Heated Area and Well Performance Analysis of Injection N2 and CO2 in Cycle Steam Stimulation Process

Abstract

Application of steam injection technology to heavy oil reservoirs is the most commercially successful EOR method. Cycle steam stimulation (CSS) is known as the most widely used and mature technology compared with various thermal methods. Because of various reasons, such as too high initial oil viscosity, excessive overburden heat loss and so on, in CSS, the radius of heated zone is small and the viscosity of heavy oil still cannot be lowered effectively, which leads to the low oil productivity and poor oil well performance. A variation on CSS process is to add N₂ and CO₂ in steam injection. Because of the influence of the N₂ and CO₂, the heated area and well performance of N₂ and CO₂ assisted CSS are different from that of steam stimulation. Therefore, this paper describes a detailed study of N₂ and CO₂ influence to cycle steam stimulation. In this paper, the physical simulation experiments of N₂ and CO₂ influence to the mixture of heavy oil are carried out at first. Through physical experiments, the enhancing oil mechanisms of N₂ and CO₂,the recovery mechanism of reducing oil viscosity by CO₂ dissolving, reducing interfacial tension between gas and heavy oil, which are different from the steam, are described respectively. Based on this, a numerical simulation model with a single horizontal well is built to carry out the quantitative and comparative study of heated area of formation. Results show that the development effect of N₂ and CO₂ assisted CSS is better than that of conventional steam stimulation in porous media. Next, the different well performance of the N₂ and CO₂ assisted CSS and conventional CSS are compared by numerical results. Finally, on the basis of the field data of two different heavy oil field, two typical wells of CSS and N₂ and CO₂ assisted CSS are analyzed in detail. Consequently, the N₂ and CO₂ injection together with steam is helpful to improve development effect in CSS process.