Volume 43: Energy Transitions toward Carbon Neutrality: Part VI

Study on microscopic development characteristics and influencing factors of nanopores during CO2 huff-n-puff in Lucaogou Formation shale oil reservoir Kun Yang, Shenglai Yang, Xinyue Liu, Jiyu Chen, Jiayi Yu

Abstract

The characteristics of low porosity, low permeability, and small pore radius of the shale oil reservoir in the Lucaogou Formation make it difficult to develop and result in low production. CO2 huff and n-puff can effectively improve the recovery rate of shale oil in the Lucaogou Formation. In this study, high-pressure mercury injection technology was used to characterize the pore distribution of the core samples. Combined with nuclear magnetic resonance technology, the recovery of different pores during CO2 huff and n-puff were studied, and the effects of injection pressure and soaking time on core recovery were analyzed. Parallel CO2 huff and puff experiment were conducted to investigate the influence of reservoir heterogeneity on the recovery of CO2 huff and n-puff. The results showed that the pores in the Lucaogou Formation shale oil reservoir were mainly nano-pores, and small pores with a pore radius between 2nm and 50nm accounted for more than 70% of the total pores. The content of large pores in the core samples increased with the increase of core permeability. CO2 huff and n-puff can effectively extract shale oil from the Lucaogou Formation. When the injection pressure is higher than the minimum miscibility pressure, increasing the injection pressure can increase the oil recovery efficiency of large pores in the core samples. The higher the content of large pores, the greater the oil recovery rate of the core samples. When the soaking time is increased from 5h to 10h, the dissolution and diffusion of CO2 in small pores are enhanced, and the recovery rate of small pores increases by about 10%. However, the recovery rate of small pores no longer increases when the soaking time exceeds 10h. The presence of fractures can effectively enhance the recovery of small pores around the fractures. However, the presence of fractures increases the heterogeneity of the reservoir, and the greater the difference in core permeability, the lower the recovery rate of small pores. The recovery rate of small pores in the matrix core samples is only 12.17%.

Keywords CO2, Huff and n-puff, Shale oil reservoir, Reservoir heterogeneity, Nanopores