An isenthalpic-based compositional framework for nonlinear thermal simulation [electronic resource]
Keywords: Energy Resources Engineering.

Thermal enhanced oil recovery (EOR) involves the complex interplay of mass and energy transport with phase behavior. Hydrocarbon and water components partition across multiple fluid phases as a function of composition, pressure and temperature. Thermal compositional simulation thus requires phase-equilibrium calculations for at least three fluid phases: an oleic phase, an aqueous phase, and a vapor phase. The use of precomputed equilibrium ratios (K-values) remains popular in industry, where it is justified on the basis of efficiency. However, this method greatly simplifies the underlying phase behavior and has been shown to lead to incorrect predictions of oil recovery. This dissertation describes the development of a thermodynamically consistent framework for thermal simulation using an equation of state (EOS). The mass and energy conservation laws are solved with a molar variables formulation, in which enthalpy is a primary variable. Isenthalpic flash provides high integrity coupling of the local thermodynamic constraints to the solution of the equations at the global level. Phase equilibrium calculations are well-developed for hydrocarbon fluids in isothermal compositional simulation. In contrast, EOS representation of phase behavior in thermal recovery has not received the same attention. In non-isothermal reservoir processes, water is the thermodynamically dominant species. In this dissertation, we describe a suite of phase equilibrium algorithms developed for hydrocarbon-water mixtures in thermal simulation. We introduced the steam saturation curve to guide the initial selection of K-values in phase stability testing. This approach requires far fewer initial estimates than existing methods. In isothermal flash, the presence of trace components in the aqueous phase precludes the use of a Newton solver with some conventional formulations. We implemented a new reduced parameter formulation, for which performance is agnostic to the presence of near-pure phases. We demonstrated performance of both our stability testing protocol and reduced phase-split formulation through comprehensive testing across an extensive parameter space. Isenthalpic flash is far more challenging than isothermal flash. Temperature is unknown a priori and the convergence of conventional algorithms is hindered by K-value sensitivity to enthalpy. We pioneered the extension of reduced variables beyond the isothermal domain with the development of a reduction method for rapid isenthalpic flash. Through a series of examples we demonstrated rapid convergence of our novel implementation, even for narrow-boiling point mixtures. The compositional framework developed in this research was brought to fruition in our implementation of a thermodynamically consistent thermal simulator. In contrast to conventional thermal simulation models, we allowed for arbitrary component partitioning across phases. The implementation uses overall molar composition, pressure and enthalpy as the principal unknowns. Convoluted derivatives of secondary variables are generated in AD-GPRS using the implicit function theorem. In this dissertation we use several case studies to show that the molar formulation outperforms the natural formulation when phase behavior is the principal source of nonlinearity.