Ji, Bingyu (SINOPEC Exploration & Production Research Institute) | Lv, Chengyuan (SINOPEC Exploration & Production Research Institute) | Wang, Rui (SINOPEC Exploration & Production Research Institute) | He, Yingfu (SINOPEC Exploration & Production Research Institute)
CO2 flooding, which owns better injectivity and displacement effect, becomes one of the most effective EOR methods for low permeability reservoirs. However, there is a big difference for CO2 flooding between low permeability reservoirs and higher permeability reservoirs, which implies the limitations of conventional theories for CO2 flooding in higher permeability reservoirs. In view of large pressure differential and uneven pressure distribution between injection well and production well in CO2 flooding for low permeability reservoirs, the miscible volumetric coefficient, the semi-miscible volumetric coefficient and the immiscible volumetric coefficient were defined to characterize the miscible state of CO2 flooding, and the phase sweep efficiency and the compositional sweep efficiency were proposed to describe the displacement front of CO2 flooding. And then, the influence factors of the miscible volumetric coefficient and the compositional sweep efficiency in CO2 flooding for low permeability reservoirs were obtained through reservoir simulation. Furthermore, the concepts mentioned above were applied in oilfields of SINOPEC, which can reasonably illustrate displacement characteristics of CO2 flooding for low permeability reservoirs. Currently, the pilot tests of CO2 flooding have been going into effect, which indicates good prospects.
Generally, the CO2 flooding processes can be divided into two categories, miscible flooding and immiscible flooding. The minimum miscibility pressure (MMP) used to describe miscibility is generally measured through slim-tube apparatus. Miscible flooding will be achieved at pressures higher than MMP [1,2] . However, miscible or immiscible could not be distinguished by using MMP alone, as the effect of in-situ pressure distribution on miscibility cannot be neglected. Moreover, the obscure boundary between the miscible state and the immiscible state indicates that most of CO2 EOR projects should be considered as near-miscible or partially miscible [3-7] . Especially, the uneven distribution of reservoir pressure and the huge pressure drop from injector to producer in CO2 flooding for low permeability reservoirs will affect miscibility and oil recovery [8-10] . Based on the above understanding, miscible volumetric coefficient, phase sweep efficiency and compositional sweep efficiency were defined and the concept of non complete miscible displacement was proposed for CO2 flooding in low permeability reservoirs, which were used for the design of CO2 EOR projects of SINOPEC.