Method tab

Use Grid Size of (XY units)

This option sets grid spacing to a user-input X and Y grid size in map units (feet or meters).  A good rule of thumb here is to use ½ the average well spacing.

To calculate average well spacing, select the “Well Dist…” button on the Data tab (highlighted below on the left).  The Well Dist… button on the gridding screen (on the right) gives statistical information on distances between wells, including average distances.  This tool will calculate distance statistics between all wells displayed on a map without discerning if they have data or not.  In other words, if you have a large number of closely spaced wells but only a few of those wells have data, this well distribution box will give you an average distance that is too small

Estimate Grid Size From Z Data

During gridding, Petra will compute a grid size based on the well data distribution.

Set Rows and Columns Slider Bar

Instead of setting a specific grid size, this option instead sets the total number of horizontal and vertical rows for the grid.  Sliding the “Coarse to Fine” slider bar changes the number of rows and columns used in the grid.  Notice that as these are changed, the x and y grid spacing sizes change accordingly.

Match Grid Size of Grid

This option sets the grid size of the new grid to exactly match the grid size of an old grid.  Select this option, and use the drop down menu to select the grid file to be matched. Note that the X and Y sizes are updated to show the grid sized used.

Mathematical calculations between two or more different grids require the size and spacing of each grid to match exactly.  As an example, creating a hydrocarbon pore volume grid (where HPV = thickness * porosity * hydrocarbon saturation) would require the isopach, porosity, and water saturation maps to all have the same grid spacing.  Using this option to match the porosity and hydrocarbon saturation grid size to the initial isopach grid will save considerable time later.

Highly Connected Features (Default)

Uses a least squares gridding algorithm that works well for most data, particularly structure maps and gently changing petrophysical data.Also works well with faulted reservoirs.  This surface style tends to not do well with rapidly changing or large contrasts between data points such as production in a closely-drilled field.

This method tends to avoid geologically unrealistic contours (or “artifacts”) on the edges of the grid, though contours can to be somewhat jagged and uneven with small grid sizes.  The application of surface flexing (“Smooth Contours Using Grid Flexing” immediately below the Surface Style drop down menu) works well with this surface style, as it tends to smooth and even out the spacing between contour lines.

Disconnected Features

This surface style uses a linear projection algorithm that tends to produce closed-off features.  This surface style can useful for mapping patch reefs or isolated channels.  The Disconnected Features surface style can be used with faults.  Contours generated from this surface style can be uneven and jagged, but this is easily remedied by adding surface flexing (“Smooth Contours Using Grid Flexing” immediately below the Surface Style drop down menu).

Since this method calculates grid values from a projected linear slope between one data point to the next, the Disconnected Features surface style is susceptible to a couple of different types of gridding artifacts.  At the edge of a map this surface style extends the nearest linear projection when calculating Z values, making it particularly prone to “runaway grid values” on the edge of the map.  The disconnected nature of the surface style also tends to make “bumpy” maps where two adjacent wells form an adjacent dome and a bowl instead of a more generalized trend.

Simple Weighting With Slopes

This surface style calculates a grid using three steps.  Petra first calculates a slope for each data point based on surrounding data points.  These slopes are then used to project the data points’ Z values out to each individual grid node.  Finally, this surface style takes the weighted average of the projected Z values.

The Distance Weighting Damping Factor on the Advanced tab can greatly affect this surface style.  This option uses any value from 1 to 8, with a recommended default setting of 2.  With a small factor, more distant data points have more influence on an individual data point which tends to average the grid node; this usually results in a smoother grid.  With a larger factor, close data points influence the individual grid much more than more distant data points.  The recommended value for this factor is 2.  For more information, see the help file on the Advanced tab.

Simple Weighting Without Slopes

This surface style applies a weighted average to the data points around each grid node.  In contrast to the Simple Weighting With Slopes surface style, no slope information is used. This option is useful for very dense control such as 3D seismic bin locations.

The Distance Weighting Damping Factor on the Advanced tab can greatly affect this surface style.  This option uses any value from 1 to 8, with a recommended default setting of 2.  With a small factor, more distant data points have more influence on an individual data point which tends to average the grid node; this usually results in a smoother grid.  With a larger factor, close data points influence the individual grid much more than more distant data points.  The recommended value for this factor is 2.  For more information, see the help file on the Advanced tab.

Distance Grid

Click to see sample

This surface style calculates the distance to the nearest data point for every grid point.  Put another way, the grid right next to a data point will have a low Z value, while a grid a great distance from any data point will have a high Z value.  Contouring this distance grid can be a useful way of visualizing drainage and bypassed parts of the reservoir.  Parts of the grid with a high distance to the nearest well are less likely to be drained than parts of the grid with a low distance.

This method only calculates distance to the nearest selected data point, which can include contour lines and control points in addition to wells.  If “Use Overlay Contour Lines” is selected on the Data tab, the surface style will calculate the distance from the nearest well or the nearest contour line.  This renders any visualization of drainage useless, so be sure to only select “Zone Data” for this surface style.

Kingdom - Flex Gridding

Flex Gridding stretches a surface across a sparse dataset.  The Flex Gridding algorithm creates an interpolated grid surface that:

·passes through, or at least is close to, the data in XYZ space.

·minimizes the RMS surface curvature without violating criterion 1.

The approach is to evaluate a series of differential equation operator located at an empty grid node. Centered at an empty grid node, two operators work only on a subset of local control points. One operator calculates minimum curvature; the other calculates minimum tension. The initial solution is a local calculator of a datum from which the grid is derived. These operators are applied to each grid node iteratively, converging to a minimum curvature surface.

Closest Point

This option simply sets each grid node to the value of the closest data point. It doesn’t interpolate between data points, and is really more useful for resampling existing grids.  It is best used with very dense data such as 3D seismic coverage or with legacy XYZ grids.

Minimum Curvature (no faults)

This surface style attempts to create a very smooth, gradual surface.  Contour lines with this method are smooth and evenly spaced, which makes this style a good choice for gently changing petrophysical properties and simple structural settings.

This method cannot be used with faults.   Since the minimum curvature algorithm is also available under the “Smooth Contours Using Grid Flexing” option (immediately below the Surface Style drop down menu), so you can use the Highly Connected surface style (which works well with faults) along with the grid flexing option.  Since this method strives to have as simple a surface as possible (one with a “minimum curvature”), this surface style tends to smooth over some of the variation in data points.  In short, this method has the potential to “not honor the data” as well as other methods.  Edge effects with “runaway Z values” are also common with this method.

The Min Curvature Tension setting on the Advanced tab can greatly affect this surface style.  Practically, high tension grids – particularly above 5 - have smoother and more even contours but may not honor the original data as well as lower tension grids.

Adjust Zero Contour for Isopach Surface

Click to see sample

This option helps to make more geologically reasonable isopach maps where there are zero-valued data points.  When creating a grid, Petra attempts to connect data points.  As an example, the grid will attempt to connect a 15’ data point with another 15’ data point, resulting in a 15’ contour line connecting the two points.

Zero-valued data points are a little different.  Since Petra attempts to connect all zero points, the grid can unrealistically oscillate around zero.  The “Adjust Zero Contour for Isopach Surface” option instead forces Petra to assume that the actual zero isopach line for the grid is midway between a zero value and a non-zero value.  Effectively, this creates a more realistic isopach map.

Smooth Contours Using Grid Flexing

This option adds an additional step after gridding to generate smoother, more even contour lines.  When this option is selected, Petra first uses the selected surface style to interpolate between the data points to create grid nodes.  Next, Petra applies the minimum curvature surface style to both the original data points and to a decimated sample of the newly-interpolated grid values.

The relative strength of the grid flexing option is set by the “Flex Grid Factor” on the Advanced tab.  This option can be set anywhere between 0 and 12.  Setting a low grid factor will keep a relatively strong primary surface style, while a high grid factor will increase the relative strength of the minimum curvature surface style.

Grid flexing is also influenced by the “Min Curvature Tension” option on the Advanced tab.  Practically, high tension grids – particularly above 5 - have smoother and more even contours but may not honor the original data as well as lower tension grids.