Arne Fylling
STATOIL
ABSTRACT
A method of estimating sand conductivity and saturation in laminated, dipping sand/shale sequences is developed. The method is particularly useful in cases of thin laminations at the orders of a few cm or less, where on increment by increment evaluation is not suitable.
In laminations with no dip relative to the borehole, induction log measurements have previously been used to estimate the sand conductivity, provided the shale volume and conductivity are known. Introducing a dip, however, the simple average of sand and shale conductivities is replaced by a more complex relation when measured by the induction log.
This complex conductivity relation is utilized to estimate the sand conductivity in dipping laminations. The results are improved by dip-dependent filtering of the shale volume curve to get it on a vertical resolution comparable to the induction log. The method is verified on formation models, applying simulation of induction log responses in dipping laminations to get the resistivity curve of each model. The method is applied in two ways:
- In case of varying sand quality and saturations, an average sand conductivity profile is estimated along the well.
- If the sand properties can be assumed to be constant, a single sand conductivity is estimated, which is less sensitive to errors in log measured data and shale parameters.
The laminated sand porosity is calculated by removing the shale effect on the density log, allowing the sand layer water saturation to be calculated by applying standard equations.
This method is applied to calculate the saturation in a laminated, North Sea reservoir. Sensitivity studies show that the method can provide good results in many cases. In some cases, however, it is sensitive to errors in shale parameters.