(559t) Phase Equilibria of Confined Fluids in Nanopores of Shale Rocks from an Adsorption-dependent Equation of State

Low-salinity water-surfactant-CO₂ (LSS-WACO₂) flooding is recognized as a promising EOR method in carbonate, sandstone and shale reservoirs due to its ability to alter wettability and decrease interfacial tension (IFT). However, the phase behavior of reservoir fluids in LSS-WACO₂ has not been well understood and analysed. This paper aims to investigate the number of phases formed, their relative amount, and compositions of the brine/CO₂/oil/surfactant systems at a particular reservoir brine concentration.

We developed a new computational model for the brine/CO₂/oil/surfactant systems based on high-quality data from combination of inhouse measurements and literature data. The Huron-Vidal mixing rule was introduced and the Peng-Robinson equation of state (PR EOS) with a modified alpha function was tuned to theoretically capture the interaction between various components of the mixture and then model phase behavior of the brine/CO₂/oil/surfactant mixtures. We also adopted group contribution model (GCM) using information from the molecular structure of the pseudo components for the estimation of their critical properties.

Phase behavior experiments were conducted at temperature of 60^oC using Alfa Olefin Sulfonate (AOS) surfactant, CO₂, n-octane and brine. Phase relative amount and compositions were thus determined. In modeling study, the Huron-Vidal mixing rule can be used to describe CO₂ and brine interactions due to its ability to capture the Van der Waals-type interactions as well as polar interactions compared to classical mixing rule, and the laboratory results were compared with the results predicted by the model. The comparison result showed the level of reliability of our new predictive model.

The model we constucted not only can be used in compositional simulation of LSS-WACO₂ operations but also be proved as an invaluable tool in planning and evaluation of LSS-WACO₂ processes. PR EOS was originally developed for non-ionic, non-polar and non-associating hydrocarbons, while this study expands its application into the system with brine and surfactant.