Displacement mechanisms of air injection in low permeability reservoirs

Abstract

By means of physical modeling and numerical simulation, this paper makes a systematic study on oil displacement mechanism by air injection in low permeability reservoirs. Based on the findings, production performance by air drive is studied through the application of a real low permeability reservoir model. Studies show that air intake capacity is far greater than water intake capacity in low permeability reservoirs. Therefore, air injection, which can effectively supplement and maintain reservoir pressure, can build effective pressure displacement system much easier than water injection. Low-temperature oxidization reaction will occur between oil and the oxygen in air, which consumes oxygen and forms nitrogen drive. At the same time, CO2 and large amounts of heat are generated, and the temperature in some parts of the reservoir can reach about 200°C. Nitrogen drive contributes 69%; temperature increasing and CO2 generation contribute 26.7% and 4.3% respectively for the total recovery of air drive. Recovery factors by air drive, nitrogen drive and water drive are also studied respectively by using the real low permeability reservoir model. Recovery factor by air drive for 30 years reaches 21.5%, and it is twice as large as that of water drive, which is 10.6% for the same period. Production performance is significantly improved. Air drive is an effective way for improving production performance and increasing recovery factor. 摘 要: 采用物理模拟和数值模拟相结合的方法，系统研究了低渗油藏注空气开发的驱油机理，在此基础上应用实际低渗油藏模型研究了空气驱的开发效果。研究表明，低渗油藏吸气能力远大于吸水能力，注空气比注水更容易建立起有效的压力驱替系统，起到有效补充或维持油层压力的作用。空气中的氧气与原油发生低温氧化反应，消耗掉氧气形成氮气驱，同时产生大量热量和二氧化碳，部分油藏温度升高到200 ℃左右。氮气驱对空气驱总采收率的贡献为69%，温度升高和二氧化碳对采收率的贡献分别为26.7%、4.3%。应用实际低渗油藏模型研究了空气驱、氮气驱和水驱的采收率，空气驱30 a的采收率达到了21.5%，较水驱30 a年采收率10.6%提高了1倍，开发效果得到明显改善。空气驱是低渗油藏改善开发效果、 提高采收率的有效方式。图11表2参13 Key words: low permeability reservoir, air injection, nitrogen drive, displacement mechanism, low-temperature oxidation, recovery factor