The Role of Oxidizing Agent in the Chemistry of In-Situ Uranium Leaching
Authors: Carson Ronald H. , Norris Robert D. , Schellinger Richard 
Published: December 2007

A one-well test to evaluate reservoir properties that govern the restorability of a zone in an underground aquifer to be solution mined for uranium has been considered. The test involves injection of a relatively small volume of leach solution and subsequent production of a larger volume of fluid. Two important parameters-cation exchange capacity and an equilibrium constant--are determined by history matching the effluent composition. Two different solution techniques are used. By neglecting dispersion and reservoir heterogeneities, an analytical solution based on the method of coherence provides considerable insight into the pertinent features of the production composition history. To include complexities such as dispersion and layering, a two- dimensional finite difference computer simulation is used. A number of different approaches have been evaluated. The most reliable approach is to use laboratory tests to determine one parameter with core samples and to use production history to determine the other parameter. Measurements of both anion and cation effluent concentrations are required to implement this procedure.

Introduction

A one-well push-pull test is one possible technique to assess the potential of in-situ uranium leaching and subsequent restoration of a contaminated mineralized zone. To address the latter, the test must be able to measure cation selectivity K12 and exchange capacity Qv in-situ from an electrolyte concentration effluent history. The adsorption parameters K12 and Qv are important factors that affect the restoration process. Other parameters that also may affect restoration are formation heterogeneities, longitudinal and transverse dispersion, injection concentration, and well placement. The purpose of this paper is to show how and to what extent the adsorption parameters can be obtained from push-pull test data and the sensitivity of the effluent history to the various factors mentioned. This is accomplished by two techniques: the coherence method, which neglects dispersion and assumes homogeneous media, and a two-dimensional finite difference solution of the appropriate concentration balance equation, which takes dispersion into account and can include heterogeneity. The one-well push-pull test consists of lixiviant injection followed by production of pregnant solution, all conducted through the same wellbore. This test is analogous to the Exxon tracer test for residual oil saturation determination. Fig. 1 shows a cross section in a three-layer reservoir. During injection (injection cycle) the fluid moves radially outward from the wellbore. At the end of the injection cycle, production may be delayed (soaking period) before production begins (production cycle). The time periods for the three cycles are referred to as tI, tS and tP in later discussions.

Mathematical Development

The mathematical formulation is restricted to restoration (cation exchange). Uranium production and history matching will be considered in a subsequent publication. The following assumptions and idealizations have been made in the development of the models.The theory is presented in radial geometry.Permeability and porosity vary with depth only.The fluid is both single phase and incompressible.The mobilities of injected and formation fluids are equal.Cation exchange is the only mechanism of adsorption and the exchange sites can be characterized by a single free energy (equilibrium constant).

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