PhD ThesisMost of the energy produced in the world comes from fossil fuels: coal, oil and
gas. Amongst them, coal is the most abundant and widespread fossil fuel in the
world. Underground Coal Gasi cation (UCG), an in situ method to extract the
calori c value of the coal, has been known for a century but has had very limited
implementation throughout the world, mainly due to the availability of cheap oil
over that period. It is now gaining relevance in order to unlock vast resources of coal
currently not exploitable by conventional mining.
However, growing concern on increased levels of carbon dioxide concentration in
the atmosphere is pointing out the necessity to reduce the use of fossil fuels. Since
alternative sources of energy (e.g. nuclear and renewables) are not in a position
to meet the constantly increasing demand in a short term, carbon capture and its
geological sequestration (CCS) is considered the best remedial option.
An environmental risk assessment framework has been developed for coupling
UCG to CCS accounting for bene ts and cost from both global and local perspectives.
A UCG site presents signi cant di erences from other typical CCS projected
scenarios, most notably the injection of CO2 into a heavily fractured zone. A model
which accounts for
ow in fractures represented by dual-porosity
ow (TOUGH2) is
coupled to a geomechanical model (FLAC3D). The impact of this fractured zone in
the CO2 injection pressure buildup and stress eld is evaluated. Furthermore the effect
of stress-dependent fracture permeability is assessed with the hydro-mechanically
i
coupled compositional simulator GEM. Simulation results suggest that in such a scenario,
CO2 injectivity and dissolution improve though con nement is compromised
and commercial injection rates seem unattainable. The e ects of miscibility and
relative permeability on pressure buildup implemented in semianalytical solutions
are also evaluated. Albeit further research is required, a UCG operation may, therefore,
not be able to accommodate the produced CO2 in the gasi ed cavity and its
surroundings in a safe and economical fashion. Rigorous studies and management
practices are needed to establish the requirements for secure long-term con nement
of the carbon dioxide in such scenario.HSBC & UK Engineering and Physical Sciences Research
Council (EPSRC) under the program EP/F013337/1. The Royal Academy of Engineering
supported my trip to Lawrence Berkeley National Laboratory with a travel
grant (ref. 09-376). Further funding was made available for extending the research by
the UK Strategy Board (STB) through the UK Transfer Partnership (KTP) scheme,
with the participation of Durham University and ERC Equipoise Ltd