Water Saturation Evaluation in Shaly Sand Reservoirs Using Advanced X-ray Diffraction and Cation Exchange Capacity Measurements

Abstract

ABSTRACT Interpretation problems are commonly associated with calculating water saturation in non-homogenous shaly sand reservoirs. Recent studies on the electrical anisotropy of shaly sands have shown that the level of our understanding our ability to correctly and accurately evaluate complex shaly sand reservoirs can be greatly improved. The model ("Total Expansible Clay Model") developed in this work is similar in form to the recent shaly sand Dual Water Model, except that the clay effect have been defined by a volume-conductivity transform rather than the usual volume-porosity transform. The model is experiment-based and designed to quantify the effects of the mineralogical and electrical properties of clay minerals on well log signatures. The Total Expansible Clay Model evaluates these effects via direct measurement of independent mineralogy and cation exchange capacity of the clay minerals within the reservoir sands. The model integrates the following as an effective basis for characterizing shaly sand reservoirs: Rietveld based siroquant assay for quantitative X-ray diffraction (XRD) used in determining mineral percentages from standard XRD trace patterns, Cation exchange capacity (CEC) used to determine the quantity of cations involved in the exchange at the shale-water interface, Porosity, permeability, density and resistivity measurements, Overall, the correlations drawn from this model yield improved results for total water saturation (especially in low porosity and highly shaly formations), which appear consistent with those calculated earlier using well known water saturation models (Dual Water and Juhasz). A total of 23 core plugs from two wells in the Cliff Head Field, Perth Basin, were analyzed for this study.