Investigation of CO2/N2 injection in tight oil reservoirs with confinement effect

This paper investigates the CO2/N-2 injection process in tight oil reservoirs considering the confinement effect. To study the microscopic physical mechanisms, the confinement effect is characterized by properties shift and capillarity and introduced into the flash calculation to obtain the phase equilibrium of mixture fluids (tight oil/CO2/N-2) in tight porous media. The results indicate that the injected nitrogen gas could effectively maintain the reservoir pressure, while it also weakens the effects of the CO2 injection recovery mechanisms, notably diffusivity and viscosity reduction. In addition, a dual-pore tight oil reservoir model is set up to investigate the CO2/N-2 injection with ultra-low permeability and hydraulic fracturing. The basic CO2 injection parameters are optimized by the orthogonal method. Based on CO2 injection process, three injection schemes of CO2/N-2 injection, which are mixed-gas injection, CO2-alternating-N-2 (CAN) injection, and N-2-alternating-CO2 (NAC) injection, were investigated and a comparative analysis was made for the pressure distribution, CO2 mole fraction distribution, and cumulative oil production. Based on this analysis, the CAN injection process proved to be the best injection scheme. A parametric analysis further suggested that the nitrogen gas injection rate was the most important factor. Besides, the effect of gravity drainage, reservoir permeability, nature fractures, and permeability heterogeneity on the oil production of CAN injection process were also investigated in detail. The results show that tight oil reservoir with better vertical connectivity, poor fracture growth, and higher heterogeneity is more favorable for the CO2/N-2 injection process.