Development of a Thermodynamically Consistent, Fully Implicit, Composittonal, Equation-Of-State, Steamflood Simulator

A thermodynamically consistent, three-dimensional, fully implicit, compositional, equation-of-state, steamflood simulator is developed. The formulation uses a single set of primary variables-overall component mole numbers, total fluid enthalpy, and pressure-to linearize the finite difference forms of the component conservation equations, the energy conservation equation, and the pore volume constraint. These primary variables constitute a thermodynamically independent set; subsequently, they may be applied throughout the reservoir, supplanting the use of variable substitution. The resulting nonlinear system of equations is solved using the Newton-Raphson method to update the primary variables. At each Newton iteration, phase equilibria is computed isenthalpically using an original Newton-type entropy maximization algorithm, combined with a Gibbs stability test, that yields the number of phases, the phase mole numbers, and temperature of the fluid mixture. The thermodynamic fluid properties such as enthalpy and density are calculated using the Soave-Redlich-Kwong cubic equation-of-state. Comparisons are made between numerical simulations and compositional steamflood experiments that illustrate the importance of phase behavior on the residual oil saturation to steam.