WELL TEST: ANALYSIS AND DESIGN
ECRIN SAPHIR TUTORIAL Dr. P. Ghahri
Prepared by: Amir Golparvar Golparvar.amir@abdn.ac.uk
Ecrin Saphir Tutorial
Initialization
This turotial is an introduction to the basic features of Saphir. It is assumed that you have installed Ecrin
workstation or Saphir stand alone to follow this session. The session can be followed in all levels of Saphir
except the Saphir reader.
The session will use the following files provided in the tutorial directory: SapGS01.rat, the rate file and
SapGS01.pre, the pressure file. The extension to the files is not important here as any ASCII file can be
loaded by the application.
Click on new project icon directly from toolbar.
This opens in succession two dialogs:
Ecrin Saphir Tutorial
This dialog allows you to choose the test type, reference fluid type, the available fluid rates, net drained
thickness, well radius and average porosity. Set the reference time to Dec 4, 1999 at 00:06:45 hours. Keep
all other parameters as the suggested default.
Click
You input the PVT characteristics; formation volumes factor, the fluid viscosity and the system
compressibility. Keep all values at their suggested defaults.
Click to create the new project. The Saphir main screen is displayed.
Ecrin Saphir Tutorial
The main screen is opened with the 'Interpretation' page active. This page (or panel) contains six icons
and by clicking consecutively on the icons from top to bottom, executing the dialogs and instructions, you
will follow exactly the default path of the basic workflow used in pressure transient analysis.
Loading Data
Click . This will initialize the load sequence which is normally a sequence of two dialogs. Specify
an ASCII file in the 'Define Data Source' dialog and click on to browse to the file SapGS01.rat in the
tutorial directory. A preview of the file will be shown in the dialog as illustrated in Figure below.
Ecrin Saphir Tutorial
Click to go to the Data Format dialog. Saphir has recognized the file as valid and has
automatically assigned the first column as 'Decimal Time' and the second as 'Oil Rate'. This is known as
'free format'. The units are correct so no need to change the formatting proposed by Saphir. See Figure
Below.
Ecrin Saphir Tutorial
Click on to load the flow rate file. A history plot with the loaded flowrate file in steps is
displayed. Double click in the title bar of the plot to maximize it and display the scales.
A click on the time button in the time scale will change the scale to real time (ToD) as defined with the reference date and time at startup. Minimize the plot.
Ecrin Saphir Tutorial
Loading Pressure Data
Click on the icon . This will initialize the load process. Specify an ASCII file in the 'Define Data Source' dialog and click on to browse to the file SapGS01.pre in the tutorial directory. A preview of the file will be shown in the dialog as illustrated in Figure below.
Ecrin Saphir Tutorial
Click to go to the Data Format dialog. Saphir has automatically recognized the file as valid, but the automatic interpretation of the format is not correct, the first column contains the date, the second column contains the real time (ToD) and the third column is listing the pressure. Thus you need to manually specify the format. Select the Field option in the Lines Format section. A spreadsheet like display is shown containing the data separated in three columns. Choose the first line in the spreadsheet and the values are displayed under
the header , now under the header you can define the type of the field by choosing from the droplists. The type of the first field is Date (DD/MM/YYY), the second is ToD - Auto and the lasts field is the value, Pressure. The pressure file is in Points mode.
Ecrin Saphir Tutorial
Click on to load the pressure file. The history plot contains now both the flowrate and the pressure file.
Ecrin Saphir Tutorial
The pressure shows that the test is a simple DST, starting with some static pressure followed by a slug as the fluid rises in the well. Then finally flowing at surface, followed by two drawdowns on different choke sizes and a final buildup. The buildup is subject to the pressure transient analysis in this session.
Extracting Delta P
The next natural step is to extract the buildup pressure to the loglog plot, compute the Bourdet derivative to be displayed together with the delta P to aid the interpreter to choose the model that may finally match the data and yield the results.
Click on the icon . The first dialog is to choose the period to be extracted, in this case 'build-up #1'. The second dialog is to set the derivative smoothing, the filtrations (points/log cycle) and the starting pressure (P at dt=0, often also called Pwf), the default value of the pressure has been chosen by Saphir as the nearest pressure point to the change in flowrate (when the flowrate goes to zero for the buildup). Stay with the default values and click OK.
Ecrin Saphir Tutorial
Now the Saphir screen contains two new plots; the loglog and the semilog plot.
The extraction process can also be done automatically, i. e. without the intervention of the two dialogs in figures above. In that case it is always the last extractable period in the history that is extracted.
To invoke automatic extraction you hold down the shift key while clicking on the icon . The red flash on the icon indicates that the extraction is in automatic mode.
Ecrin Saphir Tutorial
At the extraction Saphir makes an automatic match. Stabilization is sought in the derivative, as well as a unit slope straight line at early time in both the derivative and the delta P. This corresponds to the two most common flow regimes seen in pressure transient analysis, namely infinite acting radial flow and wellbore storage. If Saphir can find these regimes it will make the match by setting the white horizontal dotted line on the stabilization of the derivative and the unit slope white dotted line on the early time data as seen in Figure above. The lines are connected and can be moved to any other position by you (click and drag with mouse) in the event that the automatic match is not satisfactory.
Modelling Once the match is satisfactory the process changes to diagnostic mode. The diagnostic phase includes the search for all flow regimes that may be present in the response of the extracted period. This allows the interpreter to choose the most appropriate model that includes all the flow regimes identified. Then the next natural process is to run the model and obtain the match which again will yield the results. The response of this extracted flow period is straight forward with a long period of infinite acting radial flow and a small amount of wellbore storage to start with. Small wellbore storage was expected as this is a DST with downhole shut-in. The most appropriate model to use in this analysis would be a model that takes this into account, hence the use of the homogeneous model with wellbore storage and skin in an infinite system.
Click on the Model icon . This will open the Model dialog and you set the combination of wellbore, well, reservoir and boundary models. The default parameters of the model are displayed in the dialog and you can set them manually. The default values of C (wellbore storage coefficient) and kh (permeability thickness product) is deduced from the original automatic match made by Saphir. Keep the default values as seen in Figure below.
Ecrin Saphir Tutorial
Click on . The model is generated and immediately it can be seen that there is an important mismatch between the model and the data, see figure below.
Ecrin Saphir Tutorial
This mismatch is caused by not having used the correct skin. By calling the Model dialog again (and a few times) and manually changing the skin a good match will be obtained as seen in figure below. There is still a small problem with the match, the wellbore storage coefficient is not exactly right but this problem will be dealt with in the next section. It is also possible to run an automatic model. If you hold down the shift key and click on (the red flash indicates automatic mode) the homogeneous model with wellbore storage and skin in an infinite system will be run without the dialog in figure above. The skin will be estimated and the match as indicated in figure below would have been immediate. It is also possible in the Settings page (panel) to have Saphir make the automatic skin calculation the first time the model dialog is called.
Improving
Click on the improve icon . This will allow you to run a non-linear regression to improve the match between the model and data. Uncheck the permeability k and run the regression on the wellbore
storage constant C and the skin. Click .
Ecrin Saphir Tutorial
The result after regression is shown in figure below.
The Saphir screen now shows the model match on all the plots.
Ecrin Saphir Tutorial
Sensitivity
The final step in the workflow is the sensitivity. Click on the Sensitivity icon . The dialog that follows (figure below), will allow you to specify the parameters to run the sensitivity on. In this case the choice is the skin value. Move the Skin from the parameter selection area to the parameter values. And enter the following sensitivity skin values: 1, 2, 4, 6, 8, 10, 14, 20.
Ecrin Saphir Tutorial
Click and a loglog plot with all the skin models will be created.
Ecrin Saphir Tutorial
Horner Plot The Horner plot is strictly only valid for a buildup following one single constant rate drawdown. However, when you want to use such an analysis method it is possible, and it is just necessary to confirm the previous production time which is calculated within Saphir as the cumulative production divided by the last rate. The access to specialized or flexible plots is through the page (panel) 'More tools'
.
Click on . This will access the Flexible plot dialog, choose the Horner plot in the drop list and accept the default previous production time.
Click OK and the Horner plot is displayed. Right click on the plot and select 'Show Derivative' and unselect 'Show Model'.
Ecrin Saphir Tutorial
To regress or draw a semilog straight line on the plot use the button or the plot popup menu (right click in the plot). Use the regression method and click the beginning and the end of the line, the line will be
displayed and the line results are accessible by clicking on the Results icon . The derivative curve (red markers) can be used as an indication of the best possible straight line by selecting the interval where the derivative is constant. The method of drawing a straight-line on the semilog plot in the main Saphir screen is identical.