Effect of the Sodium and Ammonium Content and the pH of Carbonate Uranium Leachates on the Permeability of a Core From the Catahoula Formation in South Texas

Abstract

ABSTRACT This paper reports a laboratory study of the effect of the Na+ and NH4+ content and the pH of carbonate uranium leachates on formation permeability. The purpose of the work was to define conditions such that the leachate would not appreciably decrease formation permeability or cause significant formation damage. The core used in this study was obtained from a uranium zone in the Catahoula formation. It contained about 3% of the expandable clay, smectite, and had a permeability of 2000 md. Increasing the Na+ content of a leachate 3-fold by a 10-fold increase of the CO3= content from 600 to 6000 ppm by NaHCO3 addition at pH 8 caused a 10% decrease in permeability presumably because more of the smectite had been converted to the swelling Na+ form by ion exchange. Displacement of the 6000 ppm CO3= as NaHCO3 leachate by a 6000 ppm CO3= as (NH4CO)3 leachate at pH 9.4 caused a reversal of the loss in permeability and even a slight increase over the initial permeability presumably because the smectite had been converted to the non-swelling NH4+ form. The effect on formation permeability of Na+ and NH4+ leachates at pH 8 to 9.4, therefore, appears to be reversible. Reversibility of permeability suggests that these leachates do not cause formation damage. In contrast, a 0.3M NaCl-0.2M NaOH, pH 13, restoration solution used for NH3 removal from formations caused an immediate, irreversible 40% decrease in permeability presumably because the highly caustic solution caused formation damage by chemical reaction with the formation.