Abundant pore space in coal is not only the place for the accumulation of coalbed methane (CBM), but also the tunnel for gas migration. In this study, five sets of coal samples before and after the second coalification were selected from the eastern margin of Ordos Basin to simulate supercritical CO2 (Sc-CO2) extraction in supercritical extraction equipment. The evolutions of pore structure and porosity were tested by mercury intrusion porosimetry and nuclear magnetic resonance spectroscopy to compare the changes of pore structure and porosity due to the Sc-CO2 extraction, and to explain the related mechanism. The results show that: (1) Pore volume, pore specific surface area, and connectivity characteristics changed significantly due to Sc-CO2 extraction, and the increment of pore volume and pore specific surface area presented a law of increase–decrease–increase with the increase in the coal rank, and the turning point was near the second coalification. (2) The porosity increment change trend due to Sc-CO2 extraction was increase–decrease–increase with increasing coal rank, and the turning point was again near the second coalification, which supports the mercury intrusion porosimetry results. (3) The changes were observed in the porosity characteristics due to Sc-CO2 extraction through pore-increasing and expanding effects. Before the second coalification, the pore-increasing and expanding effects co-existed in the micropores, and after the second coalification, the pore-expanding effect mainly existed in the transitional pores and above. (4) The variation model for the pore structure of coal due to Sc-CO2 extraction was established. The conclusions offer not only important theoretical significance for the CO2-enhanced CBM (CO2-ECBM) mechanism but also important significance for CO2-ECBM engineering.

Coal measure energy production and reservoir space utilization are both important for energy security and can assist in achieving carbon neutrality. Coal measure energy production and reservoir utilization are therefore of scientific significance. This study focuses on the security of coal supply by discussing the possible technical approaches of combining low-carbon mining of coal measure resources and coal reservoir space utilization. Our study found that 1) the integration of underground coal gasification and carbon capture and sequestration (UCG-CCS) is an effective technology to improve energy efficiency and achieve the carbon neutral target in China; 2) CO2-enhanced coalbed methane (CO2-ECBM) recovery is an important way to reduce CO2 emissions during coalbed methane recovery; and 3) the integrated technology of solid waste filling (WF), coal bioconversion (CB), gas production (GP), and CO2 capture and sequestration (CCS) in coal goafs and abandoned coal mines can improve the energy production efficiency and reservoir space utilization and reduce geological disasters due to surface subsidence. These integrated technologies will become increasingly important for coal measure energy production and efficient reservoir space utilization as they will provide scientific theory and technical support to achieve carbon neutrality.