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Tips

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Tips Refining the grid resolutionIt 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 to1. 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 faultingThe 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 to1. 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 modelIf 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 To1. 2. 3. 4. 5. Define the time depth relationship through the wells settings. Create a new well top attribute for the zones corresponding to those in the 3D grid. Sample the velocity log (created during the sonic calibration step) into the zones. Generate a surface from the velocity attribute for each of the modeled zones using the Make/edit surface process. Use the resulting average velocity maps as input to the depth conversion process.

Removing areas in the well logIt 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 logUsing 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 nonreservoir. 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) propertyNtG (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 logInteractively 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 nonreservoir (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

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