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The Log Statistics function computes simple statistics on a selected log for one or more zones. The calculation includes all currently selected wells and the results will be stored in the zone items database.
To open the compute log statistics tools:
Main module: Compute>From Logs>Statistics
The Compute Log Statistics dialog box contains two tabs:
·Data tab selects the digital log curve and type of statistics to derive from the log
·Options tab provides zone, log and summation calculation options
Data tab
The Data tab is used to select the digital log curve and which statistics to derive. By default, Petra will name each data item by the statistic and log curve name. For example, calculating the sum on the ABHV curve creates a default data item Sum_ABHV. To change a name, highlight the existing name and type over it.
Click to enlarge image.
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Select Log Data From
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Selects which digital log curve in the current project will be used for statistical calculation.
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Use Alias Logs
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Allows log curve aliases to be used when a selected curve is absent.
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Compute Items
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The Compute Items grouping selects the desired statistical calculations, controls a couple of options on the statistics, and sets the data item that will store the result in each well. By default, Petra names each data item by the statistic and the curve name. Calculating the Sum on the ABHV curve (as in the screenshot above) creates a default data item Sum_ABHV. These names are defaults only; to change the name, simply overwrite the existing
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Sum
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Adds all the curve values together. Note that this calculation can be affected by the curve's sample rate - the "Compute 'Sum' Using" option on the Options tab helps to normalize sums by sample rate.
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Minimum
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Finds minimum value for the curve.
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Maximum
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Finds maximum value for the curve.
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Mean
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Finds mean or average value for the curve. The "Mean Calculation" option below sets whether the mean is Arithmetic, Geometric, or Harmonic.
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Std Dev
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Finds the standard deviation for the curve.
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Percentile N =
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Finds the value of the curve at the selected percentile. By default, Petra looks for the 10th and 90th percentiles. To change the percentile, simple type in the desired number between 0 and 100.
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Depth
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Calculates the depth at which the Minimum or Maximum log value occurs in the zone.
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View Zone Data...
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Opens the View/Edit Zone Data tool. This can be useful for checking computed results without closing the Log Statistics Calculation Tool.
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Mean Calculation
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Sets whether the mean is Arithmetic, Geometric, or Harmonic.
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Compute For Zones
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This list selects the zones used in the calculation. Petra uses Zones to both set the data item's location and the depth interval over which the log statistics are computed. As an example, selecting the "D_Sand" zone will put the calculated data items into the D Sand Zone, and use the zone's interval definition to limit the calculation to only the log curve inside the D Sand zone. This tool requires the Zone's interval definitions to be set a range between -99999 to +99999.
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Source Code For Results
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This option adds a source code to the statistics data items.
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Options tab
Click to enlarge image.
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Compute "Sum" Using
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Multiplies the computed curve sum values by the curve sample rate. A curve with a sample rate of 0.5' has twice as many values as a curve with a 1' sample rate. Even with identical curve profiles, the 0.5' sample rate curve will have a sum twice as high as the 1' sample rate curve.
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Zone Item Discriminator
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Uses a numerical zone data item to filter the calculated statistics. When selected, Petra will check every curve value against this number and ignore values that meet the criteria. As an example, this tool can set an upper limit on porosity values, so wash-out induced porosity spikes are ignored.
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Log Curve Discriminator
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Uses a discriminator curve to filter the statistical data. At depths where the discriminator curve contains a 0 or null value, the corresponding depth will be skipped in the statistical calculation. If this box is checked and the log is absent, the statistics will not be calculated.
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