Critical properties and phase behavior of confined fluids in irregular nanopores of shale gas reservoir

Real pore size and shape can induce critical properties shifts and phase behavior alterations for confined fluids in nanopores and impact the gas storage and transport mechanisms in shale gas reservoirs. Therefore, accurate calculations of phase behavior parameters in irregular nanopores are of great significance for the efficient development of shale gas reservoirs. No publications so far have been found to study the critical properties shift of confined fluids in irregular nanopores. In this paper, first, a quantitative description method for calculating the adsorbed volume of a multicomponent confined fluid in irregular nanopores is proposed, coupled with scanning electron microscope (SEM) images of shale cores and a semi-empirical formula for adsorbed thickness. Second, an effective molar volume correction method is extended to non-circular pores. Meanwhile, a new parameter is introduced to modify a molar volume term. Third, a novel correlation is proposed to predict the critical properties shifts in irregular nanopores considering pore shape and mixed fluid composition. Finally, a modified PREOS (Peng-Robinson equation of state) model is proposed, solved, and validated. Moreover, the critical parameters of pure component fluids and phase behavior characteristics of mixed hydrocarbons under different pore sizes, pore shapes, and hydrocarbon components are investigated, which reveals that a real pore structure and mixed fluid composition in shale gas reservoirs can change phase behaviors of confined fluids in nanoscale pores.