Principles of GeomodelingGeological models are created for different purposes, but common to all of them is the desire to build a representation of the subsurface. Depending on the purpose, different aspects of the model can be important.

In the case of a regional exploration model, the shape of the structures may be the most important aspect. Geological models can be used to achieve accurate volume calculations or to test the effect of different depositional regimes against observed data. With simulation models, the size and complexity can be the limiting factor for achieving a model that has a good history match.

Petrel uses a 3D grid to supply the building blocks for the user to create representations of reality.

3D Grid ConceptIn simple terms, a 3D grid divides a model up into boxes. Each box is called a grid cell and will have a single rock type, one value of porosity, one value of water saturation, etc. These are referred to as the cell's properties. This is a simplification of the true case, but allows us to generate a representation of reality that can be used in calculations, etc.

Parent topic: Principles of Geomodeling

Grid resolution

The resolution of the grid will be a key decision when building the model. A high resolution grid (many cells) will allow the modeler to create great spatial complexity, but will result in a model which has many cells and may be cumbersome(( بطئ - to use مرهقwith each process taking a long time. A lower resolution grid will have less scope for complexity, but will be quick to work with and will allow the user to test many possibilities quickly.

The decision will depend on the purpose of the model, the detail and amount of data available. There is little point in creating a model with higher resolution horizontally or vertically than the data available for modeling. It is often wise to begin with a coarse model, testing the effects of changes and then increase the resolution as parameters become more certain.

Grid Structure

The inclusion or exclusion of faults is another key decision in the model building process. When dealing with simulation, the faults may be critical as flow barriers or conduits and could be the key control on results. For volume calculations they may also be important in defining the geometry of the reservoir, however, including faults requires a number of decisions to be made regarding their inclusion in the grid and will increase the time taken to create the model.

Once faults are included there is also the question of where to stop. Including every discontinuity in the model would make it unmanageable, and at some point fractures are better modeled as modified properties as opposed to breaks in structure.

3D Grids in Petrel

3D grids are created in the Structural Modeling processes and appear on the Models tab. A 3D grid represents one version of the reservoir geometry but it can contain as many different properties as required. For example, it may have 5 different porosity property models, each representing a different interpretation of the reservoir.

Structural Modeling (Petrel Workflow)Structural modeling consists of fault modeling, pillar gridding and vertical layering. All three operations are tied together into one single data model - a three dimensional grid.

The resulting grid is a full corner point 3D grid. A model created in time can be depth converted by any Velocity model created in Petrel. The procedure of building the 3D grid is divided into 3 main steps:

1. Fault Modeling:

Generation of fault pillars known as Key Pillars, are lines defining the slope and shape of the fault. There are up to five so called Shape Points along each of these lines to adjust the shape of the fault to perfectly match your input data. The Key Pillars are generated based on input data such as fault surfaces, fault sticks, fault lines, fault polygons, structural maps, interpreted seismic lines, etc. This step involves manual work in the 3D window. See Fault Modeling for further details.

2. Pillar Gridding:

Pillar Gridding generates the 3D framework. The grid is represented by pillars (coordinate lines) that define the possible position for grid block corner points. The user can define directions along faults and borders to guide the gridding process.

This process step involves user settings for an automatic Pillar Gridding algorithm. See Pillar Gridding for further details.

3. Vertical Layering:

When defining the vertical layering, the layers are inserted into the set of pillars generated in step 1 and 2. Where each pillar intersects each layer, a node in the 3D grid is defined. Faulted areas are treated separately to ensure proper fault implementation.

Input for the vertical layering can be lines, seismic interpretation, points and surfaces. By using any of these input types, Petrel will perform a 2D gridding. The resulting 2D grid is an integral part of the 3D grid and can be extracted and exported as a regular 2D surface grid.

Make HorizonsThe Make Horizons process step is the first step in defining the vertical layering of the 3D grid in Petrel. The vertical layering of the 3D grid is defined in three process steps:

Work flow of Vertical layering generation in Petrel.

Normally, the seismic interpretation is used to define the main vertical architecture of the reservoir model. When introducing the horizons to the set of pillars generated in the Pillar Gridding process, all intersections between the pillars and the horizons become nodes in the 3D grid.

General overview of Make Horizons Make Horizons Process dialog settings Settings for specific cases Quality Control of Make Horizons process

Parent topic: Structural Modeling

General overview of Make HorizonsThe 3D grid will have as many main layers as number of horizons inserted into the set of pillars. In the Petrel Explorer this is shown as Horizons in the Models window.

This is a true 3D approach in the generation of 2D surfaces; all are gridded in the same process, taking the relationships between the surfaces into account (erosion, on-lap, etc), honoring the fault model to ensure proper fault definitions in the surfaces and keeping the well control (well tops).

For the faulted areas, the horizons are blanked (deleted) in a user given area around the faults and an extrapolation(تقدير) is performed to "stretch" the surface back onto the fault plane. This will ensure that rollovers or pull-ups near faults are eliminated((ازيلت and a high quality layering of the 3D grid is preserved.

It should be noted that the skeleton grids are modified if the top and/or base of the input data extend above or below the Top and/or Base Shape Points respectively. The Key Pillars should extend above top horizon and below base horizon to avoid negative volumes in the 3D grid. If you have horizons above or below your fault model, add them to the model in Make Horizons. The existing pillars will be extended until they meet these new horizons, and in some cases it can cause the pillars to cross. Thus, an initially perfect grid may become twisted after Make Horizons.

Settings tab (Make Horizons) The Settings options include a selection of algorithms and techniques when generating horizons. The tab consists of three main parts with options for separate elements of the Make Horizons process.

The inserted horizon becomes part of a cell layer in the 3D grid. The gridding has a true 3D approach since all horizons are gridded at the same time, taking relationships between the horizons (On-lap, truncations, etc.) into account and at the same time honoring the fault model. This ensures a consistent set of horizons with a perfect fault implementation.

Parent topic: Make Horizons Process dialog settings

Convergent (تقارب) Gridder Method

This method is always used when conforming يوافق او to another horizon. It is a ((يطيعfast and general-purpose algorithm with good extrapolation. It adapts to sparse او خفيف

and dense data distributions through converging iterations at successively finer ((ضئىلgrid resolutions. This means that general trends are retained in areas with little data while detail is honored in areas where the data exists. For further details see Make Horizon Algorithms.

Minimum Curvature Method

If this option is selected, Petrel will create the horizon in two stages. First the points will be sampled to the surrounding pillars, and then this data will be extrapolated to fill in any gaps in the grid that did not receive any data using the global extrapolation.

For both gridding methods, when surfaces are used as input, the data can be converted to points prior to processing

Temporary pre-smoothing: this will smooth the local data prior to extrapolation and thereby avoiding small local irregularities affecting the extrapolation. After the global extrapolation the original values are put back.

If points around the same node are more variable than the Max difference in Z values then the node will be estimated by the global method rather than the local one.

Step 1. Local interpolation التوليد او Points and Lines only (expert settings) - ((االستيفاء

Under local interpolation the user can set the local influence radius of the point data, and the local interpolation algorithms to be used. The user can control how the input data is sampled on to the adjacent pillars. With few data points, it is possible to let the interpolator work with a higher Local Interpolation radius. This may enable the interpolator to find input data.

The available options for local influence radius are:

1/2 cell: This option is best for low density of points. 1 cell: This option is best for high density of points.

The available options for local interpolation method are:

Moving average: This algorithm calculates the average of the points near the grid node, and works best for low density of points or point data with bad quality.

Plane: This algorithm makes a linear plane, which represents data points near the grid node.

Parabolic: This algorithm makes a 3D parabolic surface to represent the points near the grid nodes, and works best for high density of points and points with good quality.

Step 2. Global extrapolation (expert settings)

There are a number of options for global extrapolation:

Extrapolation Method: choose the algorithm to use. Minimum Curvature, for trend following results, or Full Tension for a more linear (flat) result. For further details see Make Horizon Algorithms. Click on Minimum Curvature settings to open the global setting of Minimum Curvature.

Extrapolation to faults: There are a number of options for different types of faults, see the tool tip for details.

Other Settings

Locked horizon nodes: Useful when regenerating horizons. If some nodes are not to be changed, they can be locked (in the Edit 3D Grid process) and when the horizon is regenerated the locked nodes are unchanged. You can iconize the locked nodes by right clicking on the horizon and selecting Convert locked horizon nodes to points.

Force horizons to be calculated: this will ensure that a horizon is created even if there is no input data (NB the horizon may be eroded). If the option Influence radius is not checked, locked nodes will be overwritten in the Make Horizon process.

Collapse the zones to zero thickness: Use this option to collapse cells less than the minimum thickness specified in project units. Cells are collapsed towards the eroded horizon. If no horizon is set to erosional the cells are collapsed at the mid-point. See Tool Tip for more details.

Iconize all points used: Select this option to generate a point set describing the data points used in the Make Horizons Process. Useful for establishing what data is outside of the fault influence radius and thus used as input for gridding.

Cross section through model: Collapse the zones to zero thickness set to 1m tolerance.

Note that locked nodes will be overridden by well adjustment if well tops are used in the process. Locked nodes inside or at the well influence radius will not be calculated in the local interpolation, only in the global interpolation. Locked nodes will be kept locked during smoothing.

Fault Re-sampling from the Fault Model

These settings are only relevant when Use Horizon Lines is checked on in the Horizons tab. There are 2 ways to sample the fault-horizon intersection from the fault model:

By Name: Petrel searches for a match between the name of the horizon lines stored on the fault model and the name of the horizon and faults in the 3D grid.

By ID Number (recommended): If the name of the fault(s) has been changed, or you are unsure of the history of the project, the ID Number should be used to match the faults. The ID Number is hidden from the user but is used internally in the Fault Model and the 3D grid.

Lock all Re-sampled Horizon Nodes: Select this option if you do not want the nodes on the horizon lines to be changed during the Make Horizons processing (smoothing etc).

How to generate Fault-Horizon intersections for the 3D GridThe fault modeling process is also used to create/edit fault-horizon intersections for use in the 3D gridding processes. The geometrical relationships are stored on the fault model as Horizon Lines. These Horizon Lines can be used as input for the Make Horizons

and Scale Up Structure processes to ensure structural consistency between different 3D grids belonging to a common fault model. There are several ways to generate Horizon Lines on the Fault Model.

From input polygons describing the intersection between the horizon and each side of the fault.

By digitizing them directly on to Fault key pillars in a 3D window By re-sampling from a previously defined 3D grid

Folder structure containing Horizon Lines.

The different sides of the fault are distinguished by dotted lines (back) or solid lines (front)

How to generate new horizon lines from polygons

1. Activate the Select Horizon Nodes icon from the function bar 2. Display the desired polygon in the 3D window

3. Select Create Horizon lines from selected polygon 4. A horizon line is generated on the Fault Model.

If you want to add to existing horizon lines, you can activate the object (make it bold)

How to generate new horizon lines without input data

1. Activate the Select Horizon Nodes icon from the function bar

2. Select Add/move horizon nodes on the active horizon

3. Select either to Add/move horizon node at the front of the fault or back of

the fault 4. Select a key pillar in the appropriate position to add the point and continue to the next

key pillar 5. Repeat the process until you have completed the fault-horizon intersections for all

faults. 6. Edit the horizon nodes accordingly to refine the horizon-fault relationship

Select Auto-select Horizon nodes to add nodes between the picked and selected horizon node.

How faults are incorporated into the grid?

Parent topic: Fault Modeling

Why use Horizon Lines in modelingHorizon lines are extremely useful as they represent the desired relationship between a fault and a horizon. Because they are stored on the Fault Model they can be used to generate multiple 3D grids with consistent structural relationships. Typically, you would

use the horizon lines stored on the Fault model during Scale Up Structure or to refine the geological grid to ensure that the structural relationship remains constant between the fine and coarse grids.

How to generate Horizon Lines from 3D grid

A common situation is to go through the Fault modeling, Pillar Gridding and Make Horizon processes using the available input data, and then to use the quality control tools available in Petrel (see General Intersection.) to check the quality of the resulting 3D grid. In most situations, the workflow runs fine and no editing is required. However, in certain situations, for example where there is a complex fault-horizon relationship, it may be necessary to fine-tune the model before property modeling and Up-Scaling:

1. Run through Fault Modeling, Pillar Gridding and Make Horizons processes. 2. Quality check the model. There can be areas where the fault-horizon relationship is not

as desired . 3. Open the settings for horizons or the chosen horizon and choose the Operation tab.

4. Choose the appropriate settings and sample the lines to any fault model, or resample them from a fault model.

5. Click on the Resample icon, icon and use the pick tool to edit the desired points

6. After editing, select the option to Resample from fault model on the horizon object to update the changes

7. Quality control the result and continue with the further modeling steps

8. During Scale Up Structure select to resample the horizon-Fault intersections from the Fine Grid (default)

Note that the re-sampling from the fault model uses one grid cell interpolation.

You can re-run Make Horizons to use a greater fault stepping distance.

You can also choose to use the horizon lines stored on the fault model when you upscale the fine geological grid to a coarser simulation grid in the Scale Up Structure

process. Alternatively, you can use the horizon-fault intersections directly.

Horizon from fine grid.

Horizon from fine grid (red) and up-scaled horizon (pink) with no re-sampling - note the difference in horizon intersection with the fault.

Horizon from fine grid (red) and up-scaled horizon (pink) using re-sampling technique

How to generate Horizon Lines using polygons

In situations were it is necessary to avoid the extrapolation that will take place during Make Horizons, it is possible to use polygons as input for horizon lines. Use the following workflow:

1. Display polygons representing offset horizons in 3D.

2. Display the corresponding faults from the Fault Model folder. Make the fault that the horizon line is to be attached to active (bold).

3. Activate Fault Modeling and click on the Select Horizon Nodes icon from the function bar (red circle).

4. Select the polygon to be used to generate the horizon line (green circle). 5. Click on the Create horizon lines from a selected polygon on the active fault icon

(yellow circle). If you do not already have an active horizon line, a dialog will open up, asking if you want to generate a new one. The Horizon line is displayed on the

fault and also placed in the Horizon Lines folder in the fault model. 6. To make a horizon line for a different horizon, de-activate the horizon line before you

repeat the steps above. 7. In order to be used in the Make Horizons process, the horizon lines must be renamed to

be the same as the output horizon.

Horizon lines are organized according to the horizon's placement relative to the orientation of the fault. Horizon lines located on the 'front' of the fault are solid and horizon lines located on the 'back' of fault are dotted.

Fault from fault model and fault polygon displayed in 3D window. The two circled icons are used in the process to generate the horizon lines.

How faults are incorporated into the grid? In general, it is the shape points in a fault that are honored when you build the model grid and not the pillars. For example, although a pillar is curved, grid pillars at the same location in the grid will not necessarily have the same shape.

Curved pillars in the center of a fault are ignored in the 3D grid, although all the shape points are honored.

However, there are certain instances where the pillar is honored as well as the shape points:

1. At the end of a fault. 2. At the connection between two faults. 3. If there is a trend attached to the fault pillar.

Curved pillars at the end of a fault result in curved pillars in the 3D grid.

Parent topic: Structural Modeling - Basic TheoryParent topic: How to generate Fault-Horizon intersections for the 3D Grid

How can I QC the grid? There are a number of tools for QCing the grid.

1. Examine the top, middle and bottom skeleton grids 2. Examine the pillars using I and J intersections 3. Build horizons in your model and display gridlines on the horizons 4. Insert horizons and layers (at a relatively fine scale) in your model, and use

geometrical modeling to create a bulk volume property model, then check the statistics and filter on negative volumes to identify problem areas.

What should I look for? Points describing the geological horizons in your model can only be located along the pillars in the model grid. It is desirable to have an even distribution of pillars, with enough pillars in all areas of the model to adequately describe the horizons in the model. If one area of the model has very few pillars, then the resolution of the horizon in this area will be limited.

Grid pillars, together with the model horizons and layers, form the cells for the property modeling and volume calculation. It is important that the pillars do not cross as this will result in cells with negative volumes. Look for crossing grid lines in the skeleton grids and on horizons and crossing pillars in the grid intersections. This usually occurs at the end of steeply dipping faults, or faults with trends that contrast with the local grid direction.

Here a fault in the grid (which must be aligned with a grid line) cuts a grid intersection, indicating negative cells in the model.

What causes poor grids? When you run pillar gridding, Petrel will first place pillars in the model to honor the faults and trends in the fault model. Once this is done, pillars will be placed between these defining pillars throughout the model. When placing these pillars, the aim is to have as smooth a transition as possible, both in the pillar orientation and in pillar shape.

When building the fault model, try to imagine where Petrel is going to put the pillars and what they will look like. Try to avoid situations where the angle of pillars varies dramatically over a short distance, or where the pillars will be forced to diverge or converge. In both of these situations there is a much higher risk of pillars crossing (and therefore having negative cells) and also of having areas with few pillars and therefore poor resolution on the horizon.

In some situations, complex geometries will be unavoidable. When these cases arise, pay particular attention when you QC the result. If the pillar gridding fails, it is highly probable that the problem is a result of complex geometries. If the model is too complex, try to increase the distance between the two key pillars that are creating the problems.

SimplicityIn all cases, the grid geometry should be kept as simple as possible. Use as few pillars and shape points as possible to model the shape of the fault as this will make pillar gridding and fault editing easier. Using five point fault pillars instead of three point pillars may match the input data slightly better, but your model is more likely to have crossing pillars and negative grid cells. Choose the top and bottom of your reservoir model carefully and make sure the top of the faults are smooth. By minimizing vertical thickness you can often remove the need for truncating faults and will also have a much more manageable model when you come to property modeling.

Settings for specific cases

There are some input data sets that need some particular settings in the Make Horizon process step to be able to get the best possible result.

Here we have listed examples where the best result requires experienced use of Petrel.

How to keep a hole in the internal Petrel horizons in case of e.g. a salt dome

When modeling salt domes in Petrel, we recommended using different settings in the Make Horizon process for the tops of the salt domes and the areas around them.

1. Create a fault around the hole - make sure that data from the input surfaces does not protrude through this fault. The Key Pillars of this fault should extend above and below the top and the base surface forming the hole. To be able to close the circular fault, it is necessary to create two faults and then connect them. Do not merge them!

2. Go through the setup for the Make Horizon process step, as you normally would do. 3. Under the Stratigraphy tab, deselect the option Force the horizons to be calculated for

small segments. 4. If the hole is not created then - Check which segment represents the hole on the

horizons by turning off the segments in the segment folder one by one. It is probably the last one since they are sorted by size.

5. Go back to the Make Horizon process dialog and under the Segments tab, turn off the input for that specific segment - to do this click on the tick mark.

6. Run Make Horizon again.

How to use Line data in complex fault systems

When using line data, like seismic interpretation, as input for creating horizons in the 3D grid, there is an option to make sure that they are accurately defined towards the faults.

The option is cut by faults and can be found on the right mouse button menu for line data. This operation will cut the line data towards the modeled faults. This option is particularly useful in complex fault systems, where it helps the Petrel algorithm to better define the created horizons towards the faults.

When using this option, remember that the distance to faults in the Fault tab in the process dialog has to be zero. The option also requires the line data to extend beyond the fault limit (through the fault).

1. Model the faults based on your input data and create a grid. 2. On the line data set that is input for the Make Horizon process step, click with the right

mouse button and select cut by faults.

3. In the Make Horizon process dialog in the Horizon tab, select the line data set as input and in the Faults tab set zero distance to all faults.

4. Run Make Horizon.

How to use line data as input in Make Horizons

Depth contours and line data sets can have large increments between the data points. Petrel will only consider the data points themselves as input. To be able to grid these data as accurately as possible, there are two different approaches in Petrel.

1. Use the lines or contours as input in the Make Surface process step, but in the Settings tab in this process dialog, check Use in If input is contours or 2D lines.

2. In the Make Horizon process dialog, select the contours or 2D lines as input in the Horizons tab.

3. In the Settings tab, check the Use option in If input is contours or 2D lines, and select max and min search radius.

4. Run Make Horizon with the above surface as input. 5. Use the Refine options in the Polygon operations folder under the Operations tab in the

lines data settings to add new points to the line.

6. Use the Make Surface utility with a small increment to create a 2D gridded surface and use this as input for the Make Horizon process. This way data points from the input data are sampled in two steps and will generally give a very accurate vertical layering of the 3D grid.

7. Run the Make Surface Process; see Make/Edit Surface for the input data with a very small increment, e.g. 50x50 and ensuring that the pre-processing, Refine polygons options are set.

8. In the Make Horizon process use the produced surface as input. 9. Run Make Horizon.

Note that you will have to test several settings for the Make Horizon process dialog in order to find the best settings for your data set.

Parent topic: Make Horizons

Visual quality control

The best way to check the quality of the 3D grid that has been created in Petrel is by viewing the grid with different visual settings. View the horizons in the grid and check that they look as expected. Particularly towards the faults, it is important to visually check the horizons in case the fault settings in the Make Horizon process step need to be changed.

How to generate a traditional 3D grid display

1. Open the Models window of the Petrel Explorer. 2. Display the top horizon with grid lines. See the Style tab of the Settings window for the

Horizons. 3. Display the zones. Use Edges to switch on/off display. 4. Zoom in on a fault to get a good view of the quality of the faulting in the 3D grid.

How to use the Segment Filter when visualizing the grid

1. This requires that fault segments have been defined in the Fault Modeling and Pillar Gridding process steps.

2. Display a horizon and open the Segment Filter folder. 3. Switch Off all Segments and switch On one by one. 4. Observe the change in the Display window.

How to use the Zone Filter when visualizing the grid

1. This requires that more than one zone have been defined in the Make Horizons process and later in the Make Zones process steps.

2. Display the top horizons and the Edges. 3. Open the Zone Filter and switch Off zone by zone. 4. Observe the change in the Display window.

How to use the General Intersection on the grid

1. Open the Models window of the Petrel Explorer and open the Intersections folder. 2. Use the option Insert General Intersection on the menu on the right mouse button on

the Intersections icon. Observe that a new General Intersection icon has been added to the Intersections folder.

3. Display and move the Intersection in the Display window. 4. Double click on the General Intersection icon to access further display settings.

For further information on how to use the General Intersection, see General Intersection.

A General Intersection as seen in Petrel.

How to generate a difference grid between input and generated data

1. Convert a horizon from the 3D grid to a regular surface. 2. Double click on the horizon to open the Settings window. 3. Select the Output tab and define area of interest and XY resolution. 4. Press the Make surface button. 5. Note that the converted surface is placed in the Input window of the Petrel Explorer,

and is given the name of the horizon. 6. Subtract the input surface to get the difference between the input and the horizon in

the 3D grid. 7. Use the Operations tab in the Settings window for the converted surface. 8. Select the surface in the Petrel Explorer and drop it in the area for A= by clicking on the

blue arrow .

9. Press the button.

Parent topic: Quality Control of Make Horizons process

Flatten Model

After building a faulted 3D model, it can be difficult to understand how the sediments were deposited as the structural changes have altered the original model. In Petrel, a very useful process has been implemented which gives the possibility to select a horizon and flatten it by removal of fault throws on the horizon. All other horizons in the 3D grid will be changed according to the flattened horizon.

The flattened model will set the model back to the situation when the layers were deposited. This gives a very good quality control of the model with respect to the depositional environment. This view of the 3D model gives very good quality control of the thickness of the different zones in the 3D grid.

Flatten Model is NOT a reversible process. It is NOT possible to go back to the 3D grid. Remember to always make a copy of the active 3D grid before running this process.

How to Flatten the Model on a specific horizon

1. Make a copy the active 3D grid by selecting the grid, click on the Copy item

icon in the Tool bar menu and then on the Paste item icon. The copied 3D grid will be added to the bottom of the Models tab in Petrel Explorer.

2. Go to the copied 3D grid and select the horizon that you want to flatten. 3. Click with the right mouse button on the selected horizon and select Flatten model. 4. In the pop-up dialog, click on Yes. The Horizons in the 3D grid have now been altered.

Volume quality control

One very dangerous error in building 3D models, particularly in simulation grids, is the presence of negative volumes. Negative volumes can be present in Petrel when the grid has been build on a poor fault model or with poorly defined directions and trends in the gridding process. The most common cause of negative volumes in Petrel is faults that cross each other without being connected or truncated.

After the Make Horizon process it is important to check for negative volumes before doing more work on the model. If negative volumes are present, we highly recommend that the user go back to quality control the 3D grid and the fault model.

Note that the grid cells at this stage are normally large. Negative volumes can be generated after further vertical layering when the cells are divided into smaller parts even if no negative volumes were present after the Make Horizon process.

How to check the model for negative volumes

1. Go to the Geometrical Modeling process step in the Process diagram and open the process dialog by double clicking.

2. In the process dialog, under Settings, select Geometrical and click OK. The property Bulk Volume, which is default, will be calculated for the grid. For more information about this process step, see Geometrical Modeling .

3. In Petrel Explorer go to the Property folder in the active grid. The property Bulk Volume will be listed in this folder. Open the Settings window for the property Bulk Volume by double clicking on the property.

4. In the Settings window go to the Statistics tab. The max and min values for the property are listed in the upper part of the window. If the minimum value is above zero, no cells with negative volume are present in the 3D grid.

5. If the minimum value is below zero, there are cells with negative volume in the grid. To locate these cells, open the Settings window for the Property folder and go to the Filter tab

6. Select Use value filter from the Filter settings list, and select the Bulk Volume from the Value filter list. Select the Use filter check box. The max and min values for the Bulk Volume will be shown. Change the max value to zero and click OK. For more information about the filter option, see Filtering of Property Models.

7. In the Display window visualize the Bulk Volume property. Only cells with negative volume will be visualized. To better understand where the cells with negative volume are, visualize the faults at the same time.

8. Status difference between zones and layering

9. When creating zones in Petrel each zone will be defined by two horizons. These intermediate horizons inserted into a Petrel project (disregarding of the input for generation of the zone) will have the same status as horizons in the continuing work in Petrel (also when exporting the 3D grid). These horizons can also be edited in the Edit 3D Grid process step.

10. Layering however, will not be defined by enclosing horizons. Layering is defined as the internal layering reflecting the geological deposition of a specific zone. They are only sub-dividing the grid between the zone-related horizons.

11. The layers are not affected by editing in the Edit 3D Grid process step. The Make Layering process must be run again after editing has been performed on the grid. Open the Layering Process diagram and click OK without changing anything to correct the layering towards the edited grid.

12.13. Parent topic: Make Zones and Layering

Make Zones ProcessThis dialog has three tabs. In principle it works in the same way as the Make Horizon dialog - a spreadsheet with the geological zones and horizons as rows and the specific user settings as columns. Additional options are found in the Settings and Wells tabs.

The Make Zones process is calculated one stratigraphical interval at a time. Each horizon delimits a stratigraphic interval. The program will also allow for a stratigraphical interval above and below the top- and base- horizon respectively.

Example: If there are three horizons, called Top, Mid and Base horizon, in a model, there will be four stratigraphical intervals that the Make Zones process can be applied to:

Above Top Top - Mid Mid - Base Below Base

Parent topic: Make Zones and Layering

Eroded horizons

If you are working with eroded horizons and they are not being properly eroded in the Make Horizon and/or the Depth Conversion process, this should be corrected before running Make Zones. Remember that this is an expert operation and is seldom needed.

To do this, first go to the Output2 tab of the Zone filter Settings window. Generate output isochores and choose the most appropriate erosion tolerance for your data set.

Then go to the Settings window of the 3D grid . In the Operations tab - enter the erosion tolerance and click on the Make consistent button.

The horizons should now be correctly eroded and the grid ready for the Make Zones process.

The Petrel Explorer after inserting zones to the model

How to make Zones

1. Double click on Make Zones in the Process diagram. A process dialog will pop up.

2. Select the Stratigraphical Interval for the definition of the zones, from the pull-down menu in the upper left part of the dialog. These main intervals are the intervals defined in the Make Horizons process. Note that only one interval can be calculated at a time.

3. Click the Add Zone button to add rows to the spreadsheet. Note that one click inserts three rows with two zone icons and one horizon icon (if the chosen stratigraphic

interval are either Top - Mid or Mid - Base). The zone icons represent the isochores used for calculating the new intermediate horizon.

4. Click on the Set number of items in table button and specify how many zones you want in the current stratigraphic interval.

5. Define input type. If a data object (isochore or well points) is used for defining the zones, select it in the Input window in the Petrel Explorer.

6. Click on the blue arrow next to the input field called Input and make sure that the name of the active data object is inserted. Several data objects can be inserted simultaneously but remember to check the Multiple drop in table first.

7. Continue inserting the data objects into the spreadsheet until you have filled all Input Fields.

8. Select Build from Top or Build from Bottom. This is important when working with erosional surfaces or with on-lapping geological sequences.

9. Select whether to use a Volume Correction or not. When adding isochores to a base or a top surface, the sum very seldom matches the top or base surface, respectively. The error normally is distributed proportionally or equally among the various sub intervals. By selecting No correction all of the volume error will be added to the last zone that is built.

10. The thickness calculation can be preformed as True Stratigraphic Thickness (TST), True Vertical Thickness or Along pillars. When the pillars in a data set are vertical, it is advised to perform the thickness calculation along pillars, as this is a much faster operation in comparison with the calculation of TST and TVT.

11. When clicking OK the intermediate Horizons and Zones will be generated. The new Horizons and Zones are now available from the Horizons and Zone Filter folders in the Petrel Explorer.

If you want to make zones for several intervals simultaneously (i.e. without closing the Make Zones process dialog), make sure that you click on the Apply button before moving on to the next interval.

Make Zones settings There are three tabs with various settings in the process dialog for Make Zones:

Zones - This is where the input data such isochores or well tops for the Make Zones process is entered. In cases with no input, a numerical setting can be entered.

Settings - Some settings with regards to well correction, erosion and cell thickness. Well Adjustment - well adjustments and reporting. Uncertainty - Is activated once the uncertainty icon has been toggled on. The settings

for the stochastic uncertainty surface may be changed here.

Settings tab (Make Zones) Well Adjustment tab (Make Zones) Uncertainty tab (Make Zones)

Parent topic: Make Zones and Layering

Zones tab (Make Zones)

Stratagraphic Interval (Make Zones)

The Stratagraphic Interval defines on which interval the spreadsheet of isochores is to be applied. Each interval is defined from the main zones generated in the Make Horizons process.

Example: If two horizons (Top Reservoir, Base Reservoir) were generated in the Make Horizons process, the following stratagraphic intervals can be used in the Make Zones process:

Above Top Reservoir Between Top Reservoir and Base Reservoir Below Base Reservoir

Separate zone matrices are set up and defined for each interval.

Zone definition (Make Zones)

Name - Name of the zone/horizon.

Color - Color of zone for visualization. Click on the Color legend icon to apply a rainbow color scale to your zones. Click again on the icon to reverse the color scale.

Input type - Different Input types are available for the Zones:

Constant - A user defined constant value for the thickness of the zone. The thickness value is entered in the input column.

Isochore - An isochore surface grid defining a variable thickness of the zone. Conformable - The values are gridded in Petrel to generate a horizon based on well tops

only.

Percentage - A percentage value for the thickness of the zone, 100% being the total thickness. The thickness value is entered in the input column.

Rest - The residual thickness. No thickness specified. Petrel calculates the residual based on the other zones making up the total thickness. When one of the zones (independent on which one) is defined as being Rest, the other zones are built towards that zone and whatever is being left over will become the "rest" zone. This zone will disappear if there is no rest.

Input - Set automatically when the input type is defined as an isochore, well top or rest. Otherwise define percentage or constant thickness here.

Volume Correct - Toggle on to perform the volume correction. When working with volume correction this option allows the user to toggle off the tick for special zones that should not be volume corrected. Note that this column is not accessible when "None" is selected for volume correction.

Status - New or Done, depending on whether the process has been performed or not.

Multiple drop in table - Allows the user to drop several files, like isochores,

simultaneously. Add the number of zones needed first by clicking to add several

zones with the same settings in a single operation. Alternatively, use to add single zones. Then click on the first isochore to use in the isochore folder in Petrel

Explorer. Click on the blue arrow next to the top zone. The selected isochore will be inserted, followed by the rest of the isochores within the isochore folder. Select

to set in a number of zones with the same settings.

Uncertainty icon

Clicking the Uncertainty icon will insert an extra column where the user can specify the value or surface representing one standard deviation on that particular horizon. Used in connection with the Uncertainty tab, see Uncertainty tab (Make Zones).

Build From (Make Zones)

When Building from Top the isochores are added from the top of the stratigraphic interval. Depending on the Input type used, this option can be used to let the calculated horizons be parallel or conform to the top. The same applies when Building from Bottom. Build From Both Top and Base is only relevant when there is a zone defined as Rest. For

example, if you select a rest zone to be somewhere in the middle of the zone table, all zones above and below will be built towards it respectively.

Volume correction (Make Zones)

When adding e.g. isochores to a base or a top surface, the sum will normally not match the top/base. Therefore, a correction of the zone volumes can be applied differently.

Proportional correction will split the error proportionally into the zones according to its relative thickness. This option is useful when the zones have a broad range in the thickness variation.

Equal correction will split the error into equal proportions for each zone. This option is useful when the zones have little thickness variation.

None correction will not make volume corrections for all zones. All of the volume error will be added to the last zone that is built, irregardless of its input type (even if that last zone has been defined as constant). If the zones are built from the top, the bottom zone will incorporate all of the thickness rest and vice versa. It is a good idea to use this option together with defining the last zone as Rest, see input types described in Zone definition (Make Zones).

Thickness (Make Zones)The calculation of zone thickness can be done as True Stratigraphic Thickness (TST), True Vertical Thickness (TVT) or Along Pillars:

TST – Thickness of a zone, measured perpendicular to the upper and the lower horizon of the zone.

TVT – Thickness of a zone, the vertical distance between the upper and the lower horizon of the zone.

Along Pillars – Vertical thickness of a zone along the pillars. This procedure should be used when the pillars are vertical or close to vertical, because this calculation is much faster compared with the calculation of TST and TVT.

When TVT or Along Pillars have been selected, it is possible to select an option called Horizon with steep slopes. The algorithm used in this option can handle very steep horizons, such as those found around a salt dome.

How to build layers

1. Double click Layering in the Process Diagram. The process dialog for Layering will pop up.

2. The dialog lists the zones generated in the Make Horizons and Make Zones processes. For each zone select the desired resolution and layering layout.

3. Make sure you have one or both of the I and J intersections displayed in the graphics. Zoom in to see the full vertical interval.

4. Create the internal layering of the zones by clicking OK, and observe the results.

The Horizons, Zones and Layering can be removed by clicking with the right mouse button on the 3D grid icon and selecting one of the Remove options.

A cross section with layering displayed.

Parent topic: Layering Process

Layering settings

There are some options available for the process of making the sub-zones. The two most important are resolution and type of layering. The options make up the columns in the spreadsheet for Make Sub-zones.

Common Settings (Layering)

Build Along: Select how the thickness is to be calculated. Choose from TST, TVT or Along Pillar. See tool tip for further details. The Horizons with steep slopes option is only available if TST or TVT build selection is chosen.

Use minimum cell thickness: It is possible to collapse layers below a specified threshold. This setting is especially useful in erosional or onlap settings where thin cells accumulate at the unconformities. All cells below the threshold thickness are collapsed and set to Undefined. If Proportional or Fractions laying methods are selected the direction must be specified.

Name - Name of the zone.

Color - Color of zone for visualization.

Calculate - Useful when regenerating only a few selected zones.

Zone division - Options on how to subdivide the zone into cell layers (For further details, See Zone Division). The zone division is always done along pillars in the 3D grid.

Proportional - Constant number of cell layers at every pillar of the grid. The cell layering will be somewhat conformed to both the top and base of the zone.

Follow top - Cell layering parallel to the top of the zone. Follow base - Cell layering parallel to the bottom of the zone. Fraction - Layering, with user controlled proportional thickness of each cell layer.

Label - Number of cells, cell thickness or how cells are divided within a zone. This is dependent on what is selected in the Zone division option.

Input - Resolution either in cell thickness, number of layers or division coding. Depends on the previous selections.

Reference Surface - Input a surface to control the orientation of the layers. This surface represents the orientation at the time of deposition and layers generated will therefore not have the same inclination as the input surface.

Restore Eroded/Base - Option to correct for erosion.

Zone Division

Building layering proportionally

Divides the zone into a given number of layers of the same thickness. Figure below shows 5 cell layers.

Building layering from top and downward (Follow top)

Divides the zone into cell layers with a constant user controlled thickness. The cell layers are parallel to the top of the zone. Figure below shows 5 cell layers.

Building layering from base and upward (Follow base)

Divides the zone into cell layers with a constant user controlled thickness. The cell layers are parallel to the base of the zone. Figure below shows 5 cell layers.

Building layering by using Fractions

By using fractions you may divide each sub-zone proportionally into smaller units. The figure below shows 5 layers with a division coding of 31122 (SUM of 3+1+1+2+2=9). This means that the first sub-zone is divided in 3/9 of the zone thickness, the second and third in 1/9, the forth and fifth into 2/9.

Quality Control of Make Zones and LayeringAfter the Make Zones and Layering processes, the Horizons and Zone Filter folders of the 3D grid will contain new horizons and zones. Several visualization tools can be used to quality control the result:

Segment, Zone and Fault Filter. I- and J- Intersections. General Intersection.

The zones will be stored together with the horizons in the Horizons folder in Petrel Explorer, however, the layers will not be found as objects in Petrel Explorer, but will simply be a finer division of the zones.

The Sub-Zone lines can be switched on/off in the Settings dialog for the Edges icon. Switching them off may improve the speed of the graphics display.

Structural modeling in Petrel is very strong and delivers 3D grids with a high quality. These grids can be exported on file formats like ECLIPSE and VIP for transfer to other applications. The individual main layers (Horizons) can also be exported as 2D surfaces on a range of different mapping system formats (See Export Data).

When visualizing only the Edges, the display is showing the "walls" along the Area of Interest and the fault planes. The display along the faults shows hanging wall and foot wall lines for both sides of the fault and can be messy to look at. Improve the view by displaying a horizon in addition.

See How to use the General Intersection for information on using the general intersection.

How to use the Segment Filter

1. Display selected zones and one or two horizons. 2. Switch off all segments and switch on the ones you are particularly interested in taking a

closer look at. Note that the speed of the graphics improve when displaying fewer segments.

When viewing and zooming inside a zone or segment use the Target Zoom button for improved control on the zooming.

How to use the Zone Filter

1. Display selected zones or Intersections. 2. There are two levels of icons in the Zone Filter folder. Level one consists of the main

zones generated in the Make Horizons process. Level two consists of the zones generated in the Make Zones process. When opening a zone folder for one of the main zones the zones are automatically displayed. Each zone can also manually be switched On/Off.

How to use the I and J Intersections

1. I- and J- intersections are intersections along the grid lines. Visualize one of the intersections in the Display window. Note that if you have your zone folders in the Zone Filter open, all zones are displayed in the intersection.

2. If one of the intersections is active (bold font), utilize the "play" tools at the base of the Petrel window. These tools are very useful when stepping through the various intersections in the 3D grid.

For further information on how to use the I- and J-Intersections, seeI- and J- grid intersections.

A display with intersections.

Parent topic: Make Zones and Layering

Volume quality control (Make Zones)

The 3D model should be checked for negative volumes after both the Make Zones and the Make Layering processes. The earlier negative volumes are detected, the less work

will have to be repeated. Since the Make Layering process divides the cells into smaller parts, negative volumes can be present in the 3D grid after the Make Layering process, even if no negative volumes have been detected after the Make Zones or the Make Horizon processes.

Negative volumes can be present in Petrel when the grid has been built on a poor fault model or with poorly defined directions and trends in the gridding process. The most common cause of negative volumes in Petrel is faults that cross each other without being connected or truncated.

For a description on how to check for negative volumes, see Volume quality control.

Flatten Model

After building a faulted 3D model it usually is difficult to see how the formations were deposited as the structural changes have altered the original model. In Petrel, a very useful process has been implemented which enables the user to select a horizon and flatten it by removing fault throws on the horizon. All other horizons in the 3D grid will be changed according to the flattened horizon.

The flattened model will enable you to see the model as it was when the layers were originally deposited. This gives a very good quality control of the model with respect to the depositional environment. This view of the 3D model gives very good quality control of the thickness of the different zones in the 3D grid.

To flatten a model on a particular horizon, right click on the horizon and choose flatten model.

Flatten Model is NOT a reversible process. It is NOT possible to go back to the 3D grid. Always make a copy of the active 3D grid before running this process.

Zone remapping

After importing 3D model data from another 3D modeling software, or import of 3D simulation results, all layers are listed as horizons because the file does not hold any information about which layers are horizons. As a result, the horizons and zones need to be re-arranged to match the original zonation and layering. In Petrel, the Zone remapping process has been implemented which enables the user to remap the zones and layers.

If you need to transfer a model from an external 3D modeling software into Petrel, it is compulsory that you know the layering and zonation for that model.

To remap the zones and layers of a 3D grid, right click on the 3D grid and choose Zone remapping from the context menu. In the dialog window that appears the user can create

new zones and horizons by disabling or enabling horizons from the original horizon layout.

On the left hand side all original horizons are listed with the intermediate horizons (layers).

To stack layers into zones simply select all intermediate horizons for the zone and click

the Disable/Enable button . Only enabled horizons (in black) will be in the Horizons tree of the 3D grid.

On the right hand side is the resulting zone/horizon mapping. Here you can also click on a zone/horizon to rename it. Type in the new name in the field above the list.

After importing data you may end up with a very large list of horizons in your model, and no intermediate horizons. To quickly organize the layers and zones in Zone remapping, start by selecting all the horizons on the left hand side (don't forget to scroll down) and click the Disable/Enable button. This will create 1 zone with a top and bottom horizon on the right hand side. Now, you can make up all the intermediate zones by selecting the pair of horizons on the left hand side that makes up the top/bottom of those zones and again pressing the Disable/Enable button.

The Zone remapping process is also useful if you wish to remove a few layers from within a zone (and not the entire zone) to clean up the 3D model, or before using the Output for the grid (Copy 3D grid) functionality.

This process can be repeated at a later stage should you wish to enable a horizon again.

If you have nested zones, and do something with the zone mapping here, you will lose the sub zonation.