Jing Sun

The pore mobilization characteristics of CO₂ when in shale reservoirs is an important indicator for evaluating the effectiveness of its application for enhanced recovery in shale reservoirs, and it is important to develop a comprehensive set of physical simulation methods that are consistent with actual field operations. This has underscored the need for efficient development techniques in the energy industry. The huff-n-puff seepage oil recovery method is crucial for developing tight oil reservoirs, including shale oil. However, the small pore size and low permeability of shale render conventional indoor experiments unsuitable for shale oil cores. Consequently, there is a need to establish a fully enclosed experimental method with a high detection accuracy to optimize the huff and puff process parameters. The NMR technique identifies oil and gas transport features in nanogaps, and in this study, we use low-field nuclear magnetic resonance (NMR) online displacement technology to conduct CO₂ huff and puff experiments on shale oil, covering the gas injection, well stewing, and production stages. After conducting four rounds of huff-n-puff experiments, key process parameters were optimized, including the simmering time, huff-n-puff timing, number of huff-n-puff rounds, and the amount of percolant injected. The findings reveal that as the number of huff-n-puff rounds increases, the time required for well stabilization decreases correspondingly. However, the enhancement in recovery from additional huff-n-puff rounds becomes negligible after three rounds, showing only a 1.16% improvement. CO₂ re-injection is required when the pressure falls to 70% of the initiaformation pressure to ensure efficient shale oil well development. This study also indicates that the most economically beneficial results are achieved when the injection volume of the huff-n-puff process is 0.44 pore volumes (PVs).

Enhancing oil recovery in shale is a critical technology for improving shale oil extraction efficiency. It is essential to develop a comprehensive set of physical simulation methods that are coherent and aligned with practical field operations. This paper establishes an integrated experimental approach, encompassing the entire Injecting-Stewing-Producing cycle, to simulate the actual Huff-n-Puff process accurately. Initially, the fracturing and flowback states are simulated by injecting an imbibition fluid, followed by a 48 h well-soaking process using CO₂. The extraction is then carried out under various pressures. The microtransportation of crude oil across different pore sizes and the extent of extraction during shale oil Huff-n-Puff are investigated using Nuclear Magnetic Resonance technology. The results suggest that there was an initial increase in crude oil within pores smaller than 20 nm at the beginning of the Huff-n-Puff process. In Contrast, crude oil in pores larger than 200 nm was preferentially extracted, with oil in smaller pores (<200 nm) migrating to larger pores before extraction. After the initial Huff-n-Puff cycle, the extraction efficiency of the shale oil core reaches 29.55%, constituting 63.3% of the total extraction achieved over three Huff-n-Puff cycles. This study also identifies a critical pressure drop to 60% of the initial pressure as the optimal point for injection in subsequent Huff-n-Puff cycles. These experimental insights provide valuable guidance for the practical implementation of enhanced oil recovery techniques in shale formations.