The Influence of Fractures in Shale Oil Reservoirs on CO₂ Huff and Puff and Its Pore Production Characteristics

To study the influence of fractures from shale hydraulic fracturing on the CO₂ huff and puff, low-field nuclear magnetic resonance was adopted in CO₂ huff and puff experiments with shales of different permeabilities, and the influence of fractures on the CO₂ huff and puff effect in reservoirs of different permeabilities was investigated. Experimental results show that fractures significantly improve the oil production rate and recovery at the initial stage of the CO₂ huff and puff operations. However, the influence of fractures on the recovery gradually decreases as huff and puff continues and the permeability increases. The influence of permeability on huff and puff of fractured cores is much lower than that of unfractured cores, which indicates that fractures may reduce the influence of permeability on recovery by CO₂ huff and puff. With continuing huff and puff, the increase rate in the degree of reserve recovery of crude oil from macropores contributed by fractures declines while that of crude oil from micropores increases. Despite this finding, macropores continued to be the main contributor of crude oil production. The results indicate that the quick and high production of crude oil from macropores mainly depends on volume expansion and dissolved gas drive. In contrast, the slow and low production of crude oil from micropores is primarily brought by extraction and mass transfer. The results of this study have provided a theoretical basis for evaluating the characteristics of oil production from fractured reservoirs and improving production performance.