tips for variogram modeling
DESCRIPTION
Easy variogram modelingTRANSCRIPT
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Variogram Estimation
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Types of variogramThe variogram most common that are implemented in softwares are:
Exponential
Spherical
Gaussian
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Types of variogram• The nugget can be modelled as a variogram and it can be combined with
another variogram• An example is: Exponetial Variogram + Nugget Variogram
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Sill
Geostatistical works in a normalized space.
The data are transformed in a way to have mean =0 and variance =1
Thus the sill has to be 1 because where there is not correlation we have the maximum variance
The sill can be bigger than 1 when we have few data or there is a trend or a cyclicity
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Variogram Estimation Parameters
Rule of thumb: an efficient (for CPU) Search radius has to be 2 times the correlation length
If petrophysical property are simulated after facies the search radius has to be less than
facies correlation (the facies variogram have to be calculated first)
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Variogram Estimation Parameters
the optimal Lag distance allows to have enough couples of data in each bin to have a reasonable estimation of variance
the most important bins are the first ones
in vertical direction we have a lot of data and we can use the cell thickness
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Variogram Estimation Parameters Lag distance
if we have a lot of data the Lag distance does not affect the variogram estimation
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Variogram Estimation Parameters
Lag distance27 wells
if we have a poor data set, we have more possibility to understand the variogram with a large Lag distance
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Variogram Estimation Parameters
The Lag tolerance has to be at least 50%
Increasing the Lag tolerance could help for understanding the variogram (loosing detail)
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Variogram Estimation Parameters
Tolerance angle (combined with Band with) is used to estimate variogram along a specified direction. The tolerance impact on the accuracy of the angle estimation.
The accuracy you can have is strongly dependent on the number of data
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Variogram Estimation Parameters
A large Band width and Tolerance angle =90 allow to estimate an isotropic variogram
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Variogram Estimation Parameters
Thickness
Thickness specifies a tolerance in vertical direction
Nugget is affected by the thickness