Migration and Release Mechanism of Methane Microseepage Based on Dawanqi Field Work and Physical Simulations

Methane (CH4) microseepage from petroleum basins is a significant contributor to the atmospheric CH4 budget. However, research about CH4 migration and release mechanism is still very limited. This work seeks to theorize and verify the migration and release mechanism of CH4 microseepage via field measurement and physical simulation, which, to the best of our knowledge, has not been reported in literature. Fluxes of CH4 microseepage from Dawanqi oilfield were measured, and three manifestations of release were observed, namely, continuous, flat, and episodic. Based on field observations, bench-scale physical simulation of CH4 migration through geological features of the oilfield was further conducted for 290 days. The results show that CH4 migration is mainly driven by buoyancy and diffusion. In continuous release, CH4 migration is mainly driven by buoyancy. In flat release, CH4 migration is dominated by diffusion. At low pressure, CH4 migrates upward slowly. As buoyancy increases, CH4 eventually break through the capillary pressure of the pore throat, causing spikes in CH4 concentrations in the layers above and reproducing episodic release observed during field measurement. Via field observation and verification by physical simulation, this work theorizes the migration mechanism of CH4 microseepage and its correlation with release types observed and confirms that counterbalance of buoyancy force and capillary pressure plays a critical role in episodic release of CH4 from oilfield. The findings of this study shed light on the migration mechanism and release manifestations of CH4 microseepage under different geological conditions and improve accuracy of estimating the flux of CH4 microseepage into atmosphere.