Abstract:

Two distinct effects are associated with the reduction of reservoir pressure during coal bed methane production,i.e.,the increase in effective stress,which causes the decrease in the permeability of the coal bed owing to mechanical closure of flow paths，and the release of gas leading to the coal-matrix shrinking,which widens the fractures that are primarily responsible for gas flow in coal bed reservoirs,and thus increases permeability.Based on these two simultaneously occurred effects，a general porosity and permeability model considering the effect of matrix shrinkage is developed，according to the model，some results can be attained.under the condition of constant overburden load,for the permeability change with pore pressure，there is a critical pore pressure p_c，for pore pressure p permeability decreases as the p increases，when p>p_c, the permeability increases as the p increases，and the expression of p_c is introduced.Some possible scenarios are also discussed with regard to the response of pore pressure and initial reservoir pressure to permeability.The result suggests whether the threshold pore pressure exists or not is dependent on the factors which influence the permeability，so a coupled multi-factors analysis on a coal bed methane production operation seems to be essential for optimizing the production.

Key words: coal bed methane, effective stress, coal matrix shrinkage, permeability, model