Experimental study on gas-assisted cyclic steam stimulation under heavy-oil sandstone reservoir conditions: Effect of N2/CO2 ratio and foaming agent

After several rounds of cyclic steam stimulation (CSS), the formation energy of heavy oil reservoirs is depleted. The water content in the reservoir is increased, which leads to a decrease in production. The injection of N2 and CO2 can effectively improve the recovery efficiency. In this paper, the experiments of mixed gas-assisted CSS with different N2/CO2 injection ratios and mixed gas foam-assisted CSS were performed. The temperature change, liquid production, pressure change and residual oil distribution of the sandpack were tested during the experiment, and the influencing factors of enhanced oil recovery were explored. The results show that the range of recovery variation can be divided into an increasing phase from 1:0 to 2:1 and a decreasing phase from 2:1 to 0:1 based on the different injection ratios of N2/CO2. The viscosity reduction of heavy oil under the effect of CO2 dissolution is the dominant factor in the increasing phase, while the formation energy increase under the effect of N2 expansion is the dominant factor in the decreasing phase. The recovery factor is the best when the ratio of injected N2/CO2 is 2:1. The cumulative recovery factor increases by 6.46% after adding the A-9 foaming agent solution. This shows that the mixed gas foam combines the advantages of the mixed gas and foam. It increases the flow resistance and improve the oil displacement efficiency by plugging the lager pore-throat from a microscopic perspective. Therefore, mixed gas foam-assisted CSS can effectively improve the production of heavy oil reservoirs. The research results provide theoretical guidance for improving the effectiveness of the CSS of heavy oil reservoirs.