Numerical study for CH4 production from gas hydrate-bearing sediments via CO2 injection

Abstract

CH4 production from gas hydrate-bearing sediments has been considered as a potential measure to supplement global hydrocarbon resources. Natural gas hydrates, solid compounds of gas and water, are often found in the pores of sediments under deepwater or permafrost regions, where hydrate stability conditions of high pressure and low temperature exist. CH4 production from gas hydrate-bearing sediments requires a phase change of the solid gas hydrates and has been deemed possible by depressurization through a well. However, the method tends to lower geomechanical stability of the sediments and consequently, to date, only a few short-term trials of field-scale gas production were successful. Moreover, future hydrocarbon production is overshadowed by growing concerns for its adverse environmental impacts. Therefore, there is a pressing need for an innovative energy strategy that replenishes dwindling supplies of hydrocarbons while maintaining geomechanical stability and achieving carbon neutrality. CO2 injection into gas hydrate-bearing sediments may potentially accomplish the aforementioned objective. As CO2 hydrates are usually more stable than CH4 hydrates, injected CO2 could destabilize CH4 hydrates in the sediments to release out CH4 gas for production while forming solid CO2 hydrates in the newly available pore of the host sediments. In this way, it would effectively maintain the geomechanical stability of the sediments during CH4 production and, at the same time, store CO2 as solid hydrates permanently in the sediments. However, the process is yet to be fully understood as it involves interactions of various multi-physical and chemical processes including generation of immiscible CH4-CO2 fluid mixture in sea water, evolution of chemical reaction kinetics of CH4-CO2 hydrate mixture, heat emission and absorption from hydrate formation and dissociation, respectively, and stress redistribution caused by spatial and temporal developments in CH4-CO2 mixed hydrate-bearing sediments. This research has developed a novel, coupled thermo-hydro-chemo-mechanical formulation that captures the complex processes and has investigated the behavior of CH4 hydrate-bearing sediments subjected to CO2 injection. There are mainly four contributions. Firstly, several additional processes caused by generation of CH4-CO2 fluid mixture have been incorporated such as fluid viscosity change and molar fraction induced diffusion. Secondly, formation of CH4-CO2 hydrate mixture has been taken into account, along with its effect on thermal, hydrological, chemical and mechanical processes. Thirdly, the applicability of the formulation has been validated through simulations of existing laboratory tests of CO2 injection into CH4 hydrate-bearing soil. Fourthly, the efficiency of CH4 production and CO2 storage and its geomechanical impact by CO2 injection in natural gas hydrate-bearing sediments have been thoroughly discussed. The outcome of this research provides valuable insights into the prospect of the revolutionary energy strategy that satisfies conflicting interests, unlocking the new source of hydrocarbon energy while managing geo-risk and environmental sustainability for future generations.