Study on the Difference in CO2 and Air Injection in the Fracture/Matrix Dual Medium of Continental Shale Reservoirs

To clarify the impact of different displacement mediaon the enhancedoil recovery of continental shale and realize the efficient and reasonabledevelopment of shale reservoirs, this paper takes the continentalshale of the Lucaogou Formation in the Jimusar Sag in the JunggarBasin (China, Xinjiang) as the research object and uses real coresto build the fracture/matrix dual-medium model. Computerized tomography(CT) scanning is used to visually compare and analyze the influenceof fracture/matrix dual-medium and single-matrix medium seepage systemson oil production characteristics and clarify the difference betweenair and CO2 in enhancing the oil recovery of continentalshale reservoirs. Through a comprehensive analysis of the productionparameters, the whole oil displacement process can be divided intothree stages: the oil-rich and gas-poor stage, oil and gas coproductionstage, and gas-rich and oil-poor stage. Shale oil production followsthe rule of fractures first and matrix second. However, for CO2 injection, after the crude oil in the fractures is recovered,the oil in the matrix migrates to the fractures under the action ofCO(2) dissolution and extraction. Overall, the oil displacementeffect of CO2 is better than that of air, resulting ina 5.42% higher final recovery factor. Additionally, fractures canincrease the permeability of the reservoir, which can greatly enhanceoil recovery in the early oil displacement process. However, as theamount of injected gas increases, its impact gradually decreases,and ultimately, it is consistent with the recovery of nonfracturedshale, which can achieve nearly the same development effect.