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.
Coal as a soft rock experiences compaction when pore pressure depletes and effective stress increases during Coal Seam Gas (CSG) drainage. The increase in effective stress during gas drainage causes the reservoir to undergo compaction (Schatz and Carroll, 1981). This mechanical induced compaction causes permanent deformation of coal microstructure and loss of porosity. As a result, a reduction in permeability and weaker response to gas drainage is predicted in the coal influenced by compaction creep. Also, deeper coal seams undergo less deformation owing to restriction on stress relaxation in and around the working face during mining. The impact of creep on permeability is more important for lower rank coals located at shallow depth as they are softer than higher rank coals and therefore they experience more creep. Creep compaction of rocks, as a long-term process, occurs very slowly at the equilibrium state that has been reached in millions of years. However, creep process and shrinkage of rocks are accelerated when subsurface fluids are drained from the rocks via drainage boreholes. Deformation in coal can occur much faster due to being much softer than adjacent rocks (roof and floor rocks) (Brantut et al., 2013; Kaiser and Morgenstein, 1981). Negligence of the impact of compaction creep on coal permeability may result in misestimating the level of drained gas. This may lead to a delay in mining operations due to the need for addressing residual hazards associated with methane gas such as coal and gas outburst and asphyxiation.