CO2 foam-assisted fracturing fluid flowback and CO2 sequestration in tight sandstone gas reservoirs: Experimental and numerical study

CO₂ foam fracturing is an advanced CO₂-based fracturing technique with distinct advantages over other methods, such as enhanced proppant transport capacity and reduced filtration loss, making it highly promising for applications in tight gas reservoirs. Additionally, CO₂ foam fracturing presents the potential for underground CO₂ sequestration, which contributes to the reduction of carbon emissions. During CO₂ foam fracturing, both flowback efficiency and the microscopic retention of fracturing fluid are crucial factors influencing subsequent gas production. However, limited research has specifically investigated the flowback behavior of fracturing fluids after CO₂ foam fracturing, and the potential for CO₂ sequestration during this process remains insufficiently explored. This study combines physical displacement experiments with low-field nuclear magnetic resonance (LF-NMR) techniques to investigate the flowback efficiency and microscopic retention of fracturing fluid. Additionally, the CO₂ sequestration efficiency is analyzed. Numerical simulations are performed to examine the field-scale flowback of fracturing fluid and CO₂ sequestration, focusing on the effects of foam quality, injection rate, injection volume, and soaking time. Experimental results demonstrate that increasing foam quality enhances both fracturing fluid flowback efficiency and CO₂ sequestration. Specifically, as foam quality increases from 50 % to 70 %, fracturing fluid flowback efficiency increases from 70.32 % to 87.3 %, while CO₂ sequestration efficiency rises from 32.56 % to 38.68 %. NMR test results reveal that fracturing fluid is primarily retained in small pores, and increasing foam quality reduces fluid retention across various pore sizes. In this study, increasing the foam quality from 50 % to 70 % reduces the fracturing fluid retention by 22.23 % in small pores, 6.7 % in large pores, and 20.83 % overall across all pore sizes. Numerical simulations indicate that fracturing fluid flowback efficiency increases with both foam quality and foam injection rate, while it decreases with increasing foam volume and soaking time. The CO₂ sequestration efficiency increases with foam quality, injection rate, injection volume and soaking time. Therefore, during CO₂ foam fracturing, these injection parameters should be optimized to strike a balance between maximizing fracturing fluid flowback and CO₂ sequestration. This paper provides significant insights into improving fracturing fluid flowback and CO₂ sequestration in tight gas reservoirs.