Abstract
In this study, we have built a dual porosity/permeability model through accurately expressing the volumetric strain of matrix and fracture from a three-dimensional method which aims to reveal the reservoir permeability evolution during the process of CO2-enhanced coalbed methane (CO2-ECBM) recovery. This model has accommodated the key competing processes of mechanical deformation and adsorption/desorption induced swelling/shrinkage, and it also considered the effect of fracture aperture and effective stress difference between each medium (fracture and matrix). We then numerically solve the permeability model using a group of multi-field coupling equations with the finite element method (FEM) to understand how permeability evolves temporally and spatially. We further conduct multifaceted analyses to reveal that permeability evolution near the wells is the most dramatic. This study shows that the farther away from the well, the gentler the evolution of permeability. The evolution of reservoir permeability near the injection well (IW) and the production well (PW) are very different, due to the combined effects of effective stress changes and gas adsorption and desorption. Furthermore, adsorption is the main controlling factor for the change of permeability for regions near the IW, while the change in effective stress is the main cause for the change in permeability near the PW. Increasing the injection pressure of CO2 will cause the reservoir permeability to evolve more quickly and dynamically.
Highlights
Coalbed methane (CBM) recovery from coal seams, as an unconventional resource, is a growing contributor to the energy supply for the energy-hungry world [1,2,3]
It is the purpose of this research, to consider volumetric strain induced by the mechanical process and adsorption/desorption process accurately and to establish a porosity/permeability model for the CBM reservoir to reveal the reservoir permeability evolution during the process of CO2 -ECBM recovery
Based on the combination of mechanical deformation and adsorption/desorption-induced swelling/shrinkage, and considering the fracture aperture and effective stress difference of matrix and fracture, a dual porosity/permeability model is established through accurately expressing deformation of each medium from a three-dimensional method to reveal the reservoir permeability evolution during the process of CO2 -ECBM recovery
Summary
Coalbed methane (CBM) recovery from coal seams, as an unconventional resource, is a growing contributor to the energy supply for the energy-hungry world [1,2,3]. Zhang et al [28] established a general porosity/permeability model porosity/permeability model based on the poroelasticity theory In their model, the pore volume change induced by the change in the effective stress and gas adsorption were both considered. Using one point or an average of reservoir permeability evolution to investigate the evolution behavior of an entire reservoir during the process of CO2 -ECBM recovery is not profound enough It is the purpose of this research, to consider volumetric strain induced by the mechanical process and adsorption/desorption process accurately and to establish a porosity/permeability model for the CBM reservoir to reveal the reservoir permeability evolution during the process of CO2 -ECBM recovery. Based on the combination of mechanical deformation and adsorption/desorption-induced swelling/shrinkage, and considering the fracture aperture and effective stress difference of matrix and fracture, a dual porosity/permeability model is established through accurately expressing deformation of each medium from a three-dimensional method to reveal the reservoir permeability evolution during the process of CO2 -ECBM recovery.
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