Comparative study of enhanced oil recovery efficiency by CO2 injection and CO2 huff-n-puff in stimulated shale oil reservoirs

Multi-stage fracturing applied in horizontal wells is considered as the most effective method for producing shale gas/oil reservoirs. However, the primary oil recovery from natural depletion is only few percent. The low recovery rates and the abundance of shale resource plays provide huge potential for enhanced oil recovery. Gas or oil production performance from nano-Darcy permeable shale rocks is characterized by steep decline rates. The primary shale oil recovery from hydraulically fractured horizontal wells is around 5%. Without enhancement of oil production from secondary recovery by solvent or gas injection, the huge cost of hydraulic fracturing treatment makes it economically unviable to produce shale oil reservoirs. It is imperative that we implement an enhanced oil recovery method in going beyond conventional natural depletion means to achieve profitable production. In this paper, we will compare the improved oil recovery efficiency in shale oil reservoirs by designed cyclic gas injection in a fractured horizontal well and gas flooding in two fractured wells. The biggest challenge for gas flooding in fractured shale oil reservoirs is that the injected solvent may break through to production well via the fracture network or fissures. In contrast, cyclic gas injection is a good candidate for fractured reservoirs that is not subject to earlier breakthrough. The oil recovery factor obtained from these two injection approaches were compared under both miscible and immiscible displacements. Our composition simulation results showed that under miscible flooding condition cyclic gas injection is more effective than gas flooding given at the same injection pore volume and identical fracture spacing. One reason for low recovery in gas flooding is the loss of injected fluids and decrease of displacement efficiency due to earlier breakthrough to the producer via natural or hydraulic fractures. Oil response from cyclic gas injection well performed steadily even after many cycles in shale oil reservoirs, but oil rate from gas flooding falls off rapidly. Considering the existence of natural fractures, the oil recovery by cyclic CO2 injection increased from primary 5% to ultimate 18% after 10 cycles in two hydraulically fractured horizontal well. On the other hand, the enhanced oil recovery by CO2 flooding is only 11%. The disadvantage of cyclic gas injection is that it consumes more time than gas flooding because injection and production processes are separate. If the injection time is too long, oil will be pushed far away from the fractures that makes it more difficult to produce back. The objective of this paper is to evaluate the IOR performance of cyclic gas injection and gas flooding approach in shale oil reservoirs.