Abstract

Sustained propagation of a combustion front is necessary for improved recovery of oil during an in-situ combustion process. The front is a sharp moving boundary layer and involves the complexities of combustion reactions. In this work the combustion front is represented by two oxidation reactions: (high-temperature) fuel-burning reaction and (low temperature) fuel-generating reaction. Due to distinct reaction kinetics and stoichiometry, the fuel-generating and -burning reactions occur in sequential regions within a finite separation distance in the reservoir. Strongly nonlinear interaction of these regions and its overall influence on the combustion front propagation are investigated using an analytical approach based on large activation energies of the reactions. Reservoir conditions under which the regions could travel in the formation with a common propagation speed is identified and the limits of their coherence in the presence of external heat losses are investigated. Consequently, we have found and formulated a new intricate relationship between the reservoir heat loss rate and separation distance of the reaction regions: the regions propagate closely spaced, thus minimizing the influence of deleterious external heat losses and maximizing the process performance. This two-reaction self-sustainability mechanism keeps the combustion front propagating steadily even though under the same conditions front extinction has been predicted for the equivalent single-reaction problem. The work is essential for improved oil and in-situ bitumen recovery using air/oxygen and emphasizes the importance of local chemical processes during the injection.

Introduction

Propagation of combustion fronts in porous media has been studied extensively in the filtration combustion literature; it is a subject of interest to a variety of applications, ranging from insitu combustion for the recovery of heavy oil to catalyst regeneration, coal gasification, smoldering, waste incineration, ore calcination or the high-temperature synthesis of powdered materials1. The fuel may pre-exist as part of the solid matrix or, as in the case of in-situ combustion, it may be created by processes, such as pyrolysis and low-temperature oxidation reactions.

The dynamics of filtration combustion are influenced by the flow of injected and produced gases, the heat and mass transfer in porous media and the rates of reactions. An analytical treatment of the fronts can be accomplished assuming a sharp exothermic oxidation front by using large activation energy asymptotics, a technique widely considered to investigate laminar flames in the absence of a porous material2,3.

Akkutlu and Yortsos4 considered application of the technique for modeling forward in-situ combustion fronts in porous media. They extended their approach to study the effects of reservoir heat losses5 and the impact of reservoir heterogeneity6 on sustained front propagation and extinction. Their investigations were based on a single oxidation reaction, however. In this paper, we will consider the presence of an additional oxidation reaction occurring at lower temperatures. The latter precedes the main combustion region and generates the fuel necessary for the main combustion reaction. Under certain conditions, the two reaction regions become coupled, in which case they propagate coherently, albeit at a finite distance from each other. Otherwise, they become thermally uncoupled, with the region of low-temperature oxidation traveling ahead at higher velocities.

This content is only available via PDF.
You can access this article if you purchase or spend a download.