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When we start to use our computer modeling software to generate a contour, the first question always arises as to what are the appropriate choices for my circumstances, or do I just use the software's default suggestions. We are going to look with this article at the selection of grid size and what effect that can have on your contour results and its interpretation.
The grid size is the distance between interpolated grid node values in both the X and Y direction. Rectangular grids have different X and Y grid dimensions. The grid size determines the number of rows and columns in the gridded surface. Smaller grid sizes produce more rows and columns resulting in larger grid file sizes and slower contouring. The grid size should be such that it is no larger than the smallest closure required to be seen on the contour map.
During gridding, most competent industry software will compute a grid size based on the well data distribution. This is a good selection for initial maps, and in general will suffice for all mapping. Most software will allow the user to set the number of rows and columns to set an approximate square grid size, as well as to set the grid size to match that of another grid file. This selection is important if "grid-to-grid" operations are to be performed. Grid-to-grid operations require each grid to have the same number of rows and columns.
The optimum grid size would exactly match the data spacing, if the data spacing were evenly distributed. The rule of thumb is half the data spacing.
We also want our contouring software to honor the data as no one wants to see a contour line on the wrong side of a posted well value. The grid size is a major factor in determining whether the contours will honor the wells or not. The problem is magnified when the wells are highly clustered.
The first graphic we show is one where we have chosen to set the grid size to as large as possible (690 x 690), which is an example that comes from Vance Hall and Jewel Wellborn, who are my trusted geologic experts in advanced computer-generated mapping techniques.
The second graphic is one where we have chosen to set the grid size to as small as possible (173 x 173).
Notice that the smaller grid size clearly honors the data, while the larger grid size averages through the data while smoothing the contours. While a decrease in grid size may honor the data, too small of a grid can take a significant amount of time for the computations.
You can improve honoring the data without reducing the grid size by using a technique called flexing, that adds a second step to the gridding process in which the original surface is smoothed in areas of sparse data control and the grid is forced to "fit" the well control. Flexing results in smoother contours while still honoring the original data points.
Most industry software flexes a grid by applying a minimum curvature algorithm to the original data points and samples from the grid produced by the initial gridding method. The "flex factor" controls the decimation of the original grid. For example, a flex factor of 4 takes every 4th grid node from the original grid, along with the well data, as control points for the smoothing. The higher the flex factor becomes, the more the grid will look like a straight minimum curvature method.
Grid flexing can be CPU intensive and is highly sensitive to the number of data points and number of grid rows and columns, so be careful with its use.
The following is an example of how when flexing is applied using the same data and grid spacing mentioned above.
The surface style, or algorithm, determines the shape and characteristics of the gridded surface by applying different mathematical functions to the original data. The surface gridding style chosen by the user determines the mathematical model used to generate the rectangular or triangular grid.
The bottom line is that industry software exists that can enable you to obtain unbiased interpretations and accurate visualization enabling you to better understand the reservoirs you are trying to evaluate and interpret as well as assist with illustrating convincing arguments to management. CEC Energy Consultants is available to assist you with the setting up, maintaining and evaluating your exploration and development projects.
Go out and see what new applications can be enhanced using industry-mapping software in the identification of both acquisition and drilling prospects.
Back to January 2004 Newsletter
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