The Effect of Heterogeneity on In-Situ Combustion: Propagation of Combustion Fronts in Layered Porous Media
- I. Yucel Akkutlu (U. of Alberta) | Yannis C. Yortsos (U. of Southern California)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- December 2005
- Document Type
- Journal Paper
- 394 - 404
- 2005. Society of Petroleum Engineers
- 5.1.1 Exploration, Development, Structural Geology, 4.1.5 Processing Equipment, 5.3.4 Integration of geomechanics in models, 5.7.2 Recovery Factors, 4.1.2 Separation and Treating
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In-situ combustion is a potential method for the recovery of heavy oil. Theeffect of reservoir heterogeneity, a ubiquitous feature of oil reservoirs, onin-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 wherethe reservoir consists of two layers of different permeability and thickness,separated by nearly impermeable shales. We investigate the conditions for thepropagation of steady combustion fronts as a function of some key parameters,including the permeability-thickness contrast R between the layers, thethickness ratio ?, and the external heat loss coefficient h.
We find that heterogeneity acts in two distinct ways: It reduces thetemperature 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 smallerthan a critical value Rc. The first effect may lead to low-temperatureoxidation conditions, and therefore to the effective extinction of the front inthe high-permeability layer. The second leads to a reduced sweep efficiency(and early breakthrough). However, if R exceeds the critical value, the frontsin the two layers travel coherently (with the same speed). This coherence isidentified for the first time. The resulting thermal coupling greatly retardsthe front in the more permeable layer, and accelerates only slightly that inthe 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 ?.The coupling is aided by moderate heat losses (small h), and smaller ?, whichaffect the critical value Rc. As in the homogeneous case, at sufficiently highheat loss rates, steady front propagation cannot be sustained and thecombustion process becomes extinct.
The work is useful for the understanding of the viability of in-situcombustion process in heterogeneous layered reservoirs and the effect of anumber of injection, combustion, and reservoir parameters.
The sustained propagation of a front is necessary for the recovery of oilusing in-situ combustion. Compared to other recovery methods, in-situcombustion involves the added complexity of exothermic chemical reactions andtemperature-dependent reaction kinetics. Combustion is influenced by a numberof processes, including the fluid flow of injected and produced gases, the heattransfer in the porous medium and the surroundings, the kinetics of combustionreaction(s), and the heterogeneity of the porous medium. In the presence ofexternal heat losses, there exists the possibility of extinction (i.e.,quenching). This paper focuses on the effect of heterogeneity, which has notbeen systematically addressed before.
Combustion fronts in porous media have been studied extensively in thecontext of filtration combustion, which is the combustion of a stationary solidfuel in a porous medium by an injected gas oxidant (typically air). Ananalytical treatment of the front dynamics is possible using methods similar tothe analysis of laminar flames in the absence of a porous medium. Britten andKrantz,1,2 for example, provided an asymptotic analysis of in-situ coalgasification using the property that the activation energy of the overall(rate-limiting) reaction is large in comparison with the thermal enthalpy.3 Indetailed studies, Schult et al.4,5 investigated filtration combustion in ahomogeneous porous medium, in the different contexts of fire safety and thesynthesis of compacted metal powders (SHS processes). More recently, themicroscale mechanisms of forward and reverse filtration combustion in a porousmedium were studied by Lu and Yortsos6,7 using a pore-network model.
The study of planar forward filtration combustion fronts in a homogeneousporous medium was undertaken by the present authors using a continuumapproach.8 They addressed the issue of steady-state propagation under bothadiabatic and nonadiabatic conditions. External heat losses were modeled byconduction or convection modes (the former being more appropriate forsubsurface applications). A number of important results were obtained, whichfor the benefit of the reader will be briefly summarized in the Preliminariessection. In particular, the dependence of the combustion front velocity oninjection and combustion parameters was investigated in detail.
In this paper, we extend the asymptotic approach in Ref. 8 to modelcombustion fronts in heterogeneous, and specifically in layered, porous media.Layered systems are prototypical of heterogeneous reservoirs as they capturethe channel-like features of streamtubes.9 In typical fluid displacements, theeffect of reservoir heterogeneity interacts with the fluid mobility: thedisplacement in a more permeable layer is accelerated in the case ofunfavorable mobility ratio, and retarded in the case of favorable mobilityratio. In such processes and in the absence of cross-flow, the coupling betweenthe layers only enters through their common inlet and outlet pressureconditions. This leads to the so-called Dykstra-Parsons regime (e.g. see Yanget al.10). In combustion, however, the propagation of combustion fronts isaffected by the local thermal coupling between the two layers, not only bycommon inlet and outlet conditions. Typically, the front in thehigh-permeability (hence high-flow rate) layer will move faster than that inthe lower permeability layer. Heat transfer preheats the porous medium in thelow-permeability layer, and increases the heat capacity encountered by thehigh-permeability front. This results in thermal coupling, which will retardthe high-permeability front and accelerate the low-permeability front. Animportant question is whether or not and under what conditions the two frontseventually reach a common velocity and propagate coherently. In addition, ofsignificant interest is the possible lowering of the temperature in thefast-moving front, as it may essentially extinct the combustion process.
In this paper, we provide answers to these questions for the case offiltration combustion in two layers, thermally coupled by a simple convectiveheat transfer mode. We assume that the layers do not otherwise communicate(e.g., they may be sealed from one another through an intervening shale, ortheir fluid mobility may remain constant, which is the case when the net rateof gas generation because of reaction is small11). Under these conditions, theinjection rate in each layer is constant in time, and proportional to itspermeability-thickness product. Both adiabatic and nonadiabatic (externalreservoir heat losses) conditions are studied. We focus on how the thermalcoupling affects the steady combustion front propagation in the adjacentlayers, on whether or not a state of frontal coherence develops to maintain thesystem stable and sulf-sustaining, and on conditions of extinction. The maintool of our analysis is the large activation energy asymptotics method derivedin Refs. 8 and 12 for a single layer. Because of the relevance of those resultsto the present case, they are briefly summarized below.
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