This dissertation addresses fundamental issues of reactive flow instabilities in homogenous porous media with important applications in numerous engineering processes, natural phenomena and environmental issues. Such instabilities, generically referred to as fingering instabilities, are responsible for the fact that recovery and displacement processes operate only at a fraction of their theoretical efficiencies. Thus understanding the origin and mechanisms of these instabilities is a key to their control. The reactive process is modeled through a generic bimolecular chemical reaction (BCR), A+B → C. A mathematical model is adopted for the transport of the three species through porous media.

The first part of the thesis is devoted to the horizontal displacement of a reactive interface in porous media with important applications in the understanding and optimization of heavy oil recovery techniques. Depending on the relative effect of A, B and C on the viscosity as well as on the rate of the reaction and the speed of injection, different viscous fingering (VF) instabilities can be observed. In this context, in the limit of small time scales, a linear stability analysis (LSA) of this problem is performed. The LSA allows to determine the importance of each parameter in the process and classifies the various possible instability scenarios. Unlike the nonreactive systems, it is found that in the presence of a chemical reaction, injecting a more viscous fluid into a less viscous fluid can also lead to instabilities. Nonlinear interactions between chemistry and hydrodynamics are studied by direct numerical simulations. The simulation results reveal that the development of frontal instabilities and slower injection rates are associated with a larger rate of chemical production.

In the second part of this dissertation, one of the challenges in chemically based remediation of underground soil and water is addressed. Nonlinear simulations are performed to study the transport of a reactive pollutant slice by a reactive carrier solution in underground reservoirs. It is found that the extent of spread of the pollutants and the efficiency of their remediation depend on the physical properties of the solutions. Indeed, the solution slice loses its original shape as a result of diffusion, reaction and VF instabilities. It is revealed that the rate of consumption of the reactive pollutant is the highest when the chemical product is the most or the least viscous solution. It is also found that displacements in which the pollutant viscosity is the smallest or the largest of all three species, lead to the widest spread of the pollutant. In addition, the most complex pollutant distribution structures were observed when the carrier solution has the smallest or largest viscosity in the flow.

Finally, the analysis is extended to investigate aspects of environmental issues related to geological storage of carbon dioxide. In a gravity field, the instability properties of a BCR interface between a solution of reactant A on top of another solution of reactant B are investigated. Inspired by underground natural flows, a transverse flow is introduced parallel to the reactive interface. The LSA results reveal that in spite of known stabilizing effects of transverse flow on nonreactive systems, it is shown here that it has a destabilizing effect on a reactive front. In the nonlinear regime, higher rates of chemical production are obtained when transverse flows are introduced in the system. Moreover, a special tuning of the transverse velocity to ensure maximum or minimum chemical production is proposed.