CO₂ miscible flooding is an environmentally friendly and effective method to enhance oil recovery in ultra-low permeability reservoirs. Critical well spacing is one of the key issues in reservoir development and basis for new well site deployment and old well location adjustment. Many researches about well spacing of water flooding have been discussed; however, little work has been conducted on critical well spacing of CO₂ miscible flooding. In this paper, a one-dimensional critical well spacing model of CO₂ miscible flooding in ultra-low permeability reservoirs is established based on non-Darcy miscible seepage theory, considering the reduction of crude oil viscosity, variation of the oil phase threshold pressure gradient, miscible seepage resistance, and expected yield. A new calculation method is presented based on the advancing distance of the gas displacement front and the production well pressure distribution scope, describing the change of CO₂ concentration in the CO₂-oil effective mass transfer area with analytic function. Calculation software on critical well spacing written in C# code is developed. A Shengli oil field example is calculated, and the contributions of pure CO₂ seepage area, CO₂-oil effective mass transfer area, and pure oil area to critical well spacing are analyzed. Results show that critical well spacing ranges from 153.37 m to 1003.78 m when the bottom hole flowing pressure of a production well is set to 30 MPa; the gas injection pressure is respectively set to 45, 50, and 55 MPa; and the gas injection rate is respectively set to 10, 15, 20, 25, and 30 ton·d^-1. The contribution of pure CO₂ seepage area is the biggest, CO₂-oil effective mass transfer area is less, and pure oil area is the least. Critical well spacing increases with gas injection pressure and decreases with gas injection rate. It also reveals a mechanism that the length of CO₂-oil effective mass transfer area increases with the gas injection rate at the same time and increases with elapsed time at the same gas injection rate. CO₂-oil effective mass transfer area moves forward with elapsed time, and after a certain time, the length remains unchanged.

中文翻译：

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超低渗油藏中CO ₂混相驱临界井间距的一种新计算方法

CO ₂混相驱是提高超低渗透油藏油采收率的环保和有效方法。关键井距是储层开发中的关键问题之一，也是新井场部署和旧井位调整的基础。讨论了许多有关注水井间距的研究。但是，关于CO ₂混相驱的临界井间距的研究很少。本文提出了一维CO ₂的一维临界井间距模型基于非达西混溶渗流理论，考虑到原油粘度的降低，油相临界压力梯度的变化，混溶抗渗性和预期产量，建立了超低渗透油藏的混相驱。提出了一种基于瓦斯驱替锋的超前距离和生产井压力分布范围的新计算方法，并通过解析函数描述了CO _2-油有效传质区中CO ₂浓度的变化。开发了用C＃代码编写的关键井距计算软件。以胜利油田为例，计算了纯CO ₂渗流面积CO ₂的贡献。分析了油的有效传质面积和至临界井距的纯油面积。结果表明，当生产井的井底流动压力设置为30 MPa时，临界井间距为153.37 m至1003.78 m；气体注入压力分别设定为45、50和55MPa。气体注入速度分别设为10、15、20、25、30ton·d ^-1。纯CO ₂渗流面积的贡献最大，CO _2-油有效传质面积较小，纯油面积最小。临界井距随着注气压力的增加而增加，并随注气速率的增加而减小。它也揭示了一种机制，即CO ₂的长度-油有效传质面积同时随着气体注入率的增加而增加，并且随着时间的过去以相同的气体注入率而增加。CO _2-油的有效传质面积随着时间的推移而向前移动，经过一定时间后，长度保持不变。