The broad substitution of aqueous drilling fluids for oil-based muds will require that the performance limitations of current aqueous muds be overcome. The research described in this paper is focused on the most commonly-used aqueous drilling fluids, lignosulfonate (LS03)-dispersed bentonite muds, and addresses two important issues that restrict their wider use: their limited stability at elevated temperature, and their reliance on chromium-containing additives (Cr-LSO3 and Cr-lignite) for control of their rheological properties.

In this paper, new hypotheses regarding the mode of action of Cr(VI) and Cr(III) in Cr-LSO3/bentonite muds are formulated on the basis of new and published experimental observations. Of primary importance are the ideas that (1) LSO3-bound Cr(III) is the form which enhances the thinning power of LSO3 and (2) Cr(VI) stabilizes LSO3/bentonite muds primarily by acting as a latent source of Cr(III). Acting on these hypotheses, it has been found that Cr(III) complexes of appropriate kinetic stability can duplicate the rheology-stabilizing function of Cr(VI). Further pursuit of this coordination chemical approach has led to the identification of a family of Zr(IV) complexes with which chromium-free drilling fluids of outstanding rheological stability can be prepared. The general chemical factors influencing the rheology-stabilizing activity of metal ion complexes are examined. Finally, the unexpected and remarkable thinning power of the Zr(IV) citrate complex is described and its implications for the formulation of novel drilling fluids are briefly discussed.

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