Study On Diffusion-seepage Law Of Oil-bed Gas In Coal Seam Around Abandoned Oil Well

There are many areas which coexist coal and oil resources in country.And a large amount of abandoned oil wells are left after the early exploitation of oil.The oil-bed gas accumulation areas around abandoned oil wells are formed when much oil-bed gas from oil strata flows along wells to the surrounding coal seam,which seriously threatens the safe and efficient coal mining around the wells.The study reveals that the law of oil-bed gas diffusion and seepage in coal seam around abandoned oil well is an important basis for scientific and effective prevention and control of its harmBased on the systematic analysis of the theory of coal mass structure and gas migration,the characteristics of micro-pore fracture structure of coal mass and the occurrence and migration characteristics of oil-bed gas in coal seam were obtained.It is considered that oil-bed gas occurs in the pore and fracture system of coal body in adsorbed state and free state respectively,and the migration mode in pore and fracture system obeys Fick’s law and Darcy’s law respectively.Driven by the high pressure in the oil well,the oil-bed gas in the well enters the coal fracture system through seepage,and then into the pore system through diffusion.And based on some hypothesizes such as dual-porous media of coal mass,the diffusion and seepage equations of oil-bed gas in coal seam were derived.Combined with the dynamic variation characteristics of coal permeability and the deformation mechanism of coal mass,the diffusion-seepage coupling model of oil-bed gas flowing in coal seam was established.Through the analysis of the coal sample and oil-bed gas in well in Shuangma Coal Mine,the main parameters of oil-bed gas flowing were obtained.The COMSOL Multiphysics software was used to simulate and analyze.It is found that during the flow,the increase rate of the oil-bed gas pressure in coal seam near oil well is firstly accelerated and then slowed down.The gas pressure gradually stabilizes with time.The closer to the abandoned oil well,the faster the pressure change in a short time and the greater the pressure is.The relationship between the influence radius of oil-bed gas and time is obtained by changing the pressure of oil-bed gas in oil well when the oil-bed gas pressure is constant and increases linearly at different rates.Based on the fact that the simulation results are in good agreement with the measured results of Mattan 31#oil well,the oil-bed gas flow velocity and pressure in coal seam at 30th are simulated.And it is found that with the increase of the flow distance,the oil-bed gas flow velocity and pressure decrease gradually.And the distribution relationship between gas flow velocity and pressure with flow distance in coal seam for 30th is obtained.The study result provides theoretical guidance for the prevention and control of oil-bed gas and safe and efficient coal mining in Shuangma Coal Mine.It is also of great significance for safe and coordinated mining of similar coal-oil-gas coexistence mines.