INVESTIGATION OF ELASTIC INVERSION ATTRIBUTES USING THE EXPANSIBLE CLAY MODEL FOR WATER SATURATION

J. O. Ugbo+*, C. R. Ward* and P. G. Lennox*

*School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney NSW 2052 Australia.

+corresponding author, email:  formationevaluation@gmail.com

Quantitative X-ray diffraction  has been used to characterize water saturation levels in complex shaly sand reservoirs (i.e. shaly sands with infrequent carbonates and minor proportions of iron-rich minerals such as pyrite and siderite). The results led to the design of a total expansible clay model for water saturation, which is similar in form to the Dual Water model except that the excess effect of the clay minerals has been accounted for by a volume-conductivity relationship, rather than one of the usual volume-porosity translations, effectively reducing the uncertainties in estimating water saturation. Given the ambiguities associated with predicting these petrophysical properties from data on rock properties, such as mineralogy, an investigation of the relationship of estimated water saturation based on the total expansible clay model to independently determined rock properties was undertaken using well log inversion and forward modelling techniques. The results show that there is consistency in the relationship between water saturation estimates made from the total expansible clay model and known elastic parameters such as primary and shear-wave sonic velocity (Vp , Vs), bulk density (ρ_b) and impedance (I), when the Raymer-Gardner-Hunt model is used.  Use of the Raymer-Gardner-Hunt model to reconstruct the required rock-physics relationship avoids the classic limitation of the more advanced Gassman model, which assumes that the dry shear modulus is equivalent to the wet shear  modulus (µ_dry = µ_wet).

The present work raises further questions on the application of the Voigt-Reuss-Hill (VRH) limits, or the Hashin Shtrikman bounds for averaging the effective shear modulus of the dry matrix in complex shaly sand reservoirs, where a two-mineral matrix is normally assumed.  The study shows the inapplicability of the VRH or Hashin-Shtrikman averaging techniques but provides a minor adjustment to the averaging that solves the problems faced in reconstructing the relationships between directly measured elastic properties and derived petrophysical properties for this type of reservoir rock.

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