MODELING THE THERMODYNAMIC PHASE BEHAVIOR OF HYDROCARBON MIXTURES IN NANOPORES

The global energy landscape has been reshaped by the hydrocarbon recovery from unconventional shale formations, especially the liquid-rich shale formation. Despite that liquid-rich shale formations are economically attractive, the gas-condensate flow behaviors are not well understood, which poses challenges for developing predictive models at the Darcy-scale. The thermodynamic phase behavior of a hydrocarbon mixture in the tight shale rock can significantly deviate from its bulk counterpart. Possible mechanisms responsible for the deviation include the high capillary pressure and strong sorption of hydrocarbon molecules within the extensive nanopores in shale rock. Advanced theories using molecular-level simulations can provide detailed insights for the phase behavior in a single nanoscale cylindrical or slit pore. However, they oftentimes underrepresent the complex pore geometries and are too computationally expensive to be incorporated into Darcy-scale models for reservoir simulations. To bridge the gap, we develop a new equation of state model for the phase behavior in nanopores with different geometries including cylindrical, square, irregular triangular tubes. By incorporating the effects of capillary pressure and adsorption, the model allows us to examine the nanoconfinement effects on the phase behavior in nanopores with different geometries and sizes. Our results show that: 1) The adsorption plays a much more important role than that of the capillary pressure in deviating the phase envelopes. 2) The deviated nanoconfined phase behavior is primarily governed by the pore size, while the pore geometry has a relatively minor impact. The present model can be readily incorporated into Darcy-scale models for reservoir simulations to improve the prediction of hydrocarbon recovery from shale formations.