A coupled damage-hydro-mechanical model for gas drainage in low-permeability coalbeds

- A fully coupled damage-hydro-mechanical model was established.
- Reservoir simulations were validated and applied to gas drainage.
- Quantitative evolutions of coal damage and permeability were evaluated.
- Drilling induces stress-relief and coal damage remarkably.

In the process of coal and gas mining, the excavation and drainage drilling will induce the redistribution of stress around the working face, thus damage coalbeds. In this paper, a coupled damage-hydro-mechanical model is proposed based on the elastic damage at the unloading failure stage of coalbeds. Firstly, a constitutive model for dual-porosity coal with isotropic damage is established within the framework of strain equivalence. Its damage variable is directly related to the effective strain which depends on the initial in-situ stress, pore pressure, and gas sorption induced deformation. Secondly, a dynamic model for the permeability evolution is developed with the concept of scalar damage variable. The permeability model is applied to a three dimensional elastic damage finite element program. Finally, the excavation process and gas drainage are numerically simulated. The evolutions of gas pressure, coal permeability, and gas transport for single borehole are investigated. The numerical results show that the coupled damage-hydro-mechanical model can well match and predict the field test data and the empirical data. It is useful to evaluate the drilling and gas drainage application. Drilling can remarkably relieve the stress around the borehole, induce great damage to coalbeds and thus promotes coal permeability. The numerical simulation is helpful to evaluating gas drainage and taking effective measures to prevent the occurrences of gas outbursts in coalbeds.