Abstract
The re-injection of associated sour gas, with high H2S and CO2 content, into the reservoir is proposed to be an effective development method due to its low investment cost and high oil recovery. The aim of this work is to present the phase behavior and miscible mechanism of crude oil displaced by associated sour gas. Based on the equation of state and the phase equilibrium theory, the phase behavior of crude oil mixed with various gases (associated sour gas, H2S, CO2 and CH4) have been analyzed. Then, the miscibility of associated sour gas was determined by calculating its Minimum Miscible Pressure (MMP) and the effect of sour component fraction on miscibility was evaluated. Moreover, a series of numerical simulations modeling 1D slim-tube were conducted using a compositional simulator to study the miscible mechanism in the displacement of crude oil with associated sour gas. The results show that the injection of H2S can reduce the bubble point pressure of crude oil and therefore is beneficial to prevent the crude oil degassing; nevertheless, the injection of CO2 has little effect on it. The miscible ability of associated sour gas decreases as its sour component fraction decreases. It is observed that the crude oil displaced by associated sour gas and sweet gas both show a combined condensing/vaporizing mechanism, with miscible zone in the middle of transition zone. However, the vaporizing-gas drive mechanism is slightly stronger than the condensing-gas drive mechanism during the displacement by associated sour gas while is significantly stronger during the displacement by sweet gas.
Highlights
Gas injection to improve oil recovery has been researched and successfully applied in many oil fields all over the world for almost one half century (Arne et al, 2000; Chen, 1995; Knut and Lars, 2002; Murty and Al-Khayat, 1989; Teletzke et al, 2005; Zhang et al, 2013)
The main focus of this paper is to investigate the phase behavior and miscible mechanism of crude oil displaced by associated sour gas
The mechanism of crude oil displaced by associated sour gas is not either the condensing-gas drive mechanism or the vaporizing-gas drive alone
Summary
Gas injection to improve oil recovery has been researched and successfully applied in many oil fields all over the world for almost one half century (Arne et al, 2000; Chen, 1995; Knut and Lars, 2002; Murty and Al-Khayat, 1989; Teletzke et al, 2005; Zhang et al, 2013). The crude oil displaced by injected gas may achieve miscibility in two ways: condensing-gas drive mechanism and vaporizing-gas drive. High H2S and CO2 content in associated sour gas may make the phase behavior and miscible mechanism different from usually injected gas, such as pure CO2, pure CH4 and enriched gas. The main focus of this paper is to investigate the phase behavior and miscible mechanism of crude oil displaced by associated sour gas. The phase behavior of crude oil mixed with various gases (associated sour gas, H2S, CO2 and CH4) was firstly analyzed based on the equation of state and the phase equilibrium theory. A series of numerical simulations modeling 1D slim-tube were conducted to present the miscible mechanism in the displacement of crude oil with associated sour gas. The effect of pressure and injected gas composition (associated sour gas, sweet gas and CH4) on miscibility was analyzed in detail
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