Well Segmentation
Conventional well models treat the entire wellbore as a single entity, averaging all the fluid properties in the well bore. This means that, for example, if an upper zone is flowing gas and a lower zone is flowing water, the density assumed in calculating the pressure drop in the wellbore will be incorrect.
The default segregated well model in ECLIPSE100 and ECLIPSE 300 improves on this by treating the fluid from each flowing connection separately.
However, neither an average nor a segregated well model can account for:
· Frictional pressure drops, which are dominant in horizontal wells
· Devices such as valves, pumps, etc.
· Multiple flow paths, such as those that arise from flow behind tubing or multi-lateral wells
For these effects, you need to use the multi-segmented well model. This divides the wellbore into segments, much like the reservoir is divided up into grid-cells.
FrontSim does not support the segregated or multi-segmented well models. Multi-lateral or horizontal wells, or wells with large amounts of cross flow, are likely to give different results from FrontSim than from ECLIPSE 100 or ECLIPSE 300. FrontSim does have a simpler well bore friction model for use with horizontal wells, but Petrel 2007.1 does not generate data for this option.
Overview of How to set up multi-segmented wells:
1. In the Processes pane, Open Well engineering and then open the Define well segmentation process
2. Add a well, or folder of wells, from the Input pane to the list using the Add well or folder.
3. Define the segmentation parameters as appropriate.
4. Repeat steps 2 & 3 for as many additional wells or well folders as required. Note that parameters are initialized to the previous well/folder added, and that if a well appears twice, the last occurrence will be used.
5. Click OK to finish the process.
6. To use the well segmentation process in simulation, insert the segmentation set into the drop site on the Strategies tab of the Define simulation case process.
Adding wells to well segmentation:
How to add a well to well segmentation:
1. Select the well from the Input pane.
2. Click the blue arrowon the left hand side of the wells list in the Define well segmentation dialog.
How to remove a well from well segmentation:
1. Select the well in the list.
2. Click the red crosson the left hand side of the wells list in the Define well segmentation dialog, OR press the DELETE key.
A well may appear in the list more than once, either explicitly or, more likely, as a member of a folder. In this case, when exported to the simulator, it will use the segmentation parameters for the last or lowest appearance in the list.
Thus, a set of default parameters can be defined for the Wells folder, to apply to all wells, and then over-ridden by including particular wells or well folders further down the list.
Move wells up and down the list by selecting them and using the upand down arrows on the left hand side of the wells list in the Define well segmentation dialog.
Pressure drop model
There are four main contributors to the pressure drop in the wellbore that can be modeled:
· Hydrostatic: the pressure drop due to the weight of the fluid. This is always included in the calculation.
· Phase slip: if this is enabled, fluids are allowed to flow at different velocities, including in opposite directions. This gives an improved representation of phase segregation at low flow rates, especially when the well is shut in.
· Friction: pressure drop due to friction between the fluid and the tubing or casing. The roughness of the tubing or casing is entered as a parameter of the casing or tubing.
· Acceleration: accounts for the pressure drop caused by acceleration of the fluids
Alternatively, the pressure drop in segments at particular inclinations may be modeled using a flow performance table. (Flow performance tables for devices are defined in the Make development strategy process using rules, as they vary with time).
How to use a flow performance table to model the pressure drop in a segment:
1. Calculate the flow performance table in a program such as PIPESIM and export in ECLIPSE VFP format
2. If you do not already have a folder in the Input pane in Petrel, create one by selecting Insert new folderfrom the toolbar menu. Double-click the inserted folder and rename it in the Info tab.
3. Right-click on the folder and select Import (on selection) from the context menu.
4. Select the VFP file from the Import file dialog and press OK.
5. In the Define well segmentation process, select the well or well folder to use the table in.
6. In the Flow performance tables list, select to Append item in the tableto add a row to the flow performance tables list.
7. Define the range of inclination through which to use the table. Angles should be between 0 and 180 degrees, with 90 being horizontal and 0 being downward
8. Select the flow performance table in the Input pane, and click the blue arrow buttonin the table:
Petrel 2007.1 assumes that tables used in this way include both the frictional and hydrostatic pressure drops. If the table only includes the frictional pressure drop and you want ECLIPSE to calculate and add the hydrostatic pressure drop, after exporting your model use the Keyword editor to edit the WSEGTABL keyword and change item 5 from ‘FH’ to ‘F-‘
When the flow is in the opposite direction to that assumed when calculating the table, the simulator reverses the flow for “horizontal” segments, and uses the minimum flow rate (to approximate a hydrostatic only pressure drop) for “vertical” or inclined segments. Petrel treats segments between 45 and 135 degrees as horizontal (reversing the pressure drop) and other segments as vertical.
Flow paths and Segments
Petrel analyzes the well completions to determine the path along which fluid will flow. The flow path may be displayed in the well section window by pressing the toggle flow path displaybutton on the Well completion design or Define well segmentation function bar.
If a well segmentation set is selected for display on the input tree, and the well being viewed is included in that segmentation set, then the display of well segments will override display of the flow path. The segments are displayed along the flow path, so the display is very similar: the main difference being that the flow path stops at the well head, whereas the segmentation stops at the bottom hole pressure reference depth, and obviously that the segmentation display shows the segments:
ECLIPSE supports only a tree like structure for the flow path – loops are not supported.
A single well may have two flow paths to surface: flow up the tubing, and flow behind the tubing in the annulus. Petrel 2007.1 does not support more than one tubing at a time in the well. Wells with more than one flow path to surface are modeled as two separate wells in the simulator.
Multiple well bores in Petrel may be connected together to form a single multi-lateral well; either by creating them in Petrel using the Well path design process, or by specifying them as laterals when importing the well path – see Formats for Well data. A multi-lateral well will be modeled as a single well in the simulator.
Segmentation Parameters
The segmentation parameters are used to control how the well is divided up into segments. Generally speaking, the more segments you use, the more accurate the model of the well (up to a point) but the slower your simulation will be. Segments are always created to represent devices such as valves, pumps, measuring devices, etc.
The parameters are:
· Segment up to (TVD)
· This sets the depth in the well below which segments will be created. The pressure drop above this depth may be modeled with a flow performance table, assigned using a rule in the Define development strategy process.
· This depth is also the Bottom Hole Pressure (BHP) reference depth. To ensure that all BHPs reported by the simulator are reported at a common datum, set this depth the same for all wells.
· If a branch of a multi-lateral joins the well above the specified depth, Petrel will generate an error message when exporting the well to the simulator.
· If this depth is not set, Petrel will segment up to the top change in flow path diameter, typically the bottom of the production tubing, or the top point at which a device exists, or the top point at which a lateral joins the well bore. Note that if the tubing extends below the top perforation or multi-lateral join, the BHP reference depth may be set below the top segment in a true vertical depth sense, although it is nearer the wellhead along the flow path:
· Maximum delta TVD
· This sets the maximum vertical distance between two segment nodes. The hydrostatic pressure drop is governed by the vertical distance, so this parameter may be set to ensure the hydrostatic pressure drop in a segment is not too large.
· Maximum delta inclination
· This sets the maximum difference in inclination between segments. It can be used to ensure additional segments are created in sections of the wellbore with high curvature.
· Create segment when inclination crosses horizontal
· Undulations in the wellbore can create U-bends, which act as traps for high density fluids, such as water, that can prevent flow of lighter fluids such as gas or light oils. Ensuring segment boundaries at this points allows this effect to be modeled accurately
· Segment per cell within a perforation
· This creates a separate segment for each grid cell that connects to the wellbore. This allows more accurate modeling of flow from the grid into the well. However, it can create many more segments.
· If this option is enabled, all other parameters are ignored within perforations.
· Maximum segment length within a perforation
· If the segment per cell within a perforation option is disabled, this parameter sets the maximum length along the wellbore of a segment within a perforation.
· Minimum segment length outside perforations
· This sets the minimum length of a segment outside perforations.
· This constraint may be violated if the distance between two completion items such as valves is less than this value.
· The minimum segment length within a perforation is set by cell size, since no cell may connect to more than one segment.
· Maximum segment length outside perforations
· This sets the maximum length of a segment outside perforations.
Technical Documentation and known issues
This section describes some of the technical details regarding segment generation nodes and their subsequent export. It also details current known issues with creating, visualizing and exporting segments.
Terminology. A fixed segment node is a node that represents a connection or change in topology. Examples are connection nodes that are connected to the reservoir and generate COMPSEGS/COMPSEGL keywords, the node at the end of tubing where diameter changes and device nodes that represent (for example) a choke in a valve. These nodes are will always exist (except under exceptional circumstances where branch nodes are very close)
Segment display. For speed we do not calculate connection properties when generating multi-segmented wells segments for display. Therefore it is possible in the well section window to get segment nodes for cells that do not flow. When exporting the case, we calculate these properties and only generate segments for cells that will flow. It is therefore possible for the number of segments written to the deck to be less than the number shown on the well section window.
Open hole properties. Open hole wells now have default properties for diameter (7.5"), roughness (0.006") and skin (0.0). These values may be overridden on the Global completion settings Properties tab or individually via a well's Completions settings.
Convergence issues. If your segmented wells have trouble converging, try adding WSEGITER to the deck as a User keyword using the Keyword Editor. This forces the use of a more expensive but more robust algorithm in Eclipse, which should enable quicker convergence, resulting in less time steps. No additional parameters are required - see the Eclipse documentation for more information.
Accounting for segment’s roughness when behind tubing. If a segment is in annulus and we have coincident tubing then, to account correctly for roughness, we export the Hydraulic (or Equivalent) diameter and the diameter-weighted-average roughness of the casing and tubing. These values are used by Eclipse to calculate the Fanning friction factor.
Known issues
· If properties of segments (diameter or roughness) change over time, these will not be reflected in the exported deck. The segment properties will reflect the completion item that originally generated the segment. This issue will be addressed in a future release.
· If a cell is connected to more than one branch of a well (for example, at an open hole lateral kick off), we currently only export the connection for one of the branches.
· Roughness cannot currently be set for valves or pumps. Instead they inherit the roughness from their associated tubing.
· The default roughness for casing and tubing pre-2007.1 was 0.0077" which is unrealistic. Although casing and tubing roughness could be set prior to 2007.1, the values were never used. We have taken the decision to auto-correct this value on project import to the PVTi default of 0.0006". This conversion will only take place the first timea pre-2007.1 project is loaded.