Characterisation of creep in coal and its impact on permeability: An experimental study

Abstract

ABSTRACT:Creep is a time-dependent deformation that affects coal permeability and should be considered in the prediction of Coalbed Methane (CBM) production. This study experimentally characterises and quantifies the impact of creep on coal permeability. The experiments were conducted on a bituminous coal sample, excavated from Bowen Basin, Australia, using a triaxial gas rig equipped with strain and displacement transducers. Two different types of gasses (helium and methane) were injected into the sample under various stress and pore pressure conditions. It was found that for the experiments with helium, creep caused permanent partial closure of cleats and pathways under constant effective stress, and hence a reduction in permeability. Under hydrostatic stress only, a Residual Deformation Ratio (RDR) of 14.1% and a Permeability Loss Ratio (PLR) of 71% were found following the removal of the axial load. This can be due to the damage to coal microstructure along with closure of cleats. For the experiments with methane, coal experienced an instantaneous elastic deformation, at the onset of pore pressure depletion, followed by consolidation and matrix shrinkage. Then, creep occurred when gas desorption ceased. A total permeability loss of 26% was achieved due to an increase of 1.91 MPa in effective stress caused by gas desorption. In addition, the model previously developed by authors was validated against the experimental permeability data. A good agreement was found between the model-predicted permeability data and the experimental permeability data, particularly for higher pore pressure ranges.Keywords: Coal permeability; creep; consolidation; triaxial test; gas desorption

Seepage and Damage Evolution Characteristics of Gas-Bearing Coal under Different Cyclic Loading–Unloading Stress Paths

“…With the increase of peak stress and the accumulation of damage in coal samples, the sensitivity of the permeability of coal samples to stress gradually declined. The relationship between the damage variable and the number of cycles conformed to the Boltzmann function.Processes 2018, 6, 190 2 of 20 has a significant influence on the mechanical properties and energy dissipation characteristics of coal [17][18][19][20][21][22][23][24]. Moreover, the mechanical properties and seepage characteristics of coal under different loading-unloading paths greatly differ from those under the conventional loading pattern.…”

The impact of time-dependent deformation on coal permeability and gas drainage efficiency​​​​​​​

“…However, visco-elastic and visco-plastic deformations of rocks as time-dependent mechanisms occur due to change in deviatoric effective stress during gas production. The impact of time-dependent deformation on permeability evolution and gas drainage efficiency (Danesh et al, 2016;Danesh et al, 2017) and permeability around roadways (Xue et al, 2017) has recently come to researcher's attention.…”

“…However, experimental results from triaxial tests, under deviatoric stress conditions, reveal that coal reflects a strain rate dependency and therefore visco-elastic and visco-plastic deformations around the drainage boreholes are deemed to exist during pore pressure depletion (Danesh et al, 2017).…”

“…Meaning, the coal in each zone cannot be transformed to its succeeding deformation state. Figure 6.3b shows the ideal representation of deformation states around drainage boreholes, based on the findings of recent studies on time-dependent deformations and their impact on coal permeability (Danesh et al, 2017;Danesh et al, 2016;Xue et al, 2017). (Li et al, 2016;Li, Li and Yifeng, 2015;, and b) deformation mechanisms accounting for time-dependent deformation around drainage boreholes Figure 6.4.…”

“…In addition, creep behaviour of coal under different loading conditions (Hol, Zoback and Spiers, 2013;Yang and Zoback, 2011;Pomeroy, 1956;Yin et al, 2009;Yin, Zhao and Zhang, 2008;Wang et al, 2011;Singh, 1975;Li et al, 2011) has been studied. However, limited studies (Danesh et al, 2016;Danesh et al, 2017;Yang and Zoback, 2011)…”