Study of Fluid Displacement Processes and Sequestration of CO2 in Coal Reservoirs Using Nuclear Magnetic Resonance Spectroscopy.

Geological sequestration of carbon dioxide (CO₂) is one of the key methods to attain peak CO₂ emissions and carbon neutrality in unworkable or abandoned coal seams. To study displacement characteristics and effect of geological sequestration of CO₂ in water-saturated coal seams, the pore structure of coal samples from three different mines was first investigated by nuclear magnetic resonance technique. Then, the CO₂ was injected into water-saturated coal sample, and helium was used as the control group. Next, the CO₂-injected coal sample was sealed with water. Finally, water was returned by unloading pressure. The results showed that the pores of the three types of coals were mainly micropores and transition pores. Moreover, the fractal dimension of the pores was distributed in the range of 2.236–2.655. The displacement experiment found that the water displaced by CO₂ and helium was mainly movable water in macropores, but there were significant differences in the displacement characteristics of the two. On the whole, the amount of water displaced by CO₂ was more than that by helium, and the difference between the water volume displaced by CO₂ and helium was reflected mainly in the adsorption of CO₂ on the coal surface. CO₂ occupied the adsorption sites on the coal matrix surface, which reduced the interaction between water and coal matrix surface and improved the displacement efficiency. In contrast, helium did not show adsorption capacity on the coal surface. In the water injection stage, water can only enter the larger pores due to the retention and adsorption of CO₂ in the coal. The water saturation of the coal sample after the return drainage increased by 10.24% compared with that before water injection, indicating that the water injected into the coal sample was not discharged completely. This study provides an experimental basis for investigating the characteristics of pore structure in coal, CO₂ displacement in water-saturated coal seams, and CO₂ geological sequestration. [ABSTRACT FROM AUTHOR]