In China, cyclic steam stimulation is the most widely applicable method for the development of common and ultra-heavy oil, and about 80% of thermal production is by CSS. Henan oilfield is one of the main producing areas of heavy oil in China and oil viscosity mainly ranges from 10000 to 30000mPa.s, belonging to ultra-heavy oil. After 6~8 cycles' huff and puff, the heating radius reaches to its economic limit and both oil and water recovery is low due to the unfavorable reservoir pressure. Though nitrogen can enhanced common heavy oil recovery after CSS, there are few reports about nitrogen assisted CSS for ultra-heavy oil. For these problems, this paper studies the feasibility and mechanism of nitrogen assisted CSS via laboratory experiments and simulation. Simulation results showed that it was feasible to inject nitrogen before CSS. Though nitrogen has little effect on the reduction of oil viscosity, the main mechanism of enhanced oil recovery was its good ability of heat insulation effect, so the heat loss of injected steam could be reduced and the heating radius was increased. Besides, compressibility coefficient of nitrogen was very large, so it could supply energy during exploitation process and avoid the rapid reservoir pressure drop. Pilot tests were conducted to several wells in Jinglou ultra-heavy oil reservoirs in Henan oilfield. Most wells achieved good effect. With the ratio of nitrogen to steam (volume ratio at standard conditions) increasing, the oil recovery got better. When the ratio was larger than 30, it had no better effect on enhanced oil recovery. These results are of significance for the development of ultra-heavy oil reservoirs.

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