Migration mechanisms of leaking hydrogen sulfide in inter-stratified coal-petroleum basins and mitigation with alkali injection

In inter-stratified coal-petroleum basins, the integrity of oil/gas wells penetrating coal seams is frequently compromised by longwall coal mining-induced disturbances, promoting leakage of highly toxic hydrogen sulfide (H₂S) gas from below. This released H₂S migrates via mining-induced fractures into underground workings like longwall faces and roadways, posing acute exposure risks for miners. This study investigates H₂S leakage, migration, hazard, and mitigation methods using a case study of the Shuangma coal mine (Ordos Basin western margin, China), where upper coal seams are mined above deep oil reservoirs. We developed a novel coupled mechanical-hydraulic-chemical model simulating H₂S migration and coal seam alkali injection for sulfur immobilization. Field measurements and simulations reveal that: (1) Leaking H₂S primarily adsorbs in coal matrix pores, forming enrichment zones until saturation, then distributes into free and water-soluble states in dynamic equilibrium; (2) H₂S pressure increases exponentially near wellbores, with radii of influence after 30 years measuring 238–536 m for wellbore pressures of 0.32–1.52 MPa; (3) Optimal alkali injection parameters for effective H₂S mitigation are 10 MPa pressure, 10 m borehole spacing, and 30 h grouting duration. These parameters suppressed H₂S concentrations below the safety threshold. The results: (1) elucidate coupled transport-immobilization mechanisms governing H₂S behavior in fractured coal-reservoir systems, and (2) provide a validated engineering protocol for abandoned well remediation in inter-stratified coal-hydrocarbon basins. This work advances fundamental understanding and practical solutions for H₂S risk management in mining overlying oil/gas reservoirs.