Abstract: To more accurately characterize the complex geomechanical effects in coalbed methane reservoir,a fully coupled triple porosity dual permeability fluid flow and geomechanics model was established with consideration of poroelastic properties and multi-process transportation. The corresponding numerical solver was constructed by the fully implicit finite volumetric method,and the impacts of geomechanics on porosity and permeability and methane production were investigated. The results show that both effective stress effect and matrix shrinkage can significantly affect fracture permeability,but in opposite direction. The dominate factor is effective stress effect compared with matrix shrinkage at the early stage of primary production,but latter turns to matrix shrinkage,which makes fracture permeability firstly decreases and then rebound. The final permeability can even reach several times of its original value. As Young's modulus increases,the effective stress effect recedes,which creates bigger and earlier peak gas production. As the Langmuir strain increases,the matrix shrinkage is strengthened,which creates bigger and earlier peak gas production. The fully coupled fluid flow and geomechanics model can describe the complex fluid-structure interaction of coalbed methane reservoir more accurately,which is of great significance to the prediction of CBM productivity.

Key words: coalbed methane, coupled fluid flow, geomechanics, matrix shrinkage, triple porosity dual permeability