Dual Poroelastic Response Of Coal To CO₂ Sequestration

Geological sequestration of CO₂ in coal seams shows great potential to reduce Greenhouse gas emissions and has been studied worldwide in recent years. The typical dual-porosity property and organic component of coal together with the liquid state and steady property of CO₂, as well as methane production make coal seams a promising target. However, the CO₂ sequestration in coal seams involves a serial of mechanical problems such as coal deformation, the adsorption, seepage and diffusion of gas, which restricted the implement of this technology. This paper studied the multi-physics system which coupled the coal deformation, gas adsorption, seepage and diffusion equations on the basis of poroelastic medium, theory analysis and numerical simulation, and the following conclusions are obtained,(1) Based on the interaction of two systems, the porosity models for both matrix and frature are developed. The coal deformation model has been developed on the basis of effective stress in poroelastic medium, in which the gas flow effect is included. A set of interacted three-equation system including the multiphysics effect of the solid deformation, gas flow and adsorption is developed in which the flow of gas in coal and the adsorption capacity of coal are studied.(2) Given the effect of the residual water on the gas adsorption, permeability models including the moisture effects for fracture and matrix system are developed on the basis of Ettiger model, to which the wettability is introduced.(3) The coupled models are implemented in the COMSOL Multiphysics, in which the CO₂ sequestration process is simulated and the relative factors are analysed. The injection rate of CO₂ is directly decided by the fracture development. The more fractures in the coal seams, the larger the initial permeability of the fracture, and the faster of the gas transportation in them. In the meanwhile, the more cleats, the larger contact area of gas and coal will be. When the ground stresses are not equal in the horizontal directions, the gas permeability is directional. Gas permeates fastest in the direction of the largest geostress. The injection rate of gas is faster when residual water exists in the coal seams. The larger the moisture content, the smaller of the CO₂ sequestration.(4) The initial fracture distribution has been studied by image processing and numerical generation. The image processing consists of digital image processing technique and X-ray computerized tomography technique. By using the former method, two dimensional space distributions of the initial fracture can be obtained. While by using the latter, a set of CT scanning pictures can generate the 3D fracture distribution model of the core sample. The fracture distribution in the core sample is constructed and the effect of different homogeneity indices on the fracture distribution is studied in the numerical generation method on the basis of the Weibull distribution model.(5)The dual-porosity model is applied to the CO₂-ECBM, and the porosity and permeability models for both fracture and matrix systems are generated during the competitive adsorption of binary gas. The coupled coal deformation equation is derived on the basis of effective stress theory and the deformation relation caused by the competitive adsorption of binary gas. In addition, the coupled gas convection and diffusion equations are attributed to the mass conservation law, hydraulic diffusion equation as well as the Fick's law.(6) The CO₂-ECBM model is implemented into a field case in Qinshui CBM field. The simulation is conducted under different situations of gas producing without CO₂ injection and gas producing by various injection pressures. The simulation results demonstrate that the gas production is enhanced by CO₂ injection effectively.