<p>The phase behavior and critical properties of fluids confined in nanopores deviate significantly from bulk-phase behavior due to confinement effects, making classical equations of state (EOS) inadequate for accurate predictions. Both the adsorption layer thickness and the free state of fluid molecules within nanopores jointly influence the molar volume, complicating the characterization of fluid properties at the nanoscale. Existing EOS models fail to capture these phenomena, particularly at nanoscale dimensions where critical points shift, making predictions of phase behavior challenging. This study aims to address these limitations by modifying the Peng–Robinson (PR) EOS to more accurately describe the molar volume and critical properties of confined fluids. Empirical corrections were introduced, including adsorption layer thickness, effective molecular volume coefficients, molecular quantity coefficients, and confinement indices, extending the PR-EOS from bulk to nanoscale conditions. A dimensionless pore size correlation was developed, using both experimental and molecular simulation data, to adjust the shifted critical properties. Results show that confinement effects lead to a shrinkage of the phase diagram, and the modified PR-EOS accurately predicted the phase behavior of CO₂ mixtures in 20&#xa0;nm pores, aligning well with experimental data. These findings highlight the significant influence of critical property shifts on the phase behavior of confined fluids, offering valuable insights for optimizing production in shale oil and gas fields.</p>