6. Flare: Hydraulic Parameter Analysis
6.1 Algorithm
6.2 User Interface
6.2.1 Command Summary
6.2.2 Flare Walk-through
6.3 Flare Verification
Just as heterogeneous fractured rock masses are not limited to integer flow dimensions, a series of well tests from different locations in a fracture networks may exhibit a distribution of flow dimensions rather than a single, characteristic flow dimension. This distribution of flow dimensions is thus a valuable measure of rock mass heterogeneity and connectivity. Flow dimension distributions from well test analyses of large data sets from Japan and Sweden (Geier et al., 1992; Winberg et al., 1996) are shown in Figure 6-1. Each of these sites shows a unique distribution of transmission and flow dimension which is indicative of the rock mass heterogeneity and connectivity.
Figure 6-2 illustrates the approach developed to use well test results in terms of transmissivities and flow dimension distributions to develop DFN models with consistent fracture network connectivity and hydraulic properties.
The analysis starts with:
As discussed above and by consequence of Equations 5-6, the flow dimension is a measure of the power law variation of flow area or conductance with radial distance. The relationship between the variation in flow path area, (which is an analog for conductance) with radial distance and the flow dimension is illustrated in Figure 6-3. As a result, Flare is able to derive flow dimension from the functional relationship between slow path conductance and radial distance, without requiring hydraulic simulations. This avoids many of the difficulties associated with simulated hydraulic tests, including boundary condition and local disrealization artifacts.
C _ Af _ RD-1 (6-1)
In Flare, a graph theory search is used to proceed outwards from the well into the fracture network to calculate the variation in conductance with distance from the well. Flare analysis is carried out as follows:
1. DFN Simulation: A series of realizations of a discrete fracture network model are generated, using assumed distributions for parameters based on initial data analysis. The same wells used in the field testing are "completed" into each of the DFN simulations.
2. Cluster Analysis: A cluster analysis is used to identify all the fractures which exceed a specified size or transmissivity threshold and which are connected to well test interval in the simulated well. The result of the cluster analysis may contain the entire network or it may be only a few fractures depending on the connectivity of the fracture network.
3. Graph Analysis: The fracture pattern is converted to a pipe network graph, with each graph element i assigned a length Li and pipe conductance Ci.. The pipe conductance is calculated as
Ci = Wi Ti (6-2)
where Wi is the flow width achieved in the fracture, and Ti is the transmissivity of the fracture containing the pipe.
A number of algorithms are available to calculate the flowing width in the fracture from the geometry of fractures and fracture intersections. In the current version of Flare, the width is calculated based on the geometry of the traces formed by fracture intersections, with an applied channeling factor Fi,
Wi = ½ Fi (L1i +L2i) (6-3)
L1i and L2i are the lengths of the two traces which define the fracture intersections (Figure 6-4).
4. Flow Dimension: Using this approach, a plot of radial distance from the well against conductance can be derived for any borehole configuration and DFN model. The slope S of this relationship on a log-log plot provides an estimate of the flow dimension as,
D = 1+S (6-4)
where S is the non-linear regression fit to the radial distance vs. conductance plot.
By carrying out this analysis on a series of stochastic realizations of the DFN model, one can obtain a distribution of packer test flow dimension.
5. Packer Test Transmissivity: The packer test transmissivity Tpi for each network realization can be approximated by,
Tpi = f ( _ Tfj, Di) (6-5)
where Tfj is the transmissivity of each fracture intersecting the interval and Di is the packer interval flow dimension calculated by Equation 6-4 above.
6. Comparison and Optimization: The distributions of simulated and measured packer test transmissivity and flow dimension can then be compared to determine the match between the hydrogeological heterogeneity and connectivity of the simulated DFN and the in situ rock mass. The DFN can then be calibrated or conditioned to match the observed behavior.
Flare includes the following features:
These features make it possible for reservoir engineers to determine whether the flow dimensions which might be observed in a fracture network would be consistent with those observed in situ.
Figure 6-5 illustrates the Flare user interface. To execute Flare, select the Flare icon, and double click with the mouse. The general procedure for analysis is summarized in Table 6-1. The operation of the individual Flare menu items is described in the next section. Navigation through Flare is done using Microsoft Windows mouse conventions. In general, the left hand mouse button is used for making selections.
Table 6-1 Flare Analysis Sequence
| Command | Action | |
| 1. | Run FracMan/FracWorks to produce the fracture pattern to be analyzed. Save the file in .FDT binary or .FAB ASCII format. | |
| 2. | File/Open *.FAB | Load the *.FAB format fracture data file |
| 3. | File/Open*.SAB | Load a .SAB object containing geometries for boreholes and analysis regions |
| 4. | Edit/Exploration | Modify the geometry of the wells to be tested |
| 5. | Edit/Analysis | Define and modify the analysis parameters as necessary. Select the wells to be included in the current analysis, and the test interval(s) in each well |
| 6. | Analysis/Dimension | Calculate the conductance as a function of radial distance from each of the wells |
| 7. | File/Print | Print selected analysis statistics |
| 8. | File/Exit | Leave Flare |
New: Begin a new analysis, without closing the current analysis. Opens a window for the new analysis.
Open: Open fracture geometry (.FAB) and borehole object (.SAB) files. The standard Microsoft Windows File Open menu syntax and assumptions are used.
Close: Close the current active analysis and all windows related to that analysis.
Save As: Saves statistical analysis .STS file for current analysis.
Exit: Exit Flare. Warns the user if statistical reports have not been saved.
Undo: Not currently implemented
Cut: Not currently implemented
Copy: Not currently implemented
Paste: Not currently implemented
Exploration: Edit the currently active exploration program containing borehole and analysis region specifications. The Edit/Exploration menu uses an object approach with a tree structure to describe tunnels. Only linear boreholes are currently supported. Analysis regions can be specified as boxes, slabs, or cylinders. The Exploration Menu provides the following options;
Add: Add an additional exploration object. A submenu provides the option to select the type of tunnel object.
Remove: Remove the selected object
+ : Expand the current selection to display specific objects
- : Collapse the display to show only object types
Edit: Double click on a selected object to edit its properties.
When editing or adding an object, three property tabs are available. The General tab always provides the object name. The Geometry tab requires different input for each object type. The Boundary Conditions tab is not active for Flare.
Analysis Options: Define the analysis parameters:
Wells selected for testing
Tested intervals in each well
Maximum radial distance to calculate away from each tested interval
Number of radial distances to calculate
Toolbar: Display or hide the toolbar display
Statusbar: Display or hide the line providing status information.
Fractures: Display or hide fractures.
Boreholes: Display or hide boreholes
Graphs: Select graphs to display
Stats: Select statistics to display
Dimension: Simulate well tests from selected boreholes, and report the resulting distance vs. conductance, and distance vs. flow area in tabular and graphical form. The middle window in Figure 6-5 shows the user interface in the Dimension Analysis menu.
New Window: Open a new window for the current analysis
Cascade: Cascade currently open windows
Tile: Tile currently open windows
Arrange Icons: Neatly arrange icons for minimized windows
Close: Close the current window
Close All: Close all the windows in the current analysis.
Help Index: Not currently implemented
Using Help: Not currently implemented
About Flare: Flare QA, copyright, and license information.
After launching Flare, the user is confronted with the information and licensing screen. Upon accepting the license agreement by clicking on "OK", the user comes to the main window (top window in Figure 6-5). Under the Files menu, the user should use the mouse to highlight the discrete fracture network (DFN) files to be processed.
Returning to the main menu, the user next selects Edit, to define or modify the exploration objects and set the analysis parameters for the analyses to be carried out. Once these have been set, the user can continue to Analysis, to run the simulated well tests.
After completing analyses, the user can use the View menu to obtain graphical displays desired (e.g., graphs at bottom of Figure 6-5), and File/Save to save statistical summary (.STS) files.
Figure 6-6 presents a simple DFN model used to verify Flare. This model consists of a series of 5 fractures, intersecting a well. The fractures have varying size and transmissivity, to produce a variation in both flow area and transmissivity with distance from the well. Figure 6-7 presents the verification results for the comparison of Flare results against the values obtained for this simple fracture network by hand calculations.
