Study on Phase Behavior in Pores of Different Types of Shales and Its Impact on CO2 Huff-Puff

Shale oil resources have significant strategic value, and CO₂ huff-n-puff technology is a key means for developing shale oil reservoirs. Due to the extensive development of nanopores in shale reservoirs and the complex and diverse mineral compositions, there are significant differences in the oil production laws in different pores during the CO₂ huff-n-puff process. To address this issue, starting from the core factor affecting production─phase behavior─this paper first constructs a phase equilibrium model considering the influence of nanopore size and wettability; then, based on the principle of material conservation, it establishes a CO₂ huff-n-puff numerical model. Through simulation calculations, it analyzes the production laws of multicomponent hydrocarbons in different CO₂ huff-n-puff cycles under the conditions of different pore sizes and wettabilities. The results indicate that the bubble point pressure decreases as the pore size reduces. During pressure depletion production, more liquid is produced from small pores, resulting in a higher content of heavy components. In the early stage of CO₂ huff-n-puff, more methane (CH₄) is displaced by CO₂ in small pores; as the number of cycles increases, the production of light and medium components in pores of different sizes tends to be consistent, with the heavy component production in 10 nm pores being slightly higher. The reduction of contact angle enhances the fluid–solid interaction, lowers the bubble point pressure, increases the liquid-phase yield, and thus increases the yield of heavy components. With increasing huff-n-puff cycles, pores with smaller contact angles contain fewer heavy components in the remaining fluid, leading to higher bubble point pressure and increased gas-phase production.