Model study of the influence of matrix shrinkage on absolute permeability of coal bed reservoirs

Abstract The matrix volume of coal shrinks when occluded gases desorb from its structure. In coalbed gas reservoirs, matrix shrinkage could cause the fracture aperture width to increase, causing an increase in permeability. A computer model was developed, based on elastic rock mechanics principles, to evaluate the potential effect of matrix shrinkage on the absolute permeability of coalbed reservoirs as fluid pressure is drawn down during gas production. The model predicts that the fracture width can potentially increase, depending on the combined influence of a number of parameters, particularly Young’s modulus of elasticity, Poisson’s ratio, fracture spacing and matrix shrinkage parameters. Each of these parameters vary depending on coal composition, so each individual coal will behave differently. A sensitivity study was conducted to evaluate the influence of each model parameter using a geologically reasonable range of input values. ‘Base case’, upper and lower limits were selected, based on published data. Gas production was simulated by reducing fluid pressure from 1290 PSI (8.89 MPa) to 100 PSI (0.70 MPa). The matrix shrinkage parameter ε max was found to produce the largest effect on permeability. Permeability changes as large as +250 mD are predicted for the upper case value of ε max . If ε max is small, however, the predicted permeability change will be negligible. An increase in Young’s modulus, Poisson’s ratio and fracture spacing each cause a predicted increase in permeability. Results of this model study should be verified through additional modelling, laboratory and field-based studies.