Surface steam injection is the most common enhanced oil recovery (EOR) technology used in heavy oil production. Nevertheless, limits exist due to heat loss for deep reservoirs. In the process of steam injection, adding a certain amount of nitrogen, which can reduce heat loss, maintain formation pressure and expand steam chamber, can effectively improve the development efficiency of steam injection.
In this research, high-temperature-pressure pressure/volume/temperature (PVT) measurements and sand pack flooding experiments have been operated for heavy oil of the target reservoir in Bohai Oilfield, China, observing effect of nitrogen on property of heavy oil and steam and nitrogen co-injection on displacement efficiency and optimizing gas-water ratio during the co-injection.
The results of PVT measurements indicate that the change of heavy oil viscosity is complex with the varying amount nitrogen dissolved. The reason is that dissolved nitrogen can reduce heavy oil viscosity on one hand; on the other hand, nitrogen can compress oil increasing its viscosity. The viscosity of heavy oil with dissolved nitrogen decreases significantly under low temperature and pressure (80°C, 15MPa), and the maximum viscosity reduction rate is 28%. Along with increase in temperature and pressure (150°C, 25MPa), the viscosity reducing effect becomes worse, especially at 150°C, the viscosity of heavy oil with dissolved nitrogen increases, on the contrary.
The results of sand pack flooding experiments reveal that adding nitrogen during stream injection can improve oil displacement efficiency significantly. However, the increase of oil displacement efficiency is different with the varying gas water ratio; therefore, optimizing gas water ratio has a great influence on displacement effect in steam-nitrogen co-injection process. There is an optimal gas water ratio in the process, which is correlated with temperature. For example, the optimal gas water ratio is around 50 and 100 respectively while temperature of 200°C and 300°C.
The key of mid-deep heavy oil reservoirs development using steam-nitrogen co-injection technology is gas water ratio optimization, and while optimizing gas water ratio properties of heavy oil, formation conditions and injection conditions, and many other factors should be taken into account. This research provides a direction to choose the optimal gas water ratio in the steam-nitrogen co-injection process.