Dynamic Influence Of Gas Diffusion In Matrix On Coal Permeability

The coupling of coal skeleton deformation and gas adsorption-desorption-diffusion-seepage processes in coal bed gas mining and greenhouse gas sequestration directly affect reservoir permeability.Study of coal pore structure variation patterns is essential for evaluating coal permeability.It is generally believed that the permeability of coal is closely related to its geomechanical conditions.The permeability test is usually performed in the laboratory only after the pore pressure of the coal body has reached equilibrium state,and the relevant theoretical model is also based on the pressure equilibrium assumption to describe the law of coal porosity variation.Although these methods are relatively easy to apply,from laboratory studies to theoretical analyses,they ignore the effects of non-stationary gas flow on coal pores and do not fully reflect the evolutionary characteristics of coal permeability,and the uncertainty of coal crack opening changes often affects the reliability of predictive models.In order to study the influence of dynamic gas flow on the fracture-matrix structure,this paper uses a combination of indoor experiments and numerical simulations to study the mechanism of coal permeability changes during gas flow seepage-diffusion,and to analyze the influence of fracture-matrix interaction on fracture permeability due to dynamic changes in pore pressure caused by expansion deformation of coal matrix.The main elements and conclusions of the study are as follows:1)To study the effect of effective stress on permeability during non-steady state gas flow.Permeability and strain evolution laws over time were analyzed by gas injection experiments under different stress conditions on different coal samples.The results indicate that the distribution of pore pressure in coal samples is a dynamic mechanical process from fracture pressure equilibrium(local equilibrium state)to fracture-matrix pressure equilibrium(overall equilibrium state).Permeability tends to decrease over time due to compression of the fracture by matrix expansion deformation;macroscopic expansion during the post-injection period leads to a rebound in permeability.It was also found that increasing the injection pressure under constant differential pressure reduced the permeability,while increasing the injection pressure under constant circumferential pressure increased the permeability.2)To study the effect of gas transport-induced fracture-matrix interactions on coal permeability.In this paper,the volume deformation of the coal matrix is divided into internal and external deformation,where internal deformation compresses the crack opening and external deformation increases the coal volume.The interaction relationship between the coal matrix and the fracture is quantified by the ratio of internal deformation to total matrix expansion deformation defined as the coefficient of expansion within the coal matrix.The mechanism of the influence of internal expansion coefficient on permeability is explored by building a coal body permeability model that takes into account matrix-fracture interactions.The results show that the coefficient of internal expansion under constant differential pressure increases with pore pressure.3)To study the influence factors of influencing fracture-matrix interactions.Numerical simulation was used to establish a discrete model of double pore size and double permeability of coal rock,calculate the evolution process of matrix and fracture permeability coefficient of coal,and analyze the physical processes and significance of the different stages of the evolution curve;the influence of initial fracture pore size and initial matrix permeability on fracture matrix interaction was analyzed in conjunction with numerical simulation.It was found that decreasing the initial fracture pore size resulted in reduced compression of the fracture by matrix expansion;decreasing the initial permeability of the matrix resulted in an increase in the fracture matrix interaction time and a delay in the overall time to reach equilibrium.