Estimation of Critical Temperatures Based on New Viscosity Model for HPAM Polymer Flooding in High-Temperature Reservoir

Polymer flooding in high temperature reservoirs usually has shown poor performance due to severe thermal degradation, leading to and ineffective in-situ sweep behavior. Even though the thermal degradation is important characteristic of polymer, accurate viscosity models have not been implemented in the conventional reservoir simulation. This paper presents new viscosity model as a function of temperature and time which can describe the thermal decomposition as an irreversible process. The temperature-dependent viscosity model is developed by using half-life decomposition of synthetic polymer. The new viscosity model can reflect well-characterized chemical degradation sequence for transported polymer. Having established the viscosity model, comparison to the conventional Flory-Huggins' equation. The results that temperature and initial injected concentration have more massive impact on viscosity, long-term stability, compared to conventional Flory-Huggins' model were obtained. The new viscosity model helps to evaluate accurately the application of polymer flooding in high-temperature reservoirs. From simulations including heat transfer within reservoir by adopting new viscosity model, temperature limitation was deduced in terms of various parameters such as reservoir temperature, polymer concentration, and oil viscosity. Even the hottest reservoir with 200C can be exploited by low-temperature fluid injection which provokes heat loss by convection and conduction. This heat loss occurring during polymer injection with 120C into the hot reservoir can make reaction rate of thermal decomposition slow down and long-term stability can be maintained. According to the acquired results, high reservoir temperature causes low oil recovery efficiency without guarantying long-term stability. The critical temperature for application of polymer flooding was calculated as about 160C. Above the critical range, polymer flooding is expected to show poor performance. Also, the viscous oil lowers the effectiveness of polymer flooding not only at high-temperature, but also at low-temperature reservoirs. Effects of injection fluid temperature showed even at the hottest reservoir (200C), severe thermal degradation can be avoided by injecting lower-temperature fluid, which leads to slow down decomposition rate. Heat loss from cooling water injection is able to increase oil recovery in high-temperature reservoirs. Therefore, the critical reservoir temperature estimated previously as abandonment condition can be taken into accounts and extended.