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With the increasing demand of energy, extra-heavy oil has attracted more and more attention at present. Cyclic steam stimulation (CSS) process is one of the main methods to produce extra-heavy oil, however with the increase of stimulation cycle, the periodic oil production of well decreases obviously and the water cut increases significantly. As non-condensable gas, surfactant, high-temperature foam, horizontal well technology, and other assisted CSS methods have been applied in heavy oil reservoirs, it indicates that CSS process has been developing toward diversification. Air-assisted cyclic steam stimulation (AACSS) process has become a more and more popular technique for developing extra-heavy oil due to its abundant sources and low cost. In this paper, static oxidative experiments were carried out to study the mechanism of low-temperature oxidation (LTO) reaction of extra-heavy oil and the effect of temperature, pressure, and catalyst on the LTO reaction. In addition, four groups of CSS experiments were conducted to compare the development performance of different stimulation methods, such as conventional CSS, first-injection AACSS, simultaneous-injection AACSS, and later-injection AACSS. The results show that the system pressure and O 2  content decreased, but carbon oxides increased after LTO reaction. The residue O 2  content decreased from 21% to below 12% under the safety risk of explosion caused by the mixture of O 2  and hydrocarbon gases. During LTO process, both cracking reaction and polymerization reaction of molecule occurred in the extra-heavy oil, which increased the content of saturates and aromatics, while decreased the content of heavy components. LTO reaction also increased the oil viscosity because of the increase of asphaltene content. But pyrolysis reaction would upgrade the extra-heavy oil and decrease the asphaltene content of extra-heavy oil under high-temperature condition. Besides, the higher the reaction temperature and pressure, the faster the reaction and the lower the O 2  content. Moreover, the catalyst could enhance the LTO reaction and O 2  consumption. Based on the larger scale sand pack experiments, the results indicate that AACSS could effectively improve the oil recovery compared with conventional CSS, and air injection after injecting steam during each cycle could achieve the highest recovery.

Experimental study on the mechanism of enhanced oil recovery by air assisted cyclic steam stimulation process in extra-heavy oil reservoirs