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This function allows you to compute a log curve from one or more logs by specifying a generalized equation transform. After the equation is written, you assign equation variables to database variables. Equation variables may be logs, zone data items (including fm tops), numeric constants, or curve depths.
To open the log equation transform tool:
Main module: Compute>From Logs>Equation Expression
The Log equation dialog box contains two tabs:
·Equation tab used to enter, save and load the equation
·Options tab limits the calculation to a set depth or to a subset of wells in the project
Equation tab
In the Equation tab, you can enter an equation using variables and mathematical operators. The variable on the left side of the equal sign is the result variable.
Variables
Variables are defined with text and can be specific (GAMMARAY or FORMATION_TEMPERATURE) or general (A or B).
Operators
Petra recognizes a wide variety of mathematical operators when creating equations:
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OPERATOR
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DEFINITION
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OPERATOR
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DEFINITION
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OPERATOR
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DEFINITION
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+
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Addition
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ATAN(x)
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Arctangent of x ( in radians)
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MIN (x,y)
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Minimum of x and y
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-
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Subtraction
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COS(x)
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Cosine of x in radians
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ROUND (x)
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Rounded value of x
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*
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Multiplication
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COSH(x)
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Hyperbolic cosine of x (radians)
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SIN (x)
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Sine of x in radians
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/
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Division
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EXP(x)
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e to power of x
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SQRT (x)
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Square root of x
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**
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Exponent
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INT(x)
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Truncated value of x
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SQR (x)
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x squared
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ABS(x)
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Absolute value of x
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LOG(x), LN(x)
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Natural Log of x
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SINH (x)
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Hyperbolic sine of x (radians)
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ACOS(x)
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Arccosine of x (in radians)
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LOG10(x)
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Log based 10 of x
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TAN (x)
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Tangent of x in radians
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ASIN(x)
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Arcsine of x (in radians)
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MAX(x,y)
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Maximum of x and y
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TANH (x)
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Hyperbolic tangent of x (radians)
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Operator precedence
The order of operations when writing an equation are:
1.Exponent
2.Multiplication and division
3.Addition and subtraction
Note: Use of parentheses can change the order of precedence if needed.
Example Equations
Listed below are sample equations:
·GRN = GR - MEAN_GR + MEAN
·LOGNRM = LONORM + (LOG - PICKLO) * (HINORM - LONORM) / (PICKHI - PICKLO)
·LOGNRM = LOG + LOGPICK - GOODPICK
·SW = (RW / (RT * (PHI**M))) ** (1/N)
·R = ((A + B) * (C - 1.0)) / 100
·R = X*SIN(A) + Y*COS(B)
·R = LOG10(X)
·R = X*X or R = SQR(X)
·R = X**Y (x raised to the y power)
Options tab
The Options tab allows the user to set limits for depth or to a subset of wells in the project.
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Restrict Depths
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You can restrict the operation to be performed only on a certain depth range based on tops or on depths.
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Null Results if Unable to Evaluate
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This option will tell Petra to overwrite an existing log with null values if it is unable to evaluate the equation.
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Apply Log Transform To
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This simply chooses whether the log calculation will be performed for all selected wells in the main module, or only for the currently selected well in the main module.
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Once the Equation and Options tabs are filled out, click the Next button at the bottom of the dialogue box. This causes Petra to parse the equation, and bring up the Log Equation Variable Assignments tab.
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