The Compute Pay Flag Curve function generates a binary log curve that shows where log curves meet a set of user-defined criteria.  In other words, Petra creates a log that distinguishes between pay and non-pay based on log parameters.  In the curve, pay and non-pay can be defined as any number or null values.  Usually, pay is defined as 1 and non-Pay as 0 or Null.

As an example, Petra can distinguish pay in the D sand by GR below 60 API, porosity above 10%, and resistivity above 10 ohmm between the top and bottom of the D sand.  As shown in the following log, only the interval from 6346' to 6353' fits all three criteria.  The pay curve on the right demonstrates this interval.

In addition to simply showing pay intervals, these curves can be useful in log calculations or to show contacts.  For example, to selectively erase a curve everywhere but over a pay zone you can multiply porosity by a pay curve where pay is 1, and non-pay is null.

To open Compute Pay Flag Curve Tool, select Compute>From Logs>Pay Flag Curve from the menu bar at the top of the Main Module.

Input Curves tab

Curves and Cutoffs  This section is used to select the log curves and their cutoff criteria for the curve calculation.  Select the curve name and establish the condition and cutoff value.

Available conditions are:

EQUAL TO

LESS THAN

LESS THAN OR EQUAL TO

GREATER THAN

GREATER THAN OR EQUAL TO

NOT EQUAL TO

For each log selected for use, enter the cutoff limits as the minimum, maximum, or exact value.  Log curves can also be selected as a variable cutoff.  As an example, a “gas effect” cutoff can be established so pay is only where neutron porosity is less than density porosity.  Unselected boxes are not used as cutoff criteria.

Use Log Curve Aliases - This option causes log aliases to be used.  When a specified curve is missing, an alias log will be used.  This is very useful for large projects.  Log curve names are aliased under the Log tab in the main module.

All/Any Condition Met - In the example above, the pay curve criteria is GR below 60, LLD above 10, and DPHI above 10%.  An interval that has fits all these criteria will be flagged as pay.  You also have the option to select whether any conditions are met.  This would flag any interval where any of these criteria pass as pay.

Output Curve tab

The Output Curve sets the name and specific values of a pay curve.  

Output PAY Flag as Curve - By default, Petra will create a log curve called PAY.  Alternatively, enter a new  curve name.

Curve Value when PAY - This entry sets the curves value when the criteria on the Input Curves tab is satisfied. By default, pay is 1.

Curve Value When NON-Pay - This entry sets the curves value when the criteria on the Input Curves tab isnot  satisfied.  By default, non-pay is zero

Set to NULL - This entry box will assign null values to non-pay.

Update/Replace Existing Pay Curve – When the pay curve named above already exists and you are replacing it inside a certain depth range (see below in the DEPTH TAB), you have the option of updating the curve or replacing the curve.  Updating the curve will merge the new pay curve into the existing one, leaving the pay curve outside of the depth limits untouched.  Replacing the curve, on the other hand, will null the rest of the pay curve outside of the depth limits.

Depths tab

The Depths tab allows you to set limits on the extents of the pay curve calculation.

Set Depths… - This button opens the to set limits based on discrete MD or TVDSS depths, fm tops, or from zone definitions. In the Set Depth Range box, select the relevant top, MD, or TVD button.  For MD and TVD, select the relevant button and enter the adjacent depth in the entry field.  For tops, select the desired top from the “Fm Top Name” dropdown box.  Notice that an offset can also be added or subtracted to the fm top; this offset will include data points above or below the actual fm top depth.

In the example above, the pay curve will only be calculated from the top of the D sand to the base of the D sand.

Filter tab

This sets a minimum thickness filter both for pay and non-pay.

Apply Minimum Bed Thickness Filter - This option enables and disables the minimum bed thickness filter.

In the example above, any interval that calculates as pay that is below 1' thick will be set to non-pay.  Likewise, any non-pay intervals thinner than 1' will be set to pay.  This is useful for “chattering” logs and washouts.

Pay Data tab

This option allows you to save the pay curve-calculated intervals to the raster pay database.

Wells tab

The calculation can be done for all selected wells in the main module or on only the currently selected well in the main module.