Performance evaluation of CO2 EOR in tight oil formation with complex fracture geometries



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The recent development of tight oil reservoirs has led to an increase in oil production in the past several years due to the progress in horizontal drilling and hydraulic fracturing. However, the oil recovery factor expected is still very low even after the wells have been fractured and therefore, tight formations are considered good candidates for enhanced oil recovery (EOR). One of the most suitable solutions to improve the oil recovery is the carbon dioxide (CO2)-based EOR. Although the injection of CO2 is not new for conventional oil reservoirs, its practice in tight oil formations is still a relatively novel idea. Two injection-production strategies are often employed: continuous CO2 injection or flooding and CO2 Huff-n-Puff. However, it is not clear which scenario is the best strategy to achieve an optimal recovery, which highly depends on many uncertain reservoir and fracture parameters and it is not clearly understood until recently. Another challenge of the estimation of the incremental recovery of these injection approaches is to properly model the hydraulic fractures and CO2 transport mechanism. The actual hydraulic fracturing process often creates complex fracture networks, especially when the fracture propagates in a formation with a large amount of pre-existing natural fractures. In this study, the CO2-EOR effectiveness is simulated and analyzed by comparing the Huff-n-Puff and the continuous injection scenarios. The effect of matrix permeability on the comparison of well performance of these two scenarios was investigated. Subsequently, Design of Experiment and Response Surface Methodology is used to perform sensitivity studies with four uncertain parameters including matrix permeability, number of wells, well pattern, and fracture half-length to determine the best injection approach. In addition, an efficient methodology of embedded discrete fracture model (EDFM) is introduced to explicitly model complex fracture geometries. The effects of complex fracture geometries on well performance of CO2 Huff-n-Puff and CO2 continuous injection were also investigated as well as the effect of natural fractures. The analysis of the CO2-EOR effectiveness confirms that the appropriate modelling of the complex fractures geometry plays a critical role in estimation of the incremental oil recovery. This study provides new insights into a better understanding of the impacts of reservoir permeability, complex hydraulic fractures and natural fractures on well performance during CO2-EOR process in tight oil reservoirs and in the determination and design of the optimal injection-production scheme to maximize the oil recovery factor for multi-fractured horizontal wells.