Zeolite dissolution phenomena.

Abstract

The goal of this research is to advance a fundamental understanding of the principles governing zeolite dissolution---a scientifically challenging problem that is of fundamental importance to a variety of environmental processes including wastewater stream treatment, radioactive waste immobilization, and petroleum reservoir acidization. The stimulation (i.e., acidization) of petroleum reservoirs is the process of injecting acids into a porous, low permeability, geologic formation in order to dissolve aluminosilicate minerals and thereby increase near-well bore permeability. The new knowledge developed in this dissertation is intended to forecast the mechanisms associated with zeolite dissolution processes and provide a basis for designing the acid stimulation of petroleum reservoirs. The objectives of this research are the following: (1) elucidate the dissolution kinetics of zeolites in hydrochloric acid by formulating a rate law model from first principles, (2) establish the influence of silicate precipitation on zeolite dissolution kinetics, (3) identify the underlying dissolution mechanisms and limitations, and (4) develop a universal understanding of zeolite dissolution by applying new knowledge to synthetic zeolites. Laboratory dissolution experiments are performed using a batch reactor apparatus in order to obtain experimental values of zeolite dissolution rates. A surface reaction mechanism is examined and used to develop a fundamental rate law model for zeolite dissolution. Experiments undergird that silicate precipitates completely from solution during zeolite dissolution in hydrochloric acid, and that the precipitation does not influence the dissolution kinetics. Comparison of the measured initial silicon and aluminum dissolution rates demonstrates that aluminum atoms are selectively removed from zeolites. A selective removal rate parameter is defined as the ratio of the measured silicon dissolution rate to the stoichiometric silicon dissolution rate. A new concept is introduced of using a selective removal rate parameter to delineate the mechanism of zeolite dissolution by demonstrating the universal influence of the silicon to aluminum framework ratio. The mechanism comprises the removal of silicon facilitated by the selective removal of aluminum, leading to the formation of un-dissolvable silicate particles. General guidelines are set forth to predict the dissolution behavior of zeolites existing in reservoir formations.