Thermodynamic modeling for confined fluids in nanopores using an external potential-advanced equation of state

Abstract

Quantitatively understanding thermodynamic properties of confined fluids in nanoporous media is of great significance to the development of shale gas. Due to the complex intermolecular forces in the nanopore, it is difficult to accurately predict the thermodynamic properties of fluid molecules. Focusing on the fluid-fluid and fluid-wall molecules interactions in shale reservoir system, the thermodynamic model of fluid molecules in pores is constructed, and a modified equation of state is proposed by systematically coupling the original Soave-Redlich-Kwong equation of state with Tjatjopoulos-Feke-Mann potential model in this work. The advanced EoS could facilitate a good prediction on thermodynamic properties of confined fluids without any introduction of new empirical parameters. For verification, fluid density as the important thermodynamic property was targeted and pure methane at a wide pressure range was employed to represent the fluid. The results indicated that the calculated densities accord well with the reported ones in the free gas zone. The deviation of discrete density ranges from 0.239% to 1.7329%. The fluid density distribution in the nanopores is found to be nonuniform, exhibiting a greater value near the wall than that in the pore center, which would be ascribed to the more dominant fluid wall molecule interaction. For example, the local density is 16.90 kg/m3 in the pore center, while it increases to 26.67 kg/m3 in the region which is 0.76 nm to the wall at 350 K, 3 MPa, and 5 nm (radius). Moreover, effects of other critical factors on fluid density distribution were also conducted, and it was indicated that higher pressure, lower temperature, and smaller pore size could be favorable for the occurrence of confined fluid. In general, the novel EoS could provide a quantitative and simple method in predicting the thermodynamic properties of confined fluids relating to applications of shale gas storage and exploitation.