Reactive transport modelling of high pressure gasflow in coal

This thesis describes a study of reactive transport processes in fractured rock in response to high pressure gas injection and displacement. This is achieved through the development and application of a theoretical and numerical modelling platform. A dual porosity, dual permeability framework has been formulated based on a mechanistic approach, which considers the coupled hydraulic, gas/chemical and deformation behaviour of fractured rock. The fracture network and porous rock matrix were treated as overlapping continua with distinct transport and storage properties. Flow in each continuum was considered by advection, diffusion and dispersion mechanisms, and a sink/source term was included for the kinetically controlled sorption of multicomponent gas. A mass exchange term was introduced to couple the continua and allow pressure and concentration differences to develop. The transport properties of non-ideal gas mixtures at high pressure were characterised by appropriate constitutive relationships. The developed model has been incorporated in an existing coupled thermal, hydraulic, chemical and mechanical framework. A numerical solution was obtained using the finite element method for spatial discretisation and the finite difference method for temporal discretisation. Verification of the approach proposed has been addressed via a series of benchmark tests. The results obtained provide confidence in the accuracy of the numerical implementation of the dual porosity governing equations, including a time splitting approach used to couple the transport module with the mass exchange and geochemical reaction modules. Key theoretical features have been included to enhance the model capabilities and enable application of the model to study species dependent coal-gas behaviour, especially in relation to carbon dioxide sequestration in coal and enhanced coal bed methane displacement. The development of constitutive relationships describing the feedback of dual porosity physico- and chemo-mechanical deformation on gas transport in coal was considered in detail. Furthermore, a combination of two first-order rate models was used to include the specific gas sorption behaviour in coal. A detailed validation of the model using high resolution experimental data on gas interactions, transport and displacement in coal has been included. The theoretical models developed for coal-gas interactions were first evaluated, providing a platform to facilitate numerical simulations of gas injection and displacement experiments, performed on intact samples of anthracite coal from the South Wales coalfield. Under the conditions considered and for two injection scenarios, namely, nitrogen and subcritical carbon dioxide injection, it was demonstrated that the model is capable of simulating the salient physical and chemical phenomena involved in gas transport and methane displacement in coal. More advanced simulations have been performed to study the behaviour for a larger sample size and different gas injection pressures and compositions. The injection of supercritical carbon dioxide and two carbon dioxide-rich gas mixtures at high pressure was considered. It is claimed that a substantive insight has been gained into the coupled behaviour of the material at the laboratory scale. Overall, the analysis carried out in this research indicated that species dependent chemo-mechanical deformation was the dominant factor in smaller core samples. Fracture-matrix exchange and preferential methane desorption by carbon dioxide only became more apparent in larger samples. An appreciation of the effects of sample size on the behaviour observed is therefore important when interpreting experimental data, and implies that due care must be taken in interpreting laboratory scale results towards larger scale applications. In this work, the capabilities of the new model have been showcased with regards to the study of coal-gas systems. Importantly, the developments presented are more generally relevant and thus enable the study of a broad new range of applications involving multiphase, multicomponent gas/chemical transport in fractured rock.