Mechanism Of Coupled Thermal-Hydrologic-Mechanical Processes For Exploiting Coal Bed Methane In Low Permeability Reservoir And Its Applications

The storage of coal bed methane (CBM) is rich in our country. At present, the characteristics of "abundance CBM reservoir with low permeability" has restricted the development of Chinese CBM industry a lot. Therefore, aiming to improve the economic benefits of CBM exploitation, improving the permeability of coal bed becomes an important and difficult topic for CBM exploitation. According to theory of porous medium elasticity mechanics, seepage mechanics and thermodynamics, this dissertation systematically studied the occurrence migration law of CBM under temperature effects. The coupled thermal-hydrological-mechanical (THM) mathematical model including coal deformation, gas diffusion, gas seepage, gas adsorption and temperature effects has been established. The model has been used in the engineering practices of heat injection and enhances coal bed methane (ECBM), the following conclusions are obtained:(1) The relationship between temperature and adsorption constants can be obtained through the experimental data, thereby the adsorption equation under temperature and pressure is developed. Permeability model including the temperature effects is developed for the structural feature of fracture and matrix system.On that basis, the equations including coal deformation equation, seepage equation and temperature field governing equation are developed respectively for dual porosity media, Simultaneous above equations won the THM multiphysics coupled mathematical model.(2) The coupled THM control equations are solved by COMSOL Multiphysics, having CBM exploitation as the background. It can be found that the temperature of coal can influence the production rate of CBM directly. The higher temperature of coal seam the higher production rate of CBM. In-situ stress can lessen the production rate of CBM by lowering the permeability. The smaller elastics modulus of coal seams the higher production rate of CBM. The smaller space of coal fractures the higher CBM exploration efficiency. The smaller modulus reduction ratio the faster gas output rate since it can enhance the permeability.(3) Three-dimensional physical model has been established by appling the THM coupled mathematical model to CBM exploitation, and then the impacts on production rate, pressure and permeability have been analyzed since injecting heat to the coal seam. It can be found that improving injection temperature can promote process of gas desorption and permeability of coal bed, consequently improve production rate of CBM.(4) Dynamic permeability model of coal bed has been researched under systerm of dual gas with competition adsorption effect, the coupled THM equations for CO2-ECBM are developed and been numerically solved. Correctness and rationality of the coupled model has been verified compared with experiment results.(5) The coupled THM equations are used to simulate the processing of CO2-ECBM in Qinshui CBM field. The simulation results are consistent with in-situ test data, demonstrate that CO2 injection can improve the production of CH4. Studies indicate that increase the temperature and the injection pressure are all contributing to increase production of CH4.