Characteristics and CO_2 flooding enhanced oil recovery experiments of Chang 6 in Ordos Basin
JIN Xiangchun；DU Meng；HAO Chunlian；XIN Fubing；KANG Ping；LüWeifeng；YAO Lanlan；
Chang 6 reservoir in Ordos Basin is an essential tight oil accumulation area. This paper comprehensively utilized core data，thin section identification，mercury injection，and high-precision scanning electron microscopy（SEM）and studied the reservoir characteristics in the study area from multiple aspects，including petrological properties，physical properties，pore-throat matching development characteristics，and pore structure features. On this basis，CO_2 flooding physical simulation experiments were conducted by the online nuclear magnetic resonance（NMR）at high temperatures and high pressures，and the multiple phase flow and migration behavior of crude oil were monitored during various stages of CO_2 flooding in real-time. The fluid saturation，ultimate recovery，residual oil distribution，and microscopic producing characteristics of crude oil in pores with different sizes are quantitatively studied， and the effects of injection pressures on the ultimate recoveries and recoveries of crude oil in microscopic pores are explored. The results show that the characteristics of Chang 6 reservoir in the study area are significantly different. The reservoir lithologies are mainly fine-grained feldspar sandstones and lithic feldspar sandstones， with an average porosity of 10.2% and an average permeability of 0.79 mD，predominantly low-porosity to ultra-low permeability tight reservoirs. The pore types are mainly intergranular pores，feldspar-dissolved pores，and zeolite-dissolved pores. According to the capillary pressure curve characteristics of the reservoir，Chang 6 reservoir can be divided into I，II，and III types from good to poor，with a good correlation between the pore-throat structure parameters and permeability. The crude oil in the study area is mainly stored in three types of pores（< 0.1μm，0.1-1μm，and 1-10μm）. There are differences in the microscopic production characteristics of CO_2 flooding in pores with different sizes. The recoveries of crude oil in macropores and medium pores show a gradual increase trend，while the recovery of crude oil in large and medium pores shows an increasing trend， while that in small pores shows a trend of decreasing first and then increasing. The injection pressure is positively correlated with the recovery of CO_2 flooding. High-pressure CO_2 flooding can enhance the mass transfer between oil and gas，reduce interfacial tension， and then improve the overall recovery. The research results can provide a reference for the efficient development of tight oil reservoirs in the study area.
