Environmental Engineering Reference
In-Depth Information
1 Introduction
The in-situ combustion (ISC) is one enhanced oil recovery method. It is applied
mainly in heavy-oil reservoirs. In the ISC method, standard or oxygen enriched
air are injected into the reservoir to promote the oil ignition and propagation of a
combustion front from the injections wells to the producer wells. Thus, the high
temperatures taking place inside the reservoir improve the oil mobility due to the
reduction of its viscosity and fluid (steam, combustion gases, water) flooding. In
ISC there are several complex mechanisms driving the momentum, energy and mass
transport. The modelling of such mechanisms plays a crucial role for understanding
the physics behind the process parameters.
Currently, a large amount of petroleum is in naturally fractured reservoirs. This has
leaded to develop theoretical and experimental works for gaining knowledge about
the effects that fractures have over the thermal and hydrodynamic performance of
the ISC (Awoleke et al.
2010
; Greaves et al.
1991
; Schulte and de Vries
1985
). The
most of these works agree that the oxygen transport is one of the main features to
sustain and propagate the combustion front. Thus, the oxygen dispersion from the
fracture to the matrix plays a crucial role for the ISC inside the porous medium.
In an ISC process, the availability of oxygen is a key factor because several chem-
ical reactions involve it with further heat releasing. The oxygen might react directly
with some oil components, or with coke, which is produced by a pyrolysis reaction.
In this work, we focus specifically on the oxygen transport from fractures to
porous medium at pore-scale, and eventually the influence of the fracture width,
Peclet number and oxygen-coke reaction rate were studied. With these aims, we
avoid the real complexity of an ISC process and we consider a two-phase (rock and
gas), three-component (N
2
,O
2
and coke), isothermal system. The porous medium
has an idealized 2D microstructure, which is initially saturated with nitrogen.
2 Physical and Mathematical Models
An ISC process involves several phenomena interacting simultaneously in a mul-
tiphase mixture. For instance, we can quote: (1) the existence of several chemi-
cal reactions, (2) multicomponent and multiphase equilibrium, (3) multiphase flow,
(4) thermal expansion, etc. In order to study such phenomena at lab-scale, combus-
tion tube experiments are carried out, where air is injected to promote and propagate
a combustion front (Mamora
1993
; Cazarez-Candia et al.
2010
).
The predominance of some phenomena over others yields characteristic zones
in an ISC as: the burned, combustion, coking, condensing steam, water, oil and
undisturbed zones (starting from the injection to the production well). A detailed
explanation of each zone can be found in the work of Sarathi (
1999
). In Fig.
1
a
simplified scheme of the coke and surrounding zones are depicted. There, the picture
corresponds to a hypothetical combustion tube where the porous core is surrounded
by an annular space representing the fracture. In our study we are interested only
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