The optimization of pre-slug size in CO2-N2 compound flooding

In order to enhance oil recovery in low and ultra-low permeability layer, both of the numerical simulation and physical model experiment have been researched. First, the dynamic distribution of CO1 and N1 in the oil and gas phase in the CO2 -N2 compound flooding process was numerically simulated by using the long slim-tube model. The results show that the CO2 slug should have at least 0.3 PV to prevent the impact of N2 channeling effectively. Second, under the experimental conditions of complete miscibility of CO2 -crude oil, the two types of natural cores including low and ultra-low permeability, respectively, are used for experimental study on oil displacement. The results confirm that CO2 -N2 compound flooding with 0.3 PV CO2 pre-slug can achieve a good result. Finally, a five-point well pattern element model is established by CMG. The recovery and the gas cost of per ton of oil are calculated respectively for CO2 -N2 compound flooding and full CO2 flooding at 300 m well spacing of low and ultra-low permeability layer. According to the simulation results, the optimal CO2 pre-slug size in CO2 -N2 compound flooding under the condition of low and ultra-low permeability layer five-point well pattern is 0.4 PV.