SCM Mapping Workflow Feb 09[1]

K n o w l e d g e W o r t h S h a r i n g

Petrel Tips & Tricks from SCM, Inc.

Petrel is a mark of Schlumberger

4801 Woodway Drive, Suite 150W • Houston, TX 77056 • www.scminc.com • [email protected]

Copyright © 2000-2009 SCM, Inc. – All Rights Reserved

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The Mapping Workflow in Petrel Many who move to Petrel from another mapping program are confused as to how to do in Petrel the functions they did in that other program. The Mapping Workflow is a common activity that must be done and yet is not intuitive to new Petrel users. Petrel has all the tools to execute the Mapping Workflow and those tools work very well. Learning which tools to use, where those tools are located, and in what order to execute them is the Petrel learning curve. This Tips & Tricks article describes what the Mapping Workflow is and walks you through the main steps of that workflow. The article is too short to go into detail on all aspects of the Workflow. You can learn those details through trial and error, by asking other users, or by taking SCM’s Mapping Workflow course. The author’s hope this brief introduction will jumpstart your Petrel mapping experience and provide the foundation you need to take advantage of further training, regardless of where you get it.

The Mapping Workflow

The term Mapping Workflow means different things to different people. Mapping Workflow as used in this document starts with structure data (tops, seismic events, digitized contours…) and zone‐average petrophysical data, carries that data through the mapping process, and ends with volume calculations. The Mapping Workflow described here does not focus on data generation, on building polished base, contour, or property maps, nor on the generation of polished cross sections. The steps in the Mapping workflow are:

1. Import or create data 2. Build 2D structure Grid for each horizon 3. Build structural framework (3D Grid) 4. Build average 2D petrophysical Grids for each zone 5. Build petrophysical properties (3D Grid) 6. Create fluid contacts 7. Calculate volumes

Figure: Graphic images from Petrel showing the Mapping Workflow.

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Import or Create Data

Data used in the Mapping Workflow are typically related to culture, structure surfaces, or petrophysical properties. The procedures used to import ASCII files or to transfer from a data base using Open Spirit are not discussed here. You will need to refer to the Petrel Help Manual or to a Petrel Introductory or Mapping Workflow course for import details.

Culture

Culture data usually consists of polygons representing leases and features such as roads, streams, shore lines, pipe lines, platforms, and buildings. Sometimes the polygons have Z‐values but often they do not, in which case a value of 0.0 is automatically assigned by Petrel. The polygon’s Z‐values can be reset at any time using an operation that assigns a constant or snaps to a surface.

The Make/edit Polygons process can be used to create polygons. This is often done when bitmaps, showing culture features, are available but polygon files are not. The bitmaps are imported into Petrel and the polygons digitized from those bitmaps. Methods for digitizing from a bitmap are described in the December 2008 Tips & Tricks article.

Figure: Lease polygons are typical culture features used in the Mapping Workflow.




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Figure: Bitmap imported and displayed in Petrel (left) and the digitized fault block polygon within which volumes are calculated (right).

Structure Data

Structure data relate directly or indirectly to the horizons being modeled. Data relating directly are top picks from wells, interpreted seismic events (time or depth), and digitized contours. Indirect data include fault polygons or sticks usually from seismic interpretation and fault cuts from wells.

Top picks and fault cuts are sometimes interpreted in other programs. They can be imported into Petrel as X‐Y‐Z point files or as points with attributes, in which case there can be many Z‐values linked to one X‐Y location. More commonly, the tops are imported and linked to well bores. To do this requires that the well bores exist in Petrel. Creating well bores involves:

1. Creating a wells folder.

2. Importing the Well Headers which contains: X‐Y, well name, KB and other support information about the well.

3. Importing the deviation survey for the well.

Once well bores exist then a tops folder is created and the tops, fault cuts, or both are imported. Critical parameters for this import are the well name (exactly as the well bore is named), the name of the surface or fault, the measured depth (MD) and (optionally) the data type (horizon or fault). MD is almost always used rather than X‐Y‐Z (another choice for importing tops) since MD will link the top to the well bore, while X‐Y‐Z data will force the pick to be located in that position regardless of whether the well bore actually passes through the location.




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Figure: Points as a single X‐Y‐Z file (left), with multiple attributes (center), and linked to a well bore (right).

Tops and fault cuts are often picked in Petrel. To do this requires that logs have already been imported into Petrel. The logs are displayed in a Well Section window and the desired tops or cuts named and picked using the Make/edit well tops process. The picks can easily be reset to a different top pick or fault cut using the Well Tops spreadsheet.

Figure: Tops and fault cuts displayed in the Well Section window in which they were interpreted (left) and the Well Tops spreadsheet used to QC, edit, import, and export the tops from one or many wells.

Seismic horizons can be interpreted in either time or depth within Petrel. Often these data are interpreted in other programs and moved into Petrel. Sometimes grids are built from the interpreted seismic data and those grids brought into Petrel. The authors have found it is best to bring both the original seismic interpretation and the grids built from the interpretation into Petrel. This way fault gaps in the original data can be seen and used to check fault polygons and the structure grid can be recreated in case the original grid was overly smoothed or the wrong increment used.

Fault polygons, sticks, and cuts can be brought into Petrel. All are useful for building fault models in the Petrel Modeling Workflow but generally only fault polygons are used when working in the Petrel Mapping Workflow. If the seismic interpretation is fully picked and terminates cleanly at fault gaps then fault polygons are not really needed to build a reasonably good quality grid of the surface. The grid will walk down the fault face like a very tight membrane stretched over the seismic interpretation.




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Figure: Fault polygons imported and displayed in a 3D window with seismic interpretation (left), a 2D Grid built using the polygons and the seismic interpretation (center), and a 2D Grid built using only the seismic interpretation (right).

Digitized contour data are used to build both structure and petrophysical 2D Grids. The data may come from another program or be created in Petrel. A file of contours contains many lines. The lines have Z‐values and the Z‐values for one line are all the same. The Make/edit polygons process is used to create contours (actually polygons with constant Z‐values) in Petrel. The method used to create and edit digitized contours in Petrel differs significantly from that used in other programs. See SCM’s September 2008 Tips & Tricks for hints and methods for digitizing and gridding contour data.

Figure: Digitized contours (left) and the grid built from them (right). Note that the points on the contours are widely spaced. This spacing is important when working with contours in Petrel to allow easy editing but does not impact the quality of the grid.

Petrophysical Data

Petrophysical data representing the average petrophysical value at the location where a well penetrates the zone can be calculated in Petrel or by other programs and then imported into Petrel. Usually the calculated value is stored at the location where the well penetrates the top of the zone for which the value is being calculated. If these values are calculated in Petrel they are stored with the top picks as zone attribute data. They can be extracted from the Tops file as a separate point file for each zone. If these data are moved into Petrel from another program they will likely be X‐Y points with one or more Z values per point.




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Figure: Petrophysical data in the Tops file displayed in a spreadsheet (left) and as a points file (right).

The creation of zone average petrophysical values in Petrel is not intuitive and would be a good subject for a future Tips & Tricks article. The general approach is:

1. Have a log of the property to be averaged

2. Have a tops file containing tops between which the zone average values are to be calculated (be sure there are no missing tops)

3. Create a new attribute in the Tops file (continuous or discrete depending on what you are calculating)

4. Go to Attribute operations tab and calculate the value:

a. Check the radio button To the zones at level:

b. Check the radio button Sample from : Well logs

c. Select the Log to be used:

d. Select the Averaging method:

e. Adjust other parameters as needed

f. Click on the Run button

5. Rename the attribute if desired

6. QC the values in a Well section




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Figure: Attribute operations tab used to calculate zone average values (left) and Well section showing the original log and calculated average value (right).

Build 2D Structure Grid for each Horizon

2D structure grids are built for each horizon to be modeled. The Make/edit surface process is used to build these. Several data scenarios are used for this work and some of the most common are summarized below. Release 2009 of Petrel has new functionality that allows several files representing the same surface to be blended together by the Make/edit surface process with each file being given a different weight.

Seismic Interpretation and Tops

Seismic interpretation in depth is gridded and that grid tied to top picks in one pass of the Make/edit surface process. Seismic interpretation data is the Main input. The Algorithm tab parameters control building the grid and are usually allowed to default. The Geometry tab controls the X‐Y increments, rotation, and X‐Y limits and can be automatically set using the input data. The Well adjustment tab allows top picks related to the seismic data to be used to tie the grid. The influence radius for the correction can be controlled and the calculated error data and error grid output along with a report to understand how closely the original seismic was tied to the tops.




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Figure: Gridding seismic interpretation and tying to top picks: General parameters and Algorithm tab parameters (left), Geometry tab parameters (center), and Well adjustment tab parameters (right).

Structure Grid and Tops

Often a structure grid will have been built in another program or built in Petrel and then edited. In either case the grid may or may not tie to the top picks for that surface. It is easy to use the Make/edit surface process to tie the grid to its top picks. The grid is the main input, the algorithm can be allowed to default (Convergent interpolation) or the resampling algorithm used, the geometry is made to match the input grid or altered if desired, and the Well adjustment tab used to point to the top picks.

Figure: Tying an existing grid to top picks while using or changing the existing grid geometry: General parameters and Algorithm tab parameters (left), Geometry tab parameters (center), and Well adjustment tab parameters (right).




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Digitized Contours and Tops

Digitized contours are often used to precisely define the form of a structure surface. Digitized contours can be gridded and that grid tied to top picks in one pass of the Make/edit surface process (Sept. 2008 Tips & Tricks). The digitized contours are the Main input. Algorithm tab parameters control building the grid and are usually allowed to default. The Pre processing tab is used to resample the digitized contour points from a very sparse spacing (needed for quick editing) to a very tight spacing (needed to ensure the line form is honored by the grid). The Geometry tab parameters are adjusted to be reasonable for the input data. The Well adjustment tab allows top picks related to the digitized contours to be used to tie the grid.

Figure: Gridding digitized contours and tying to top picks: General parameters and Pre Processing tab parameters (top left), Algorithm tab parameters (center right), original contours and tops (top right), Geometry tab parameters (bottom left), Well adjustment tab parameters (bottom center), and constructed grid, contours, and tops (bottom right).




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Often, the digitized contours are edited using the Make/edit polygons process after the initial grid has been created. When editing is done the grid can be updated by simply right clicking on the grid and selecting Regenerate. This will rebuild the grid using all the original parameters and avoids having to open the Make/edit surface process.

Figure: Edited contours in blue (far left), Regenerate option selected as a means to rebuild a grid when data content changes but the parameters and names of the files do not (left center), grid before regenerate (right center), and grid after regenerate (far right). Regenerate avoids opening the Make/edit surface process just to rebuild the grid with the same parameters.

Mix of Contours, Seismic Interpretation, and Tops

Often several data sets are needed to fully define a structure surface. For example seismic may cover a part of a surface, hand drawn contours may be needed to define the rest of the surface, and well tops need to be tied to. This little scenario is best handled by building a small workflow (see workflow in September 2008 Tips & Tricks). You would create the main input to the Make/edit surface process by copying the contours, refining by spline interpolation (add more points to contours), converting to points, and appending the seismic data with the contour points. From that point on the same process that was used to grid seismic interpretation and tops above is used. The example in the figure below is for sand thickness and adds an additional modification step for the point data (eliminate zero valued data) but is basically the same. Note that the files are always copied before they are changed.




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Figure: Portion of a workflow used to merge digitized contours with points (example is for thickness data).

Use of Faults When Building Grids

Often fault polygons are available for use when building grids. These polygons may or may not have Z‐values that relate to the surface being constructed. The Faults are input to the Make/edit surface process by highlighting the file name and then

clicking on the to the right of the words Fault center lines/polygons. The parameters for controlling how faults are used are found on the Algorithm tab Convergent interpolation Settings sub‐tab and on the Expert sub‐tab. Faults are not used by any other algorithm. Four parameters influence fault use:

Influence – A range of 1% to 100% is the percent of the gridding iterations that use the faults. The early iterations will not use the faults while later iterations will, which allows the regional form to carry across the fault and the local form to be disrupted by the fault. This parameter does not work in Petrel 2008 but has been fixed in Petrel 2009.

Use Z‐values – If the fault has values that represent the surface then these can be used during gridding when this parameter is checked.

Fill inside – The fault gap associated with closed fault polygons is filled when this parameter is checked.

Specify initial coarsening factor – This is set to a small multiple of the grid increment (e.g., 4 times the grid increment) and defines the starting grid increment. If not set, the Influence parameter will not work. Again you will not see any impact using this parameter in Petrel 2008 because the Influence parameter is not working but should work in Petrel 2009.

Often, when using seismic data, fault polygons are not used. The project goal is usually volumetrics which requires the surface be filled in the fault gap. Not using fault polygons will fill the gap left in the interpretation when the grid is built. If data are sparse then fault polygons are often used as constraints during the gridding process. In this case the faults are usually filled inside during gridding. Whether or not to use fault Z‐values depends upon whether the values represent the surface or have some other value (e.g., zero).




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Figure: Make/edit surface parameters used when gridding with faults. General parameters and Algorithm tab Settings sub‐tab parameters (left), Algorithm tab Expert sub‐tab parameters (center), and resulting grid (right).

Baselap and Truncation Relationships

When structural surfaces intersect due to baselap or truncation it is best to allow the surfaces to cross one another, in fact it is desirable. Crossing means to allow the surface that “doesn’t exist” to project past and stay past the other surface. In the next step of the Mapping Workflow, 2D horizon Grids will be intersected with one another. The surfaces input to this step should be made to cross. If instead they are made coincident by performing an operation, then the tool that combines all the surfaces into one framework may, due to re‐interpolation, create a pair of surfaces that are almost coincident but not quite and that crisscross back and forth slightly.

Figure: Surfaces crossing at a truncation showing how the truncated 2D Grid is allowed to cross the truncating 2D Grid (left) and what sometimes happens when the two are made coincident too early in the Mapping Workflow (right). In this case the truncated grid was edited slightly before being linked with the truncating structures and this allowed the two surfaces to separate in the area of truncation.

Build Structural Framework (3D Grid)

The Mapping Workflow has not traditionally been thought of as a 3D Modeling process. However, Petrel has a number of tools used for 3D Modeling that can be used for 2D Mapping. These tools make the incorporation of geologic relationships, generation of isochores and displays, and calculation of volumes easy and quick (hours instead of days). To use these tools requires that the 2D Mapping elements be moved into the 3D Modeling world. Petrel has made it easy to do this using the Make simple grid process.




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The 2D structure Grids built earlier in the Mapping Workflow are linked together into a structural framework using the Make simple grid process. How this is done and the parameter controls for doing it are described in detail in SCM’s June 2008 Tips & Tricks. To quickly summarize that document, you do the following steps:

1. Create 2D structure grids that all cover the same area, have acceptable geologic form, and cross in areas where baselap or truncation occur.

2. Open the Make simple grid process.

3. Insert the 2D Grids in top down stratigraphic order

4. Define their geological relationships

5. Use one of the grids to define the X‐Y limits and grid increments to use

6. Build the 3D Grid (structural framework)

Figure: Parameters of the Make simple grid process used to build the structural framework.

Figure: The independent 2D structure Grids (left) and the horizons linked in a 3D grid and cut by a general intersection (right).




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Once in a 3D Grid, cross sections can be generated in any direction through the framework, isochores can be generated as 2D Grids for each zone with a push of a button, and gross rock volumes are calculated in a matter of minutes.

Build Average 2D Petrophysical Grids for Each Zone

Often in the Mapping Workflow a constant average petrophysical value is used for an entire zone. This allows you to move quickly into the volumetrics portion of the workflow. Statistics on all log values in all wells for a zone are calculated and the average used (if net‐to‐gross is used then care is needed to ensure that porosity and saturation logs used for averaging represent only net, otherwise a double dipping effect will be seen and result in lowered volumes).

In projects where petrophysical properties vary laterally across the field, zone average grids are commonly used. Building these grids is similar to building structural grids except you have some additional parameters that are used. The input data are typically zone average points from a Tops or point file or they are zone average contours, or they are both. For this discussion average values in a tops file will be used to build the zone average grid. The general approach to build a zone average porosity grid using the Make/edit process is:

1. Highlight the zone name from the Stratigraphy folder in the Tops file and arrow it into the Main input: parameter.

2. Select the Attribute to be gridded.

3. Because Petrel will name all grids built using this approach the same (for the attribute) you must check the box in front of Name: and enter a unique name for the grid (the authors have always considered this a bug in Petrel and perhaps in some release it will be fixed).

4. Go to the Geometry tab and set the parameters as desired.

5. Go to the Algorithm tab and adjust the parameters.

a. Usually use the Convergent interpolation algorithm

b. Usually check the Maximum value: and allow it to go +10% of input data

c. Usually check the Minimum value: and make it to go ‐10% of input datta

6. Go to the Post proc tab and adjust the parameters.

a. Usually alter the Min Z‐value: parameter to be Truncated and set the value to 0.0.

b. Sometimes alter the Max Z‐value: parameter to be Truncated and set the value to a reasonable upper limit (e.g., 1.0 for net‐to‐gross).

Using these parameters allows the grid to extrapolate (in Z direction) some but prevents it from violating reasonable limits for the type of data being gridded.




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Figure: Parameters used to build a zone’s porosity grid.

Figure: 3D View of the porosity grid with data posted. Note that a little transparency has been added to see the bottom portions of the text.

Build Petrophysical Properties (3D Grid)

If all the structure and petrophysical grids for each zone are used as individuals then a considerable amount of work is required to combine structures with fluid contacts to create isochores and then to discount those isochores by N:G, Porosity, 1 – Sw, and so on (volume processing). Each of these operations is prone to errors and each set of operations must be performed for each zone, a time consuming process. If these structure and petrophysical elements are combined in one 3D Grid then Petrel automatically handles all the volume processing.

Since the structures were linked together in a 3D Grid in a previous step, it is a simple process to link the zone average petrophysical grids to the zones of that 3D Grid. This is done using the Geometrical modeling process:




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1. Make sure the correct 3D Grid is active

2. Open the Geometrical modeling process

3. Check Create new property radio button

4. Set Select method: equal to Constant or surface in segments and zones

5. Select the Template that matches the property you are building

6. Uncheck Same for all zones

7. Check All surfaces

8. Highlight the 2D petrophysical Grids and use the blue arrow to insert them

9. Click OK to create a petrophysical property.

Figure: Parameters used to create the petrophysical property (left), location in the data tree of the Models tab where the property is stored (center), and 3D fence view of the resulting property (right).

Create Fluid Contacts

Fluid contacts start out as either constants (if horizontal) or 2D Grids (if tilted). These contacts are linked to the 3D grid using the Make contacts process. You will need to determine the values to use or build the 2D Grids prior to calling this process. The steps in creating a fluid contact set are:

1. Make sure the correct 3D Grid is active

2. Open the Make contacts process

3. Select or add the desired contact

4. Set the Contact type:

5. Enter the Contact name:

6. Insert the constant or grid representing the contact (note: it can vary for each zone, you did not use faults so it cannot vary by segment)

7. Click OK to build the contact.




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8. The contact is stored in the 3D Grid on the Models tab.

Figure: The parameters used to build the fluid contacts (top left), the contacts draped over one of the horizons (top right), and the contacts displayed in cross section (bottom).

Calculate Volumes

Volume calculations are easy in Petrel because you have built all the files and they are linked in the 3D Grid. Use the following steps to calculate volumes.

1. Open the Volume calculation process

2. Check Create new case and enter a name with no spaces or special characters

3. Select the 3D Grid you want to calculate volumes for

4. Walk through the sub‐tabs associated with the Properties tab

a. Contacts sub‐tab

i. Check whether working oil, gas, or both

ii. Highlight fluid contacts and enter them using blue arrows

b. General properties sub‐tab




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i. Select the Net/Gross property or enter a constant for all zones

ii. Select the porosity property or enter a constant for all zones

c. Oil properties sub‐tab

i. Select or enter desired values for Saturations (Sw and Sg),

ii. Select or enter desired values for Surface conditions (Bo, Rs),

iii. Select or enter desired values for Recovery factor (REC)

d. Gas properties sub‐tab (same process as Oil properties sub‐tab)

5. Walk through the sub‐tabs associated with the Results tab

a. Output sub‐tab

i. Don’t usually check the Make property items

ii. Often check a few of the Make volume height map (grids) and set the grid increment appropriately

iii. Check the box in front of Make spreadsheet report

iv. Click on the Report settings button

1. Under the Cases select what volumes are to be reported and number format

2. Under the format specify how the report is to look

b. Facies sub‐tab is not used in the Mapping Workflow

c. Boundaries sub‐tab

i. Enter the lease polygons if you have any

6. Click Apply button to save the parameters with the case name

7. Click Run button to calculate volumes

The results are printed to a report and written to the case. The case is stored in the Cases tab in the Petrel explorer. You can open the case at any time, right click on Volume calculation and select Make volumetric report to have the report regenerated with different formats, etc. The requested thickness grids will be in the Input tab in a folder named for the case.




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Figure: Parameters used to run Volume calculation process.




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Report: Volumes report for this example.

Petrel 2008.1 Schlumberger User name dhamilton Date Monday, March 02 2009 15:49:37 Project HGF.pet Model Class Model Grid 3D grid Input XY unit m Input Z unit m HC intervals Includes oil and gas interval. Gas oil contact: Gas oil contact Lower oil contact: Oil water contact General properties Porosity: Zone‐BCU (Porosity) Net gross: 0.80000000 Properties in gas interval: Sat. water: 0.20000000 Sat. gas: 1‐Sw‐So Sat. oil: 0.00000000 Bg (formation vol. factor): 0.00800000 [rm3/sm3] Rv (vaporized oil/gas ratio): 0.00000000 [sm3/sm3] Recovery factor gas: 0.80000000 Properties in oil interval: Sat. water: 0.20000000 Sat. oil: 1‐Sw‐Sg Sat. gas: 0.00000000 Bo (formation vol. factor): 1.23000000 [rm3/sm3] Rs (solution gas/oil ratio): 535.00000000 [sm3/sm3] Recovery factor oil: 0.52000000 Boundaries used Project Boundary Case Bulk volume[*10^6 m3] Net volume[*10^6 m3] Pore volume[*10^6 rm3] HCPV oil[*10^6 rm3] HCPV gas[*10^6 rm3] STOIIP[*10^6 sm3] GIIP[*10^6 sm3] Recoverable oil[*10^6 sm3] Recoverable gas[*10^6 sm3] Folder Mapping_example 5595 4476 615 363 129 295 174175 154 139340




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Totals all result types Zones Zone 1 1862 1490 138 81 29 66 38940 34 31152 Zone 2 2044 1635 271 160 57 130 76833 68 61466 Zone 3 1689 1351 206 122 43 99 58401 52 46721 Boundaries Project Boundary 5595 4476 615 363 129 295 174175 154 139340 Detailed results Zones Boundaries Bulk volume[*10^6 m3] Net volume[*10^6 m3] Pore volume[*10^6 rm3] HCPV oil[*10^6 rm3] HCPV gas[*10^6 rm3] STOIIP[*10^6 sm3] GIIP[*10^6 sm3] Recoverable oil[*10^6 sm3] Recoverable gas[*10^6 sm3] Zone 1 1862 1490 138 81 29 66 38940 34 31152 Project Boundary 1862 1490 138 81 29 66 38940 34 31152 Zone 2 2044 1635 271 160 57 130 76833 68 61466 Project Boundary 2044 1635 271 160 57 130 76833 68 61466 Zone 3 1689 1351 206 122 43 99 58401 52 46721 Project Boundary 1689 1351 206 122 43 99 58401 52 46721

Figure: Some displays generated by the Volume calculation process. Note that although the template says volume the grid actually represents thickness.

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SCM Mapping Workflow Feb 09[1]