A permeability model including effective stress and coal matrix shrinking effect

To recover a large percentage of coalbed methane in coalbeds, reservoir pressure must be reduced significantly in the primary production. Two distinct effects are associated with the reduction of reservoir pressure during methane extraction: (1) the release of gas, and (2) an increase in effective stress. The increase in effective stress causes a decrease in the permeability of the coal owing to mechanical closure of flow paths. However, an opposing effect of coal-matrix shrinkage can occur, one that is attributed to a desorption phenomenon. This shrinkage widens the fractures that are primarily responsible for gas flow in coalbed reservoirs, and thus increases permeability. Some permeability models such as Shi-Durucan model, Palmer-Mansoori model and Gray model incorporating the effect of matrix shrink and effective stress influence were developed. These models were based on two key assumptions: coalbed is under uniaxial strain condition and constant vertical stress. According to this, a coupled seepage-stress coalbed methane flow model considering matrix shrinkage was used to examine the accuracy of these assumptions; then this model was used to analyze permeability change during primary production; the result suggested that the assumption of uniaxial strain appears valid, while constant vertical stress may introduce error; it varies as gradients in strain are induced towards the production well; and its contribution to permeability can be significant; then that existing coal permeability models could underestimate permeability change.