Summary
In-situ combustion is a potential method for the recovery of heavy oil. The
effect of reservoir heterogeneity, a ubiquitous feature of oil reservoirs, on
in-situ combustion has not been systematically addressed in prior studies,
however. In this paper, we present analytical models for filtration combustion,
namely the combustion of a stationary solid fuel, in the specific case where
the reservoir consists of two layers of different permeability and thickness,
separated by nearly impermeable shales. We investigate the conditions for the
propagation of steady combustion fronts as a function of some key parameters,
including the permeability-thickness contrast R between the layers, the
thickness ratio η, and the external heat loss coefficient h.
We find that heterogeneity acts in two distinct ways: It reduces the
temperature of the leading front in the high-permeability layer in all cases,
and uncouples the propagation of the fronts in the two layers if R is smaller
than a critical value Rc. The first effect may lead to low-temperature
oxidation conditions, and therefore to the effective extinction of the front in
the high-permeability layer. The second leads to a reduced sweep efficiency
(and early breakthrough). However, if R exceeds the critical value, the fronts
in the two layers travel coherently (with the same speed). This coherence is
identified for the first time. The resulting thermal coupling greatly retards
the front in the more permeable layer, and accelerates only slightly that in
the less permeable one, until the two fronts reach a common velocity.
We study the effects of R, the heat loss rate and the ratio of thickness η.
The coupling is aided by moderate heat losses (small h), and smaller η, which
affect the critical value Rc. As in the homogeneous case, at sufficiently high
heat loss rates, steady front propagation cannot be sustained and the
combustion process becomes extinct.
The work is useful for the understanding of the viability of in-situ
combustion process in heterogeneous layered reservoirs and the effect of a
number of injection, combustion, and reservoir parameters.
Introduction
The sustained propagation of a front is necessary for the recovery of oil
using in-situ combustion. Compared to other recovery methods, in-situ
combustion involves the added complexity of exothermic chemical reactions and
temperature-dependent reaction kinetics. Combustion is influenced by a number
of processes, including the fluid flow of injected and produced gases, the heat
transfer in the porous medium and the surroundings, the kinetics of combustion
reaction(s), and the heterogeneity of the porous medium. In the presence of
external heat losses, there exists the possibility of extinction (i.e.,
quenching). This paper focuses on the effect of heterogeneity, which has not
been systematically addressed before.
Combustion fronts in porous media have been studied extensively in the
context of filtration combustion, which is the combustion of a stationary solid
fuel in a porous medium by an injected gas oxidant (typically air). An
analytical treatment of the front dynamics is possible using methods similar to
the analysis of laminar flames in the absence of a porous medium. Britten and
Krantz,1,2 for example, provided an asymptotic analysis of in-situ coal
gasification using the property that the activation energy of the overall
(rate-limiting) reaction is large in comparison with the thermal enthalpy.3 In
detailed studies, Schult et al.4,5 investigated filtration combustion in a
homogeneous porous medium, in the different contexts of fire safety and the
synthesis of compacted metal powders (SHS processes). More recently, the
microscale mechanisms of forward and reverse filtration combustion in a porous
medium were studied by Lu and Yortsos6,7 using a pore-network model.
The study of planar forward filtration combustion fronts in a homogeneous
porous medium was undertaken by the present authors using a continuum
approach.8 They addressed the issue of steady-state propagation under both
adiabatic and nonadiabatic conditions. External heat losses were modeled by
conduction or convection modes (the former being more appropriate for
subsurface applications). A number of important results were obtained, which
for the benefit of the reader will be briefly summarized in the Preliminaries
section. In particular, the dependence of the combustion front velocity on
injection and combustion parameters was investigated in detail.
In this paper, we extend the asymptotic approach in Ref. 8 to model
combustion fronts in heterogeneous, and specifically in layered, porous media.
Layered systems are prototypical of heterogeneous reservoirs as they capture
the channel-like features of streamtubes.9 In typical fluid displacements, the
effect of reservoir heterogeneity interacts with the fluid mobility: the
displacement in a more permeable layer is accelerated in the case of
unfavorable mobility ratio, and retarded in the case of favorable mobility
ratio. In such processes and in the absence of cross-flow, the coupling between
the layers only enters through their common inlet and outlet pressure
conditions. This leads to the so-called Dykstra-Parsons regime (e.g. see Yang
et al.10). In combustion, however, the propagation of combustion fronts is
affected by the local thermal coupling between the two layers, not only by
common inlet and outlet conditions. Typically, the front in the
high-permeability (hence high-flow rate) layer will move faster than that in
the lower permeability layer. Heat transfer preheats the porous medium in the
low-permeability layer, and increases the heat capacity encountered by the
high-permeability front. This results in thermal coupling, which will retard
the high-permeability front and accelerate the low-permeability front. An
important question is whether or not and under what conditions the two fronts
eventually reach a common velocity and propagate coherently. In addition, of
significant interest is the possible lowering of the temperature in the
fast-moving front, as it may essentially extinct the combustion process.
In this paper, we provide answers to these questions for the case of
filtration combustion in two layers, thermally coupled by a simple convective
heat transfer mode. We assume that the layers do not otherwise communicate
(e.g., they may be sealed from one another through an intervening shale, or
their fluid mobility may remain constant, which is the case when the net rate
of gas generation because of reaction is small11). Under these conditions, the
injection rate in each layer is constant in time, and proportional to its
permeability-thickness product. Both adiabatic and nonadiabatic (external
reservoir heat losses) conditions are studied. We focus on how the thermal
coupling affects the steady combustion front propagation in the adjacent
layers, on whether or not a state of frontal coherence develops to maintain the
system stable and sulf-sustaining, and on conditions of extinction. The main
tool of our analysis is the large activation energy asymptotics method derived
in Refs. 8 and 12 for a single layer. Because of the relevance of those results
to the present case, they are briefly summarized below.
© 2005. Society of Petroleum Engineers
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History
- Original manuscript received:
29 May 2002
- Revised manuscript received:
2 June 2005
- Manuscript approved:
10 July 2005
- Version of record:
15 December 2005
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