Experimental study of impact of creep on coal permeability

Drainage of fluids (hydrocarbon or water) from coal seam via drainage boreholes brings about compaction creep and permanent loss of porosity and permeability over time. In this paper, the impact of creep (time-dependent deformation) on coal permeability has been experimentally studied using a triaxial rig. The bituminous coal sample used for the triaxial creep test was excavated from Bowen Basin, Australia. Methane gas was used in the triaxial test to investigate the effect of creep triggered by pore pressure depletion on permeability during desorption under constant hydrostatic and axial stresses. The results show that the creep induced to coal results in a significant drop in coal permeability when approximately zero creep rate is reached. Permeability measurements show that drop of pore pressure in a step manner from 2.5 MPa to 1.5 MPa, 1 MPa, and 0.5 MPa leads to decrease in permeability loss ratio of 16.8%, 6% and 5.4% for each step change. Permeability rebound was not obtained in our experiments for this range of pressures. The experimental results of this study can shed light on complexity of interaction of gas transport and timedependent deformation of coal during gas drainage.