The Flow Mechanism And Numerical Simulation Research Of Gas In Coal-bed With Low Permeability

For a long time, China’s energy consumption has been dominated by coal and oil, energystructure is single. Coupled with soaring international oil prices in recent years and increasingfactors of instability, China’s energy safety has been under serious threat. Coal-bed methane isnot only the most important reason for mine accident, but also its content of more than95%isgreenhouse gas, which causes global warming. China is a CBM resource-rich country, but itsproduction was not sufficient. The main reason is that China coal is "high permeabilityreservoir". Coa l core has low permeability and its pore size is the number of micro-nano level,the gas flow mechanism in the coal micro-nano pores differs from conventional reservoirs.For better characterizing the adsorption of coal-bed methane on coal surface, isothermallangmuir adsorption model is introduced into mass conservation equation. Knudsen numberis used to decide the flow regime of gas flow in porous media with low permeability. Thenthe molecular diffusion and slippa ge effect are considered, according to which themathematical mode l of coal-bed methane flow is established. In the mod el, gas adsorption,molecular diffusion and slippa ge effect are included. Using the finite difference method,solutions for the mathematical model are solved. By a one-dimensional core model and atwo-dimensional single well production mode l, the flow mechanisms of coal-bed methane arediscussed in detail. At the same time, the sensitivity analysis of coal-bed methane productioncapacity is carried out. The main results are as follow:(1) The gas adsorption on coal surface is a key factor to the flow and prod uction ofcoal-bed methane.(2) Molecular diffusion is an important flow mechanism of coal-bed methane seepage.Molecular diffus ion can increase the flux of gas flow and the prod uction rate. Whenmolecular diffusion coefficient is very small, the molecular diffusion flux is much smaller than the Darcy flux, the influence can be neglected. When the diffus ion coe fficient is large,the diffusion flux and Darcy flux is compa rable, molecular diffusion must be taken intoaccount.(3) It’s important to decide the flow regime when considering the flow mechanism ofcoal-bed methane. The slippage effect must be included in coal-bed methane flow model oflow permeability. Beskok-Karniadakis permeability correction factor considers the collisionof gas and the collision between gas and wall. It can be used to represent gas flow in detailand fully. When the gas flow belongs to slip flow regime, Beskok-Karniadakis permeabilitycorrection returns to Klinke nbe rg correction.