liquid density as 45.11 lb/ft3.

63. Click on Next to create the experiment. 64. Click Close.

Simulating all the experiments

All the experiments have now been entered. 65. PVTi: Run | Simulate A simulation report, showing information on all the experiments, is displayed in the Output Display panel. 66. Output Display: File | Close

Plotting all observations for an experiment 67. PVTi: View | Observations... 68. Select the Differential Liberation (DL1) experiment. 69. Click OK. This plots each observed data set (as points) for the differential liberation experiment and each calculated data set (as lines) generated by simulation. Double-clicking on one of the small plots swaps it with the large plot. 70. Examine each of the plots and note how well (or badly) the simulation has

matched the data. Saving the project for future use

71. PVTi: File | Save (concise)... 72. Call the file SIMULATE_SECTION.PVI.

Fitting an equation of state to experimental results This part of the exercise shows how a fluid definition can be fitted, by regression, to describe experimental results. The equation of state is fitted to the observation data to produce a better representation of the fluid. A sensitivity analysis is carried out to determine which attributes of the fluid components improve the solution by the smallest change. The most sensitive attributes are then adjusted slightly by regression to improve the equation of state model of the fluid. Sensitivity analysis Sensitivity analysis is used to establish which fluid properties most affect the difference between the observed and simulated values.

73. PVTi: Run | Regression... opens the Regression panel.

74. Select Normal as the Type of regression variables in the Variables section of the panel.

75. Click Variables.

The regression variables are numbered for each property. Entering 1 in the critical pressure (Pcrit) column in the rows corresponding to C3, IC4, NC4, IC5, NC5 and C6 groups those components into the first regression variable. 76. Fill in the table in the Select EOS parameters for regression panel with the

following data:

77. Leave the second part of the Select EOS parameters for regression panel blank.

78. Click on OK. 79. Click Regression in the Report section of the panel

80. The Regression Report panel provides several views of the regression

problem, designed to give the best possible insight into creating a fluid model.

• Select the Sensitivities folder. The sensitivities for the first Pcrit parameter are generally lower than for the other regression variables.

• Select the Hessian folder. The values in the leading diagonal dominate the matrix, except in the first row, the row relating the first Pcrit parameter.

• Select the Covariance folder In this table the largest value is for the first Pcrit parameter, indicating that it is the least well determined by the regression.

• Select the Correlation folder. There is a strong negative correlation between the two Pcrit parameters, indicating that the regression would proceed better if only one of those two parameters were used.

From an examination of the information in the Regression Report panel, it can be seen that the first Pcrit parameter is not likely to aid the regression, and it may hinder it. Consequently that regression variable is removed before regression is started.

81. Close the Regression Report panel. 82. Click Variables in the Regression panel. 83. In the Select EOS parameters for regression panel click on Reset to clear

all the cells in the table. 84. Fill in the columns to describe the reduced set of regression variables with

the following data:

85. Click on OK 86. Repeat steps 90-91 and compare.

Viewing the regression progress

87. Right-click on experiment DL1 in the project tree-view and select Plot from the pop-up menu.

88. Click Run in the Regress section of the Regression panel. This starts the regression.

89. Click on Regression in the Report section of the Regression panel. 90. Select the Modifiers folder. 91. The difference between the final and initial value of each regression

variable is displayed.

92. Select the Details folder. 93. An observation-by-observation breakdown of the final fit is shown, along

with the total fit to all data (both unweighted and incorporating the observation weights).

94. Examine the plots in the main window. The observed data are plotted as points and the simulated data before and after regression are plotted as lines. The regression has improved the equation of state model, so the regression results can be accepted. 95. Click Accept in the Regress section of the Regression Panel. 96. Close the Regression Report panel. 97. PVTi: Save (concise)... 98. Call the file REGRESS_SECTION.PVI

Exporting ECLIPSE Black Oil PVT tables Once the fluid description has been fitted to the experimental observations, it may be used in a reservoir simulation. PVTi facilitates the transition between a fluid description and the PVT keyword description required by the ECLIPSE family of simulators. Information can be exported either to the blackoil or compositional simulator. Exporting water properties The water properties exported from PVTi are generated by correlation. This is effectively separate from the fluid model.

99. PVTi: File | Export Keywords | Water... 100. Enter a reservoir temperature of 220 F and an initial reservoir

pressure of 2500 psig.

101. Click on OK 102. Enter the filename PVTW.PVO for the water keyword 103. Close Output Display panel.

Generating ECLIPSE Black Oil PVT tables In order to generate ECLIPSE BlackOil simulation PVT tables, PVTi requires either a Differential Liberation experiment or a Constant Volume Depletion experiment to be simulated from the fitted equation of state. The PVT tables are generated off either of these experiments.

104. Right-click on experiment DL1 in the sample tree and select Export Keywords...from the drop-down menu.

105. Select PVTO and PVDG (Live oil and dry gas) on the radio button menu.

106. Click OK 107. In the File Selection box, enter ECLIPSE100 as the name of the

export file. The keywords are generated and the Display Output module shows the generated keywords.

Generating Compositional Fluid Model (Optional) In this part of the exercise, full compositional model and composition versus depth table (ZMFVD) are exported if a compositional model is required. Exporting the fluid model

108. PVTi: File | Export Keywords | ECLIPSE Compositional Fluid Model...

109. Select the fluid {None}. This means that PVTi does not write out a ZI keyword for the ECLIPSE Compositional fluid model. This is the correct selection in this case as the equilibration (RSVD) is used to create a composition versus depth table (ZMFVD). 110. Enter the reservoir temperature as 220o F.

111. Click OK. 112. Export the fluid model to FLUID.PVO

113. PVTi: File | Exit (There is no need to save the PVI file as it can be created from the ECLIPSE Office case).

Exporting equilibration keywords

114. In PVTi, right-click on the composition versus depth experiment COMPG1.

115. Select Export keywords... from the drop-down menu. 116. In the COMPG1 export panel, select ZMFVD (Compositional) on

the radio button.

117. Click OK. 118. Export the keyword to the file ZMFVD.PVO.

119. Save the file and Exit.