## 1 Introduction  
As oil and gas exploration transitions from conventional to unconventional fields, tight oil and gas have emerged as key resources for boosting China's oil and gas production. China's tight oil geological resources are estimated at approximately 200 \(\times\) 10\({}^{8}\) t, highlighting their abundance and potential, which have garnered increasing attention in recent years [1, 2, 3]. A defining characteristic of tight oil reservoirs is their extremely small pore size [4]. While conventional oil reservoirs typically have pore sizes exceeding 2 \(\upmu\)m [5, 6], tight oil reservoir pores range from 5 to 750 nm, with 20-40% measuring below 10 nm [7, 8]. This nanometer-scale pore size amplifies capillary forces, significantly altering fluid phase behavior compared to conventional reservoirs [9, 10, 11]. Phase behavior in conventional reservoirs is typically described by traditional equations of state (EOS), such as Redlich-Kwong equation of state (RK-EOS), Soave-Redlich-Kwong equation of state (SRK-EOS), and Peng-Robinson equation of state (PR-EOS) [12, 13, 14]. These EOS assume that phase behavior is independent of pore size and provide sufficient accuracy for conventional oil and gas calculation [15]. However, in tight reservoirs, the nanometer-scale throats and strong interactions between pore walls and fluids significantly influence fluid phase behavior, deviating from the phase behavior observed in conventional reservoirs. Consequently, the phase state theory for fluids in large conventional pores becomes inadequate for tight reservoirs [16]. Numerous studies have confirmed that the phase behavior of fluids in nanometer pores cannot be accurately described by traditional equations of state [17; 18]. [PERSON] [19] and [PERSON] [20] observed through adsorption-desorption experiments that the saturation pressure of fluids in nanometer pores is lower than in larger pores. [PERSON] [21], [PERSON] [22], and [PERSON] [23] further demonstrated that the critical temperature of fluids in nanometer pores is reduced compared to larger pores. Similarly, [PERSON] [24] found via diffusion methods that the critical temperature of n-pentane in 6 nm and 15 nm pores is lower than in larger pores. To directly observe and measure these phenomena, [PERSON] [25] employed high-pressure nanofluidic chips and showed significant deviations in the dew point pressure of propane and carbon dioxide within 17-33 nm porous media compared to calculated values. [PERSON] [26] used microfluidic chips to simulate shale matrices and found that decreasing pore size significantly impacts the bubble point properties of fluids. [PERSON] [27], using precision-fabricated 8 nm silicon nanochannels, demonstrated that nanoscale pore and throat interactions influence fluid dew point and bubble point pressures. These experimental findings collectively indicate that pore size significantly affects the phase behavior of tight oil reservoirs, rendering traditional EOS inadequate for describing phase behavior in nanometer pores. To accurately calculate the fluid phase behavior in tight reservoirs, improvements to traditional equations of state are essential. [PERSON] [28] developed a new EOS for predicting the critical pressure of multi-component hydrocarbon systems and used numerical simulations to study the impact of capillary forces in porous media on dew and bubble points. [PERSON] [29] analyzed the influence of capillary action on fluid saturation pressure in shale gas and oil reservoirs, while [PERSON] [30] incorporated critical temperature and pressure variations, along with capillary force effects, into the Pressure Volume Temperature (PVT) phase equilibrium of micropore fluids. [PERSON] also utilized an improved EOS and multi-component mixing algorithm (MMC) to calculate the minimum miscibility pressure for Bakken crude oil and carbon dioxide mixtures.