Experimental study on characteristics of phase transitions and pore-scale mobilization in shale volatile oil reservoirs based on online nuclear magnetic resonance

Shale reservoirs are characterized by well-developed nanopores, where the nano-confinement effect is significant. To address the unclear phase transition mechanisms and the significant heterogeneity in oil and gas mobilization characteristics caused by the nano-confinement effect during the development of shale volatile oil reservoirs, the study comprehensively considers pore-scale effects and CO₂ diffusion dynamics. Physical simulation experiments of elastic depletion and CO₂ huff and puff were conducted using online nuclear magnetic resonance (NMR) technology. Based on T₂ spectrum calibration, a pore size characterization relationship was established, and pores were classified into macropores, mesopores, and micropores. Subsequently, the phase transition characteristics during elastic depletion and their influence on the degree of crude oil mobilization were investigated. In addition, the recovery effects of different CO₂ injection durations and soaking times and their pore-scale oil mobilization characteristics during CO₂ huff and puff were analyzed. Experimental results indicated that the nano-confinement effect significantly suppressed the bubble point pressure in shale volatile oil reservoirs. Specifically, compared with the bulk bubble point pressure of 36.66 MPa, the bubble point pressures in cores with different pore structures were reduced by 13.66 MPa and 18.66 MPa, respectively. This indicated that the reduction in bubble point pressure became more pronounced as pore size decreased. During elastic depletion, the exsolution of gas bubbles facilitated oil and gas recovery. However, when pressure continued to decline below the bubble point pressure, the Jamin effect occurred, which ultimately reduced the recovery efficiency. During the CO₂ injection stage, dissolution and diffusion were the primary mechanisms for mobilizing crude oil within shale nanopores. CO₂ preferentially entered larger pores and gradually diffused into smaller pores with increasing injection time. A longer CO₂ injection duration resulted in a more significant oil recovery enhancement. During the soaking stage, crude oil within the core underwent redistribution, and CO₂ and oil gradually reached a dynamic equilibrium under the combined influence of capillary forces and diffusion. Small pores were the primary zones for residual oil enrichment and represented the key challenge for efficient oil recovery in subsequent development.