Petrophysical Evaluation of Kaolinite-bearing Sandstones: I: Water saturation (Sw), an example of the Tirrawarra Sandstone reservoir, Cooper Basin, Australia
Authors: Rezaee M.R. , Lemon N.M. 
Published: December 2007

Water saturation (Sw) in shaly sand formations has been a subject of debate for many years and several equations have been introduced for the estimation of Sw. In the present study, several equations for the estimation of Sw are applied in the Late Carboniferous - Early Permian kaolinite-bearing Tirrawarra Sandstone, an important oil reservoir in the Cooper Basin. The results from Archie and Waxman-Smits equations display a good correlation with measured core water saturation.

Plots of water saturation from the Archie equation with water saturation from the other equations show that the Archie and Waxman-Smits equations are approximately the same (r2=98%). As can be expected, due to the low cation exchange capacity (CEC) of Tirrawarra Sandstone, the second part of the Waxman-Smits equation concerning shale conductivity is very small. As a consequence, the values of Sw from the Waxman-Smits equation are very close to those from the Archie equation. Petrographical studies indicated that kaolinite patches in the Tirrawarra Sandstone are water wet. In this case, the formation resistivity is a function of formation water in macro-porosity and irreducible water associated with kaolinite micro- porosity. In other words, when kaolinite is electrically inert, the whole rock can be considered as a clean sandstone which obeys the Archie law. Because resistivity logs cannot recognise free water within the macroporosity from bound water associated with clay minerals, calculated water saturation includes both free and irreducible water. As the irreducible water associated with kaolinite minerals is not expelled during production, some intervals of kaolinite-rich sandstones, which may never produce water, may be bypassed as non-productive zones.

A new equation is introduced for estimation of effective water saturation. The equation is based on the integration of resistivity and sonic logs with image analysis data for wells in the Moorari and Fly Lake fields, where obtaining the volume of clay is difficult and unreliable. This equation, which reduces calculated water saturation by about 10% for the Tirrawarra Sandstone, is likely to be applicable to other kaolinite-bearing sandstones.

Introduction

Water saturation (Sw) is one of the most important petrophysical parameters required for reserve calculation. In clean sandstones, estimation of water saturation is rather simple due to the lack of electrically-conductive materials such as clays. The presence of clays in the shaly sandstones which display high conductivity (Ref. 1) makes estimation of water saturation complicated. In this paper, factors affecting conductivity of shaly sandstones are reviewed and some of the common equations which are used for calculation of water saturation are applied to the Tirrawarra Sandstone and a new equation to estimate water saturation in kaolinite-rich sandstones is introduced.

Basic Concepts

Electrical conductivity of a rock depends on several variables including, conductivity of rock components, conductivity of pore fluid, fluid saturation and formation factor (F). In clean sandstones, the conductivity of rock components is zero and electrical conductivity is a function of conductivity of pore fluid, fluid saturation and formation factor. In shaly sand formations, the electrical conductivity of rock components can not be considered zero, as clay minerals provide additional conductivity (Ref. 1). In these sorts of formations, conductivity of the clay minerals should also be estimated.

Formation Factor. Formation factor was first introduced by Archie (Ref. 2) and defined as the ratio of the conductivity of brine to the conductivity of fully saturated clean sandstone:

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