Modelling of gas transport in coal - a hybrid coupled dual porosity and discrete fracture approach

This thesis describes an investigation of coupled thermal–hydraulic-chemical–mechanical (THCM) behaviour of rock with a complex fracture system, which has been achieved through the advancement and application of a theoretical and numerical modelling platform of THCM processes. A hybrid coupled dual continuum and discrete fracture model has been developed based on the thermoporoelastic theory, which considers multiphase fluid flow, heat transfer, geochemical processes, and deformation in discretely fractured rock such as coal. The uniformly distributed natural fracture network, composed of micro-fractures, and the rock matrix itself have been modelled using a dual porosity approach. Large-scale fractures have been represented explicitly as discrete fractures. The coupling between the different media has been achieved via a mass and heat exchange processes. Appropriate relationships were incorporated in the model to accurately describe the major thermodynamic properties of compressible fluids and the chemo-mechanical deformation of coal during carbon dioxide (CO2) sequestration and coalbed methane (CBM) recovery. Furthermore, a new porosity-permeability model was derived in this work describing feedback of the physical and chemical mechanisms of deformation on coal structure and fluid flow, which provides an improved physical interpretation for changes of the pore space. The developed model has been implemented in an existing coupled thermal, hydraulic, chemical, and mechanical framework. A numerical solution was presented using the Galerkin finite element method for the spatial discretisation and the finite difference method for temporal discretisation. The discrete fractures were idealised as lower-dimensional geometric objects with the discrete fracture elements located on the edges of continuum elements sharing the same nodes. The coupling between the two flow systems was achieved by using the principle of superposition. Verification of the approach proposed has been addressed via a series of benchmark tests. The results obtained provided an evaluation of the numerical implementation of the theoretical framework for non-isothermal unsaturated flow in deformable fractured rock, which established a good level of confidence in the accuracy of the implementation of the theoretical and numerical formulations. Validation of the model using experimental data on coal-gas interactions and field data of CBM production has been included to examine the capacities of the proposed model to interpret processes in the material behaviour and various physical mechanisms involved. These tests have indicated that the model is capable of simulating the key physical and chemical processes occurring during CBM recovery and CO2 storage into coal seams. A set of simulations have been performed to study the coupled THCM behaviour of low permeability coal reservoirs during CBM recovery and CO2 injection, and the effect of hydraulic fracturing stimulation on the CBM production and CO2 injectivity has been considered. A substantive insight into the coupled behaviour has been gained. Overall, analysis of the evolutions of the primary field variables indicated that the fluid pressure, temperature, and pore structure in coal seams were influenced by CBM recovery and CO2 injection. CO2 injection induced swelling can cause loss of permeability and eventual rebound of coal permeability during CO2 sequestration with enhanced CBM recovery. Comparison of cumulative gas production and injection shown that CO2 injection together with hydraulic fracturing treatment was an effective solution to increase the CBM production and improve the injectivity of CO2 in low permeability coal seams. In this work, the new capabilities of the model have been demonstrated with regards to the investigation of the coupled processes of a coal-gas system. In particular, the model can be further utilised in the study of a broad new range of applications involving non-isothermal, multiphase, multicomponent gas/chemical transport and deformation in unconventional gas reservoirs