A Numerical Model to Characterize the Flow and Heat Transfer Behaviors of Compressed Air in High-Pressure Air Injection Wells

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Summary High-pressure air injection (HPAI) is a significant EOR technology of light oils especially in deep, thin, low-permeability reservoirs. With the rapid development of technology, concentric dual-tubing injection technique was employed in multi-layer HPAI wells to overcome the influence of heterogeneity and to adjust the uneven suction in each layer. The objective of this study is to better characterize wellbore pressure and temperature distribution of compressed air along wellbore in HPAI wells with concentric dual-tubing injection technique. Based on mass, momentum and energy balance equations, mathematical model was established and solved by finite difference method and iterative technique. The pressure drop in both inner tubing and annulus is calculated based on the momentum balance equation, and the temperature drop along wellbore is calculated based on the energy balance equation. The heat conduction between inner tubing and annulus, and the dynamic behaviors of injected air are taken into consideration. The effect of injection temperature on distribution of air temperature and pressure in inner tubing are conducted. It is found out that: (1) . As well depth increasing, temperature difference between formation and wellbore tends to become constant, and the radial heat transfer tends to reach an equilibrium state. The lower the injection temperature is, the deeper the equilibrium depth is. (2) . The air pressure in both inner tubing and annulus is mainly dominated by the hydrostatic pressure and increases with well depth. The pressure gradient in the annulus is larger than that in inner tubing. (3) . Decreasing the injection temperature can increase the temperature gradient increases in inner tubing, which is caused by the increasing of heat transfer rate between formation and wellbore fluid. (4) . Increasing the injection temperature can decreases the air pressure in inner tubing. This is because the air density decreases with increasing of injection temperature, which causes the decrease of hydrostatic pressure and increase of friction losses. This paper proposed a novel model to predict the pressure and temperature distribution along wellbore in HPAI wells. The theoretical studies in this paper provides following researchers with the very basic theory for the application of concentric dual-tubing injection technique in HPAI wells and can be taken as a reference for engineers in optimization of injection parameters.