Thermodynamic Characteristics of cold and hot non-condensable gases simultaneously flowing along vertical wellbore

Abstract

In recent years, thermal non-condensable gas injection was widely used in petroleum industry to enhance oil recovery. However, the research on the heat transfer process and thermodynamic characteristics analysis along wellbore is almost in blank under the condition of cold and hot non-condensable gases simultaneously flowing in wellbore. In this paper, a complicated mathematical model was established considering simultaneous hot and cold gases injected into tube and annulus. Meanwhile, a coupling calculation method was introduced based on temperature equations, pressure equations, and thermodynamic properties. The model can be used to analyze temperature and pressure distribution, the depth of phase transition, the heat transfer pattern and the thermodynamic characteristics. The accuracy of the model was verified and sensitivity analysis of some parameters was conducted. It is found that: (a) The injection method (hot N2 is injected into tube and cold CO2 is injected into annulus) is optimal, which produces fainter heat loss and maintains higher total energy; (b) Some factors, such as, the injection temperature, the injection mass ratio of hot and cold gases, and the injection velocity, have a dramatic effect on temperature and pressure distribution, fluid thermophysical parameters, and fluid phase state. (c) According to the calculation results, the optimal temperature of nitrogen is 350 °C, the optimal injection mass ratio of N2 to CO2 is 15:1, and the optimal injection velocity of CO2 is 120 t/d; (d) The temperature along wellbore changes intensively at the beginning of injection while it becomes stable when the injection time exceeds 10 days. The ingenious coupling model with intricate iterative processes can perfectly simulate the injection process of hot and cold non-condensable gases and obtain the temperature and pressure distribution along wellbore, which can provide tremendous guidance and assistance for injection operations in the oilfield.