Permeability and porosity models considering anisotropy and discontinuity of coalbeds and application in coupled simulation
The permeability of coalbeds plays a paramount role for the success of coalbed methane (CBM) development. Due to the characteristic of stress sensitivity the permeability of coalbeds is not constant during CBM production but varies with the changes of in situ conditions, such as the change of pore pressure and the desorption/adsorption of gases. In order to simulate the inﬂuence of these alterations in predicting or evaluating CBM production, new porosity and permeability models used for reservoir and geomechanical coupled simulation have been established in this paper. In the formulation discontinuous coal mass (containing cleats and matrix) is considered as an equivalent continuum elastic medium and the anisotropy of coalbeds in permeability, matrix shrinkage/swelling due to gas desorption/adsorption, thermal expansion due to temperature change and mechanical parameters, are included. The implementation procedure of an explicit-sequential coupled simulation using the developed models and industrial simulators is detailed and the application of coupled simulation to pressure depleting CBM production is demonstrated. Important issues, such as the length of time step, the relation between permeability and pore pressure or in situ effective stress, the relation between permeability and porosity, and the inﬂuence of initial water saturation are discussed. The limitations of the proposed models and industrial simulators applied are also commented.