Multiphysics of coal-gas interactions

Abstract

Advances in our understanding of the geomechanics of coal-gas interactions have changed the manner in which we treat coal seam gas: from mitigating its dangers as a mining hazard to developing its potential as an unconventional gas resource recovered as a useful by-product of CO sequestration. When coal seam gas is recovered, complex coal-gas interactions have strong controlling effects on the extraction efficiency of coal or gas. These include influences on gas sorption and flow, coal deformation, porosity change and permeability modification. This chain of reactions can be defined as “coupled processes” implying that one physical process affects the initiation and progress of another. Therefore, the inclusion of cross couplings is the key to rigorously formulate the geomechanics of coal-gas interactions. A wide variety of coal permeability models incorporating the correct physics have been proposed to simulate the interactions of multiple processes triggered by the CO injection or production of methane. Coal permeability models, representing the effects of sorption, swelling and effective stresses on the dynamic evolution of permeability, are normally required to define the mechanics of coal-gas interactions. The inclusion of the effective stress concept led to a generalized coal permeability model being developed. This contribution transformed the pore pressure-based models into a new generation of effective stress-based model. Consistent efforts have done to include that the impact of coal matrix-fracture compartment interactions into the mechanics of coalgas interactions. A series of permeability models have advanced this subject from the pore pressure-based theory to the effective stress-based theory, to the effective stress transfer-based theory. These models couple the transport and sorption of a compressible fluid within a deformable medium where the effects of deformation are rigorously accommodated. This paper reports this novel framework on geomechanics of coal-gas interactions and its applications primarily in the field of coal seam gas extraction.