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Easy variogram modeling
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Variogram Estimation
Types of variogramThe variogram most common that are implemented in softwares are:
Exponential
Spherical
Gaussian
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
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)
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
Variogram Estimation Parameters Lag distance
if we have a lot of data the Lag distance does not affect the variogram estimation
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
Variogram Estimation Parameters
The Lag tolerance has to be at least 50%
Increasing the Lag tolerance could help for understanding the variogram (loosing detail)
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
Variogram Estimation Parameters
A large Band width and Tolerance angle =90 allow to estimate an isotropic variogram
Variogram Estimation Parameters
Thickness
Thickness specifies a tolerance in vertical direction
Nugget is affected by the thickness
