# Numerical Validation of Analytical Estimates for Propagation of Thermal Waves Generated by Gas-Solid Combustion

Authors: Weslley da Silva Pereira; Grigori Chapiro
Journal: Geofluids
 (2017)

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## Abstract

Gas-solid combustion appears in many applications such as in situ combustion, which is a potential technique for oil recovery. Previous work has analyzed traveling wave solutions and obtained analytical formulas describing combustion wave temperature, velocity, and gas velocity for one-dimensional gas-solid combustion model using geometrical singular perturbation theory. In the present work these formulas are generalized. Using numerical simulation we show that they can be adapted and then applied to describe more general two-dimensional models for in situ combustion in a nonhomogeneous porous medium.


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## Body

## 1. Introduction

There is a renewed interest in using combustion to recover medium- or high-viscosity oil. In situ combustion (ISC) is the oldest thermal recovery technique and it has been considered a potential method for the recovery of heavy oil [1, 2]. In this method, air is injected in the porous medium. Heavy and immobile components of the crude oil are used as fuel, producing in-place heat and consequently reducing the viscosity of the oil. Most of the oil is driven toward the producers. An operational advantage of this technique is the abundance of injection gas regardless of location.Modeling the combustion in porous media presents a number of research challenges, mainly because of the physical complexity of the process. The crude oil is composed of many substances which directly affect the combustion process. Furthermore, the combustion reaction and other thermal reactions, like pyrolysis, change the chemical structure of the components along the process. Due to these reasons, a good model for this technique generally consists of many equations, which hinders its analytical and numerical study. That is why a number of works simplify ISC process to one of its main components, gas-solid combustion; see [3–7] and references therein.Analytical solutions and estimates are simple and cheap tools which can be used for validation of existing numerical solutions and to obtain information about some aspects of oil recovery before investing in computational codes. They also help to better understand the physical process. Usually the combustion front is regarded as a traveling wave. For one-dimensional models there are two main methods used to study such wave: (1) the first method explores the strong nonlinearity of the Arrhenius factor in the reaction rate, which allows neglecting the reaction rate as soon as the temperature decreases [8]. Other works use this method to obtain estimates on the combustion wave speed, temperature, and oxygen concentration; see [3–5, 9] and references therein. (2) The second method considers that the reaction is active for all temperatures but heat losses are negligible; see [7] and references therein. The last method also allows obtaining physically consistent process parameters such as combustion wave speed and combustion front temperature. The previously mentioned works consider one-dimensional models that do not take the pressure equation into account. None of these is directly applicable to two-dimensional models, nor to models containing the pressure equation. The main goal of this work is to verify whether the analytical estimates proposed in [7] can be applied to more general two-dimensional nonhomogeneous models.Other works address two-dimensional combustion. For example in [10], two-dimensional simulations were performed and it was observed that the combustion front assumes a curved profile when the domain has strongly spatially correlated characteristics, whereas it is almost linear if heterogeneity is random. In [11] the compositional flow in porous media was considered to model forward ISC. The viscous fingering effect was observed for heavy oil oxidation. There is a number of other works that study the filtration combustion in forward (e.g., [12]) or reverse (e.g., [13, 14]) regimes. Although many papers deal with multidimensional combustion models, there is a lack of analytical studies on this topic. Similar to [10], fingering instabilities observed in the present work appear only for correlated nonhomogeneous medium, when fingers increase in length over time.In this work we follow [3, 7] and consider the combustion of immobile fuel. In Section 2 we present a two-dimensional gas-solid model containing an equation for pressure, balance laws for energy, oxygen content, fuel molar mass, and total gas mass, where the gas density is described by the ideal gas law and the average gas speed by Darcy’s law. In Section 3 we show that the model proposed in this work is indeed a generalization of the one-dimensional model proposed in [3]. In this section we also review and generalize some analytical results obtained in [7]. In order to simulate the gas-solid process we use the finite element method described in Section 4. Section 5 presents numerical results, which are compared with analytical estimates. Further numerical examples are performed in order to validate the numerical model. Finally, Section 6 summarizes final conclusions.


## 2. Model

Air is injected into a porous medium initially filled with a nonreactive gas and immobile fuel, which can be solid or liquid at low saturations. We consider a combustion reaction that consumes oxygen and fuel, generating heat. We assume that only a small part of the void space is occupied by the fuel, so its consumption does not affect the porosity of the matrix. We consider the local thermal equilibrium resulting in equal temperatures for all phases (fuel, gas, and rock). Heat losses are neglected, which is a reasonable hypothesis for ISC in field conditions.Some other assumptions are considered: heat transfer due to radiation and energy source due to pressure variation are neglected; the work related to surface and body forces is also neglected; the ideal gas law is the equation of state for the gas phase; gas heat capacity is negligible when compared to the heat capacity of the rock. To simplify the model, we neglect changes in the effective molecular weight of the gas phase due to the reaction by approximating this quantity by a constant. We do not consider the effect of gravity in the flow and the dispersion terms in the diffusion-dispersion tensor. Thermodynamic and transport properties such as thermal conductivity of the medium, heat capacity of the rock, and specific heat capacity of the gas are considered constants. Generally, the porosity is related with permeability through polynomial relations, it also changes within the combustion reaction. For simplicity, we consider isotropic porous medium with constant porosity and simplified combustion reaction:C+O2→CO2.Lett∈R+ and x∈R2 be the time and space coordinates. The primary dependent variables are the temperature, T(x,t) (Kelvin), the oxygen concentration in terms of molar fraction, Y(x,t) (moles of oxygen per moles of gas), the molar concentration of fuel, ρf(x,t) (moles of fuel per cubic meter), and molar density of gas, ρ(x,t) (moles of gas per cubic meter).The system of equations extends the one-dimensional model from [6] by considering variable pressure, two-dimensional domain, and Darcy’s law, based on [15]. Parameter names and typical values are given in Table 1. We present the following equations under the assumptions described above. The energy balance law is(1)Cm∂tT+cg∇·ρTu=kTΔT+QW,where Cm is the volumetric heat capacity of the rock, cg is the specific heat capacity of the gas, u is the average gas speed, kT is the thermal conductivity, Q is the heat released due to combustion of each mole of fuel, and W is the combustion rate. The molar mass balance of oxygen in the gas phase is(2)ϕ∂tYρ+∇·Yρu=ϕdm∇·ρ∇Y-W,where ϕ is the porosity of the medium and dm is the molecular diffusion coefficient. The molar mass balance of fuel is(3)∂tρf=-W.The molar mass balance of total gas is(4)ϕ∂tρ+∇·ρu=0.The reaction rate is described by the Arrhenius law combined with the linear Law of Mass Action [16].(5)W=kpYρfppatmexp⁡-ERT,where patm is the reference pressure, E is the activation energy, kp is the preexponential factor, and R is the ideal gas constant. The gas phase equation of state is the ideal gas law:(6)p=ρRT.The average gas speed is given by Darcy’s law:(7)u=-κμ∇p,where μ is the dynamic gas viscosity and κ is the absolute permeability of the porous medium.Table 1
Dimensional parameters for ISC and their typical values.
Symbol Description Value Unit R Ideal gas constant 8.314 J/(mol⋅K) C m Heat capacity of the rock 2 × 106 J/(m3⋅K) c g Specific heat capacity of the gas 27.42 J/(mol⋅K) k T Thermal conductivity of the medium 0.87 J/(m⋅s⋅K) Q Heat released due to combustion 4 × 105 J/mol ϕ Porosity of the medium 0.3 ⋅ d m Molecular diffusion coefficient 2 × 10−6 m2/s k p Preexponential factor 500 1/s E Activation energy 58000 J/mol p a t m Atmospheric pressure 1 atm T r e s Initial temperature of the reservoir 370 K Y i n j Molar fraction of oxygen in the air 0.21 ⋅ ρ f r e s Initial molar concentration of fuel 372 mol/m3 p r e s Reference pressure in reservoir 1 atm κ - Average permeability of the medium 10−12 m2 μ - Average gas viscosity 2 × 10−5 Pa⋅s


### 2.1. Dimensionless Equations

In order to simplify the mathematical analysis, we introduce dimensionless dependent and independent variables (denoted by tildes)(8)t~=tt∗,x~=xx∗,u~=uu∗,Θ=T-T∗δT∗,Y~=YY∗,ρ~f=ρfρf∗,p~=p-p∗δp∗,ρ~=ρρ∗,and some reference quantities (denoted by stars)(9)T∗=Tres,Y∗=Yinj,ρf∗=ρfres,p∗=pres,ρ∗=p∗RT∗,δT∗=Qρf∗Cm,u∗=κ-μ-δp∗x∗,t∗=patmkpY∗p∗exp⁡ERT∗,δp∗=pres,v∗=Y∗ρ∗u∗ρf∗,x∗=v∗t∗,where Tres, ρfres, and pres are the initial reservoir conditions for temperature, molar density of fuel, and gas pressure, respectively; Yinj is the oxygen fraction in the injected air; and κ¯ and μ¯ are the average permeability of the porous medium and average dynamic gas viscosity, respectively.Using (8), (9) and omitting tildes,  (1)–(7) are written in dimensionless form as(10)∂tΘ+VT∇·ρΘ+Θdu=1PeTΔΘ+Φ,(11)∂tYρ+σ∇·Yρu=1Pe∇·ρ∇Y-σΦ,(12)∂tρf=-Φ,(13)∂tρ+σ∇·ρu=0,(14)p=pdρ1+ΘΘd-1,(15)u=-λ∇p,(16)Φ=t∗ρf∗W=ρfY1+ppdexp⁡EΘd-EΘ+Θd,where the dimensionless constants are(17)Θd=T∗δT∗,pd=p∗δp∗,Pe=v∗x∗dm,PeT=v∗x∗CmkT,VT=cgρf∗CmY∗,σ=ρf∗ϕρ∗Y∗,E=ERδT∗,where Θd and pd are the scaled reservoir temperature and pressure, PeT and Pe are the Péclet numbers for thermal and mass diffusion, VT is the dimensionless thermal wave speed, σ is the fuel to oxygen concentration rate, and E is the scaled activation energy. In this work we consider that permeability κ depends on x and viscosity μ depends on Θ; thus the scaled gas mobility function (see [[17], Sec. 4.9]) is defined as λ(Θ,x)=(μ¯/μ(Θ))(κ(x)/κ¯). In this work μ is considered constant, the general case was left for the future work.


## 2.1. Dimensionless Equations

In order to simplify the mathematical analysis, we introduce dimensionless dependent and independent variables (denoted by tildes)(8)t~=tt∗,x~=xx∗,u~=uu∗,Θ=T-T∗δT∗,Y~=YY∗,ρ~f=ρfρf∗,p~=p-p∗δp∗,ρ~=ρρ∗,and some reference quantities (denoted by stars)(9)T∗=Tres,Y∗=Yinj,ρf∗=ρfres,p∗=pres,ρ∗=p∗RT∗,δT∗=Qρf∗Cm,u∗=κ-μ-δp∗x∗,t∗=patmkpY∗p∗exp⁡ERT∗,δp∗=pres,v∗=Y∗ρ∗u∗ρf∗,x∗=v∗t∗,where Tres, ρfres, and pres are the initial reservoir conditions for temperature, molar density of fuel, and gas pressure, respectively; Yinj is the oxygen fraction in the injected air; and κ¯ and μ¯ are the average permeability of the porous medium and average dynamic gas viscosity, respectively.Using (8), (9) and omitting tildes,  (1)–(7) are written in dimensionless form as(10)∂tΘ+VT∇·ρΘ+Θdu=1PeTΔΘ+Φ,(11)∂tYρ+σ∇·Yρu=1Pe∇·ρ∇Y-σΦ,(12)∂tρf=-Φ,(13)∂tρ+σ∇·ρu=0,(14)p=pdρ1+ΘΘd-1,(15)u=-λ∇p,(16)Φ=t∗ρf∗W=ρfY1+ppdexp⁡EΘd-EΘ+Θd,where the dimensionless constants are(17)Θd=T∗δT∗,pd=p∗δp∗,Pe=v∗x∗dm,PeT=v∗x∗CmkT,VT=cgρf∗CmY∗,σ=ρf∗ϕρ∗Y∗,E=ERδT∗,where Θd and pd are the scaled reservoir temperature and pressure, PeT and Pe are the Péclet numbers for thermal and mass diffusion, VT is the dimensionless thermal wave speed, σ is the fuel to oxygen concentration rate, and E is the scaled activation energy. In this work we consider that permeability κ depends on x and viscosity μ depends on Θ; thus the scaled gas mobility function (see [[17], Sec. 4.9]) is defined as λ(Θ,x)=(μ¯/μ(Θ))(κ(x)/κ¯). In this work μ is considered constant, the general case was left for the future work.


## 3. Analytical Estimates for a Simplified Model

In this section we simplify system (10)–(16) in order to obtain analytical estimates. Following [6, 7] we restrict ourselves to the one-dimensional case and consider small pressure variation when compared to reservoir reference pressure. Mathematically, this is equivalent to considering the constant prevailing pressure p¯ and removing Darcy’s law from the system. To simplify notation we denote P=1+p¯/pd. The new model considers gas density as a function of temperature given by the ideal gas law. Under these hypotheses, the dimensionless system can be rewritten as(18)∂tΘ+VT∂xρΘ+Θdu=PeT-1∂xxΘ+Φ,(19)∂tYρ+σ∂xYρu=Pe-1∂xρ∂xY-σΦ,(20)∂tρf=-Φ,(21)∂tρ+σ∂xρu=0,(22)ρ=PΘdΘ+Θd,(23)Φ=PρfYexp⁡EΘd-EΘ+Θd.When the prevailing pressure is considered to be equal to the atmospheric pressure, we obtainP=1. In this case the model is equivalent to ones studied in [3, 6, 7]. Differently from [7] we do not restrict ourselves to the limit case with large values of σ. We consider(24)Initial  conditions:t=0,x⩾0:Θ=Y=0,u=ρf=1,(25)Injection  conditions:t>0,x=0:Θ=ρf=0,Y=1,u=uinj.The solution of system (18)–(22) with general boundary conditions can be described as a sequence of waves: fuel concentration wave, thermal wave, combustion wave, and gas composition wave separated by constant states; see [6]. Considering conditions (24)-(25) after a sufficiently large time only thermal and combustion waves are presented. We consider the case in which the thermal wave is slower than the combustion wave, which is reasonable in ISC.We consider the combustion wave as a traveling wave solution of system (18)–(22) moving with constant speed V>0. We indicate the combustion wave neighbor constant states as burned (indicated by letter b) and unburned (indicated by letter u) and define the traveling variable ξ=x-Vt. The left (burned) limit states for the combustion wave are(26)ξ⟶-∞:Θb>0,Yb=1,ρfb=0,ub=fρb,uinj,where f is a known function that can be obtained from the analysis of the thermal wave; see Appendix A. Variable ρb is obtained by substituting the unknown Θb into (22). The right (unburned) states corresponds to(27)ξ⟶+∞:Θu=Yu=0,ρfu=1,uu>0.Only uu is unknown. The variable values at these states can be determined by analysis of the initial and boundary conditions; see Appendix A.Under condition (24) the sequence of waves moves in a positive direction of the x-axis; then the conditions to the left of the thermal wave are given by the injection conditions. Since the thermal wave is slower than the combustion wave, the conditions to the left of the combustion wave are equal to the conditions to the right of the thermal wave.The equations describing the traveling wave solution are obtained from (18) to (22) substituting the derivatives ∂x and ∂t by d/dξ and -Vd/dξ:(28)-VΘ′+VTPΘdu′=PeT-1Θ′′+Φ,(29)-VYρ′+σYρu′=Pe-1ρY′′-σΦ,(30)-Vρf′=-Φ,(31)-Vρ′+σρu′=0,where the prime (′) means derivative in ξ. We substitute (30) into (28) and (29) obtaining(32)VΘ-VTPΘdu+PeT-1Θ′+Vρf′=0,VYρ+Pe-1ρY′-σYρu-σVρf′=0,Vρ-σρu′=0.Integrating these equations in(-∞,+∞), substituting the limit states (26)-(27), and considering that derivatives vanish in the boundary Θ′=Y′=0, we obtain the estimates for the combustion wave temperature Θb, combustion wave speed V, and gas speed uu:(33)Θb=11-VT,V=ubp¯,uinj1/ρbp¯+1/σ,uu=1Pp¯+1σVp¯,uinj.Analytical formulas in (33) are generalizations of equivalent ones present in [7], where was considered atmospheric pressure p¯=0 and σ≫1. In Section 5, the values of parameters Θb, V, and uu, from (33), are compared with values obtained from numerical simulations using the general model described by (10)–(16). These parameters are also used in Appendix B to validate the numerical model from next section.


## 4. Numerical Model

In this section we present the numerical model used to solve system (10)–(16) in an open bounded domain Ω∈R2 with boundary Γ. The spatial discretization uses FEM; time discretization uses Crank-Nicolson method and the resulting implicit scheme is solved using Newton’s method.In order to allow the implementation of the numerical method we use (13) to obtain a nonconservative form of (11):(34)∂tY+σu-1Pe∇ρρ·∇Y=1PeΔY-σΦρ.In what follows we shorten the notation by grouping the variables as U=(Θ,Y,ρf,ρ) and consider Ω boundary as union of Dirichlet type boundary Γ1 and zero-flow Neumann type boundary Γ2 resulting in Γ=Γ1∪Γ2¯. We always use boundary conditions compatible with the initial condition:(35)Ut=0=U0=U01,U02,U03,U04=Θ0,Y0,ρf0,ρ0.Implemented boundary conditions for t>0 are(36)Ux,t=gx,∀x∈Γ1,(37)∇Ux,t·nx=0,∀x∈Γ2.In this work we apply the standard FEM on system (10)–(16). This simple approach meets our goal, although the use of specific methodologies could improve the precision and convergence of the numerical procedure. In Appendix B.2 we show that the numerical instabilities appearing in the simulations do not affect the structural stability of the combustion front and do not compromise present results.


### 4.1. Weak Formulation and Discretization

LetV={v=(v1,v2,v3,v4)∈[H1(Ω)]4;v|Γ1=0} be the space of test functions. We use 〈u,v〉 to denote the inner product between u,v∈L2(Ω). In the weak formulation, solving the system described in (10)–(16) is equivalent to find U such that for all v∈V and t>0 it satisfies(38)∂tΘ,v1-VTρΘ+Θdu,∇v1+PeT-1∇Θ,∇v1-Φ,v1=0,∂tY,v2+σu-1Pe∇ρρ·∇Y,v2+1Pe∇Y,∇v2+σΦρ,v2=0,∂tρf,v3+Φ,v3=0,∂tρ,v4-σρu,∇v4=0,together with boundary conditions (36) and initial conditions(39)Uit=0,vi=U0i,vi,i=1,2,3,4.LetVh∈V be a finite element space with basis β={φi}i=1m. After applying Standard Galerkin and Crank-Nicolson methods to system (38)-(39), the matrix formulation of the weak problem reads: for each k=0,1,…,n, determine αk∈Rm such that(40)fkαk=Mαk-αk-1-Δtk2Fαk+Fαk-1=0,k>0,Mα0=F0,where M∈Rm×m and F(α),F0∈Rm are described in Appendix C and(41)Uhk=∑i=1mαikφi+g.For each time stepk>0 we use Newton’s method to obtain zeros of the function fk considering the previous time step αk-1. We use the adaptive time step to speed up the simulations, where the size of next step was based in the convergence of Newton’s method with tolerance equal to 10-6 for the relative error using the Euclidean norm.


## 4.1. Weak Formulation and Discretization

LetV={v=(v1,v2,v3,v4)∈[H1(Ω)]4;v|Γ1=0} be the space of test functions. We use 〈u,v〉 to denote the inner product between u,v∈L2(Ω). In the weak formulation, solving the system described in (10)–(16) is equivalent to find U such that for all v∈V and t>0 it satisfies(38)∂tΘ,v1-VTρΘ+Θdu,∇v1+PeT-1∇Θ,∇v1-Φ,v1=0,∂tY,v2+σu-1Pe∇ρρ·∇Y,v2+1Pe∇Y,∇v2+σΦρ,v2=0,∂tρf,v3+Φ,v3=0,∂tρ,v4-σρu,∇v4=0,together with boundary conditions (36) and initial conditions(39)Uit=0,vi=U0i,vi,i=1,2,3,4.LetVh∈V be a finite element space with basis β={φi}i=1m. After applying Standard Galerkin and Crank-Nicolson methods to system (38)-(39), the matrix formulation of the weak problem reads: for each k=0,1,…,n, determine αk∈Rm such that(40)fkαk=Mαk-αk-1-Δtk2Fαk+Fαk-1=0,k>0,Mα0=F0,where M∈Rm×m and F(α),F0∈Rm are described in Appendix C and(41)Uhk=∑i=1mαikφi+g.For each time stepk>0 we use Newton’s method to obtain zeros of the function fk considering the previous time step αk-1. We use the adaptive time step to speed up the simulations, where the size of next step was based in the convergence of Newton’s method with tolerance equal to 10-6 for the relative error using the Euclidean norm.


## 5. Numerical Experiments

Using parameter values given in Table1 and (9)–(17) we obtain the value of the characteristic quantities and dimensionless parameters: ρ∗=32.94mol/m3, δT∗=74.40m, t∗=1.470×106s, u∗=4.306×10-4m/s, x∗=11.77m, Θd=4.97, pd=1, PeT=216.56, Pe=47.10, VT=0.0243, σ=179.27, and E=93.77.System (10)–(16) was solved using FEM in a rectangular domain discretized using square elements with 200 divisions in horizontal x axis and 50 divisions in the vertical y axis. The algorithm uses an adaptive time step with the initial time step value equal to Δt=4×10-7. The right and left sides are modeled using the Dirichlet boundary conditions gi=U0i, for each i=1,2,3,4. The upper and lower boundaries are modeled using Neumann zero-flow conditions. The initial conditions are(42)Θ=0,Y=1,x⩽0,0,x>1,ρf=0,x⩽0,1,x>1,p=4-x2,and the initial value of ρ is obtained by substituting (42) into (14). Air is injected from the left side corresponding to zero on the horizontal axis.Generally, the permeability functionκ depends on space coordinates. In this paper we study two possibilities for κ. The first case corresponds to a homogeneous porous medium with constant permeability κ=κ¯ given in Table 1. The second kind of permeability field is more realistic from the physical point of view and represents typical reservoir conditions [18]. A number of works use similar permeability fields in numerical experiments [11, 19, 20]. This case corresponds to a nonhomogeneous porous medium with log-normal permeability field, similar to the one used in [11]. Permeability values vary from 0.40×10-12[m2] to 9.97×10-12[m2] and the mean value is equal to κ¯, given in Table 1. This field is given by(43)κx=ea+bZx,where a=-27.693 and b=0.35297 are the mean and standard deviation of the permeability’s natural logarithm, respectively and Z is a standard normal distribution with constant values in each element; see Figure 1. Figure 2 shows the histogram and probability density function for this distribution.Figure 1
Standard normal distributionZ correlated by formula (43) to the log-normal permeability field in a 200×50 grid.Figure 2
Histogram of the log-normal permeability fieldκ and respective probability density function fκ. The “Freq.” function shows the frequency of elements that has permeability value occurring in each range of permeability.


### 5.1. Parameter Values from Numerical Simulations

In order to obtain the combustion wave speed for two-dimensional simulations we assume that the combustion wave moves only in thex direction. This is exactly the case for homogeneous porous media. For the nonhomogeneous case, the x component of gas velocity ux is more than 11 times greater than |uy| as can be seen in Figure 9.At timet we track the position of the combustion wave for each value of y by the value of x for which ρf=0.2. This approach is accurate since there are no diffusion nor advection terms in the fuel balance equation (12). The combustion wave speed VS(t) is the average of the velocities of each y and σ(VS)(t) denoting the standard deviation of these velocities.Analogously to the combustion wave speed, at timet for each value of y, we consider the combustion wave temperature ΘSb(y,t) as the maximum value of Θ varying x. In this case, the combustion wave temperature, ΘSb(t), is the average of the values ΘSb(y,t) for all y and the standard deviation of such values is denoted by σ(ΘSb)(t). Finally, we consider that the gas speed downstream the combustion wave, uSu(t), can be obtained by the average of u over the right side of the spatial domain.


### 5.2. Parameter Values for Analytical Formulas

In order to use (33) we need the value of the prevailing pressure, p¯, and the injection gas speed, uinj, which are not constant for system (10)–(15). Here we study three different choices for the prevailing pressure:(1)
Minimum pressure in the reservoir, which coincides with the native reservoir pressure.(2)
Maximum pressure in the reservoir, which coincides with the injection pressure.(3)
The average pressure measured at the top of the combustion front, which is a numerical estimate for the pressure in the burned state of combustion wave.We estimate the parameteruinj in two different ways:(A)
Injection gas speed is equal to the average injection gas speed (uinjS):(44)uinjt=uinjSt.(B)
Injection gas flux is equal to the average injection gas flux:(45)uinjt=pinjSt+pdp¯t+pduinjSt,wherepinjS(t) is the average pressure of injection in the simulation. This equation was obtained using (14) and (22).BothuinjS and pinjS(t) are obtained as the uSu parameter described in Section 5.1. In the simulations we use constant injection pressure, so estimates (A2) and (B2) are equal.


### 5.3. Simulation Results: Homogeneous Porous Medium

In the homogeneous case, the solution is constant in they direction. In order to improve visibility we plot the numerical result corresponding to the cross-section at y=0. The simulation results for dimensionless variables Θ, Y, ρf, ρ, p, and ux corresponding to 10 and 50 days are plotted in Figures 3 and 4, respectively. The analytically estimated parameter Θb, given in (33), is also plotted showing good agreement with the value of Θ obtained numerically. The combustion and thermal waves are present for both plots. After 50 days one can observe small numerical oscillations arising in variables Y and ρf as shown in Figure 4. In Appendix B.2 we show evidence that these oscillations do not affect the structural stability of the combustion front.Figure 3
Simulation results for the homogeneous permeability field at 10 days. Dimensionless variables values forΘ, Y, and ρf are on (a) and for ρ, p, and ux are on (b). The figure corresponds to the cross-section y=0. The analytically estimated parameter Θb is plotted on (a). The space variable x corresponds to the horizontal axis.
(a) (b)Figure 4
Simulation results for the homogeneous permeability field at 50 days. Dimensionless variable values forΘ, Y, and ρf are on (a) and for ρ, p, and ux are on (b). The figure corresponds to the cross-section y=0. The analytically estimated parameter Θb is plotted on (a). The space variable x corresponds to the horizontal axis.
(a) (b)In Figures5, 6, and 7 we compare the values of parameters ΘSb, VS, and uSu obtained from numerical simulations described in Section 5.1 with analytical estimates from (33). Parameters uinj and p¯ were chosen as described in Section 5.2. Combustion wave temperature in the simulated model is the same as in the simplified one. This means that parameter Θb can be used to predict combustion wave temperature with good accuracy. Notice that this estimate does not depend on either uinj or p¯.Figure 5
Combustion wave temperature for the homogeneous porous medium: analytically estimatedΘb and numerically obtained ΘSb; see Section 5.1.Figure 6
Analytically estimated combustion wave speedV (see Section 5.2) and numerically obtained combustion wave speed VS corresponding to the homogeneous porous medium.Figure 7
Analytically estimated gas velocityuu (see Section 5.2) and numerically obtained gas velocity uSu corresponding to the homogeneous porous medium.In Figure6, we see that the estimates corresponding to case (B) are more accurate than the ones corresponding to case (A). All estimates (B1), (B2), and (B3) give good approach to the combustion wave velocity. Among all considered cases, only case (B1) presents good estimates for uu, as shown in Figure 7. We conclude that the approximation (B1) provides a better description of the simulations presented in this work.


### 5.4. Simulation Results: Nonhomogeneous Porous Medium

The simulation results for the nonhomogeneous case at 10 days and 50 days are shown in Figure8 for variables Θ, Y, ρf, ρ, and p and Figure 9 for variables u=|u|, ux, and |uy|. One can observe the fast propagating combustion wave, corresponding to sudden variation in variables Θ, Y, and ρf and the slow thermal wave corresponding to variation in variable Θ. Interface instabilities (fingering) can be seen in the combustion front of the solution for Θ, Y, and ρf. We stress that since we simulate single phase flow, these instabilities are not related to the process called viscous fingering, which occurs when a less viscous fluid meets a more viscous one. The fingering instabilities seem to increase in length from 10 to 50 days, as can be seen in Figure 8. This effect is expected to be even greater in more realistic models since gas speed increases from upstream to downstream of the combustion wave. This relation between gas speed and temperature is due to the dynamic gas viscosity, which increases as temperature decreases; see Sutherland’s formula in [21].Figure 8
Simulation results in a log-normal permeability field for the dimensionless variables from top to bottom:Θ, Y, ρf, ρ, and p. The solution is shown at 10 days (a) and 50 days (b). Space variables x and y are plotted in meters.
(a) (b)Figure 9
Simulation results in a log-normal permeability field for the dimensionless variables from top to bottom:u=|u|, ux, and |uy|. The solution is shown at 10 days (a) and 50 days (b). Space variables x and y are plotted in meters.
(a) (b)We can see that horizontal velocityux is higher than vertical velocity uy indicating that the flow occurs mainly in the horizontal direction. The results present numerical instabilities near the top of the combustion wave. This kind of error is due to the heterogeneity of the medium and diminishes if we use a more refined mesh. Although it cannot be clearly seen in Figure 8, numerical instabilities similar to those observed for the homogeneous porous medium appear near the right side of the domain. The numerical results from Appendix B.2 show evidence that these instabilities do not compromise the solution.As done in the previous section, in Figures10, 11, and 12 we compare parameter values obtained from the numerical simulations with the corresponding analytical estimates obtained from (33), where uinj and p¯ are chosen as described in Section 5.2.Figure 10
Analytically estimated combustion wave temperatureΘb, numerically obtained temperature ΘSb, and its standard deviation σ(ΘSb) in the nonhomogeneous porous medium.Figure 11
Analytically estimated combustion wave speedV (see Section 5.2) and numerically obtained combustion wave speed VS corresponding to the nonhomogeneous porous medium.Figure 12
Analytically estimated gas velocityuu (see Section 5.2) and numerically obtained gas velocity uSu corresponding to the nonhomogeneous porous medium.In Figure10 we plot the analytical estimate of Θb, numerically obtained temperature ΘSb, and its standard deviation μ(ΘSb). The relation |Θb-μ(ΘSb)|<σ(ΘSb) is achieved and after some time the average of combustion wave temperature remains near Θb.In Figure11 we see that combustion wave speed estimates corresponding to case (B) are more accurate than ones corresponding to case (A). All estimates (B1), (B2), and (B3) approach the combustion wave velocity. Furthermore, relation |V(B)-VS|<σ(VS) is satisfied for all cases (B) and (A2). Figure 12 shows that only case (B1) presents good accuracy estimating uSu. As in homogeneous case the approximation (B1) describes better the simulation results.We present the relative error for main parameters and cases in Table2. The errors were calculated at each time step for t>10 days. Each set of errors of the average and standard deviation were evaluated. This data evidences the conclusion that (B1) is a good parameter choice.Table 2
The relative error (its mean value and standard deviation (std)) for all estimates for homogeneous and nonhomogeneous porous medium.
Relative error of estimate Θ b V ( B 1 ) V ( B 2 ) V ( B 3 ) u u ( B 1 ) Homogeneous case: Mean 0.05% 4.04% 3.08% 3.35% 2.00% std 0.07% 0.73% 0.72% 0.76% 0.45% Nonhomogeneous case: Mean 4.99% 3.91% 3.03% 3.21% 1.70% std 1.11% 1.85% 1.72% 1.78% 0.42%


## 5.1. Parameter Values from Numerical Simulations

In order to obtain the combustion wave speed for two-dimensional simulations we assume that the combustion wave moves only in thex direction. This is exactly the case for homogeneous porous media. For the nonhomogeneous case, the x component of gas velocity ux is more than 11 times greater than |uy| as can be seen in Figure 9.At timet we track the position of the combustion wave for each value of y by the value of x for which ρf=0.2. This approach is accurate since there are no diffusion nor advection terms in the fuel balance equation (12). The combustion wave speed VS(t) is the average of the velocities of each y and σ(VS)(t) denoting the standard deviation of these velocities.Analogously to the combustion wave speed, at timet for each value of y, we consider the combustion wave temperature ΘSb(y,t) as the maximum value of Θ varying x. In this case, the combustion wave temperature, ΘSb(t), is the average of the values ΘSb(y,t) for all y and the standard deviation of such values is denoted by σ(ΘSb)(t). Finally, we consider that the gas speed downstream the combustion wave, uSu(t), can be obtained by the average of u over the right side of the spatial domain.


## 5.2. Parameter Values for Analytical Formulas

In order to use (33) we need the value of the prevailing pressure, p¯, and the injection gas speed, uinj, which are not constant for system (10)–(15). Here we study three different choices for the prevailing pressure:(1)
Minimum pressure in the reservoir, which coincides with the native reservoir pressure.(2)
Maximum pressure in the reservoir, which coincides with the injection pressure.(3)
The average pressure measured at the top of the combustion front, which is a numerical estimate for the pressure in the burned state of combustion wave.We estimate the parameteruinj in two different ways:(A)
Injection gas speed is equal to the average injection gas speed (uinjS):(44)uinjt=uinjSt.(B)
Injection gas flux is equal to the average injection gas flux:(45)uinjt=pinjSt+pdp¯t+pduinjSt,wherepinjS(t) is the average pressure of injection in the simulation. This equation was obtained using (14) and (22).BothuinjS and pinjS(t) are obtained as the uSu parameter described in Section 5.1. In the simulations we use constant injection pressure, so estimates (A2) and (B2) are equal.


## 5.3. Simulation Results: Homogeneous Porous Medium

In the homogeneous case, the solution is constant in they direction. In order to improve visibility we plot the numerical result corresponding to the cross-section at y=0. The simulation results for dimensionless variables Θ, Y, ρf, ρ, p, and ux corresponding to 10 and 50 days are plotted in Figures 3 and 4, respectively. The analytically estimated parameter Θb, given in (33), is also plotted showing good agreement with the value of Θ obtained numerically. The combustion and thermal waves are present for both plots. After 50 days one can observe small numerical oscillations arising in variables Y and ρf as shown in Figure 4. In Appendix B.2 we show evidence that these oscillations do not affect the structural stability of the combustion front.Figure 3
Simulation results for the homogeneous permeability field at 10 days. Dimensionless variables values forΘ, Y, and ρf are on (a) and for ρ, p, and ux are on (b). The figure corresponds to the cross-section y=0. The analytically estimated parameter Θb is plotted on (a). The space variable x corresponds to the horizontal axis.
(a) (b)Figure 4
Simulation results for the homogeneous permeability field at 50 days. Dimensionless variable values forΘ, Y, and ρf are on (a) and for ρ, p, and ux are on (b). The figure corresponds to the cross-section y=0. The analytically estimated parameter Θb is plotted on (a). The space variable x corresponds to the horizontal axis.
(a) (b)In Figures5, 6, and 7 we compare the values of parameters ΘSb, VS, and uSu obtained from numerical simulations described in Section 5.1 with analytical estimates from (33). Parameters uinj and p¯ were chosen as described in Section 5.2. Combustion wave temperature in the simulated model is the same as in the simplified one. This means that parameter Θb can be used to predict combustion wave temperature with good accuracy. Notice that this estimate does not depend on either uinj or p¯.Figure 5
Combustion wave temperature for the homogeneous porous medium: analytically estimatedΘb and numerically obtained ΘSb; see Section 5.1.Figure 6
Analytically estimated combustion wave speedV (see Section 5.2) and numerically obtained combustion wave speed VS corresponding to the homogeneous porous medium.Figure 7
Analytically estimated gas velocityuu (see Section 5.2) and numerically obtained gas velocity uSu corresponding to the homogeneous porous medium.In Figure6, we see that the estimates corresponding to case (B) are more accurate than the ones corresponding to case (A). All estimates (B1), (B2), and (B3) give good approach to the combustion wave velocity. Among all considered cases, only case (B1) presents good estimates for uu, as shown in Figure 7. We conclude that the approximation (B1) provides a better description of the simulations presented in this work.


## 5.4. Simulation Results: Nonhomogeneous Porous Medium

The simulation results for the nonhomogeneous case at 10 days and 50 days are shown in Figure8 for variables Θ, Y, ρf, ρ, and p and Figure 9 for variables u=|u|, ux, and |uy|. One can observe the fast propagating combustion wave, corresponding to sudden variation in variables Θ, Y, and ρf and the slow thermal wave corresponding to variation in variable Θ. Interface instabilities (fingering) can be seen in the combustion front of the solution for Θ, Y, and ρf. We stress that since we simulate single phase flow, these instabilities are not related to the process called viscous fingering, which occurs when a less viscous fluid meets a more viscous one. The fingering instabilities seem to increase in length from 10 to 50 days, as can be seen in Figure 8. This effect is expected to be even greater in more realistic models since gas speed increases from upstream to downstream of the combustion wave. This relation between gas speed and temperature is due to the dynamic gas viscosity, which increases as temperature decreases; see Sutherland’s formula in [21].Figure 8
Simulation results in a log-normal permeability field for the dimensionless variables from top to bottom:Θ, Y, ρf, ρ, and p. The solution is shown at 10 days (a) and 50 days (b). Space variables x and y are plotted in meters.
(a) (b)Figure 9
Simulation results in a log-normal permeability field for the dimensionless variables from top to bottom:u=|u|, ux, and |uy|. The solution is shown at 10 days (a) and 50 days (b). Space variables x and y are plotted in meters.
(a) (b)We can see that horizontal velocityux is higher than vertical velocity uy indicating that the flow occurs mainly in the horizontal direction. The results present numerical instabilities near the top of the combustion wave. This kind of error is due to the heterogeneity of the medium and diminishes if we use a more refined mesh. Although it cannot be clearly seen in Figure 8, numerical instabilities similar to those observed for the homogeneous porous medium appear near the right side of the domain. The numerical results from Appendix B.2 show evidence that these instabilities do not compromise the solution.As done in the previous section, in Figures10, 11, and 12 we compare parameter values obtained from the numerical simulations with the corresponding analytical estimates obtained from (33), where uinj and p¯ are chosen as described in Section 5.2.Figure 10
Analytically estimated combustion wave temperatureΘb, numerically obtained temperature ΘSb, and its standard deviation σ(ΘSb) in the nonhomogeneous porous medium.Figure 11
Analytically estimated combustion wave speedV (see Section 5.2) and numerically obtained combustion wave speed VS corresponding to the nonhomogeneous porous medium.Figure 12
Analytically estimated gas velocityuu (see Section 5.2) and numerically obtained gas velocity uSu corresponding to the nonhomogeneous porous medium.In Figure10 we plot the analytical estimate of Θb, numerically obtained temperature ΘSb, and its standard deviation μ(ΘSb). The relation |Θb-μ(ΘSb)|<σ(ΘSb) is achieved and after some time the average of combustion wave temperature remains near Θb.In Figure11 we see that combustion wave speed estimates corresponding to case (B) are more accurate than ones corresponding to case (A). All estimates (B1), (B2), and (B3) approach the combustion wave velocity. Furthermore, relation |V(B)-VS|<σ(VS) is satisfied for all cases (B) and (A2). Figure 12 shows that only case (B1) presents good accuracy estimating uSu. As in homogeneous case the approximation (B1) describes better the simulation results.We present the relative error for main parameters and cases in Table2. The errors were calculated at each time step for t>10 days. Each set of errors of the average and standard deviation were evaluated. This data evidences the conclusion that (B1) is a good parameter choice.Table 2
The relative error (its mean value and standard deviation (std)) for all estimates for homogeneous and nonhomogeneous porous medium.
Relative error of estimate Θ b V ( B 1 ) V ( B 2 ) V ( B 3 ) u u ( B 1 ) Homogeneous case: Mean 0.05% 4.04% 3.08% 3.35% 2.00% std 0.07% 0.73% 0.72% 0.76% 0.45% Nonhomogeneous case: Mean 4.99% 3.91% 3.03% 3.21% 1.70% std 1.11% 1.85% 1.72% 1.78% 0.42%


## 6. Conclusions

Previous work analyzed traveling wave solutions and obtained analytical formulas describing combustion wave temperature, velocity, and gas velocity for the one-dimensional gas-solid combustion model. In order to apply these estimates from [7] some assumptions on the reservoir prevailing pressure and gas injection speed needed to be considered. The best fit was obtained using average gas injection flux and minimum reservoir pressure measured on the production side. For numerical validation we presented a two-dimensional generalization of the gas-solid model studied in [3, 6, 7]. The numerical implementation used the finite element method for space discretization and Crank-Nicolson scheme for time discretization.For simulations using the nonhomogeneous porous medium, fingering instability phenomena were observed. In order to verify that these phenomena are not generated by numerical errors simulations using perturbed homogeneous porous medium were performed.



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