Flue gas injection for methane recovery from gas hydrate reservoirs and geological Storage of CO2
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The global energy system has been gradually de-carbonised over the years, from wood to coal, coal to oil, and then natural gas. Natural gas hydrates with their abundance in nature, therefore represent a potentially significant new clean energy source for the future. A few field trials have been conducted to recover natural gas (methane) from gas hydrate reservoirs. While the outcomes of these trials offer a glimmer of hope on the possibility of methane production from gas hydrate resources, there remains the nagging question of production sustainability as most field trials are short-lived due to high energy penalty, sand management issues, excessive water production, and potential environmental risks. This thesis reports the development of a novel technique for methane recovery from natural gas hydrate reservoirs by flue gas injection. Compared to the existing methods, the principal concept of the technique is to break the thermodynamic equilibrium of methane hydrate by flue gas injected, causing a shift in the equilibrium phase boundary to accommodate the presence of flue gas while releasing methane from hydrate dissociation. A series of experiments were conducted at different simulated hydrate reservoir conditions to demonstrate the feasibility of the technique vis-à-vis understanding how methane hydrate decomposes in the presence of flue gas, the impact of flue gas on the depressurisation process, and the possibility of the CO2 component in the flue gas being sequestered as CO2 or CO2-mixed hydrates. Furthermore, the impact of the excess aqueous phase, salinity, and sediment mineralogy on methane recovery were also investigated. Finally, peculiarities of gas flow in hydrate-bearing sediments were also investigated and modelled with existing permeability models. Results indicated significant dissociation of methane hydrate by a shift in the methane hydrate equilibrium phase boundary leading to a rise in methane concentration in the vapour phase. Enhanced methane recovery by depressurisation in the presence of flue gas generated a methane-rich vapour phase of up to 80 mol% methane at experimental conditions within the methane hydrate stability zone (HSZ). CO2 hydrate, N2-CO2-CH4 hydrate, and CO2-CH4 were formed simultaneously alongside methane recovery after flue gas injection. Up to 70% of CO2 in the vapour phase was captured and retained in the hydrate phase. Increased aqueous phase salinity enhanced methane recovery and increased CO2 capture and storage in excess water environments. Extension of the concept to air and nitrogen injection showed enhance depressurisation compared to flue gas injection with up to 90 mol% methane in the vapour phase at conditions still within the methane HSZ. It is also flexible, with the possibility of stepwise depressurisation with continuous and incremental methane recovery. Potentially these techniques are economically feasible as they save on costs in terms of thermal energy supply and chemical additives. On the operational front, it is not subject to injectivity constraints due to secondary hydrate formation. It also has the capacity to maintain reservoir energy, limit water production, and deliver better sand management. Additionally, direct capture and storage of CO2 from flue gas could provide huge savings in carbon capture and storage processes.