Title of Invention  MODELING SIMULATION AND COMPARISON OF MODELS FOR WORMHOLE FORMATION DURING MATRIX STIMULATION OF CARBONATES 

Abstract  1. A method of modeling a stimulation treatment involving a chemical reaction in a porous medium including describing the chemical reaction by coupling the reactions and mass transfer occurring at the Darcy scale and at the pore scale and considering the concentration Cf of a reactant in the pore fluid phase and the concentration of said reactant Cj at the fluid solid interface of a pore. 
Full Text  Modeling, Simulation and Comparison of Models for Wormhole Formation during Matrix Stimulation of Carbonates REFERENCE TO RELA TED PRO VISIONAL APPLICA TION This application claims the benefit of United States Provisional Application Serial No. 60/384,957. BACKGROUND OF THE INDENTION Field of the Invention. [0001] The present invention is generally related to hydrocarbon well stimulation, and is more particularly directed to a method for designing matrix treatment. The invention is particularly useful for designing acid treatment in carbonate reservoirs. Discussion of the Prior Art. J0002J Matrix acidizing is a widely used well stimulation technique. The primary objective in this process is to reduce the resistance to the flow of reservoir fluids due to a naturally tight formation or damages. Acid dissolves the material in the matrix and creates flow channels that increase the permeability of the matrix. The efficiency of this process depends on the type of acid used, injection conditions, structure of the medium, fluid to solid mass transfer, reaction . rates, etc. While dissolution increases the permeability, the relative increase in the permeability for a given amount of acid is observed to be a strong function of the injection conditions. [0003] In sandstone reservoirs, reaction fronts tend to be uniform and flow channeling is not observed. In carbonate reservoirs, depending on the injection conditions, multiple dissolution patterns may be produced, varying from uniform, conical and wormhole types. At very low flow rates, acid is spent soon after it contacts the medium resulting in face dissolution. The dissolution patterns are observed to be more uniform at high flow rates. At intermediate flow rates, long conductive channels known as wormholes are formed. These channels penetrate deep into the formation and facilitate the flow of oil. Experiments conducted in carbonate cores have shown that the relative increase in permeability for a given amount of acid injected is observed to be higher in wormholes. Thus, for optimizing a stimulation treatment, it is desirable to identify the parameters (e.g: rate of injection, acid type, thickness and permeability of the damaged zone etc.) that will produce wormholes with optimum density and penetrating deep into the formation. [4] It is well known that the optimum injection rate depends on the reaction and diffusion rates of the acid species, concentration of the acid, length of the core sample, temperature, permeability of the medium etc. The influence of the above factors on the wormhole formation is studied in the experiments. Several theoretical studies have been conducted in the past to obtain an estimate of the optimum injection rate and to understand the phenomena of flow channeling associated with reactive dissolution in porous media. However, the existing models describe only a few aspects of the acidizing process and the coupling of the mechanisms of reaction and transport at various scales that play a key role in the estimation of optimum injection rate are not properly accounted for in these models. [5] Several models have been proposed that are based on the assumption of an existing wormhole. Reference is made for instance to Wang, Y., Hill, A. D., and Schechter, R. S.:"The Optimum Injection Rate for Matrix Acidizing of Carbonate Formations" paper SPE 26578 presented at 1993 SPE Annual Technical Conference and Exhibition held in Houston, Texas, 3 6 October 1993; Buijse, M. A.:"Understanding Wormholing Mechanisms Can Improve Acid Treatments in Carbonate Formations," SPE Prod. & Facilities, 15 (3), 168175, 2000; and Huang, T., Zhu, D. and Hill, A. D.: "Prediction of Wormhole Population Density in Carbonate Matrix Acidizing," paper SPE 54723 presented at the 1999 SPE European Formation Damage Conference held in The Hague, 31 May01 June, 1999. [0006J These models are used to study the effect of fluid leakage, reaction kinetics etc., on the wormhole propagation rate and the effect of neighboring wormholes on growth rate of the dominant wormhole. The simple structure of these models offers the advantage of studying the reaction, diffusion and convection mechanisms inside the wormhole in detail. These models, however, cannot be used to study wormhole initiation and the effect of heterogeneities on wormhole formation. [0007] Network models describing reactive dissolution have been presented in Hoefiier M. L. and Fogler. H. S.: "Pore Evolution and Channel Formation During Flow and Reaction in Porous Media," AIChE J, 34, 4554 (1988); and Fredd, C. N. and Fogler, H. S.: "Influence of Transport and Reaction on Wormhole Formation in Porous Media," AIChE J, 44, 19331949 (1998). These models represent the porous medium as a network of tubes interconnected to each other at the nodes. Acid flow inside these tubes is described using HagenPoiseuille relationship for laminar flow inside a pipe. The acid reacts at the wall of the tube and dissolution is accounted in terms of increase in the tube radius. Network models are capable of predicting the dissolution patterns and the qualitative features of dissolution like optimum flow rate, observed in the experiments. However, a core scale simulation of the network model requires huge computational power and incorporating the effects of pore mergingand heterogeneities into these models is difficult. The results obtained from network models are also subject to scale up problems. [8] An intermediate approach to describing reactive dissolution involves the use of averaged or continuum models. Averaged models were used to describe the dissolution of carbonates by Pomes, V., Bazin, B., Golfier, F., Zarcone, C., Lenormand, R. and .Quintard, M.: "On the Use ofUpscaling Methods to Describe Acid Injection in Carbonates," paper SPE 71511 presented at 2001 SPE Annual Technical Conference and Exhibition held in New Orleans, Lousiana, 30 September3 October 2001; and Golfier, F., Bazin, B., Zarcone, C., Lenormand, R., Lasseux, D. and Quintard, M.: "On the ability of a Darcyscale model to capture wormhole formation during the dissolution of a porous medium," J. Fluid Mech., 457, 213254 (2002). Unlike the network models that describe dissolution from the pore scale and the models based on the assumption of existing wormholes, the averaged models describe dissolution at a scale much larger than the pore scale and much smaller than the scale of the core. This intermediate scale is also known as the Darcy scale. [9] Averaged models circumvent the scaleup problems associated with network models, can predict wormhole initiation, propagation and can be used to study the effects of heterogeneities in the medium on the dissolution process. The results obtained from the averaged models can be extended to the field scale. The success of these models depends on the key inputs such asmass transfer rates, permeabilityporosity correlation etc., which depend on the processes that occur at the pore scale. The averaged model written at the Darcy scale requires these inputs from the pore scale. Since the structure of the porous medium evolves with time, a pore level calculation has to be made at each stage to generate inputs for the averaged equation. [10] Averaged equations used by Golfier et al. and Pomes et al. describe the transport of the reactant at the Darcy scale with a pseudohomogeneous model, i.e., they use a single concentration variable. In addition, they assume that the reaction is mass transfer controlled (i.e. the reactant concentration at the solidfluid interface is zero). [11] The inventors have found that most systems fall in between the mass transfer and kinetically controlled regimes of reaction where the use of a pseudohomogeneous model (single concentration variable) is not sufficient to capture all the features of the reactive dissolution process qualitatively and that 'a priori' assumption that the system is in the mass transfer controlled regime, often made in the literature, may not retain the qualitative features of the problem. [12] It would be therefore desirable to provide an improved model for predicting the dissolution pattern during matrix stimulation of carbonates. SUMMARY OF THE INVENTION [13] The present invention proposes to model a stimulation treatment involving a chemical reaction in a porous medium including describing the chemical reaction by coupling the reactions and mass transfer occurring at the Darcy scale and at the pore scale and considering the concentration Cf of a reactant in the pore fluid phase and the concentration of said reactant a, at the fluid solid interface of a pore. [14] The present invention is particularly suitable for modeling acidizing treatment of subterranean formation, in particular matrix acidizing and acid fracturing. Apart from well stimulation, the problem of reaction and transport in porous media also appears in packedbeds, i pollutant transport in ground water, tracer dispersion etc. The presence of various length scales and coupling between the processes occurring at different scales is a common characteristic that poses a big challenge in modeling these systems. For example, the dissolution patterns observed on the core scale are an outcome of the reaction and diffusion processes occurring inside the pores, which are of microscopic dimensions. To capture these largescale features, efficient transfer of information on pore scale processes to larger length scales becomes important. In addition to the coupling between different length scales, the change in structure of the medium * adds an extra dimension of complexity in modeling systems involving dissolution. The model of the present invention improves the averaged models by taking into account the fact that the reaction can be both mass transfer and kinetically controlled, which is notably the case with relatively slowreacting chemicals such as chelants, while still authorizing that pore structure may vary spatially in the domain due for instance to heterogeneities and dissolution. According to another embodiment of the present invention, both the asymptotic/diffusive and convective contributions are accounted to the local mass transfer coefficient. This allows predicting transitions between different regimes of reaction. BRIEF DESCRIPTION OF THE DRA WINGS [16] Figure 1 is a schematic diagram showing different length scales in a porous medium. [17] Figure 2 is a plot of permeability versus porosity for different values of the empirical parameter P used in Equation (7) [18] Figure 3 is.a plot showing the increase in pore radius with porosity as a function of p. [19] Figure 4 is a plot showing the decrease in interfacial area with porosity as a function of [20] Figure 5 is a plot showing the pore volumes required for breakthrough computed from the 1D model versus Damkohler number for cp2 = 0.001 and Nac  0;0125. [21] Figure 6 is a plot showing the dependence of optimum Damkohler number on the Thiele modulus (p2. [22] Figure 7 is a plot showing the dependence of pore volumes required for breakthrough on the acid capacity number Nac [23] Figure 8 is a plot showing the dependence of pore volumes to breakthrough and optimum Damkohler number on the parameters cp2 and Nac [24] Figure 9 is an experimental plot of pore volumes required for breakthrough versus injection rate for different core lengths. Figure 10 is an experimental plot showing the decrease in optimum pore volumes required for breakthrough with increase in acid concentration. [26] Figure 11 shows the simulation results of 1D model according to the invention, illustrating the shift in the optimum injection rate with increase in the Thiele modulus cp2. [27] Figure 12 is an experimental plot of pore volumes required for breakthrough versus injection rate for different acids. [28] Figure 13 shows the increase in the optimum injection rate predicted by the 1D model according to the present invention with increase in the Thiele modulus [30] Figure 15 shows the correlated random permeability fields of different correlation lengths X generated on a domain of unit length using exponential covariance function. TwoScale Continuum Model [31] Convection and diffusion of the acid, and reaction at the solid surface are the primary mechanisms that govern the dissolution process. Convection effects are important at a length scale much larger than the Darcy scale (e.g. length of the core), whereas, diffusion and reaction are the main mechanisms at the pore scale. While convection is dependent on the larger length scale, diffusion and reaction are local in nature i.e., they depend on the local structure of the pores and local hydrodynamics. The phenomenon of reactive dissolution is modeled as a coupling between the processes occurring at these two scales, namely the Darcy scale and the pore scale as illustrated figure 1. The twoscale model for. reactive dissolution is given by Eqs. (15). [32] Here U=(U, V, W) is the Darcy velocity vector, K is the permeability tensor, P is the pressure, £ is the porosity. Cf is the cupmixing concentration of the acid in the fluid phase, Cs is the concentration of the acid at the fluidsolid interface, De is the effective dispersion tensor, kc is the local mass transfer coefficient, av is the interfacial area available for reaction per unit volume of the medium, ps is the density of the solid phase and a is the dissolving power of the acid, defined as grams of solid dissolved per mole of acid reacted. The reaction kinetics are represented by R(CS). For a first order reaction R(CS) reduces to ksCs where ks is the surface reaction rate constant having the units of velocity. [33] Equation (3) gives Darcy scale description of the transport of the acid species. The first three terms in the equation represent the accumulation, convection and dispersion of the acid respectively. The fourth term describes the transfer of the acid species from the fluid phase to the fluidsolid interface and its role is discussed in detail later in this section. The velocity field U in the convection term is obtained from Darcy's law (Eq. 1) relating velocity to the permeability field K and gradient of pressure. Darcy's law gives a good estimate of the flow field at low Reynolds number. For flows with Reynolds number greater than unity, the Darcy Brinkman formulation, which includes viscous contribution to the flow, may be used to describe the flow field. Though the flow rates of interest here have Reynolds number less than unity, change in permeability field due to dissolution can increase the Reynolds number above unity. However, the Darcy's law, computationally less expensive than the DarcyBrinkman "formulation is preferably used for the present invention, though the model can be easily extended to the Brinkman formulation. The first term in the continuity Eq. (2) accounts for the effect of local volume change during dissolution on the flow field. While deriving the continuity equation, it is assumed that the dissolution process does not change the fluid phase density significantly. [34] The transfer term in the species balance Eq. (3) describes the depletion of the reactant at the Darcy scale due to reaction. An accurate estimation of this term depends on the description of transport and reaction mechanisms inside the pores. Hence a pore scale calculation on the transport of acid species to the surface of the pores and reaction at the surface is required to calculate the transfer term in Eq. (3). In the absence of reaction, the concentration of the acid species is uniform inside the pores. Reaction at the solidfluid interface gives rise to concentration gradients in the fluid phase inside the pores. The magnitude of these gradients depends on the relative rate of mass transfer from the fluid phase to the fluidsolid interface and reaction at the interface. If the reaction rate is very slow compared to the mass transfer rate, the concentration gradients are negligible. In this case the reaction is considered to be in the kinetically controlled regime and a single concentration variable is sufficient to describe this c situation. However, if the reaction rate is very fast compared to the mass transfer rate, steep gradients develop inside the pores. This regime of reaction is known as mass transfer controlled regime. To account for the gradients developed due to mass transfer control requires the solution of a differential equation describing diffusion and reaction mechanisms inside each of the pores. Since this is not practical, we use two concentration variables Cs and Cf, one for the concentration of the acid at fluidsolid interface and the other for the concentration in the fluid phase respectively, and capture the information contained in the concentration gradients as a difference between the two variables using the concept of mass transfer coefficient. Mathematical representation of the idea of transfer between the fluid phase and fluid solid interface using two concentration variables and reaction at the interface is shown in Eq. (4). The l.h.s of equation represents the transfer between the phases using the difference between the concentration variables and mass transfer coefficient kc. The amount of reactant transferred to the surface is equated to the amount reacted. For the case of first order kinetics (R(CS) = ksCs) Eq. (4) can be simplified to Cf C>=JR K [36] In the kinetically controlled regime, the ratio of kj/kc is very small and the concentration at the fluidsolid interface is approximately equal to the concentration of the fluid phase (Cs ~ Cf). The ratio of kj/kc is very large in the mass transfer controlled regime. In this regime, the value of concentration at the fluidsolid interface (Eq. (6)) is very small (Cs ~ 0). Since the rate constant is fixed for a given acid, the magnitude of the ratio kj/kc is determined by the local mass transfer coefficient kc. The mass transfer coefficient is a function of the pore size and local hydrodynamics. Due to dissolution and heterogeneity in the medium, the pore size and fluid velocity are both functions of position and time. Thus, the ratio of ks/kc is not a constant in the medium but varies with space and time leading to a situation where different locations in the medium experience different regimes of reaction. To describe such a situation it is essential to account for both kinetic and mass transfer controlled regimes in the model, which is attained here using two concentration variables. A single concentration variable is not sufficient to describe both the regimes simultaneously. [37] The twoscale model can be extended to the case of complex kinetics by introducing the appropriate form of reaction kinetics R(CS) in Eq. (4). If the kinetics are nonlinear, equation (4) becomes a nonlinear algebraic equation which has to be solved along with the species balance equation. For reversible reactions, the concentration of the products affects the reaction rate, thus additional species balance equations describing the product concentration must be added to complete the model in the presence of such reactions. The change in local porosity is described with porosity evolution Eq. (5). This equation is obtained by balancing the amount of acid reacted to the corresponding amount of solid dissolved. [38] To complete the model Eqs. (15), information on permeability tensor K, dispersion tensor De, mass transfer coefficient ko and interfacial area av is required. These quantities depend on the pore structure and are inputs to the Darcy scale model from the pore scale model. Instead of calculating these quantities from a detailed pore scale model taking into consideration the actual pore structure, we use structureproperty relations that relate permeability, interfacial area and average pore radius of the pore scale model to its porosity. However, a detailed calculation including the pore structure could be made and the above quantities K, De, kc and av obtained from the pore scale model can be passed on to the Darcy scale model. Here, we use the structureproperty relations to study the trends in the behavior of dissolution for different types of structureproperty relations and to reduce the computational effort involved in a detailed pore scale calculation. Pore Scale Model StructureProperty Relations [39] Dissolution changes the structure of the porous matrix continuously, thus making it difficult to correlate the changes in local permeability to porosity during acidization. The results obtained from the averaged models, which use these correlations, are subject to quantitative errors arising from the use of a bad correlation between the structure and property of the medium, though the qualitative trends predicted may be correct. Pore level modeling where the properties are calculated from a specified structure of the medium obviates the use of these correlations. In the absence of reaction where the structure of the matrix does not change, the properties predicted by pore level models could be representative of the real field case provided the specified structure is reasonably accurate. However, changes in the structure such as pore merging, changes in coordination number etc., caused by dissolution are difficult to incorporate " into these models and hence the predictions may not be accurate or representative of what is observed. Since a definitive way of relating the changes in properties of the medium to the changes in structure does not exist, we use semiempirical relations that relate the properties to parameters (e.g. porosity) that are measures of the structure of the medium. These relations offer the advantage of studying the sensitivity of the results to different qualitative trends between the ' structure and properties. [40] ' The permeability of the medium is related to its porosity using the relation (7) proposed by Civan in "Scale effect on Porosity and Permeability: Kinetics, Model and Correlation," AIChE J, 47, 271287(2001). (K f £ Y ' VTfeJ (7) [41] The parameters y and P are empirical parameters introduced to account for dissolution. The parameters y and 1/p are observed to increase during dissolution and decrease for precipitation. In Eq. (7) the hydraulic diameter ((K/e)1/2) is related to the ratio of pore volume to matrix volume. The permeability, average pore radius and interfacial area of the pore scale model are related to its initial values Ko, a, r0 respectively in Eqs. (8)(10). k _[ r I g[gqg.) r8) _ f**T=f YY*qoY (9) r0 V*.* {y.Xe.ds)) ( V1 r „ ao erp UJ *„Utt*)J • [42] Figures 2, 3 and 4 show plots of permeability, pore radius and interfacial area versus . porosity, respectively, for typical values of the parameters. The increase in porosity during dissolution decreases the interfacial area, which in turn reduces the reaction rate per unit volume. The decrease in interfacial area with increase in porosity is shown in Figure 4. The model would yield better results if structureproperty correlations that are developed for the particular system of interest are used. Note that, in the above relations permeability that is a tensor is reduced to a scalar for the pore scale model. In general, permeability is not isotropic when the pores are aligned preferentially in one direction. The assumption of isotropic permeability for the pore scale model is made here based on random orientation of pores without any preference for the direction. For the case where permeability is anisotropic, extra relations for the permeability of the pore scale model in the transverse directions may be used to complete the model. Mass Transfer Coefficient [43] The rate of transport of acid species from the fluid phase to the fluidsolid interface inside the pores is quantified by the mass transfer coefficient. It plays an important role in characterizing dissolution phenomena because mass transfer coefficient determines the regime of reaction for a given acid (Eq. (6)). The local mass transfer coefficient depends on the local pore structure, reaction rate and local velocity of the fluid. The contribution of each of these factors to the local mass transfer coefficient is investigated in detail in references in Gupta, N. and Balakotaiah, V.:"Heat and Mass Transfer Coefficients in Catalytic Monoliths," Chem. Engg. Sci., 56, 47714786 (2001) and in Balakotaiah, V. and West, D.H.: "ShapeNormalization and Analysis of the Mass Transfer Controlled Regime in Catalytic Monoliths," Chem. Engg. Sci., 57,12691286 (2002), both references hereby incorporated by reference. [44] For developing flow inside a straight pore of arbitrary cross section, a good approximation to the Sherwood number, the dimensionless mass transfer coefficient, is given by 2k r (J Sh = ff£jL = SK+Mi^ R ettV2ScU3 (11) A„ v*; where kc is the mass transfer coefficient, rp is the pore radius and Dm is molecular difEusivity, Shao is the asymptotic Sherwood number for the pore, Rep is the pore Reynolds number, dh is the pore hydraulic diameter, x is the distance from the pore inlet and Sc is the Schmidt number (Sc = v/Dm; where v is the kinematic viscosity of the fluid). Assuming that the length of a pore is typically a few pore diameters, the average mass transfer coefficient can be obtained by integrating the above expression over a pore length and is given by Sh = Sh„+bRepU2Scv* (12) where the constants Sho= and b (= 0.7/m0'5), m = pore length to diameter ratio) depend on the structure of the porous medium (pore cross sectional shape and pore length to hydraulic diameter ratio). Equation (12) is of the same general form as the Frossling correlation used extensively in correlating mass transfer coefficients in packedbeds. [For a packed bed of spheres, Sh» = 2 and b = 0.6. This value of b is close to the theoretical value of 0.7 predicted by Eq. (12) for m = 1.] [0045] The two terms on the right hand side in correlation (12) are contributions to the Sherwood number due to diffusion and convection of the acid species, respectively. While the diffusive part, Sh«>, depends on the pore geometry, the convective part is a function of the local velocity. The asymptotic Sherwood number for pores with cross sectional shape of square, triangle and circle are 2.98, 2.50 and 3.66, respectively. Since the value of asymptotic Sherwood number is a weak function of the pore geometry, a typical value of 3.0 may be used for the calculations. The convective part depends on the pore Reynolds number and the Schmidt number. For liquids, the typical value of Schmidt number is around one thousand and assuming a value of 0.7 for b, the approximate magnitude of the convective part of Sherwood number from Eq. (12) is 7Rep1/2. The pore Reynolds numbers are very small due to the small pore radius and the low injection velocities of the acid, making the contribution of the convective part negligible during initial stages of dissolution. As dissolution proceeds, the pore radius and the local velocity increase, making the convective contribution significant. Inside the wormhole, where the velocity is much higher than elsewhere in the medium, the pore level Reynolds number is high and the magnitude of the convective part of the Sherwood number could exceed the diffusive part. The effect of this change in mass transfer rate due to convection on the acid concentration may not be significant because of the extremely low interfacial area in the high porosity regions. The acid could be simply convected forward without reacting due to low interfacial area by the time the convection contribution to the mass transfer coefficient becomes important. Though the effect of convective part of the mass transfer coefficient on the acid concentration inside the wormhole is expected to be negligible, it is important in the uniform dissolution regime and to study the transitions between different reaction regimes occurring in the medium due to change in mass transfer rates. The effect of reaction kinetics on the mass transfer coefficient is observed to be weak. For example, the asymptotic Sherwood number varies from 48/11 (=4.36) to 3.66 for the case of very slow reaction to very fast reaction. The correlation (12) accounts for effect of the three factors, pore cross sectional shape, local hydrodynamics and reaction kinetics on the mass transfer coefficient. The influence of tortuosity of the pore on the mass transfer coefficient is not included in the correlation. Intuitively, the tortuosity of the pore contributes towards the convective part of the Sherwood number. However, as mentioned above, the effect of convective part of the mass transfer coefficient on the acid concentration profile is negligible and does not affect the qualitative behavior of dissolution. Fluid Phase Dispersion Coefficient [47] For homogeneous, isotropic porous media, the dispersion tensor is characterized by two independent components, namely, the longitudinal, Dex and transverse, Dei, dispersion coefficients. In the absence of flow, dispersion of a solute occurs only due to molecular diffusion and Dex = Der = a0Dm, where Dm is the molecular diffusion coefficient and a0 is a constant that depends on the structure of the porous medium (e.g., tortuosity). With flow, the dispersion tensor depends on the morphology of the porous medium as well as the pore level flow and fluid properties. In general, the problem of relating the dispersion tensor to these local variables is rather complex and is analogous to that of determining the permeability tensor in Darcy's law from the pore structure. According to a preferred embodiment of the present invention, only simple approximations to the dispersion tensor are considered. [48] The relative importance of convective to diffusive transport at the pore level is characterized by the Peclet number in the pore, defined by IK (13) m where u is the magnitude of the Darcy velocity and dh is the pore hydraulic diameter. For a wellconnected pore network, random walk models and analogy with packed beds may be used to show that ^ = a0+lxPe (14) m ^ = ag+ArPe (15) m where Xx and Xt are numerical coefficients that depend on the structure of the medium (Xx~ 0.5, 1T~ 0.1 for packedbeds). Other correlations used for Dex are of the form 2sL = a0+Pe ln(—) (16) Du ° 6 2 ^L = ao+JLPe2 .(17) A„ [49] Equation (17) based on TaylorAris theory is normally used when the connectivity between the pores is very low. These as well as the other correlations in literature predict that both the longitudinal and transverse dispersion coefficients increase with the Peclet number. According to a preferred embodiment of the present invention, the simpler relation given by Eqs. (14) and (15) is used to complete the averaged model. In the following sections, the 1D and 2D versions of the. twoscale model (15) are analyzed. OneDimensional Model [50] The one dimensional version of the model is analyzed in this section for the case of an irreversible reaction assuming linear kinetics (R(CS) = ksCs). To identify the important dimensionless groups the equations are made dimensionless by choosing the length of the core L as the characteristic length scale in the flow direction, inlet velocity u0 as the characteristic velocity and the inlet concentration C0 as the' characteristic concentration of the acid species. In 1D, the dimensionless model for the case of constant injection rate is given by X u = l\DaNocacsdx (18) 0 dcf 3c, c, = —i— (19) 3/ a* R I Sh) c. (20) I Sh) = DaNaeacs (21) at where u, cfj cs and r are the dimensionless velocity, dimsnsionless fluid phase and fluidsolid interface concentrations and dimensionless pore radius, respectively. The definitions of the "three dimensionless groups in the model Damkohler number Da, Thiele modulus (p2 and acid capacity number Nac are given below: An P, where a^ is the initial interfacial area per unit volume, r0 is the initial average pore radius of the pore scale model and a is the acid dissolving power. The Damkohler number Da is the ratio of convective time L/uQ to the reaction time l/ksa [51] The velocity field in 1D is described by Eq. (18) which is obtained by combining the continuity equation with the porosity evolution Eq. (21) and integrating once with respect to x using the boundary condition u = 1 at the inlet. The integral in the equation is a correction to the velocity due to local volume change during dissolution. This term is negligible for small values of the product DaNac. For high values of DaNac this term cannot be neglected. Since the calculations performed here are to study the qualitative behavior of dissolution, dispersion term in the species balance equation is neglected. Neglecting the dispersion term does not change the qualitative nature of the solution. Equation (20) is the dimensionless form of Eq. (6). The ratio (q>2r/Sh) is equal to the ratio of kj/kc and the parameters cp2 and Sh depend only on the local reaction and mass transfer rates. This equation is called the local equation. In the following subsection local Eq. (20) is analyzed to identify different regimes of reaction and transitions between them. Local Equation [52] As mentioned earlier, the magnitude of the term (p2r/Sh or ks/kc in the denominator of the local equation determines whether the reaction is in kinetically controlled or mass transfer controlled regime. In practice, the reaction is considered to be in the kinetic regime if (p2r/Sh 10. For values of cp2r/Sh between 0.1 and 10, the reaction is considered to be in the intermediate regime. The Thiele modulus (p2 in [54] The values of (pVSh and Q/Cf in the table show that all the above acids except HC1 are in the kinetic regime during the initial stages of dissolution. The reaction between HC1 and calcite is in the intermediate regime. As the reaction proceeds, the pore size becomes larger increasing the value of cp2r/Sh leading to transitions between different regimes of reaction. For example, the reaction between HC1 and calcite will change from intermediate regime to completely mass transfer controlled regime if the dimensionless pore radius increases by a factor more than ten and the Sherwood number remains constant. However, the Sherwood number has both diffusion and convective contributions in it, and when the pore radius increases significantly, the Sherwood number also increases due to the convective contribution. This reduces the magnitude of q>2r/Sh (or ks/kc). Thus, the reaction may or may not reach a mass transfer limited regime with an increase in the pore radius. In this case, most of the reaction occurs in the intermediate regime and part of the reaction occurs in the mass transfer controlled regime because the interfacial area available for reaction is very low by the time the reaction reaches completely mass transfer controlled regime. Similar transitions between different reaction regimes can occur for the case of 0.25M CDTA which is on the boundary of kinetic and intermediate regimes initially. In addition, heterogeneity (varying pore radius) in the medium can lead to different reaction regimes at different locations in the medium. The above discussion illustrates the complexity in describing transport and reaction mechanisms during dissolution due to transitions and heterogeneities. Nonetheless, these transitions are efficiently captured using two concentration variables in the local Eq. (20). A single concentration variable is not sufficient to describe both kinetic and masss transfer controlled regimes simultaneously. Numerical Simulation of the 1D Model 10055] A parametric study of the onedimensional model (1821) is presented in this section. The results are compared to experimental observations in the next section. The three dimensionless parameters in the model are (p2, Nac and Da. Numerical simulations are performed by holding one of the parameters constant while varying the other two. A value of 0.2 is used for the initial porosity in all the simulations. The breakthrough of the acid is defined as an increase in the permeability of the core by a factor of 100 from its initial value (K/Ko=100). [56] The value of Nac is fixed at 0.0125 in the first set of simulations. The Thiele modulus is varied between (p2 = 0.001 and (p2 = 100. The plot of pore volumes injected for breakthrough versus Danikohler number Da is shown in Figure 5 for (p2 = 0.001. The plot shows an optimum Damkohler number at which the number of pore volumes of acid required to break through the core is minimum. For very large and very small Damkohler numbers, the amount of acid required for breakthrough is much higher. Figure 6 shows the pore volumes required for breakthrough for cp2 values of 0.001,1.0 and 10.0. As the value of cp2 increases the plot shows an increase in the optimum Damkohler number and decrease in the minimum pore volume required for breakthrough. In the second set of simulations presented here, the effect of acid capacity number Nac on the behavior of dissolution is investigated. Figure 7 shows the plot of pore volumes injected for breakthrough versus Damkohler number for values of acid capacity number Nac = 0.0125, Nac = 0.0625 and Nac = 0.125 for the same Thiele modulus (p2 = 0.001. The minimum acid required for breakthrough decreases with increase in acid capacity number. This decrease in the minumum pore volumes is almost proportional to the increase in Nac. Figure 8 shows the plots of pore volumes injected versus Da where both cp2 and Nac are varied. The figure shows a horizontal shift in the curves when the Thiele modulus is increased and a vertical shift for an increase in the acid capacity number. 2D Model [58] In this section, twodimensional simulations that demonstrate the wormhole initiation, propagation, density, fluid leakage and competition between neighboring wormholes are presented. The effect of heterogeneity on the wormhole structure is investigated using different kinds of random permeability fields. The dimensionless' twodimensional model and the boundary conditions for constant injection rate used in the numerical simulations are shown below: = (22) dx V dx J dy dy J dt dcr dcf dcf c, dt dx dy J (px c. = , \ x (24) I Sh) j = DaNaeaCj ' (25) cf=l @x = 0 (26) rr \p uaL L J0 dx P = 0 @x=l (28) riP = 0 @y = 0 (29) dy ^ = 0 @y = a0 (30) c/=0 @t = 0 (31) £ = £0+f @t = 0 (32) [59] Combining continuity equation with Darcy's law gives —V.(*V a. dt [0060J In Eq. (22) for the pressure field, the accumulation term 3e/3t is neglected assuming quasi steady state. The magnitude of d&/dt is equal to DaNacacs. This term can be neglected if the product of Da and Nac is small. Equation (27) describes the constant injection rate boundaiy condition at the inlet, where (q/u0L) is the dimensionless injection rate, H is the width of the domain and a0 is the aspect ratio. The fluid is contained in the domain by preventing its leakage through the side walls using no flux boundary conditions at y = 0 and y = H (Eqs. (29) and (30)). Heterogeneity is introduced in the domain as a random fluctuation f about a mean value e0. The amplitude of f is varied from 10 to 50% about the . mean value of porosity. [00611 In the first step of the solution, pressure field in the medium is obtained by solving the algebraic equations resulting from the discretization of the above equation using the iterative solver GMRES (Generalized Minimal Residual Method). The flow profiles in the medium are calculated from the pressure profile using Darcy's law. Acid concentration in the medium is obtained by solving the species balance equation using an implicit scheme (Backward Euler). The porosity profile in the medium is then updated using the new values of concentration. This process is repeated till the breakthrough of the acid. Dissolution Patterns and Dominant Wormhole Formation [62] At the inlet of the domain, injection rate of the acid is maintained constant As the injection rate is varied different types of dissolution patterns similar to the patterns in experiments are observed. In the simulations, the aspect ratio and initial porosity of the medium are maintained at 1 and 0.2, respectively. The Damkohler number decreases as the injection rate increases. For very low injection rates (high Da) facial dissolution is observed. The acid is consumed completely as soon as it enters the medium. For higher injection rates, the acid • channels through the medium producing a wormhole. In this case the acid escapes through the wormhole without affecting the rest of the medium. At very high injection rates, the acid dissolves the medium uniformly. [63] The formation of a dominant wormhole from the stage of initiation is desirable. A number of wormholes are initiated when the acid enters the medium. However, as the dissolution progresses, most of the acid is channeled into a few of these wormholes increasing their size. This preferential flow of acid into larger wormholes arrests the growth of smaller channels. Eventually, one of these three channels grows at a faster rate than the other two, drawing all the acid and thereby reducing their growth rate. In the above simulations the wormholes are initiated due to the heterogeneity in the medium and the competitive growth of wormholes can be seen from the figures. Experimental Comparison [64] The effect of core length, acid concentration, temperature, diffusion and reaction rates on the optimum injection rate are investigated in the experimental studies. The influence of the each above factors on optimum rate of injection is studied separately using the model. Core Length [65] The optimum injection rate is observed to increase with the core length. Figure 9 shows the experimental data on pore volumes required for breakthrough versus injection velocity reported in [4] for two different core lengths 5cm and 20cm. The acid used in these experiments is 7% HC1. In terms of dimensionless numbers, the acid capacity number Nac and the Thiele modulus (p2 are fixed because the quantities on which these parameters depend, acid concentration, reaction and diffusion rates are constant in these experiments. For fixed values of Nac and (p2, the theoretical prediction of the model on optimum flow rate is similar to that shown in Fig. 5, except that the Thiele modulus and optimum Damkohler number are different Since the optimum Damkohler number is fixed for fixed values of Nac and cp2, the optimum injection rates in the two experiments can be related by (Daopl\=(Daopt)2 iii (33) [66] Using Eq. (33), the optimum injection rate for a core length of 20cm can be obtained from the optimum injection rate of 5cm core. The value of optimum injection rate for the 20cm core is approximately u2 = ((20)/5)(0.15) = 0.6 cm/min, which is close to the experimentally observed injection rate. [67] The result in Eq. (33) when extended to the reservoir scale (L2/Li—>oc), suggests that the maximum wormhole length is achieved when the acid is injected at the maximum possible rate. This design of injecting the acid at maximum possible injection rate and pressure below the fracture pressure has been suggested by Williams, B. B., Gidley, J.L., and Schechter, R. S.: Acidizing Fundamentals, SPE Monograph Series, 1979, and is observed to increase the efficiency of stimulation in some field studies conducted by Paccaloni, G. and Tambini, M.: "Advances in Matrix Stimulation Technology," J. Petrol. Tech, 256263, March 1993. Bazinin "From matrix Acidizing to Acid Fracturing: A Laboratory Evaluation of Acid/Rock Interactions," February 2001, SPE Prod. & Facilities, 2229, made similar observations in experimental studies using cores of different lengths. Acid Concentration [68] Figure 10 shows the effect of different acid concentrations, 0.7%, 3.5%, 7% and 17.5% HC1, on pore volume to breakthrough observed in the experiments performed by Bazin. The figure shows a decrease in the pore volumes and an increase in the optimum injection rate required for breakthrough with increase in concentration of the acid. The change in acid concentration affects only the acid capacity number Nac for a first order reaction. For a given acid or a fixed Thiele modulus (p2, Figure 8 shows that increasing the acid capacity number or equivalently increasing the acid concentration decreases the pore volumes required for breakthrough. Temperature The optimum injection rate is observed to increase with temperature. The reaction rate constant increases with increase in temperature, thereby increasing the Thiele modulus cp2. Figure 11 shows the increase in dimensionless injection rate (cp2/Da = 2r0Uo/(DmaoL)) or different values of Thiele modulus that correspond to the same acid at different temperatures, obtained from the 1D model. The acid capacity number for all the simulations is 0.0125. The figure shows an increase in the dimensionless injection rate with increase in temperature or low to intermediate values of (p2. However, for very high values of Thiele modulus the dependence of dimensionless injection rate on the Thiele modulus is observed to be very weak. At very high temperatures (or large Thiele modulus), the reaction is completely mass transfer controlled and the surface reaction rate or Thiele modulus plays a minor role in the behavior of dissolution. Thus, the optimum injection rate is a weak function of the surface reaction rate in the completely mass transfer controlled process. Acid Diffusion Rate 10070] Fredd and Fogler performed experiments using acids with different diffusion rates with the same acid capacity number. Figure 12 shows the optimum injection rate curves for these acids as a function of the injection rate. However, in these experiments the acid reaction rates are also different, thus both the rate constant ks and molecular diffusivity Dm in the Thiele modulus are varied in these experiments. The values of Thiele modulus for different acids used in these experiments are listed in Table 1. Since the acid capacity number Nac is maintained constant in these experiments, dissolution behavior is only a function of the Thiele modulus [0071] The onedimensional model predicts qualitatively the dependence of optimum injection rate and pore volume to breakthrough on, various factors. However, the optimum pore volume required for breakthrough is over predicted when compared to the experimental results. For example, the model predicts approximately 200 pore volumes at optimal conditions for HC1 to breakthrough (Figure 13), whereas the experimental value is close to one in Figure 12. Similar discrepancy between experimental value and model prediction (approximately 500 pore volumes) is observed in the 2D network model developed by Fredd & Fogler. The reason for this difference is due to the velocity profile (Eq. (18)) used in the 1D model. During dissolution, the acid channels into the conductive regions resulting in an increase in the local velocity. For constant injection rate, if we consider a core of 3.8cm diameter used in the experiments and a wormhole of a 3.8mm diameter, the velocity inside the wormhole could be much higher than the inlet velocity as shown in the following calculation. A ( 38 Y uw~—c—uin,« = "^,=100^, (34) [72] Here, uw is the velocity inside the wormhole, Umiet is the injection velocity, Ac0re and Awormhoie are the cross sectional areas of the core and wormhole respectively. This increase in the velocity inside the domain due to channeling is not included in the 1D velocity profile Eq. (18) where the maximum velocity inside the domain cannot be higher than the inlet velocity. [73] Since the 2D model includes channeling effect on the velocity profile, the pore volume required for breakthrough is found to be significantly lower than the value predicted by the 1D model. However, the value obtained from the 2D model is still higher than the experimental result because the maximum velocity inside the domain would not increase as the square of the ratio of diameters (Eq. (34)) of the wormhole and the core, but as the ratio of diameters in two dimensions. It is believed that a complete 3D simulation would predict approximate pore volumes required for breakthrough as observed in the experiments. [74] The decrease in pore volumes to breakthrough due to channeling in 2D is shown in Figure 14. The parameters cp2 = 0.02 and Nac = 0.07 are maintained the same in both 1D and 2 D simulations. The aspect ratio (a0) for the 2D simulation is 0.37. The figure shows a factor five decrease in the optimum breakthrough volume from 1D to 2D simulation due to channeling of the flow, into the wormholes. It should be noticed that the optimum Damkohler number for the 2D case is much higher than the 1D. For the same initial conditions in 1D and 2D, increase in the Damkohler number (Da = ksaoL/u0) implies a decrease in the injection rate. Thus the injection velocity required for optimal breakthrough is much lower in two dimensions when compared to flow in 1D. Though the injection velocity is low, channeling produces much higher local velocities as given by Eq. (34). Since this effect is absent in 1D, the fluid velocity required for optimal breakthrough is much higher. [75] The above comparisons between 1D and 2D results suggest that the pore volumes required for breakthrough for a complete 3D core scale simulation would be less than the 1D and 2D simulations and probably bridge the gap between the experimental and numerically simulated pore volumes. The injection velocity for optimal conditions also would be less than that obtained from the 1 D and 2D simulations. Sensitivity of the results to various parameters in the model and their effects on wormhole structure [76] . The dependence of breakthrough time for different mesh sizes has been studied for the case Da = 100, cp2 = 0.02, Nac  0.07 and aspect ratio equal to unity. Different mesh sizes for which the simulations were carried are given below Ni*N2 = 50*50, 80*80, 80*100,100*80, 100*100. [77] Here Ni is the number of grid points in the flow direction and N2 is the number of grid points in the transverse direction .The dimensionless breakthrough time was observed to be approximately 1.5 for all the cases. Influence of the exponent (3 in the permeabilityporosity correlation on the breakthrough time in the wormholing regime is observed to weak. The breakthrough times obtained for different values of p are listed below. P Breakthrough time 0.8 1.73 1.0 1.67 1.5 1.58 2.0 1.82 Effect of Heterogeneity [78] Heterogeneity is introduced into the model as a random porosity field. The sensitivity of the results and the dependence of wormhole structure on initial heterogeneity are investigated using two types of random porosity fields. In the first case initial porosity in the domain is introduced as a random fluctuation of the porosity values about a mean value at each grid point in the domain. The amplitude of the fluctuation is varied between 10%50% of the mean value. The results obtained for fluctuations of this magnitude are observed to be qualitatively similar. On a scale much larger than the grid spacing, this type of porosity field appears to be more or less uniform or homogeneous. Numerical simulations in 2D using the above mentioned heterogeneous porosity field show that the model can capture wormhole initiation, fluid leakage, wormhole density and competitive growth of wormholes. However, heterogeneity, when introduced in the above form is observed to produce almost straight wormholes with little deviations in the path. Branching of wormholes is not observed. [79] In the second case, heterogeneity is introduced at two different scales namely (a) random fluctuation of porosity about a mean value at each grid point (b) random fluctuation of porosity values about a different mean than the former over a set of grid points (scale larger than the scale of the mesh). The simulations with different scales of heterogeneity show that branching, fluid leakage and the curved trajectories of the wormholes observed in the experiments could be a result of different types of heterogeneities present in carbonates. [80] The acid is diverted into the center of the domain and dissolution gives a straight wormhole. However, when the mean value of porosity at the center of the domain is increased to 0.4, branching is observed. During the initial stages of dissolution, the acid flows into the channel and leaks at the tip. Following this two branches evolve of which one grows much faster than the other and breaks through the core. If an additional low porosity region is introduced in the middle of the domain, the presence of a low porosity region inside the domain can be interpreted as a portion of the core with very low permeability. In this later case, the acid prefers to branch instead of dissolving the rock in the low permeability region. Since such regions of low permeability can occur in carbonates, branches might evolve from the wormhole when it comes in contact with these regions. The above simulations show that the complex structure of the wormhole observed in the experiments and fluid leakage could be a result of different scales of heterogeneity present in the core. The effect of these heterogeneities on the breakthrough time has not been investigated in a systematic way in the literature. To study the effects of heterogeneities on wormholing and the sensitivity of breakthrough time to heterogeneity, it is required to introduce different types of permeability fields as initial condition to the numerical simulation. One way to introduce different permeability fields is to increase the random fluctuation of permeability about a mean field. However, as stated earlier, this procedure always gives a.permeability field that is more or less homogeneous on a scale much larger than the grid scale. [82] The other approach to generate different permeability fields is to introduce a correlation length 1 for the permeability field. By changing the correlation length, different scales of heterogeneity can be generated. Thus, locations in the domain that are close to each other have correlated permeability values and for locations separated by distance much greater than X, the • permeability values are not correlated. The maximum amplitude of the fluctuation of permeability value about the mean at each grid point is controlled by the variance a2 of the permeability distribution. By changing the correlation length X and the variance a2 of the distribution, initial heterogeneities of different length scales can be produced. When the correlation length becomes very small, random permeability field of the first type is produced. Thus the permeability fields generated using the first approach are a special case of the random permeability fields generated using the second method. For example, Figs. 15(a)15(c) show random correlated permeability fields generated on a onedimensional domain of unit length. The correlation lengths X, for Figs. 15(a)15(c) are 0.1, 0.05 and 0.01, respectively. As the correlation length is decreased the permeability field becomes similar to that generated using the first approach. An exponential covariance function with a variance o2 of two is used to generate these 1D permeability fields. The above procedure offers the advantage of studying the effect of heterogeneities on wormhole formation and structure in a systematic way. [83] A new averaged model is developed for describing flow and reaction in porous media. The model presented here describes the acidization process as an interaction between processes at two different scales, the Darcy scale and the pore scale. The model may used with different pore scale models that are representative of the structure of different types of rocks without affecting the Darcy scale equations. The new model is heterogeneous in nature and may be used in both the mass transfer and kinetically controlled regimes of reaction. Numerical simulations of the new model for the 1D case show that the model captures the features of acidization qualitatively. Twodimensional simulations of the model demonstrate the model's ability to capture wormhole initiation, propagation, fluid leakage and competitive growth of the wormholes. The effect of heterogeneity on wormhole formation can also be studied using different initial porosity fields. The quantity of practical interest, pore volumes required for breakthrough, is found to be a strong function of flow channeling. The simulations presented here are preliminary and the effect of heterogeneity on wormhole formation and structure of wormholes e.g. branching of wormholes, fluid leakage associated with branching etc., have not been completely studied. [84] Since the model of the present invention allows accurate scaleup, stimulation treatments may be designed by first obtaining a reservoir core, obtaining a set of parameters representative of said reservoir core, said set of parameters including Darcy scale parameters and pore scale parameters and performing the method of modeling according to the present invention. Said set of parameters will preferably include the Sherwood number, the dispersion tensor, the Thiele modulus, and the Peclet number. In addition, data representative of the heterogeneities present in the reservoir core are also collected. What is claimed is: 1. A method of modeling a stimulation treatment involving a chemical reaction in a porous medium including describing the chemical reaction by coupling the reactions and mass transfer occurring at the Darcy scale and at the pore scale and considering the concentration Cf of a reactant in the pore fluid phase and the concentration of said reactant Cj at the fluid solid interface of a pore. 2. A method of modeling a stimulation treatment involving a chemical reaction in a porous medium including, quantifying the rate of transport of the reactive species from the fluid phase to the fluidsolid interface inside the pores by a mass transfer coefficient taking into account both the diffusive and convective contributions. 3. The method according to any preceding claim, wherein said porous medium is a subterranean formation. 4. The method of claim 3, wherein said stimulation treatment is acidizing. 5. The method of claim 4, wherein said stimulation treatment is selected from the group consisting of matrix acidizing and acid fracturing. 6. The method according to any preceding claims, wherein said chemical reaction involves the dissolution of the porous media. 7. The method of claim 6, wherein the model includes a description of the reactive dissolution of the porous media using coupled global and local equations. 8. The method of claim 7, wherein said equations involve the permeability, the dispersion tensor, the average pore radius and the local mass transfer coefficient 9. The method of claim 1, wherein the flow of chemical reactant is modeled using a nonzero divergent velocity field V . & 10. The method of claim 2, further including the use of correlated random fields to account for different scales of heterogeneity. 11. The method of claim 2, wherein the diffusive contribution of the mass transfer coefficient is represented by the asymptotic Sherwood number for the pore. 12. The method of claim 11, wherein the dimensionless mass transfer coefficient (Sherwood number Sh) is given by Sh = Sh„+bRepU2 Scm (12) where Shoo is the asymptotic Sherwood number for the pore, b is a constant depending on the pore length to pore diameter ratio, .Rep is the pore Reynolds number, and Sc is the Schmidt number. 13. The method of claim 12, wherein b= 0.7/m0'5, where m is the pore length to diameter ratio. 14. A method of modeling a stimulation treatment involving a chemical reaction in a porous medium including describing the chemical reaction "by coupling the reactions and mass transfer occurring at the Darcy scale and at the pore scale and considering the concentration Cf of a reactant in the pore fluid phase and the concentration of said reactant cs at the fluid solid interface of a pore and quantifying the rate of transport of the reactive species from the fluid phase to the fluidsolid interface inside the pores by a mass transfer coefficient taking into account both the diffusive and convective contributions. 15. A method of designing a stimulation treatment involving a chemical reaction in a subterranean formation including obtaining a reservoir core, obtaining a set of parameters representative of said reservoir core, said set of parameters including Darcy scale parameters and pore scale parameters and performing the method according to any of the preceding claims using said set of parameters. 16. The method of claim 15, wherein said set of parameters includes the Sherwood number, the dispersion tensor, the Thiele modulus, and the Peclet number. 17. The method of claim 15 or 16, wherein said set of parameters further includes data related to the heterogeneities. 18. A method of modeling a stimulation treatment substantially as herein described with reference to the accompanying drawings. 

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Patent Number  257463  

Indian Patent Application Number  2711/CHENP/2004  
PG Journal Number  41/2013  
Publication Date  11Oct2013  
Grant Date  04Oct2013  
Date of Filing  30Nov2004  
Name of Patentee  SCHLUMBERGER TECHNOLOGY B.V  
Applicant Address  PARKSTRAAT 8389, 2514 JG THE HAGUE  
Inventors:


PCT International Classification Number  E21B41/00  
PCT International Application Number  PCT/EP03/05651  
PCT International Filing date  20030529  
PCT Conventions:
