Two-phase flow in heterogeneous porous media II: Numerical experiments for flow perpendicular to a stratified system

In this work, we make use of numerical experiments to explore our original theoretical analysis of two-phase flow in heterogeneous porous media (Quintard and Whitaker, 1988). The calculations were carried out with a two-region model of a stratified system, and the parameters were chosen be consistent with practical problems associated with groundwater flows and petroleum reservoir recovery processes. The comparison between theory (the large-scaled averaged equations) and experiment (numerical solution of the local volume averaged equations) has allowed us to identify conditions for which the quasi-static theory is acceptable and conditions for which a dynamic theory must be used. Byquasi-static we mean the following: (1) The local capillary pressure,everywhere in the averaging volume, can be set equal to the large-scale capillary pressure evaluated at the centroid of the averaging volume and (2) the large-scale capillary pressure is given by the difference between the large-scale pressures in the two immiscible phases, and is therefore independent of gravitational effects, flow effects and transient effects. Bydynamic, we simply mean a significant departure from the quasi-static condition, thus dynamic effects can be associated with gravitational effects, flow effects and transient effects. To be more precise about the quasi-static condition we need to refer to the relation between the local capillary pressure and the large-scale capillary pressure derived in Part I (Quintard and Whitaker, 1990). Herep_c¦_y represents the local capillary pressure evaluated at a positiony relative to the centroid of the large-scale averaging volume, and {p_c}¦_x represents the large-scale capillary pressure evaluated at the centroid.In addition to{p_c}^c being evaluated at the centroid, all averaged terms on the right-hand side of Equation (1) are evaluated at the centroid. We can now write the equations describing the quasi-static condition as,, This means that the fluids within an averaging volume are distributed according to the capillary pressure-saturation relationwith the capillary pressure held constant. It also means that the large-scale capillary pressure is devoid of any dynamic effects. Both of these conditions represent approximations (see Section 6 in Part I) and one of our main objectives in this paper is to learn something about the efficacy of these approximations. As a secondary objective we want to explore the influence of dynamic effects in terms of our original theory. In that development only the first four terms on the right hand side of Equation (1) appeared in the representation for the local capillary pressure. However, those terms will provide an indication of the influence of dynamic effects on the large-scale capillary pressure and the large-scale permeability tensor, and that information provides valuable guidance for future studies based on the theory presented in Part I.Roman Letters A scalar that maps {}*/t onto - A scalar that maps {}*/t onto - A interfacial area between the -region and the -region contained within, m² - A interfacial area between the -region and the -region contained within, m² - A interfacial area between the -region and the -region contained within, m² - a vector that maps ({}*/t) onto, m - a vector that maps ({}*/t) onto, m - b vector that maps ({p}–g) onto, m - b vector that maps ({p}–g) onto, m - B second order tensor that maps ({p}–g) onto, m² - B second order tensor that maps ({p}–g) onto, m² - c vector that maps ({}*/t) onto, m - c vector that maps ({}*/t) onto, m - C second order tensor that maps ({}*/t) onto, m² - C second order tensor that maps ({}*/t) onto. m² - D third order tensor that maps () onto, m - D third order tensor that maps () onto, m - D second order tensor that maps () onto, m² - D second order tensor that maps () onto, m² - E third order tensor that maps () onto, m - E third order tensor that maps () onto, m - E second order tensor that maps () onto - E second order tensor that maps () onto - p_c=(), capillary pressure relationship in the-region - p_c=(), capillary pressure relationship in the-region - g gravitational vector, m/s² - largest of either or - - - i unit base vector in thex-direction - I unit tensor - K local volume-averaged-phase permeability, m² - K local volume-averaged-phase permeability in the-region, m² - K local volume-averaged-phase permeability in the-region, m² - {K} large-scale intrinsic phase average permeability for the-phase, m² - K–{K}, large-scale spatial deviation for the-phase permeability, m² - K–{K}, large-scale spatial deviation for the-phase permeability in the-region, m² - K–{K}, large-scale spatial deviation for the-phase permeability in the-region, m² - K^* large-scale permeability for the-phase, m² - L characteristic length associated with local volume-averaged quantities, m - characteristic length associated with large-scale averaged quantities, m - I_i i = 1, 2, 3, lattice vectors for a unit cell, m - l characteristic length associated with the-region, m - ; characteristic length associated with the-region, m - l_H characteristic length associated with a local heterogeneity, m - - n unit normal vector pointing from the-region toward the-region (n=–n) - n unit normal vector pointing from the-region toward the-region (n=–n) - p pressure in the-phase, N/m² - p local volume-averaged intrinsic phase average pressure in the-phase, N/m² - {p} large-scale intrinsic phase average pressure in the capillary region of the-phase, N/m² - p local volume-averaged intrinsic phase average pressure for the-phase in the-region, N/m² - p local volume-averaged intrinsic phase average pressure for the-phase in the-region, N/m² - p–{p}, large scale spatial deviation for the-phase pressure, N/m² - p–{p}, large scale spatial deviation for the-phase pressure in the-region, N/m² - p–{p}, large scale spatial deviation for the-phase pressure in the-region, N/m² - P_c p–{p}, capillary pressure, N/m² - {p_c}^c large-scale capillary pressure, N/m² - r₀ radius of the local averaging volume, m - R₀ radius of the large-scale averaging volume, m - r position vector, m - , m - S /, local volume-averaged saturation for the-phase - S^* {}*{}*, large-scale average saturation for the-phaset time, s - t time, s - u , m - U , m² - v -phase velocity vector, m/s - v local volume-averaged phase average velocity for the-phase in the-region, m/s - v local volume-averaged phase average velocity for the-phase in the-region, m/s - {v} large-scale intrinsic phase average velocity for the-phase in the capillary region of the-phase, m/s - {v} large-scale phase average velocity for the-phase in the capillary region of the-phase, m/s - v–{v}, large-scale spatial deviation for the-phase velocity, m/s - v–{v}, large-scale spatial deviation for the-phase velocity in the-region, m/s - v–{v}, large-scale spatial deviation for the-phase velocity in the-region, m/s - V local averaging volume, m³ - V volume of the-phase in, m³ - V large-scale averaging volume, m³ - V capillary region for the-phase within, m³ - V capillary region for the-phase within, m³ - V_c intersection of m³ - V volume of the-region within, m³ - V volume of the-region within, m³ - V() capillary region for the-phase within the-region, m³ - V() capillary region for the-phase within the-region, m³ - V() , region in which the-phase is trapped at the irreducible saturation, m³ - y position vector relative to the centroid of the large-scale averaging volume, mGreek Letters local volume-averaged porosity - local volume-averaged volume fraction for the-phase - local volume-averaged volume fraction for the-phase in the-region - local volume-averaged volume fraction for the-phase in the-region - local volume-averaged volume fraction for the-phase in the-region (This is directly related to the irreducible saturation.) - {} large-scale intrinsic phase average volume fraction for the-phase - {} large-scale phase average volume fraction for the-phase - {}* large-scale spatial average volume fraction for the-phase - –{}, large-scale spatial deviation for the-phase volume fraction - –{}, large-scale spatial deviation for the-phase volume fraction in the-region - –{}, large-scale spatial deviation for the-phase volume fraction in the-region - a generic local volume-averaged quantity associated with the-phase - mass density of the-phase, kg/m³ - mass density of the-phase, kg/m³ - viscosity of the-phase, N s/m² - viscosity of the-phase, N s/m² - interfacial tension of the - phase system, N/m - , N/m - , volume fraction of the-phase capillary (active) region - , volume fraction of the-phase capillary (active) region - , volume fraction of the-region (+=1) - , volume fraction of the-region (+=1) - {p}–g, N/m³ - {p}–g, N/m³