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Well Path Design
The Well path design process is a tool which enables users to generate well trajectories based on reservoir properties, seismic attributes or any other data. Well trajectories can be manually digitized in the 3D window. The design points can be displayed in a spreadsheet and can be easily cut and pasted between Petrel and other windows software applications. Reservoir targets defined by the user can be used as input to the Well Optimizer. This feature will, given a set of reservoir targets and a cost model, find well trajectories and platform locations that minimize the total cost of the project. Wells designed in Petrel are automatically placed under the Wells folder in the Input pane in a sub-folder called ‘Proposed wells’.
The Well path design process allows the user to plan a new well trajectory through a 3D model. Points along the trajectory can be digitized in 3D space, which gives the user control of:
•Stratigraphic targets
•Precision of fault penetration
•Property values at target
•Well performance at target
•Technical limitation of drilling operation with respect to well deviation (doglegs)
A well designed in Petrel allows the user to obtain important information such as well path coordinates, expected (synthetic) log values, expected zone penetration log and fault intersections.
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Well Path Algorithm
Petrel uses the Advanced Design Trajectory (ADT) algorithm, both for manual well design and in the generation of optimized well trajectories. This algorithm will attempt to restrict curvature to a specified dog leg severity (DLS). The resulting well paths are made up of straight sections and curves. How this works is described in more detail below.
In Petrel 2005 it used to be possible to make well traces as standard spline curves. This option is no longer available for making new well traces. However, wells in an old project that were made using spline curves, can be still be used in a Petrel 2007 project.
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ADT algorithm and settings
The Advanced Design Trajectory (ADT) algorithm that is used to draw well paths and has two user-defined settings:
Requested upper this is the dog leg severity (DLS) number that is requested for the curved sections of a new well trace. The number is defined in degrees per 30 m or 100 ft depending on the project units.
Maximum the maximum dog leg severity that can be used in the well path. If this option is deselected, the DLS must always be lower than the requested upper value.
How does the algorithm work?
The algorithm will attempt to design a well that passes through all the design points with a curvature which is smaller than the ‘requested upper’ dog leg severity (DLS) set by the user. This is done by using a series of straight sections and curves of the requested DLS.
The user can lock the inclination and the azimuth of the tangent through a design point by selecting the design point and pressing the ‘Freeze/Unfreeze design point tangent’ icon in the function bar. To see which design points have been locked, open the spreadsheet for the new well by right clicking it and selecting ‘Spreadsheet’. If the tangent has been locked, the ‘Use’ box in the ‘TANGENT’ column of the spreadsheet is checked.
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The user can edit the tangent through a design point by moving the arrow which indicates the direction of the path through a point (see illustration below)
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If the tangent is not used, the algorithm will use a straight line between point one and two, then ‘r’ curves between subsequent points (curve then straight section). Subsequent points added to the start of the well path will also be connected by ‘r’ curves. If the requested DLS can not be achieved because of the positioning of the design points, (points are too close together and at a too severe angle), then the user is prompted with the dialog:
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If you press
• ‘Yes’ – The point will be added to the spreadsheet, but either this point or the succeeding one is not included in the well path since the DLS constraint was not met. However, the point is stored in the spreadsheet. If a neighboring point is later deleted, the obsolete point will be included in the well path if the DLS constraint can be met.
• ‘No’ – The maximum allowed DLS number is increased such that the path can be constrained to the new DLS value.
• ‘Cancel’ – does not insert a new design point
If tangent is used, then the well path will be adjusted accordingly. If the requested DLS cannot be achieved, then the algorithm will incrementally increase the requested DLS for that section until a well path can be drawn. If the requested DLS reaches the Maximum DLS and the well path can still not be drawn, then the algorithm will fail. This will be shown during the edit in the info bar at the bottom left and the user will see a warning message.
When tangent is used, the algorithm must work iteratively to find a solution. If a solution cannot be found at the requested DLS, then the same calculations will be performed at each step, up to the maximum, until a solution is found. In these cases, it may take several seconds to arrive at a solution after each edit.
Well Path Design Settings
The settings controlling how a designed well is drawn are found on the Settings tab for the individual designed wells. Double click the well’s icon in the Petrel Explorer to access this tab. A number of interactive controls for designing and editing wells are found on the well design toolbar.
The draw style for wells is set on the settings dialog for the well folder, see Style (Wells).
Settings (Designed Well)
This tab is available for wells designed in Petrel and controls how the well trace is computed between the design points.
The settings for a designed well also control how the well path is drawn between the first and second designed points. Choose the Settings tab under the wells settings and select:
DLS: Requested upper: this is the dog leg severity that will be used on all the curves in the well path if possible.
Maximum: The maximum dog leg severity that can be used in the well path.
Simple: The well head is assumed to be vertical above the uppermost design point. The well trace is drawn between the design points. The well head is not included.
Stand alone well: User gives Well head, KB and MD at kickoff point. The well is assumed to be vertical from the well head to the kickoff point. The user has the option whether to generate the well from the well head or from the kickoff point.
Side track: The user must specify the main well and it’s MD at start of the side track. Entering a specified kick off depth will ensure a straight well path down to the specified depth. A kick off angle can be given interactively by editing the direction of the initial point in the well path. The user has the option to include the main well when generating the well trace. Clicking on a well when digitizing will automatically offer the user the option of creating a side track.
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Well path design toolbar icons
The following icons appear on the Well path design toolbar:
Add New Points – adds new points to the active well
Free Movement – the editor widget will be aligned vertically in space.
Vertical Plane Only - the editor widget can only be moved in the vertical plane.
Move Along Tangent – the editor widget will be aligned along the proposed well path.
Show/Hide DLS – toggles between showing and hiding the dog leg severity colors.
Show/Hide Pipe – toggles between showing and hiding well path pipe.
Show/Hide Error Cone – toggles between showing and hiding the error cones.
Freeze/unfreeze design point tangent – toggles between having the direction of the well path at a target point defined by the user or by the algorithm.
Digitizing wells
New wells can be digitized directly in 3D in Petrel. This can be done by clicking on any type of data displayed in the active window. Horizons surfaces or intersection planes are particularly useful to use when digitizing new wells. Intersection planes can be activated through any of the folders in the Input window of the Petrel Explorer (See General Intersection) or by using the Intersections option in the Model window.
Make sure none of the proposed wells are active, and press in the function bar to start a new well. Set the desired DLS settings from the Settings tab in the Well Path Design process dialog. To add points to an existing well, select that well in the Petrel explorer and click. New wells will, by default, appear under a sub folder to the wells folder called proposed wells.
Digitizing a new well
As the user digitizes points for the new well, a trajectory is drawn between the points. Digitized points will continue to be visible as long as the Well Path Design process is active. Click on one of those points to display the widget for 3D editing. How the point can be edited dependd on the selected editing mode:
Free Movement – the editor widget will be aligned vertically in space (independent of well path). The design point can be moved in any direction.
Vertical Plane Only - the editor widget can only be moved in the vertical plane (aligned through the well path).
Move Along Tangent – the editor widget will be aligned along the proposed well path.
Dragging the cylinder section of the widget will move it along the direction the cylinder points.
Dragging the rounded section of the widget will move the point in a plane perpendicular to the cylinder.
Pressing Ctrl repeatedly will move the widget round 90 degrees.
To edit the angle of the well path at a design point, simply move the edit arrow to the chosen direction. To restore the angle back to the optimum, as calculated by the algorithm, select the point and press. Clicking on a point and pressing, even without editing, will lock the angle of the well path at that point do that edits elsewhere in the well path do not disturb the path beyond this point.
To delete a design point, simply select it and press Delete. The points can also be edited in the point editor, see Editing coordinates.
Design points with an azimuth defined will have a darker color than those without an azimuth.
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It is quite common to exceed the DLS constraints while digitizing design points in 3D. A common problem for new-comers to the Well Path Design Process is digitizing design points too close together. This can lead to unwanted sinusoidal patterns in the resulting trajectory. Normally this can be overcome by spacing the design points further apart. However, in most situations crowded design points will exceed the DLS for the trajectory and the user is prompted to select an option from the following message.
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• ‘Yes’ – The point will be added to the spreadsheet, but either this point or the succeeding one is not included in the well path since the DLS constraint was not met. However, the point is stored in the spreadsheet. If a neighboring point is later deleted, the obsolete point will be included in the well path if the DLS constraint can be met.
• “No”: the target point is added to the well trajectory and the DLS constraints for the well modified accordingly. This DLS value will be the minimum required DLS to fit design points to the well, and its value will be updated in the Max field in the Settings page for the well.
• “Cancel”: no point is added.
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Remember that if the Z- Scale setting in the active viewer is set to >1 then the well path will look exaggerated in the Z direction.
How to make a new well trajectory
Highlight the Well path design process, under the Well engineering folder in the Processes pane, by clicking on the icon. The function buttons for the Well path design process will appear in the Function bar.
1. Display an object in the 3D Display window. E.g. an intersection plane or a horizon.
2. Make sure no proposed wells are active (bold). Activate the Add new point icon in the Function bar and start digitizing the new well.
3. Edit the well points as required, either interactively or in the editor (see Editing coordinates).
4. Right click the well in the Input pane and select Settings to display the draw style for the active well, and to select how to connect the well to the surface.
Note that if using an intersection plane, the position of the intersection plane can be changed while digitizing.
Note that if the Drag parallel to Intersection button is selected, the points will be moved parallel to the intersection plane.
Editing coordinates
The coordinates for a designed well can be displayed in the wells spreadsheet. Points can be added and coordinate data can be edited or removed.
The spreadsheet format makes it easy to copy data to other applications (e.g. MS Excel) for editing, and paste it back into the spreadsheet.
To lock the well path angle at one of the design points, check the ‘Use’ box in the ‘TANGENT’ column or select the design point and press in the function bar.
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How to edit coordinates
1. Right click a Proposed well in the Proposed Wells Folder and select ‘Spreadsheet’ on the drop-down menu. The spreadsheet can also be accessed from the Show spreadsheet icon under the Info tab in the settings window.
2. Make the editing and click OK when finished.
3. Coordinates from the well points are located in rows, with separate columns for X-, Y, Z, MD, INCL, Azim, DX, DY, TVD and DLS-values. Only the data in the cells for the X, Y and Z, coordinates can be edited, deleted or copied to other cells. Rows can be appended, inserted or deleted, and by clicking on the header of the columns the data will be sorted after the X-, Y- or Z-values.
Fields in the Tangent column become checked when the tool is active.
How to edit data in MS Excel
Open the pull-down menu for a Proposed well, by clicking on it with the right mouse button.
1. Select the Spreadsheet.
2. Select the cells with data to export, and copy them by using Ctrl+C.
3. Open MS Excel, and paste the data into a spreadsheet.
4. Do the required editing.
5. Copy the edited data in MS Excel and paste it into the Spreadsheet in Petrel, by using Ctrl+V.
Implementing external coordinates
External coordinates (X, Y, Z) can be imported as a new well, by importing a text file with X, Y and Z coordinates. Well Path Design X, Y, Z, (ASCII) format is described in Appendix 1.
When importing external coordinates a new well will be created and stored under the Proposed Wells Folder.
External coordinates can also be implemented on an existing designed well, by entering coordinate data from other applications (e.g. MS Excel) in the Point Editor.
How to import a file with external coordinates
1. Open the pull-down menu for the Proposed Wells Folder, by clicking on it with the right mouse button.
2. Select the option Import (on selection), and the Import File dialog will pop up.
3. Select an appropriate file type. Select the file to import.
4. Enter well name in the Import Data dialog, and press OK.
5. The coordinate data will be stored as a new well in the Proposed Wells Folder.
Note: Petrel creates the Proposed Wells Folder when a new well is being designed. The folder can also be created from the Insert pull down menu in the Menu bar.
How to import external coordinates from MS Excel
Open the pull-down menu for a Proposed well by clicking on it with the right mouse button.
1. Select Spreadsheet.
2. Open the data file in MS Excel and copy the data to import.
3. Paste the data into the Spreadsheet in Petrel, by using Ctrl+V.
4. Press OK.
Display options for well path design
The user has a number of display options aimed specifically at well design, such as Dog Leg Severity and Error Cones. These as with other well display options are set on the settings panel for the Wells folder, see Style (Wells).
Dog Leg Severity
Dog leg severity (DLS) is a measure of the degree of curvature in the designed well path. This is displayed along the length of the well by the use of a changing color representing the degree of curvature at each point. At a specific curvature, the color displayed will change dramatically highlighting areas of the well path that will cause problems during drilling.
The dog leg severity color template can be edited directly via the icon. Also, the color used to mark a maximum dog leg severity can be set directly in the Max DLS box.
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Error cone
Error cone is a display of the uncertainty that may arise during the drilling of a new well. This is specified in terms of error in distance units per 1000 distance units drilled and is drawn as a cone shaped structure.
Error can be specified separately in the vertical and the horizontal directions, as a constant value or as a log, and is always drawn perpendicular to the well path. If a log is used to define the error cone, it should describe the error at the corresponding point on the well path rather than the error propagation. The Drilled depth is always assumed to be correct, thus the error in a completely vertical well will only be in the horizontal direction and the vertical error propagation will not affect the error cone.
Use the log calculator together with zone logs to create a log with different error propagation in different zones.
The radius of the error cone in a particular direction normal to a point on the well path is given by the formula:
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Where rh is the potential error in the horizontal direction and rv is the potential error in the vertical direction. For example, error propagation divided by 1000 (propagation is specified per 1000 units) multiplied by the distance traveled vertically or horizontally.
Error cone display is set through the Wells settings or the settings for a wells sub-folder. See Style (Wells) for information on other well display options.
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Well Optimizer
Given a set of reservoir targets the Well Optimizer will calculate well trajectories and platform locations that minimize the total cost of a drilling project. The user specifies targets and a cost function as minimum input. The output is a set of optimized trajectories based on geometrical drilling constraints extending from the reservoir back to the surface location. These trajectories are automatically sorted into special folders to distinguish the optimized wells.
There are three main factors that control the optimization process:
• Optimizer settings: Targets are defined as data points for which the optimized well paths must pass. They can be digitized well trajectories, or simple point sets. A combination of the two data types is also possible. When point sets are used as input, the optimizer will work out the most optimal way to join the data points determined by the DLS constraints and the cost model used. When designed wells are used as input, the optimizer selects the first design point in the well as the attachment point. In both cases the well trajectories are designed back to the optimal surface location. Targets can be locked to platforms and target-platform sets can be constrained by closed boundaries. Any existing well in the project can be used as a platform.
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• Cost Model: The purpose of the optimizer is to generate wells at a minimum cost for the given input data. The cost model is designed up front before the optimizer can be executed. Cost is based on the Rate of Penetration (ROP) principle. That is, predicting the cost of drilling a unit length of vertical well section, normally measured in $/ft. Accelerators can be applied to increase the cost per unit length based on well inclination and curvature. In addition to the ROP costs the user can add platform and well costs.
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•Trajectory constraints: The output well trajectories are constrained by a user defined DLS. Only well trajectories that conform to the DLS settings are generated. The well optimizer utilizes the ATD algorithm and is therefore consistent with manually digitized well paths.
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How the Optimizer algorithm works
The algorithm works as follows:
• Once the ordering of targets is set, the well trajectories are computed using the ATD algorithm
• The final cost is computed. A maximum of 5 solutions may result.
Background to DDI
The Drilling Difficulty Index (DDI) provides a first pass evaluation of the relative difficulty to be encountered in drilling a well. The principle behind DDI is that the risk associated with drilling a complex well is greater than drilling a simple well due various factors (Temperature, Pressure, rig capabilities, etc). Moreover, the cost of drilling is likely to increase due to extended rig time or even abandonment for complex wells. For a detailed explanation of how DDI evolved the user is referred to the SPE paper: IADC/SPE 59196 “The Directional Difficulty Index – A New Approach to Performance Benchmarking”, Alistair W. Oag, SPE, and Mike Williams; Schlumberger.
The DDI equation is a relationship between the MD and TVD of a well:
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Where;
MD = Measured Depth
TVD = True Vertical Depth
AHD = Along Hole Displacement
Tortuosity = Total curvature imposed on a wellbore
Typical DDI values range from 5 (low risk short wells) and 7 (highly complex long wells). The range 6.0 to 6.4 is considered a medium complex well.
Cost Model
To make a cost model, press the ‘Edit/Create’ button on the ‘Cost model’ tab under the Well Optimizer tab in the Well path design process dialog. Previously generated cost models are available from the drop down list.
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Costs are split in to two categories:
Basic Costs: This part of the dialog is used to specify cost of platforms and additional costs for each well.
ROP Costs: This part of the dialog is used to specify the drilling/completion cost and to input directional constraints.
Drilling/Completion Costs: Specify the cost ($/unit) to drill a vertical section of the well.
Max incline for vertical: Specify the maximum accepted angle tolerance (degrees) for a section to be categorized as vertical. Well segments with a greater incline are considered non-vertical. Their cost is computed using the ‘Tangential’ divisor as multiplier.
Directional Divisor: Specify the multiplier used to calculate the decrease in ROP (increase in cost) for drilling curved segments. You can specify different multipliers for the first three curved segments.
Tangent divisor: Specify the multiplier required to calculate the decrease in ROP (increase in cost) for drilling non-vertical linear segments.
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The cost model is stored once the Apply button is selected and can be accessed from the Cost model tab. There is no limit to the number of cost models in a project. It may be useful to set up several cost models to analyze the most sensitive parameters in your model and rank the results accordingly. The Well path design process is available in the Process Manager making it simple to create sensitivity scenarios.
Well Optimizer tab
After generating a cost model the targets and constraints need to be specified in the Well Optimizer tab.
Select first between:
• Create New Run
• Overwrite existing
Targets tab
Datum: If a project is situated offshore, then the Mean sea level datum level should be selected. A surface can be input to represent the platform elevation using the blue arrow. A constant datum reference is also available. When Surface is used as datum, well paths will be posted back to the datum only if all targets lie within the surface boundary. If any target lies outside the surface area, MSL will be used as datum.
You can force well trajectories to a surface datum even if targets lie outside the surface area by employing a boundary.
Input Targets: To add input targets, activate the subject in the Input pane and then press to insert the data. Targets can be point sets or existing proposed wells. Any combination of the two data types can be selected. Targets can be generated in Petrel using the [pic] Make/Edit Points tool in the Make/Edit Polygons process step.
Platforms: By default the platforms column is empty. If wells are to be added to existing wells in the project they can be input using the icon. If platforms are added, the number of available slots should be specified in the Slots field. If new platforms are allowed, tick the Allow new platforms box.
Boundaries: Enter boundaries as closed polygons in this field. Platforms can only connect to targets that lie within the boundary. The list is hierarchical, so the order in which the optimizer uses the boundaries is from the top down.
The Kick-off point is an optional setting. The value should be specified in project units and represents the MD value at which the first curved well section from the platform is permitted. If the well plan is to be based entirely on the ROP cost model then the optimizer can be executed by selecting the Run button. If the Make Report icon is selected a spreadsheet will be written showing all the input parameters used in the run and the cost details.
DDI Enhanced Cost
In Petrel, there is an option to optimize wells based on minimizing risk by incorporating the DDI factor in to the calculation. When the DDI Enhanced Cost checkbox is selected, DDI is incorporated in computing the cost of each trajectory. Incorporating DDI will favor wells with lower complexity. When the checkbox is selected you must provide a value between 0 and 1. A higher value will give a relatively stronger weight to the DDI relative to the Rate of Penetration (ROP) parameters in the Cost Model. A value of 0 is equivalent to not checking the ‘DDI enhanced cost’ checkbox. The true ROP cost is always reported.
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