The evolution of closed pores induced by in situ pyrolysis of coal and its effect on volatile yield

Coal underground in-situ pyrolysis technology has opened new avenues for the clean and efficient utilization of coal resources. However, crustal stress constraints significantly affect the transport of pyrolysis tar-gas through pores, making a deeper understanding of the impact on pyrolysis tar-gas release urgently needed. This research simulated the pyrolysis behavior of coal at 500°C and 5.0–25.0 MPa, followed by a systematic investigation of the evolution of pyrolysis closed pores, trapped oil, and pyrolysis product yields under stress loading using low-temperature nitrogen adsorption and T₁-T₂ nuclear magnetic resonance. The results exhibited that the synergetic evolution of closed pores and coal pyrolysis products varied under different stresses. 5.0–17.5 MPa, the closed pore volume increased by 0.00112 cm³/g. The trapped oil signal intensity and char yield exhibited a positive correlation with closed pores, while tar-gas yield exhibited the opposite trend. The former was attributed to the enhanced retention of heavy components due to improved pore-fracture closure, while the latter resulted from restricted tar-gas release caused by insufficient pore network connectivity. 20.0–25.0 MPa, the closed pore volume further increased by 0.00047 cm³/g. The evolution of char and tar-gas yield remained consistent with those 5.0–17.5 MPa, indicating the continuity of closed pore structure affected by pyrolysis product occurrence under different stresses. However, the trapped oil signal intensity decreased with increasing closed pores, reflecting the effect of high stress: promoting secondary conversion of trapped oil into light components.