Dynamic characteristics and influencing factors of CO2 huff and puff in tight oil reservoirs

CO2 huff and puff experiments of different injection parameters, production parameters and soaking time were carried out on large-scale cubic and long columnar outcrop samples to analyze dynamic characteristics and influencing factors of CO2 huff and puff and the contribution of sweeping mode to recovery. The experimental results show that the development process of CO2 huff and puff can be divided into four stages, namely, CO2 backflow, production of gas with some oil, high-speed oil production, and oil production rate decline stages. The production of gas with some oil stage is dominated by free gas displacement, and the high-speed oil production stage is dominated by dissolved gas displacement. CO2 injection volume and development speed are the major factors affecting the oil recovery. The larger the injected CO2 volume and the lower the development speed, the higher the oil recovery will be. The reasonable CO2 injection volume and development speed should be worked out according to oilfield demand and economic evaluation. There is a reasonable soaking time in CO2 huff and puff. Longer soaking time than the optimum time makes little contribution to oil recovery. In field applications, the stability of bottom hole pressure is important to judge whether the soaking time is sufficient during the huff period. The oil recovery of CO2 huff and puff mainly comes from the contribution of flow sweep and diffusion sweep, and diffusion sweep contributes more to the oil recovery when the soaking time is sufficient.