Experimental study on CO2-EOR in fractured reservoirs: Influence of fracture density, miscibility and production scheme

The effects of fracture density, miscibility and production scheme on CO 2 enhanced oil recovery (EOR) in fractured reservoirs were investigated. A series of CO 2 injection experiments were performed in various lengths of core plugs at both fractured and unfractured condition. Results demonstrate that although presence of fractures dramatically reduces the effectiveness of CO 2 flooding, increasing fracture density clearly benefits its recovery potential. The oil recovery mechanism is mainly recognized to be oil swelling at the beginning of CO 2 continuous injection and gradually turns to extraction of lighter hydrocarbons (C 5 C 30 ) with increased injection time. First-contact miscible flooding is more effective than both multi-contact miscible and immiscible flooding and proves its feasibility when fracture density exceeded 5.0 fractures per meter, with the maximum recovery up to 68.1% of original oil in place (OOIP). CO 2 huff-n-puff is very powerful in recovering the residual oil after CO 2 flooding. Fracture density has no major effect on huff-n-puff performance while at lower fracture-density condition, more cycles are required to reach the ultimate oil recovery. Multi-contact miscible CO 2 huff-n-puff has the greatest potential of EOR ability in fractured reservoirs, which can increase the oil recovery to 86.2% OOIP on the basis of CO 2 flooding test.