缝洞型油藏氮气扩散系数测定及影响因素

针对塔河缝洞型油藏注氮气提高采收率深层次机理认识有待提高的问题,开展了该类型油藏条件下注氮气扩散系数影响因素的研究,采用压力衰竭法测定了氮气在油相和填充介质中的扩散系数,分析了原油黏度、气体组成、填充类型及流体饱和度对氮气扩散系数的影响。结果表明,氮气在稠油中的溶解度、扩散系数均低于稀油,但压力敏感性高于稀油;氮气中混合二氧化碳促进注入气在原油中的扩散。氮气在垮塌、砂泥和致密等3种常规缝洞油藏填充介质的扩散结果显示,填充介质的致密程度增加,氮气向填充介质内部扩散越困难,且致密介质中氮气的扩散系数达到固体扩散系数的级别（×10^-11 m²/s）;含水饱和度的增加减缓了氮气在填充介质中的扩散。首次测量了氮气在缝洞型油藏高温高压条件（温度>100℃,压力>20 MPa）下原油及填充介质中的扩散系数,并分析了相关影响因素,深化了缝洞型油藏注气提高采收率机理认识,为现场注气应用提供相应的理论指导。

Measurement and Effecting Factors of Nitrogen Diffusion Coefficient in Vug-fracture Reservoir

Diffusion coefficient is an important parameter for studying the mechanism of enhanced recovery by gas injection and numerical simulation. In this paper, pressure decay method is utilized to determine the N₂ diffusion coefficient under the condition of high temperature and high pressure in Tahe Oilfield. Not only the effects of oil viscosity and gas composition on N₂ diffusion coefficient in the bulk oil phase are analyzed, but the determination and comparison of N₂ diffusion coefficient in different filling model of fracture-cave carbonate reservoirs of Tahe Oilfield are studied. The solubility and diffusion coefficient of nitrogen in heavy oil are lower than in light oil, but more sensitive to pressure variation than in light oil. The mixing of CO₂ contributes the diffusion process. Diffusion results of nitrogen in the breccia, silt and tight media show that the tighter the filling model is, the more difficult it is for nitrogen to diffuse into the filling medium. The diffusion coefficient of nitrogen in the tight medium reaches the order of magnitude of solid diffusion coefficient (×10^-11 m²/s). Besides, the rise of water saturation in the filling medium also retards the diffusion process of nitrogen in the filling medium. This paper presents the N₂ diffusion coefficient in bulk oil phase and filling model of fracture-cave carbonate reservoirs under condition of high temperature and high pressure conditions (temperature >100 ℃, pressure >20 MPa) for the first time, and analyzes the related influencing factors. The results furthers understanding of EOR mechanism of gas injection, and offers strategic insights for field application of nitrogen injection.