特低渗透油藏注氮气流体相态行为特征

PHASE PERFORMANCE CHARACTERISTICS OF INJECTED N2 IN EXTRA-LOW PERMEABILITY RESERVOIR

特低渗透油藏储层孔道微细、渗透率低，油水流动时渗流阻力大，油藏注水开发存在注不进、采不出的问题。为了探索特低渗透油藏注气可行性，开展了注氮气室内实验。根据大庆油田特低渗目标油藏地层原油注N2气膨胀PVT试验、多次接触PVT实验、细管注氮气驱替最小混相压力测定实验结果，论述了特低渗透油藏注氮气气驱油藏流体相态行为特征。随着注入氮气量的增加，地层原油的膨胀能力增加，饱和压力升高。地层原油注氮气前接触地层原油气油比随接触次数的增加先增大再减小，再逐渐稳定，但变化幅度不大，地层原油密度随接触次数的增加基本保持不变。细管实验表明，氮气容易突破，气体突破后，采收率增幅变缓，综合采收率不高，表现为非混相驱特征。

Micro-pores ＆ channels and lower permeability of extra-low permeability reservoir result in the problems such as much serious flow resistance when oil and water move, and furthermore water cannot be injected and oil cannot be produced in the course of water flooding. In order to explore the feasibility of gas injection in this kind of reservoir, the laboratory experiment of N2 injection is conducted. Through PVT expanding experiment, multiple contact PVT test and minimum miscible pressure measurement in slim tube displacement in Daqing target reservoir, this paper describes the phase performance characteristics of the N2 injection in this kind of extra-low permeability reservoir. With increase of the injected N2 volume, the expanding ability of the formation oil becomes greater, and furthermore the saturation pressure is also raised. The gas-oil ratio （GOR） of the contacted reservoir increases first, then decreases with the rise of contact time before N2 injection, and then gradually stabilizes, but changes are little. On the contrary, the oil density of the reservoir almost maintains the same with the rise of contact time. Slim tube test shows that N2 breakthrough is easy. After that, the degree of enhancing oil recovery slows down and the combined recovery factor is not so high. In a word, the immiscible displacement characteristics are shown obviously.