STUDY ON THE MECHANISM OF CO2 INJECTION TO ENHANCE OIL RECOVERY IN TRIASSIC RESERVOIRS IN LUNNAN OILFIELD

Carbon dioxide oil recovery technology is to inject a certain amount of carbon dioxide into the reservoir and use carbon dioxide to dissolve the crude oil to reduce the viscosity of the crude oil, expand the volume of the crude oil, and reduce the oil-water interface. It solves the problems of difficulty in crude oil flow and insufficient formation energy in reservoir development, thereby, increasing the production of oil wells and achieving the purpose of improving oil reservoir recovery. Through the gradual practice in recent decades, the relevant oil fields at home and abroad have adopted carbon dioxide oil recovery technology, which has achieved an increase of 5-15% in crude oil recovery and achieved significant results. However, studies have found that for different reservoir environmental conditions, the results obtained by injecting carbon dioxide into oil are very different. Therefore, for different types of oil reservoirs, it is necessary to carry out targeted research on the mechanism of injecting carbon dioxide to improve oil recovery. Especially for low- and medium-permeability reservoirs, in the process of water injection and gas injection into the formation, the fluid flow mechanism in the core pores is complicated, which brings great difficulties to improving the oil recovery efficiency of the formation. This paper takes the TI2 oil sands group of the Huangshanjie Formation of Lunnan Triassic as an example. The target layer is a typical low-permeability reservoir. Through indoor core experiments, the experiment of carbon dioxide displacement of crude oil was carried out, and the temperature, injection pressure, and displacement were analyzed. The influence of speed, permeability and other factors on the oil recovery rate of core flooding. Studies have shown that the higher the injection pressure, the more the carbon dioxide dissolves in the crude oil and the better the oil displacement effect. The higher the injection formation temperature, the lower the viscosity of the crude oil and the lower the displacement effect. When the displacement temperature, displacement pressure, and permeability are the same, if the displacement rate is higher than a certain level, the higher the rate is, the more obvious gas channeling is, and the worse the displacement effect is. This result is consistent with the reservoir in the study area. The existence of some micro-cracks in the core is related to the existence of micro-cracks that provide conditions for the generation of gas channeling. Therefore, in the actual on-site gas injection practice process, it is necessary to consider higher pressure and temperature, and control the gas injection rate to avoid gas channeling and affect the gas injection effect.