Impact assessment of mineralization and cation exchange capacity on the resistivity index of clayey sandstones on the example of cretaceous deposits in Western Siberia

Background. The validity of initial deposit assessment largely depends on the accuracy of water saturation determination. The saturation exponent (n) is an important parameter included in the majority of generally accepted integrated models for determining water saturation. The error of saturation determination at the level of 0.5 may distort the final estimate of water saturation by 30%. It is believed that the Resistivity Index (RI) is affected not only by the coefficient of current water saturation (Sw) but also by such factors as wettability, reservoir water mineralization and its ionic composition, as well as the amount and mineral composition of clays.  

Aim. To assess the effect of cation exchange capacity and mineralization on the saturation exponent of Cretaceous clayey rocks from a Western Siberian field.  Materials and methods. The research objects were 23 core samples alternately saturated with model formation water of different mineralization and composition. The samples were collected from Cretaceous deposits in a Western Siberian field located in the Middle Ob oil-gas area. The experiments were conducted in the modes of full and partial water saturation.  

Results. The influence of cation exchange capacity, as well as the mineralization and composition of the model formation water, on the saturation exponent is absent. No change was found in the effect of ‘bound’ water on the rock conductivity depending on changes in the current water saturation.  

Conclusion. Knowledge of the influence of the mineralization and composition of reservoir water, as well as cation exchange capacity, on the Resistivity Index is important for assessing the risks of inaccurate determination of saturation exponent (n) during standard laboratory studies using sodium chloride models of reservoir water of a single mineralization.  

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