Improvement on CO2 Storage Efficiency by Foam Fluid

CO2 storage efficiency refers to the amount of CO2 storage in a certain volume of underground space, which is directly affected by CO2 sweeping volume. Besides, the balance between vertical and horizontal migration of CO2 is the key to increase the sweeping volume. This study focused on the influence of foam fluid on the fluidity and percolation characteristics of CO2 in porous media. The rheological properties, percolation characteristics and maximum injection volume of pure CO2 and CO2 foam were investigated by rheometer, percolation performance test and CO2 storage simulation experiment, respectively.

As the experimental results shown, the apparent viscosity of CO2 foam reached 6000 mPa·s at 85℃, and the viscosity of pure CO2 was below 0.1 mPa·s at the same temperature; the resistance factor (the ratio of the pressure difference between the two ends of the core during foam injection and the pressure difference between the two ends of the core during water injection) of foam was over 500 times that of pure CO2 in 10mD core, and the difference in resistance factors was more significant in cores with lower permeability; in a core with a pore volume of 127 ml, the CO2 storage amount of foam injection was 136% that of pure CO2 injection. Meanwhile, the impact of foam`s property, such as diameter distribution, gas-liquid ratio, on the storage efficiency was investigated by a series experiments. Firstly, the resistance factor and residual resistance factor of CO2 foam reached the highest in the cores with permeability of 110 mD class, and the second in the cores with 1 mD class. Secondly, under the condition of the same permeability, the larger the gas-liquid ratio is, the better the blocking effect is. Thirdly, under all three permeability conditions, the residual resistance factor showed a decreasing and then increasing trend at the beginning of injection.

According the results and analysis, foam injection can effectively improve the CO2 storage efficiency. The key parameters affecting the effectiveness of storage efficiency improvement are as follows. Firstly, matching of foam particle size to formation pore size. Bubbles shown a higher probability of entering narrow pore channels with pore diameters smaller than their particle sizes, resulting in a more frequent occurrence of the Jarman effect, which manifested in the increase of sweeping volume and fluidity control capacity in macroscale. Secondly, the larger the gas ratio, the more frequently the foam system is generated, and the greater the density of bubbles in the system, giving the foam system a higher chance of blocking when passing through pores and pore throats.

Although the global CO2 storage potential is more than 4 trillion tonnes, if geological sequestration becomes a routine method to reduce CO2 emissions, underground space will be used up. Therefore, improving the CO2 storage efficiency is a key choice to enhance the CO2 storage potential and extend the life of CCUS technology. This study proposed a method to improve the CO2 storage potential by changing the fluid form, which can provide a new idea for the better utilization of underground space.