Due to the vast exploration of conventional oil and gas fields, unconventional sources like ultra-deep reservoirs pose as feasible alternatives for the crescent demand for fossil fuel energy.

However, besides the challenges of exploring deep-sea areas under thick layers of salt, reservoir fluids and rock are very complex: high CO_{2} content and heterogeneous properties due to carbonate formation, respectively.

The complexity of the phase behavior of reservoir fluids increases considerably due to presence of CO_{2} and water. Besides multiphase equilibrium, at high temperatures the solubility of water in the non-aqueous phases is substantial; generating deviations in the equilibrium calculations for simplified thermodynamic models, such as cubic Equations of State (EoS).

Most of the reservoir simulators employ cubic EoS, which are not able to deal with the complex phase equilibrium. Therefore one of the goals of this project is to implement into a simulator advanced thermodynamic models, i.e. Cubic-Plus-Association EoS (CPA).

Another key point of the project is that ultra-deep reservoirs are usually under high pressure and temperature, so at these conditions the mixture may be close to the critical point. It is well known that phase equilibrium calculations near the critical point are very complex, and large deviations may be found. Thus a second objective of this work is to evaluate the capacity of the thermodynamic model to better describe the phase behavior of fluids near the critical region.

This project will be divided in three parts. First, the phase behavior of some selected systems will be evaluated by the CPA and compared to experimental data. Then, with the model tuned for these systems, this equation will be implemented into a reservoir simulator, which will be used to perform, in the last part, a series of simulations to determine the importance of applying a sophisticated thermodynamic model in reservoir simulation.