Error and impact of porosity-permeability transform in tight reservoir

Abstract

Porosity permeability transforms are used to evaluate permeability; this is a widely accepted and used method in the petroleum industry. There are number of uncertainties associated with porosity permeability transforms such as for example, biasness in the core selection, core to log depth mismatch, permeability up-scaling and geostatical error. Some of them like depth matching and up-scaling can be eliminated but the uncertainty due to biased core selection and statistical uncertainty remains a challenge even in present times. The effects of those uncertainties are even more pronounced in case of low permeabilities in a tight gas sand reservoir environment. An example for a tight gas sand reservoir field from Middle East was used for this study and it is demonstrated that the general fit to the data significantly underestimates the permeability whereas by using Swanson's mean the predicted permeability is in good agreement with the arithmetic mean of permeability. It was also demonstrated that the way transforms have been obtained has a significant impact on porosity cutoff. It is shown that in tight gas sand reservoir the best way to get porosity permeability transform relationship is by using a non-parametric estimator such as Swanson's mean. An attempt is also made to show the effect of porosity cutoffs on connected hydrocarbon in-place. Further, it has been demonstrated that how porosity cutoff will impact reserve estimation and field development planning.