Productivity Prediction Model of CO2 Huff - N – Puff in Heavy Oil Reservoirs Under Variable Diffusion Coefficient
Authors: Chen Yongjie , Wang Yunhan , Zou Xuefei , Liu Tao , Liu Wei , Xiao Jianfeng 
Published: September 2023

ABSTRACT
CO2 huff − n − puff is considered to be an effective method to improve the recovery efficiency of heavy oil reservoirs, but the solvent-assisted CO2 huff − n − puff productivity prediction mathematical model of horizontal wells in heavy oil reservoirs has not been thoroughly investigated. In this study, we propose an improved model to investigate the CO2 concentration distribution and single horizontal well productivity of heavy oil produced by the solvent-assisted CO2 huff − n − puff technology. Firstly, we establish a multi-region diffusion model based on the variable diffusion coefficient of CO2 in different regions of the formation. Here, the multi-region diffusion model refers to the region coexisting light oil region and heavy oil region. Secondly, CO2 concentration profiles in different time and different areas is solved. Finally, we derive the productivity formula of solvent-assisted CO2 huff − n − puff single well in heavy oil with variable diffusion coefficient, and discuss the influence of diffusion coefficient on oil well productivity. The research results show that the solvent-assisted CO2 huff − n − puff technology can push the CO2 diffusion front farther towards the oil layer, compared with the CO2 huff − n − puff technology. In addition, the single horizontal well productivity is positively correlated with the diffusion coefficient and the length of the horizontal section. And if the longer the horizontal section is, there is a greater impact on the single horizontal well productivity come from the range of light oil.

INTRODUCTION
With the development of conventional oil and gas resources entering the "bottleneck" stage, unconventional oil and gas resources will become the future development hotspot (Song et al., 2017). Heavy oil reservoir is an important unconventional oil and gas resource, which is the focus of current scholars (Zou et al., 2022). Due to the high viscosity of crude oil, low permeability of reservoir and serious heterogeneity of reservoir, heavy oil reservoir has poor mobility under formation conditions, making its development very difficult (Gao et al., 2022). At present, the development of heavy oil reservoirs is mainly through fracturing and production, but it has problems of short stable production period and rapid production decline (Wang et al., 2021). Therefore, reducing the viscosity of crude oil, improving the fluidity of crude oil and supplementing the formation energy have become the key to the development of heavy oil reservoirs. Practice showed that CO2 huff − n − puff technology was one of the most promising methods to improve oil recovery at present (Jia et al., 2018). Using CO2 as oil displacement medium for heavy oil displacement could effectively inhibit bottom water channeling, reduce interfacial tension and crude oil viscosity (Sohrabi et al., 2017; Dariusz et al., 2019), enable oil swelling and reservoir repressurization (Hoffman and Reichhardt, 2019; Zhang et al., 2004). Practice showed that molecular diffusion was the main mechanism of mass transfer between CO2 and crude oil (Perkins and Johnston, 1963; Shelton and Schneider, 1975), and the prediction of oil well productivity and analysis of its influencing factors are of great significance in the development of heavy oil reservoirs. Therefore, it is very important to study the diffusion of CO2 in the formation and predict the productivity of single well in the process of CO2 huff − n − puff.