Geochemical Flow Modeling

The mathematical formulation· of a one-dimensional, finite-difference compositional simulator is given in this work. The model assumes thermodynamic equilibrium between flowing electrolyte solutions and stationary mineral assemblages, and solves for the mineral identities and compositions and aqueous compositions as a function of both time and position. The development considers both dissolution and precipitation of solids and uses an activity coefficient correction to account for non-ideal solution behavior. The model features an electron balance in order to describe the equilibria in systems that include redox reactions. Several model applications are presented. All applications which included fluid flow illustrate one of the most important findings of this work: the downstream equilibrium condition. The downstream equilibrium condition states that whenever a solid disappears across a wave in the downstream direction,. the downstream aqueous phase concentrations are saturated with respect to that solid even though the solid itself is not present in the downstream region. An important consequence of the downstream equilibrium condition is that it permits an algebraic solution to equilibrium flow problems that otherwise require a finite difference approach for solution. In a sandstone acidizing application, the model showed that several insoluble species can form during HF/HCl acidizing of iv argillaceous sandstones and that the most damaging precipitates can be avoided if the HF /HCl formulation is properly matched to the mineralogy of the formation. In an uranium leaching application, the model was used to study both acid leaching using ferric sulfate solutions and alkaline leaching using carbonate solutions. For acid leaching. simulations revealed that the pyrite dissolution kinetics and potential precipitation of ferric hydroxide determine the leach solution selection. For alkaline leaching, simulations revealed that hexavalent uranium precipitation in the form of uo2(0H)2 and/or uo2co3 results if too high an oxidant or too low a carbonate concentration is used. In addition, if pyrite is present, the precipitation of uo2 results and the elution of soluble uranium is delayed. The amount of precipitation and duration of delay is proportional to the amount of pyrite initially present. In an application which studied the formation of mineralized uranium deposits. simulations showed that some of mineralogical features characteristic of uranium roll-fronts could be reproduced using geochemical flow modeling. Under suitable conditions, selenium and arsenic mineralization occurs during roll-front genesis and that the positions of these trace metals are located immediately upstream and downstream, respectively, of the uranium mineralization. The simulations predicted that roll-front formation requires about eight million years. In a final application, we studied the flow of aluminum citrate solutions in porous limestones and sandstones, and the simulations showed that, under certain conditions, the precipitation of aluminum and ferric hydroxide takes place. The model predictions compared well to the results of laboratory displacements. The model was used to design aluminum citrate formulations that, when matched to the formation mineralogy, yielded no precipitates. Laboratory corefloods confirmed the correctness of the model design guidelines.