Petrel TIPS&TRICKS from SCM

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1 Petrel TIPS&TRICKS from SCM Knowledge Worth Sharing Merging Overlapping Files into One 2D Grid Often several files (grids or data) covering adjacent and overlapping areas must be combined into one 2D Grid. Sometimes the overlapping areas of those files do not match vertically. A common situation involves the interpretation of one seismic event on several adjacent and overlapping seismic surveys. Although the event on each survey is the same, variations in data gathering, processing, velocity modeling, etc. will cause the same event in adjacent and overlapping areas to differ and sometimes by quite a bit. Many techniques have been devised to combine overlapping data or grids. This paper describes a method for combining 2D grids that overlap and have approximately the same vertical position, a method for bulk shifting one grid s vertical position to match the positions of other files, and how to use the Union operation to combine nonoverlapping grids into one. Each technique is useful by itself and is described separately in this paper. Using the three methods in combination allows merging many files with overlapping areas into one 2D Grid. If you are only merging two overlapping files then consider using only steps 1, 2, and 4 of the workflow below. If merging multiple overlapping files, you will want to consider using the entire workflow. Figure: Example showing 2D Grids that overlap one another and have approximately the same vertical position before being merged (left), showing detailed contours of the original grids (center), and detailed contours after being merged (right). The center polygon is the area of primary blending and the area between the center and outside polygon is the transition zone. 1

2 Figure: Example showing 2D grids that overlap one another but have dramatically different vertical positions (left) and after one grid has been bulk shifted to match the other (right). File Merging Workflow The workflow used here to merge a variety of files whether grid or data into one grid is: 1. Build a set of 2D Grids, one for each file or grid to be merged. 2. Trim each 2D Grid to its original input data or grid area 3. Define the merge, shift, and union order 4. Implement the Merge, Shift, and Union processes in the order defined 5. Perform extrapolation and smooth steps as needed to create the final grid. The File Merging Workflow is not trivial and requires many steps. None of the steps are difficult, they are just repeated many times during the merging process. The difficulty relates to managing the steps and files. Petrel Workflows are used for the example in this paper to merge several files. The authors suggest you study these workflows and consider using that approach for complex problems. Figure: Map views of workflow steps used to merge files: 1) Build 2D grids, 2) Trim grids 3) Define merging order, 4) Merge, Shift, and Union 5) Extrapolate and smooth the final grid

3 Build 2D Grid for Each Input File Ultimately one grid will be built which contains information from all input files. To make merging easy and avoid special processing related to combining grids having different X Y limits or grid increments, new grids are built for each file, all having the same X Y limits and grid increments. Use X Y limits large enough to cover all input data and a grid increment fine enough to preserve the detail of importance in the most detailed file. The Make/edit Surface process is used to build these grids. Occasionally, there are top picks available for depth files that are in the correct vertical position. In that case the data will need to be input to the Well Adjustment tab of the Make/edit Surface process (this was not done in this example). Often some data are seismic based and contain noise that needs smoothing. Also, the smoothness of the input files will often vary dramatically. This variation will create a patchwork quilt like look to your merged grid if not properly managed. Consider using the Smooth Operation to lightly smooth noisy input data so it more closely matches other data without destroying the information content of the file. We recommend using an operation to smooth rather than the Post Proc tab of Make/edit Surface because these steps are often placed in a workflow and the Operation can easily be edited to adjust smoothness and the tab is more difficult. Figure: Input file (left) and new grid (right) with minor smoothing. Trim 2D Grids Only portions of the initial grids that are defined by data should have values, the rest of the grid s area should be undefined. The Make/edit surface process can restrict the gridding to an area close to the data but will always make a convex perimeter. Often this results in small extrapolations away from input data. Draw a polygon around each input file with the polygon held close to the edge of the data. Use the Eliminate Outside operation to blank the grid outside this polygon. Look at contours from the grid after blanking. Often the person who built the input file will have allowed the interpretation or original gridding to extrapolate and there will be parts of the original file that also need blanking. Now is the time to clean up edge effects that are in the input data but clearly are not part of the true surface s form. 3

4 Figure: Initial file (left), constructed grid before blanking outside the data edge polygon (center) and after blanking (right). The smooth and blank steps are often placed in a workflow along with the gridding step so that adjustments to smoothing and blanking polygons can be made and the workflow rerun to quickly update the files. The figure below contains a workflow used to perform the gridding, smoothing, and blanking steps for this example. Figure: Workflow used to Build initial grids, smooth them (when needed) and blank them. Note that when the smoothing is not needed it is merely commented out (made inactive). Note also that the smoothing may be done differently for each input file. 4

5 Figure: Parameters used in the Make/edit Surface process of the workflow (left) and the initial and output folders with just two overlapping surfaces (right). Define Merge, Shift, and Union Order In most projects, it is clear which data are best or highest priority and which are of lower quality or lower priority. It is also obvious which files need to be shifted up to match the vertical position of others. We have found it best to merge all files that can be merged first then shift files to the merged or sometimes the original files (if an isolated file), and finally merge the remaining files and shifted files together. Any grids or groups of grids that do not overlap another grid or group are merged with that other grid or group using a Union operation with no changes made to the grids. The three examples below demonstrate the variety of merges that may be encountered. In Example A of the figure below, the file priority has been set (numbers) and number 5 needs a shift. Using the numbers as file names, the processing steps would be: Merge 1 with 2 creating M1_1 2 Merge 1 2 with 3 creating M2_1 2 3 Merge with 4 creating M3_ Shift 5 to creating S4_s5 Merge with s5 creating M5_ s5 In Example B of the figure below, the file priority has been set (numbers) and number 5 needs a shift. Using the numbers as file names, the processing steps would be: 5

6 Merge 1 with 3 creating M1_1 3 Merge 2 with 1 3 creating M2_2 1 3 Shift 5 to and 4 creating S3_s5 Merge with s5 creating M4_2 1 3 s5 Merge 4 with s5 creating M5_ s5 In Example C of the figure below, the file priority has been set (numbers), number 5 needs a shift, and number 2 is not overlapping anyone. Using the numbers as file names, the processing steps would be: Merge 3 with 4 creating M1_3 4 Shift 5 to 3 4 and 1 creating S2_s5 Merge 3 4 with s5 creating M3_3 4 s5 Merge 1 with 3 4 s5 creating M4_1 3 4 s5 Union s5 with 2 creating U5_1 3 4 s5 2 Before doing any merge or shift processing this ordered list of steps should be created. Then, it becomes a mechanical process of executing the steps. Figure: Map view of three examples of files to be merged with their priority order listed. In each case number 5 needs to be shifted vertically to the files that overlap it. A B C The figure below shows the map position of files being merged in this paper and what their priority order is. The merge order that will be used is: Merge 1 with 2 creating M1_1 2 Shift 3 to M1_1 2 creating S2_s3 Merge M1 1 2 with S2_s3 creating M3_1 2 s3 Union 4 with M3_1 2 s3 creating U4_4 1 2 s3 6

7 Figure: Files to be merged in the example being worked and their priority order. Note that Area 3 is not in the correct vertical position. Implement Merges, Shifts, and Unions Merges are used to blend the overlap regions of two overlapping files and result in both files being placed in one grid. Both files are assumed to be at the correct vertical position. Shifts are used to move one file s vertical position to match another s. A shift is done to a file before it is merged with other files. Normally the shifted file is then merged with the files used to shift it. Unions are used for files that do not overlap other files. These files are assumed to be at the correct vertical position. Values from their nodes are simply placed in the other file s grid. If a vertical correction is needed for one of these files then that correction should be performed prior to the Union using whatever technique works. These three methods are described in detail below. Merge Two Grids Given two grids that a) represent the same surface, b) overlap in some area, and c) are at approximately the same vertical position, then these two grids can be merged together easily into a single 2D grid. The resulting grid will contain values from the original files in areas away from the overlap region, will be a blend of the two files in selected portions of the overlap and will transition smoothly from the non overlap regions to the selected portions of the overlap region. The process involves many steps and is best done using a Workflow. In the Merge Two Grids procedure, one of the grids is defined as the winner and the other the loser. The winner is the grid you have more confidence in and is output in much of the area where the two overlap. The steps in the Merge Two Grids process follow. 1. Create a polygon along the edge of the winner grid where it overlaps the loser grid and call it the minus polygon. Include a 4 to 5 node wide band along the winner grid that also covers the loser grid. 2. Subtract the two grids from each other (winner loser) 7

8 3. Blank the difference grid outside the minus polygon created in step 1 above. Figure: Winner grid (red) overlapping looser grid (green) with polygon defining the overlapping edge nodes of the winner (left), the two grids have been subtracted and then blanked outside the polygon (right). 4. Create a grid that is all zeros and has the same grid increment and X Y limits as the other grids. Normally you will create this grid once (a template grid) and copy it for each merge. 5. Create a polygon named adjustment that encompasses some or both the winner and loser grids outside the minus polygon of step 1. This area will represent the transition zone where each grid will gradually move from its vertical position outside the adjustment polygon to the position of the difference grid area (minus polygon of step 1). 8

9 Figure: The adjustment polygon (larger) outside the minus polygon (smaller) and covering some of both the winner (red) and loser (green) grids. This is the transition area for the merge. Outside this area, the winner grid will be output where it exists and when it doesn t exist the loser grid is output. Note that the polygons always outline the winner grid edge, not the loser grid edge. 6. Blank the zero grid inside the adjustment polygon. 7. Combine the un blanked nodes from the difference grid and zero grid using the A Union B Keep A operation. Figure: Combined difference and zero nodes making the adjustment grid. It now needs to be filled and adjusted by the winner ratio and 1 winner ratio. 8. Fill the gap between the difference values and the zeros using the Extrapolate operation. 9. Multiply the filled difference grid by the winner ratio (value between 1.0 and 0.0) that defines how much of the difference is assigned to the winner grid, creating the winner correction grid. Commonly the winner ratio is.50 but sometimes it is set to 0.0 to preserve as much as possible of the winner grid form (zero adjustment). 10. Multiply the filled difference grid by (1 winner ratio), creating the loser correction grid. 9

10 Figure: Winner correction grid created by filling the gap between zeros and difference values and multiplying by the winner ratio (.5 in this case). 11. Subtract the winner correction grid from the winner grid 12. Add the loser correction grid to the looser grid 13. Use the A Union B Keep A operation to combine the adjusted winner and adjusted loser grids into one grid. Make the winner the A grid of the operation. Because of the large number of steps, it is best to use a workflow to do this, allowing fine tuning and rebuilding. The authors generally build a separate workflow for each merge. That way the flow can be copied and only minor edits made to change the input for each merge. The grids built by each merge are prefaced with the letters M1 if it is the first merge, M2 for the second, etc. This makes it easy to link the files to the workflow. A copy of the workflow used to perform the steps above is shown in the figure below. Figure: Winner and loser grids before merge (left) and after merge (right). Note that in this example, no smoothing was done on either grid and there is a significant difference in texture between the two. Later examples will have more smoothing of the lower left data. 10

11 Figure: Workflow used to merge the two grids shown above together. Step numbers refer to the example above. Steps 1, 4 and 5 were done outside the workflow and the results of those steps are referred to in variables C, A and D respectively. 11

12 Shift Grid Given one grid that a) is vertically in the wrong position and b) overlaps one or more files (grids or data) that are in the correct vertical position, then that grid can be shifted to match the vertical position of those other files. The resulting grid will have its original shape away from the overlap regions but be shifted vertically. In areas of overlap the grid s form will warp to the form of the files it is being shifted to. The amount of warp depends upon the number of overlapping grid nodes that are used for the shifting process. The Shift Grid process involves several steps and is best done using a Workflow. The steps in the Shift Grid process follow. 1. Convert to data any grids that are to be used to shift the primary grid. When doing this, decimate the grid points significantly (e.g., output every 200 th, 500 th, or even 1000 th point). 2. If they exist, convert top picks that must be honored to points (right click on the tops name and convert to points, you may need to then convert the Z value attribute to points to make a useable file). 3. Use the Append operation to concatenate all grid to data points and the original top picks (for the surface being processed) into one point file. 4. QC (view) the points to ensure that surfaces and data are represented (except the grid to be shifted). 12

13 Figure: The green surface needs to be shifted down to the red surface (left). The appended points (red dots) are used as data to globally tie the green surface to the red one s position (right). 5. Use the Make/edit surface process to tie the primary grid to the appended points. a. Use the primary grid as the main input b. Use the primary grid as the boundary input c. Use the Convergent Interpolation algorithm (since the nodes being calculated and the input data are at the same position nothing is changed). d. For Geometry, check the button in front of Automatic (from input data/boundary) and make sure the grid increment is correct (the same as the input grid, something that is not automatically done for you) e. For Well adjustment set Well tops to use the appended points, do a Global adjustment, and use no Influence radius. Figure: Green surface shifted to match the red data (left), the green is the surface before shifting and yellow is after shifting (right). 6. Rename the resulting grid appropriately as it is the shifted version of the primary input grid. 13

14 Figure: Workflow used to shift Area 3 to M1_1 2 grid position. The Step numbers in the workflow refer to the steps above. 14

15 Figure: Parameters used in the Make/edit surface process to shift the input grid to the appended points. The shifted grid is now ready to merge with the grids that overlap it using the Merge Two Grids process. Union Two Grids Given two grids that a) are in the correct vertical position and b) do not overlap anywhere they are defined, then these grids can be combined into one grid using the Merge A Union B, Keep A operation. The resulting grid will contain all the defined nodes from each grid. Because each grid is identical (in an earlier step we forced these grids to have the same geometry), this merge will work correctly every time. If the grids are significantly different in vertical position and close together, then adjustments may need to be made to one or both grids before the Union operation is applied. 15

16 Figure: Two grids (M3_1 2 s3 and Area 4) combined using the Union operation. See the figure below this one for the workflow steps. Clean up Final Grid Once the grids are merged the processing could be considered complete, however, often it is desired to fill in holes between the file patches and sometimes to smooth specific areas of the grid. Filling holes is done using the Extrapolation operation inside a polygon (usually one big polygon surrounding the area of the grid where the surface is desired and including the existing grid nodes). The Smooth operation is also done within a polygon or inside several polygons that are all in one file. The trick is that the polygon must be turned on and the operation performed inside that polygon (see workflow below). 16

17 Figure: Workflow showing the last merge step (actually a union) and extrapolate and smooth steps used to clean the final grid. Note the smooth step was not applied but is included for completeness. Figure: Final merged grid with the holes filled (left) and the original patches with the filled portions showing through (right). Note that in the right figure, the final grid was shifted down 50 feet temporarily so that the patches would show above it, which is why the contour position and colors vary between the two pictures. 17

18 File Management File management is important because of the number of steps and files that are involved. The authors keep files of similar type in the same folder. We have also adopted the convention of having one merge area folder that contains a set of folders, one for each merge that is performed. Often the last merge folder will also contain the final extrapolated and smoothed grids. The directory structure used for this exercise is shown in the figure below. Figure: Data folders used in the merge process described above. 18