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Analysis and Research on Influencing Factors of Coal Reservoir Permeability

The permeability of coal is an important parameter in the theory of coal reservoir gas flow. Experiments have demonstrated that after the coal matrix adsorbs gas, adsorption swelling deformation will occur and when gas desorbs from coal matrix, it undergoes a shrinkage deformation. A new formula for estimating permeability is developed. By considering the adsorption swelling and desorption shrinkage deformation in this formula for permeability, the effect of coal matrix swelling and shrinkage on coal reservoir permeability is studied. The study shows that, as gas in the coal reservoir desorbs, gas pressure decreases and the coal matrix shrinks, so as to make swelling deformation smaller and increase the permeability of the coal reservoir. In addition, the influence of effective stress, Klinkenberg effects, burial depth and natural fracture on coal reservoir permeability is analyzed. The analysis and research is of significance to coal mine gas disaster prevention and coal bed methane gas resource development. INTRODUCTION The permeability of coal is an important parameter in the theory of coal reservoir gas flow. Many gas dynamic phenomena, such as coal and gas outburst, emission etc, are dependent coal reservoir permeability during exploitation. In this paper the effect of coal matrix swelling and shrinkage on coal reservoir permeability is studied by considering adsorption swelling and desorption shrinkage deformation in a new formula for estimating permeability. The influence of effective stress, Klinkenberg effects, burial depth and natural fracture on coal reservoir permeability is also analysed. FACTORS INFLUENCING COAL RESERVOIR PERMEABILITY Shrinkage effects of coal matrix Many experiments have demonstrated that after the coal matrix adsorbs gas, adsorption swelling deformation will occur and when gas desorbs from the coal matrix, it undergoes a shrinkage deformation (Reucroft P J,Patel H, 1986; Seidle J P, Huitt L G, 1995; Levine J R, 1996). In the process of coal production the equilibrium of the original stress field and the original gas pressure field is disturbed, which lead to a stress redistribution near the mining space and results in gas flow. When the gas pressure falls below the critical desorption pressure, gas starts to desorp. With the decrease of pore pressure, gas desorption and micro-porous surface free energy increase, which results in the coal matrix shrinking and enhanced permeability of coal reservoir. The increase of coal porosity will lead to the change of the permeability of coal and this affects gas seepage in the coal reservoir. According to the Kozeny-Carman equation, the change of permeability with porosity, φ , can be expressed as (Lu Ping et al., 2002)