Abstract: Coal reservoir permeability is under very complicated change due to the combination of the effective stress effect and matrix shrinkage effect during the depletion of reservoir pressure. Nowadays a large number of models have been established to describe permeability variation with the depletion of reservoir pressure. However, no attempt has been made to draw enough attention to the difference of the effect of various factors on permeability variation in different production stages of unsaturated coalbed methane (CBM) reservoirs. This paper summaries the existing and common permeability models, and determines the relationship among various effects (effective stress effect, matrix shrinkage effect and Klinkenberg effect) and desorption characteristics of the recovery of unsaturated CBM reservoirs, then establishes two improved models to describe quantificationally permeability variation by improving the existing models, and finally discusses influence factors on permeability variation with a case study of Hancheng CBM field in Ordos Basin. Results show that the permeability variation during the recovery of unsaturated CBM reservoirs can be divided into two stages: the first one is that the permeability variation is only affected by the effective stress effect, and the second is that the permeability variation is affected by the combination of effective stress effect, matrix shrinkage effect and Klinkenberg effect. In the second stage, matrix shrinkage effect and Klinkenberg effect play much more significant role than effective stress effect, leading to the increase of permeability with depletion of reservoir pressure. A comparative study of the improved models indicates that the improved SD model is more sensitive to various parameters than the improved PM model and the improved models, and can describe dynamic permeability variation more exactly than the original ones. Thus, the case study, by improved SD model, on Hancheng CBM field shows that the increase in coal reservoir permeability is determined by gas saturation, and the damage in coal reservoir permeability is controlled by the difference between the initial reservoir pressure and critical desorbed pressure, and the descending order of the average increment is 3#, 11# and 5#.

Key words: coalbed methane reservoir, permeability, dynamic variation, Hancheng mining area