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Tips

Refining the grid resolution It is often desirable to have smaller grid cells in the center of the model (or close to the wells) than at the edges. Around the wells the resolution of the data is far higher, and during simulation this is where all the action is. Further away from the center there is much more uncertainty in the model, so small grid cells are not necessary. Increasing the size of cells away from the center will reduce the number of cells in the model, making simulation more efficient.

How to

1. Under the Pillar gridding settings, set the I and J increment to the cell spacing required at the edge of the grid. 2. Create a square of I and J trends in the center of your grid. 3. Define the number of cells on each of the four trends, remember that opposite sides should have the same number of cells. (Use the measuring

tool to estimate how many cells you will need.) 4. As the grid is being compressed in the center, you will also need to increase the total number of cells in the grid. To do this, increase the 'Edge

Growth' option under the expert settings tab of pillar gridding.

5. To generate a grid with more orthogonal grid cells, extend these trends to the boundary and generate a second set of trends along the boundary. 6. Set these outer trends as the boundary and select 'edge of grid is limited by trends and directed faults' under the settings for Pillar Gridding.

Complex low angle faulting The complexity of faulting in Petrel is not limitless, and as a general rule, a fault cannot be truncated by a fault which is itself truncated. However, there are ways of breaking down complex problems to simplify matters. In the case below, a single reservoir unit is split by a complex thrust structure. Because there is no connection between the two units, it is necessary to model them together in order to depth convert the footwall structure. The solution is to model the two reservoirs separately, but use the depth converted bottom horizon of the hanging wall structure to define the first zone of the depth conversion of the foot wall.

The figure below shows a section through the interpreted faults. The thick black line is the main thrust used to divide the model in two.

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The areas above and below the main thrust were modeled separately. In addition, faults with minimal displacement were ignored in order to simplify the gridding process as much as possible.

The final models include most of the complexity of the original interpretation and can easily be depth converted using the same velocity model.

How to

1. Build two separate fault models, one for the hanging wall and one for the footwall. Avoid complex truncations wherever possible. Remember, faults need not be defined above or below the input data for the model you are working on.

2. Build the pillar grid for each model. Make sure the Hanging wall model extends beyond the footwall model (otherwise, you might have problems when it comes to depth conversion).

3. Copy the input data for the horizons, so you have two sets of data, one for the hanging wall and one for the footwall. 4. Build a surface of the main thrust fault you want to use to split the model. 5. Use Operations - > Eliminate where to remove areas of the input data on the wrong side of the thrust for each model. (It can be useful to make

copies of the thrust surface slightly above and below the original to ensure that all extra data is removed).

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6. Build the horizons using the input data. 7. Depth convert the hanging wall model. 8. Export the bottom horizon of the hanging wall model in time (from the original grid) and in depth (from the converted grid), and use these to

create a surface of the average velocity through the hanging wall model. (be aware of the two-way time option in the depth conversion settings). 9. Use the exported time horizon from the hanging wall model to define the first zone for the depth conversion of the footwall model. Use V=V0

and drop in the velocity surface you made into the field for Vo. (Make sure the units agree with those stated in the Settings and again, be aware of the two-way time option).

10. Use any additional surfaces to model the velocity between the last horizon in the hanging wall model, the thrust surface, and the first horizon in the foot wall model.

Using a wells time depth curve for depth conversion of the model If a time depth relationship has been defined for a well via check shots, a time log or calibrated sonic during the Synthetics process, this can be used to extract average velocity information for each zone in each well. These are held on the well tops and can then be used in the Make/edit surface process to create velocity maps for depth converting the model grid.

How To

1. Define the time depth relationship through the wells settings. 2. Create a new well top attribute for the zones corresponding to those in the 3D grid. 3. Sample the velocity log (created during the sonic calibration step) into the zones. 4. Generate a surface from the velocity attribute for each of the modeled zones using the Make/edit surface process. 5. Use the resulting average velocity maps as input to the depth conversion process.

Removing areas in the well log It is often useful to remove certain areas of a well log before upscaling and modeling. For example, if you are using a net to gross property in your model and scale up porosity and saturation, you should remove the areas of the log corresponding to shale.

How to blank out areas in the log

Using the Calculator

1. Generate a new discrete log with two groups, non-reservoir (0) and reservoir (1). Interpret the areas for each of the logs as either reservoir or non-reservoir. See Generating a NtG (Net to Gross) property

2. In the Calculator, under Global well logs enter: New_property=if(reservoir = 0,U,Old_property). Note: U=undefined 3. Display the New_property log in a well section to visualize the result. 4. Then, upscale the New_property.

Using the Log editor tool

The Log editor is a tool that allows you to remove areas in the log. The options available for this process are Clip and Change undefined. For more detailed information on how to perform this processes, see Log Editor.

Generating a Net to Gross (NtG) property NtG (Net to Gross) indicates the percentage of a particular interval which is potential reservoir. On a log scale it is a discrete property, at any one depth in a well the sequence is either reservoir or non reservoir. It is useful to generate this as a discrete log in Petrel so that it can be easily edited in the well correlation panel. However, after upscaling it should be a continuous log referring to the percentage of potential reservoir in each grid cell, e.g. 0.5 for interbedded sand and shale, 0.9 for sand with little shale.

How to generate a discrete or continuous NtG log

Interactively using Well Section:

1. Copy the General discrete template and rename it as NtG_discrete, go to the Colors tab and specify two codes non-reservoir (0) and reservoir

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(1) 2. Open a New well section widow, display the wells and select the Paint discrete log class icon to make enable the Create new discrete log

icon 3. Click on Create new discrete log icon and select the template NtG_discrete. Go to the Global well logs folder and check on the NtG_discrete

log to display it on the Well section 4. Then interpret the areas of each of the wells as either reservoir or non-reservoir by positioning on the empty track for the NtG_discrete log and

select the class (code) to paint 5. Now to create a countinuous log from a discrete log: in the Global well logs for NtG_discrete log go to the Settings\Operations tab\Resample

log points, select the option As continuous log and push the button Resample. This operation ensures that the sampling points in the discrete log will also make sense in the continuos log

6. Then using the Calculator from Global well logs to generate a new continuous log reservoir by choosing a Net/Gross template and type NtG_Continuous=If(NtG_discrete=1,1,0). The new log should look identical to the old log with values of 1 or 0 along its whole length

7. Upscale the new NtG_Continuous log using arithmetic as the averaging method. Upscaled cells will have a value between 1 and 0 depending on the amount of reservoir within the log in the grid cell

8. Display the property in the Well section window together with the original log (NtG_Continuous) and use the color fill

In the figure below is shown the resultant NtG log

Note: In Well section to show the upscaled cells just for the values with Net to Gross, make a property copy and use the Property calculator Copy of NtG_Continuous=If(NtG_continuous=0,U,NtG_Continuous) and display it. Use the Show cell boundaries for properties icon to show or hide the layers on the track.

Using the Calculator with continuous logs as variables:

The NtG log can be calculated by using the Vsh (shale volume)

1. Then using the Calculator from Global well logs to generate a new continuous NtG log reservoir by choosing a Net/Gross template and type NtG=1-Vsh

2. Copy the General discrete template from the Templates pane and rename it as NtG_cutoff, go to the Colors tab and specify two codes non-reservoir (0) and reservoir (1)

3. Open the Calculator from Global well logs to make a NtG_Cutoff (discrete log) assigning the new template and type NtG_Cutoff=If(NtG<0.8,0,1). Note: the cutoff value should be defined according to your reservoir

4. Display the logs (Vsh, NtG and NtG_Cutoff) in a Well Section to compare and quality check

Modeling saturation to take account of geology and capillary effects Gas, oil and water saturation at a point within a model are dependent upon both the elevation at that point (point on the saturation curve) and petrophysical parameters, such as pore size distribution. When interpolating between well logs to generate a saturation model, it is therefore important to take account of both petrophysical effects and capillary effects.

Workflow to modeling water saturation (example)

1. Create a new function to describe the capillary component of the saturation log (for water saturation, this should go from zero at depth to 1 in the HC zone). See Function curves in a folder (Settings)

2. In the Global well logs folder Copy the Saturation logs cutting out the area where the effects of capillarity are affecting the log values. This can be done using the Calculator for Wells

3. Upscale the edited Saturation logs and generate a property through Petrophysical modeling (this is effectively a maximum potential saturation based on petrophysical parameters)

4. Use the Property Calculator to generate a new property based on depth and the function describing the saturation curve

5. Multiply the two properties together so that below the free water level, the water saturation is always 1, far above it, it depends only on petrophysical properties, and in between, it depends on petrophysical properties and the capillary curve

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Controlling zones made from well tops. When building zones using the conformable setting in Make zones, Petrel will interpolate between horizons as realistically as possible. However, you may have other ideas about what the zone should look like and want to edit the result.

How to

1. First try changing the settings used in Make zones e.g. build from base, build from top. The definition of the horizon types above and below the zone (general, erosional etc.) will also affect the way the zone is built, check these too and change them if required.

2. Choose the best model from the ones you have generated and generate an isochore from the zone you are interested in. 3. Display the isochore in a 2D window together with the well tops used to make the isochore. You can now generate a new surface with a

correction to be applied to the isochore. This should be zero at the wells (as the well tops were the only known points the isochore will be correct at these points), and equal to the isochore thickness at points where the thickness should be zero.

4. Use 'Make Polygons' to digitize a point on each well top. 5. Edit this polygon so that all of these points now have a Z-value equal to zero. 6. Digitize additional points where you would like to change the thickness of the isochore and set the Z value equal to the correction you would like

to apply. Each point you digitize will have a Z-value equal to the isochore thickness at that point, so to specify the zone limit (where the final isochore will equal zero) just digitize directly onto the isochore.

7. Generate a surface using the polygon as input and the model extent as a boundary. 8. Copy the original isochore and subtract the new surface from it to create an edited isochore. Remove areas of the isochore with negative values.

Check this and repeat the process if required. 9. Use the new isochore as input in 'Make Zones'.

Making maps for use in facies modeling Using surfaces to control facies and property modeling is a powerful way to control your modeling and ensure that your property distribution agrees with your geological conceptual model.

How To

1. Draw a polygon that extends beyond your model boundary. This will form the boundary for your surface. 2. Digitize points within the boundary using make-edit polygons. This will form the main input for the surface. You can edit the value for each

point using the Z-value selector. 3. Generate a surface using the polygon you created as input and the boundary you drew first. 4. In facies modeling dialog, choose 'surface' as the distribution for the appropriate parameter and drop in the surface you made.

It is also straightforward to generate an angle map to steer the orientation of facies bodies.

1. Generate a boundary as described above. 2. Use make-edit polygons to draw a new polygon object consisting of short lines in the desired orientation (see below). 3. Generate a surface using the polygon as input and ticking 'convert to directional values' under pre processing. 4. Use the map generated as input under 'orientation' in facies modeling.

Well design tips Wells designed for production are often placed close to, but below the top surface of the reservoir - naturally in the area of most oil. By generating a top reservoir surface and volume height map, such wells can be designed quickly in Petrel.

How to

1. Run the volume calculation process and generate a volume height map of STOIIP. 2. Choose the surface representing the top of the reservoir and generate a surface.

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3. Display the STOIIP map on the top reservoir surface. 4. Move the top reservoir surface down a few meters to ensure that the designed well is within the reservoir. 5. Digitize the well directly on the surface.

Enhancing features in a surface Small scale faults picked up by 3D autotracking often go unnoticed when viewing the interpretation, even in 3D. By generating a surface from these points and using user defined light sources, these small scale features can be enhanced significantly.

How to

1. Make the interpretation (3D autotracking is an excellent way of picking out faults). 2. Generate a surface from the interpretation. 3. Insert a new light source (Windows tab, insert new light source), and turn off the Headlight source. 4. Right click on the light source and choose Edit. A widget and arrow will appear with which you can control the direction of the light. If you

haven't moved the camera since you inserted the light source, then this will be directly in front of the camera and the view will be gray. 5. Adjust the direction of the light to enhance the features in your interpretation.

It can sometimes be useful to open up a second 3D window to edit the light source in, that way the light source is always in view, even when you zoom and pan away from the light source in your main window.

The images below show the same surface generated from a seismic interpretation, seen from above. On the left, the surface is lit with the headlight, while on the right, it is lit with a directional light of low angle. Small faults picked up using the 3D autotracking in Petrel, are much clearer when using the directional lighting.

Generating facies bodies with vertical property trends Many of the bodies modeled during the Facies modeling process will have an inherent vertical trend in grain size and, thus, will often have a similar trend with respect to physical properties. Fining and coarsening upwards sequences are common in geology and are often associated to channels and sheet sands.

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By generating a 3D trend property while building the object model, you can use this trend to determine how properties are extrapolated throughout the zone with respect to the facies bodies modeled. It is important to ensure that the upscaled cells for the property do not contradict with the vertical trend. If they do, then it is likely that the "noise" created will hide the trend completely and you will end up with a inadequate final model.

How to generate facies bodies using vertical property trends

1. Build the facies model using Object modeling and generate a Depth trend during the simulation (for example, facies of channels and levee). See Other Output (object modeling).

2. Upscale the logs for the continuous property that you want to model with the Depth trend, for example, porosity. 3. Open Data analysis and select the property representing the logs you have just upscaled. 4. Choose to condition your property model with respect to Facies and select the facies you used when you inserted the bodies in step 1. 5. Insert a 3D trend transformation (see 3D Trend), choose the Depth trend as the trend property, and choose to scale the trend with a linear or

general function 6. Insert a Normal score transformation. 7. Repeat for any other facies which you have modeled with trends 8. Open Petrophysical modeling and choose an algorithm that allows the use of trends (for example, Sequential Gaussian simulation or Kriging). 9. Condition the modeling to Facies and set the settings for each facies.

10. For the Facies that you have defined a vertical trend for, choose to take the trend from Data analysis by clicking the Use the transformation button.

11. Click OK.

Petrophysical modeling condition to Facies Object modeling using Local varying azimuth Local variations in a property within a depositional body will often be orientated along the direction of the body itself. This is especially true of meandering channel systems where high or low porosity areas will follow the channel geometry.

The problem can be addressed by generating an azimuth property while performing object modeling, and then using this property as a local varying azimuth for the variogram in a Sequential Gaussian simulation.

How to use Local varying azimuth with facies

1. While building the Facies model using Object modeling, go to the Other output tab and select the Directional trend, use the Create button to generate a new property to avoid overwriting an existing one. See Other output tab (Object modeling)

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2. Open Petrophysical modeling, select the required property and choose Sequential Gaussian Simulation, as method to model

3. Press the Facies button and select the created Facies Object model to condition the property model

4. Select the Facies type that the Directional trend was created with

5. Go to the Variogram tab, choose Locally Varying Azimuth and select the Directional trend property created in Object modeling

6. Set the Major / Minor Ranges to the required values. Tip: A typical value of Major Range may be up to 10 times the Minor Range for a channel system

7. Go to the Distribution tab and set up the Output data range

8. Click OK to run the modeling

Creating overturned structures When modeling overturned structures it is important to keep in mind that each horizon only can be represented by one node on each of the non-faulted pillars. It might be necessary to use dummy faults to control the orientation of the pillars. The dummy faults can be set to No Faults in the Pillar Gridding process and will then not be included in the 3D grid.

How to

1. Display your structural input data in a 3D Window and digitize some dummy faults. The optimal orientation of the faults is perpendicular to the bends of the overturned structure. Dipping faults may also be placed along the 3D grid boundary. Further, the faults can be used to create segments in 3D grid and it might be useful to have segment boundaries in the center of the bends.

2. Run Pillar Gridding. 3. Run Make Horizons. Under the Horizons tab append new columns in the table and use different inputs for different Segments.

Extracting 2D and 3D trends from your data Identifying and analyzing trends in your data is an important step in data analysis. The existence of a trend in your input data does not necessarily mean that it exists in reality or that it should be honored, but identification is a useful first step. The residual between the extracted trend and the input data should have a mean of zero although this may be dependant upon the distribution of the data.

Trends can be created by simply modeling the data using the Functional Interpolation algorithm with the Point Weighting option set to Equal. With these settings, all of the data is considered together and a function is created that passes through the chosen points. This approach can be used in 2D (Make/edit surface) and 3D (Petrophysical modeling). There are 4 options to choose from to control the complexity of the surface.

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The results are easiest to demonstrate with surfaces but work in the same way with a property:

Plane - Creates a simple plane

Bilinear - creates a bilinear plane (rectangular hyperboloide)

Simple Parabol - Creates a symmetrical parabol

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Parabol - Creates a standard parabol which is not constrained to symmetry

How to create a 2D trend through your data

Your input data must be in a 2D form. If you are working with properties, then you will need single values for each well representing the average. This is easiest to do by creating zone attributes based on the original well data (Calculating Well Top Attributes). If you do not have well tops, then you can generate them based on the model (Report (Wells)).

1. Open the Make/edit surface dialog and set the required size (Grid size and position) and resolution (Grid incremente) for your trend in the Geometry tab

2. Drop your 2D data (e.g. well tops) into the Main Input (Run tab) 3. Under the Algorithm tab choose Functional interpolation as the method 4. Change the point weighting to Equal 5. Choose the type of surface you require next to Fit points to a 6. Press Apply and display it to check the result

How to create a 3D trend through your data

You should have your data upscaled into the 3D grid before you begin. This process will create a 3D trend through that data.

1. Open the Petrophysical modeling dialog, choose the correct property and the correct zone 2. Choose Functional interpolation as the method for the interpolation 3. Set the Output data range to some sensible limits for the trend 4. Change the Point weighting to Equal 5. Choose the type of surface you require next to Fit points to a 6. Press Apply, the result will be a 3D property that tries to match the data points. Upscaled cell values will be unchanged, to overwrite them with

the value of the trend 7. Change the filter to display only the upscaled cells 8. On the Settings tab for the property, go to the Operations tab, choose Set undefined from the Property operations folder 9. Toggle Off the upscaled cells lock icon, so that upscaled cells can be changed

10. Toggle On the filter such that only the displayed cells are affected 11. Press Run 12. On the Settings tab for the property, choose Extrapolate from the Property operations folder. 13. Toggle Off the filter 14. Press Run

Experiment with increasing the vertical range and swapping between Follow layers and Horizontal in the settings of the Functional interpolation algorithm.

Modeling fluvial facies with crevasse Channel objects can be modeled as channels and associated levees. Crevasse splays, which are geologically tied to the channels, do not exist as such when modeling channels in Petrel. Therefore, it is difficult to incorporate crevasse splays objects closely tied to channels during object modeling. However, adding a second "crevasse splay" object to the simulation can give satisfactory results, if done with the proper geometry and rules settings.

Channel-levee-crevasse splays systems

In fluvial depositional environments, the main geological objects are channels running across a floodplain. Sandy sediments can deposit in channel beds, depending on flow energy. Channels can be bordered by levees, mud flow deposits that occur during floods as water flows overbank. At times, the levees can break during floods and water shoots out locally, carrying and depositing sediments in fans, fining from the proximal part (next to the channel) to the distal part (away from the channel.) See Figures 1, 2 and 3.

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Facies modeling of such systems

In Petrel Facies modeling, channels, levees, and crevasse splays related together can be modeled with the object simulation algorithm applying the following rules.

� Put channels first, whatever the parameters are. � Second, insert objects for crevasse splay (See figure 4). � Type: Ellipses, or fans oriented perpendicular to channel direction might be appropriate. In case of fans, two objects with two opposite directions

should be used � Rules - Insert in levees only. Overwrite only background facies (and levees according to the cases). According to the density of channels and the

size of the objects, you may want to keep the entire crevasse objects. � Trends can be applied, if necessary.

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Figure 4.- Rules set for modeling crevasse splay objects (here modeled as ellipses).

The following figures show some results using this method:

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Advantages of the method:

� Honors well data � Honors the distribution settings set in the Data analysis module, in particular the vertical distribution. � Subordinate crevasse splay to channel positions, which makes sense geologically.

Shortcomings:

� In case of isolated channels, one object could appear on both sides of its master channel, which makes no geological sense (Figure 7-1). For several overlapping channels, they could be considered as eroded by more recent channels, whereas it does not apply for isolated channels.

� When using fans, there is no way to select the origin of the fan to be the proximal part, i.e. closer to the channel (Figure 7-2). � No control on where the crevasse splay is tied to the channels (inner or outer bank).

Figure 7.- Possible problems when using fan objects (in two sets: both perpendicular to the channels, one heading west, the other heading east) are illustrated.

Note: This workflow can be applied to oxbow lakes as well. Ooxbow lakes are former dead meanders of a channel that took another course after a flood. As this object is asymmetrical, and as for fan objects, restrictions apply (Figure 8).

Figure 8.- Application of the same method to tie oxbow lakes objects, in purple, to channel objects.

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