低渗透油藏CO2变周期气水交替注入气水比理论设计

基于开发过程中饱和度、渗透率、分流率不断变化的特点,提出变周期水气交替注入(water alternating gas,WAG)气水比理论设计的概念。从油、气、水三相渗流力学原理出发,借助三相相对渗透率表征模型,建立各相饱和度与相对渗透率间的函数关系,得到各相饱和度与流度、气水比、流度偏离系数等相关参数的函数关系。流度偏离系数为引入的新参数,用于考察气水两相流动能力偏离水的流动能力幅度。采用枚举法,得到三相饱和度与气水比和流度偏离系数的关系图版,初步确定三相流动范围内气水比和流度偏离系数的取值范围。根据流度偏离系数和气水比关系曲线的特征点与含水饱和度之间的相互关系,最终确定变周期WAG气水比理论取值的合理范围。通过对实际区块气水比理论设计研究表明:合理气水比与含水饱和度是幂指数的函数关系;目标区块气水比的取值范围为0～34,流度偏离系数的取值范围为0～518。在中低含水阶段,建议WAG气水比理论设计数值为2;在中含水阶段,建议WAG气水比理论设计数值为1;在中高含水阶段,建议WAG气水比理论设计数值为0.5。该方法得到的变周期WAG气水比数值与数值模拟方法得到的结论相同,填补了变周期WAG气水比理论设计空白,对于完善变周期WAG其他参数的理论设计具有启发和借鉴作用。

Theoretical design study of gas-water ratio during CO2 flooding with variable cycle WAG in low permeability reservoirs

Based on the characteristics of constant change of saturation, permeability and shunt rate during the development process, the concept of theoretical design of gas-water ratio with variable cycle WAG was proposed. Based on the mechanics principle of three-phase percolation, the functional relationship between saturation and relative permeability of each phase was established by using the three-phase relative permeability characterization model. The functional relationship between saturation and fluidity, gas-water ratio, fluidity deviation coefficient and other related parameters was obtained. The fluidity deviation coefficient was a new parameter, which was used to investigate the fluidity deviation amplitude of two-phase flow capacity of gas and water. By using enumeration method, the relation chart of three-phase saturation with gas-water ratio and fluidity deviation coefficient was obtained, and the value range of gas-water ratio and fluidity deviation coefficient in three-phase flow range was preliminarily determined. According to the relationship between the water saturation and the characteristic points of the relationship curve of fluidity deviation coefficient and gas-water ratio, the reasonable range of gas-water ratio theoretical value of variable cycle WAG was finally determined. The theoretical design of gas-water ratio shows that the reasonable gas-water ratio and water saturation are a function of power exponent. The value range of gas-water ratio in the target block is 0~34, and the value range of fluidity deviation coefficient is 0~518. In the middle and low water-cut stage, the theoretical design value of gas-water ratio is suggested 2. In the middle water-cut stage, the theoretical design value of gas-water ratio is suggested 1. In the stage of medium and high water cut, the theoretical design value of gas-water ratio is suggested 0.5. The gas-water ratio of variable cycle WAG obtained by this method is the same as that obtained by the empirical method and numerical simulation method. The method fills the gap in the theoretical design of gas-water ratio of variable cycle WAG, and provides inspiration and reference for improving the theoretical design of other parameters of variable cycle WAG.