Abstract

Clay containing mono-and divalent exchangeable cations indicate a significantly greater amount of divalent cations on the coarser fractions while monovalent cations are concentrated in the finer ones.

The sequence of replaceability of cations present on the clay sites followed the rule Na >K >Mg >Ca.

A Tektite electrode shows a specific sensitivity to divalent cations and measures the sum of both calcium and magnesium present.

A graphical solution has been presented for determining either the Cation Exchange Capacity [CEC] of calcium or magnesium alone. The chart is valid only if the calcium-to-magnesium ratio in the investigated natural system. is known.

Introduction

The concept of cationic exchange reactions on bentonitic clays [sodium or calcium montmorillonite) is fundamental in many aspects related to drilling muds, well logging and formation properties. Drilling fluid technology has been largely influenced by the chemical reactions of montmorillonite clays. These clays may retain adsorbed cations in an exchangeable state; i.e., a calcium montmorillonite can be converted into a sodium clay, or conversely the sodium, may be replaced by calcium. The basic significance of exchange reactions of montmorillonite in drilling fluids is recognized as an important factor in the rigid requirement of mud properties for drilling deeper and deeper wells. The purpose of this investigation was to develop a method whereby the cation exchange capacity of certain clay fractions, obtained through a fractionation procedure using a supercentrifuge, could be determined through direct titration. Primarily, the study was undertaken to determine the degree of variation, if any, which might exist in the exchange monovalent and divalent cations and clay particle size.

Development of an electrode sensitive to divalent cations was recognized as a vital part of the investigation and the scope of the work was broadened the determine the utility of tektite glass [thin slices of tektite material] as a divalent sensitive electrode. The preparation and use of the tektite electrodes, for the purpose stated, was a distinct objective.

Three different clays were investigated. They represented [1] predominantly Ca++ clays, as in the case of clay No. 4; [2] heterogeneous clays, with Na+ content dominating Ca++ - Mg++ as in clay No. 11 and No. 14; and [3] heterogeneous clays with Ca++ - Mg++ content in excess of Na+ as in the case of clay No. 1. Source of the materials was clay fractions obtained in previous work by Mungan.

Localities and Descriptions

No. 1- Bentonita Blanca Vetessa. Locality:Mexico. Color: beige. Impurities:mostly talc and cristobalite.

No. 4- Bentonita Comisa. Locality: Mexico Color: greyish-white. Impurities:quartz and calcite

No. 11-Bentonita de Durango. Locality: Mexico Color: red-brown. Impurities: large quantity of feldspar and some quartz.

No. 14-Wyoming Bentonite. Locality: Wyoming Color: beige. Impurities: quartz and feldspar.

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