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

Abstract

Coal as a soft rock experiences a time-dependent deformation induced by continuous change in effective stress in reservoir during Coal Seam Gas (CSG) production and during well shut-in at static reservoir pressure. This deformation results in compaction of coal microstructure and loss of porosity and permeability over the course of gas production. Limited studies have considered the impact of time-dependent deformation (i.e. consolidation and creep) on coal permeability that may play a key role in determining the efficiency of CSG production. Negligence of the impact of consolidation and creep on coal permeability may result in overestimation of level of drained gas.This research aims to characterize time-dependent deformation in the coal injected with gas and investigate its impact on permeability and gas drainage efficiency. This has been achieved via implementation of two triaxial tests and development of governing equations (i.e. permeability models and mass balance equation) required for simulation of gas transport in coal. In the first test, the effect of the consolidation exerted by an axial load in addition to a hydrostatic load, under deviatoric stress, on permeability of the coal sample injected with helium is studied. In the second test, the effect of the consolidation and creep triggered by desorption of methane from the coal sample, under increasing effective stress, on permeability is studied. For this purpose, a triaxial rig equipped with strain and displacement transducers is employed. The sample used in the tests was a bituminous coal excavated from Bowen Basin, Australia.In the case of helium, the reduction in permeability is mainly attributed to permanent partial closure of cleats and pathways, which is reflected in higher elasto-plastic deformation than visco-elastic deformation and considering non-adsorptive helium. The results reveal that a significant amount of irrecoverable coal deformation exists after removal of the axial load and under hydrostatic stress only, which is believed to be attributed to the damage to coal microstructure along with closure of cleats. The results also show that the coal injected with methane experiences an instantaneous elastic deformation, at the onset of pore pressure depletion, followed by consolidation and matrix shrinkage. Then, a decelerating or steady-state creep initiates when gas desorption ceases. A significant permeability loss of 26% was achieved due to an increase of 1.91 MPa in effective stress caused by gas desorption. The results show that consolidation and creep can have a significant impact on permeability evolution under loading and during gas desorption for the experimental coal sample injected with gases of interest.The permeability models that account for the effects of elasticity, consolidation and creep under increasing and constant effective stress are also developed in this study. For this purpose, non-linear stress-strain equations have been extended by introducing visco-elastic and visco-plastic terms of classical Nishihara model to an existing linear stress-strain equation for anisotropic coal. Then, a Finite Element (FE) formulation of the developed models and gas transport equations is implemented in order to carry out scenario-based simulations by COMSOL Multiphysics. The numerical results show that consolidation can have a significant impact (up to 25% or more) on coal permeability and gas drainage. The developed permeability models are validated against the experimental permeability data. A good agreement is found between the model-predicted permeability data and the experimental permeability data, particularly for higher pore pressure ranges.