Feasibility Study On ISC Via Oxygen-Reduced Air Injection At High Pressure In Deep Heavy Oil Reservoirs

Deep heavy oil reservoirs have the characteristics of deep burial,high temperature and pressure,conventional steam injection for thermal recovery is of low efficiency and high development cost.In-situ combustion technique has a good application prospective in the development of deep heavy oils due to its high recovery ratio and low cost.The air with oxygen content of 21% is often used as the injected gas in traditional In-situ combustion technique.High pressure air injection has a great risk of oil and gas explosion for deep heavy oil reservoirs with high pressure,and the effect of oxygen-reduced air in suppressing combustible gas explosions provides the possibility for the application of oxygen-reduced air in deep heavy oil reservoirs.In this paper,a small batch reactor was used to conduct heavy oil oxidation experiments under conventional air and oxygen-reduced air atmospheres to study the effects of oxygen content and pressure on the oxidation reaction rate and heat release of heavy oils,and to clarify the heat release characteristics of oxidation reaction of heavy oils.Based on the experimental results,calculation models for the heat release and reaction rate of heavy oils oxidation were established.Explosion experiment of oil-gas-air mixture was carried out in the oxygen-reduced air atmosphere with different oxygen content,and the effect of oxygen-reduced air on suppressing the explosion of oil-gas-air mixture was investigated.Based on the reservoir simulation method and using reservoir simulation software,a simulation model of deep heavy oil in-situ combustion with three-dimensional,four-phase and seven components was established.The in-situ combustion process of oxygen-reduced air was simulated under different gas injection conditions,and the effects of oxygen content and gas injection rate on the front temperature and recovery efficiency of in-situ combustion process of heavy oil were discussed.Experimental results show that oxidation process is an exothermic process of heavy oil,the total exothermic heat and reaction rate during the oxidation process of heavy oil are mainly affected by the pressure and oxygen content of the gas injected.The heat release and reaction rate are linearly proportional to the partical pressure of oxygen and oxygen content in the injected gas.The higher the pressure and oxygen content of gas injection,the more oxygen molecules in the gas can participate in the oxidation reaction,and the more total heat released by heavy oil oxidation,and the faster the oxidation reaction rate.Oxygen-reduced air can restrain the explosion of heavy oil and gas.By reducing the oxygen content in the injected gas,the explosion energy can be effectively reduced to achieve the purpose of explosion reduction and suppression.And the experimental results show that the risk of oil and gas explosion can be reduce or eliminated when the oxygen content in the air is reduced to 10%.So it is feasible to implement oxygen-reduced air injection at high pressure for in-situ combustion in deep heavy oil reservoirs from two aspects of oxidation heat release and explosion reduction.The numerical simulation results of in-situ combustion using the conceptual geological model of deep heavy oil show that using oxygen-reduced air with an oxygen content of 10%for in-situ combustion can form a stably advancing combustion front with high temperature in the reservoir and achieve the effect of in-situ combustion,and the total oil recovery factor is over 66%.The oxygen content and gas injection rate affect the temperature and propelling speed of the combustion front during the in-situ combustion process.The higher the oxygen content and the gas injection rate and the pressure,the higher the temperature of the combustion front,which is beneficial to enhance the recovery factor.However,a higher gas injection rate and oxygen content will increase the total amount of crude oil consumed by oxidation,and accelerate the advancing speed of the combustion front,shorten the production time,and reduce the effective sweep volume,which will lead to a decrease in the amount of crude oil produced.