There are many areas which coexist coal and oil resources in country. And a largeamount of abandoned oil wells are left after the early exploitation of oil. The oi-bed gasaccumulation areas around abandoned oil wells are formed when much oil-bed gas from oilstrata flows along wells to the surounding coal seam, which seriously threatens the safe andefficient coal mining around the wells. The study reveals that the law of oil-bed gas diffusionand seepage in coal seam around abandoned oil well is an important basis for scientific andeffective prevention and control of its harm.
Based on the systematic ana lysis of the theory of coal mass structure and gas migration,the characteristics of micro-pore fracture structure of coal mass and the occurrence andmigration characteristics of oil-bed gas in coal seam were obtained. It is considered thatoil-bed gas occurs in the pore and fracture system of coal body in adsorbed state and free staterespectively, and the migration mode in pore and fracture system obeys Fick's la w and Darcy'slaw respectively.
Driven by the high pressure in the oil well, the oilbed gas in the well enters the coalfracture system through seepage, and then into the pore system through diffusion. And basedon some hypothesizes such as dual-porous media of coal mass, the diffusion and seepageequations of oil-bed gas in coal seam were derived. Combined with the dynamic variationcharacteristics of coal permeability and the deformation mechanism of coal mass, thedifusion-seepage coupling model of oi-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 Multiphysicssoftware was used to simulate and analyze. It is found that during the flow, the increase rate ofthe oil-bed gas pressure in coal seam near oil well is frstly acce lerated 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 relationshipbetween the influence radius of oil-bed gas and time is obtained by changing the pressure ofoil-bed gas in oil well when the oil-bed gas pressure is constant and increases linearly atdifferent rates. Based on the fact that the simulation results are in good agreement with themeasured results of Mattan 31#oil well, the oil-bed gas flow velocity and pressure in coalseam at 30th are simulated. And it is found that with the increase of the flow distance, theoil-bed gas flow velocity and pressure decrease gradually. And the distribution relationshipbetween gas flow velocity and pressure with flow distance in coal seam for 30th is obtained.