Development of a Four-Phase Compositional Simulator Using Equations of State
Creators: name: Wei, Yizheng
Publisher: name: Graduate Studies
Year: 2015
Subjects: subject: Engineering--Chemical
subject: Engineering--Petroleum
subject: Reservoir Simulation
subject: Compositional Model
subject: Equation of State
subject: Multiphase Equilibrium
subject: Multiphase Flow in Porous Media

description: Compositional models are widely used in the simulation of gas injection processes where phase mole fractions and compositions at equilibrium change with space and time. Typically, the models are capable of handling three-phase flow (oil, gas and water), where the mole fractions and compositions of the oil and gas phases are determined by performing two-phase flash calculation using an equation of state. No mass interchange between the water and hydrocarbon phases is assumed. Laboratory experiments, however, have observed that a second hydrocarbon liquid phase can coexist with oil and gas at equilibrium when CO2 or rich gas is injected into a low temperature reservoir. This second liquid phase makes most conventional compositional models no longer suitable for the simulation of these processes. Using the two-phase flash calculation in a three-phase region provides false phase equilibrium solutions that can result in either erroneous simulation results or discontinuity in the calculation of phase properties over a time step, leaving the simulation fail to converge. In this work, a new three-dimensional isothermal compositional simulator has been developed. Governing equations that describe compositional flow in porous media are reformulated for four coexisting equilibrium phases (oil, gas, 2nd liquid and aqueous). Water is treated as a component rather than an independent phase in the formulation of component flow and phase equilibrium. A component can exist in any phases as long as the thermodynamic equilibrium condition is satisfied. A robust and efficient four-phase equilibrium calculation algorithm consisting of stability analysis and phase split calculation is incorporated in the simulator to determine the number of phases, phase amounts and phase compositions. An equation of state is employed to calculate the densities and model the phase behavior of both aqueous and hydrocarbon phases. The component flow equations are discretized using a finite difference method on the basis of a block-centered grid system and solved by an implicit pressure and explicit saturation and composition (IMPES) solution scheme. The proposed model has been validated with commercial software in both stand-alone flash calculation and simulation problems with two hydrocarbon phases. The new four-phase simulator has been used to investigate the effects of complex phase behavior on displacement mechanisms in CO2 injection processes. Both the multiphase behavior phenomena of the CO2/crude oil/water mixtures and the multiphase flow during the injection have been addressed. Simulation results are in agreement with experimental observations in demonstrating the existence of a CO2-rich liquid phase above a certain pressure and high oil recovery as a result of the formation of the CO2-rich liquid phase. Compared to a conventional compositional simulator, the four-phase simulator focuses on a more realistic physical model of multiphase multicomponent flow. The simulation results not only provide more accurate predictions of reservoir performance but also promote a better understanding of the dynamic interactions between complex flow and phase behaviors.