Thermal Simulation of CO2 Storage with Generalized Cubic Equation of State

Abstract

Relationships between pressure, temperature and density are generally described by an equation of state. For CO2, the Span-Wagner equation is generally assumed to give the best fit to measured data. The high accuracy of this equation does not come without a cost. Therefore, this equation of state is mostly used as a reference for comparisons with other formulations. In reservoir simulations, cubic equations of state such as the Peng-Robinson and the Soave-Redlich-Kwong equations are widely used. They are fairly accurate, and computation of the solution is fast. In this paper, a generalized cubic equation of state is introduced. This equation is computationally precisely as efficient as the traditional equations of state. With the generalized equation of state, improved approximations of the density of CO2 in predefined temperature-pressure domains may be obtained. The parameters of the generalized cubic equation of state are determined through comparison with the Span-Wagner equation. We show applications of the generalized cubic equation of state for different temperature-pressure domains. When compared with the Peng-Robinson equation, the root mean square density deviation is reduced by a factor 2 for domains containing the critical point, and a facto 7 for supercritical domains. Similarly, thermal simulations with the generalized cubic equation of state show large improvements in density and improvements in saturation close to the CO2 front.