Description: The "guest exchange"of methane (CH4) by carbon dioxide (CO2) in naturally occurring gas hydrates is seen as a possibility to concurrently produce CH4 and sequester CO2. Presently, process evaluation is based on CH4-CO2 exchange yields of small-or medium-scale laboratory experiments, mostly neglecting mass and heat transfer processes. This work investigates process efficiencies in two large-scale experiments (210 L sample volume) using fully water-saturated, natural reservoir conditions and a gas hydrate saturation of 50%. After injecting 50 kg of heated CO2 discontinuously (E1) and continuously (E2) and a subsequent soaking period, the reservoir was depressurized discontinuously. It was monitored using electrical resistivity, temperature and pressure sensors, and fluid flow and gas composition measurements. Phase and component inventories were analyzed based on mass and volume balances. The total CH4 production during CO2 injection was only 5% of the initial CH4 inventory. Prior to CO2 breakthrough, the produced CH4 roughly equaled dissolved CH4 in the produced pore water, which balanced the volume of the injected CO2. After CO2 breakthrough, CH4 ratios in the released CO2 quickly dropped to 2.0-0.5 vol %. The total CO2 retention was the highest just before the CO2 breakthrough and higher in E1 where discontinuous injection improved the distribution of injected CO2 and subsequent mixed hydrate formation. The processes were improved by the succession of CO2 injection by controlled degassing at stability limits below that of the pure CH4 hydrate, particularly in experiment E2. Here, a more heterogeneous distribution of liquid CO2 and larger availability of free water led to smaller initial degassing of liquid CO2. This allowed for quick re-formation of mixed gas hydrates and CH4 ratios of 50% in the produced gases. The experiments demonstrate the importance of fluid migration patterns, heat transport, sample inhomogeneity, and secondary gas hydrate formation in water-saturated sediments.

Description: CO2 injection has been deemed a promising method for CH4 production from gas hydrate-bearing sediments for its potential in stabilizing the host sediments and balancing carbon emission. However, the process is yet to be fully understood, as it involves interactions of multi-physical and chemical processes including the generation of water-immiscible CH4–CO2 fluid mixtures, the evolution of chemical reaction kinetics for both CH4 and CO2 hydrates, heat emission and absorption during hydrate formation and dissociation, and stress redistribution caused by spatially evolving responses of CH4–CO2 hydrate-bearing sediments. This paper develops a coupled thermo-hydro-chemo-mechanical formulation that captures the complexity of these processes and applies it to investigate the behavior of CH4 hydrate-bearing sediments subjected to CO2 injection. The capabilities of this coupled formulation are validated through numerical simulations of laboratory experiments of CO2 injection into CH4 hydrate-bearing soil. Moreover, the application of this formulation in a field-scale scenario reveals insights into the efficiencies of CH4 production and CO2 storage and the geomechanical implications. Notably, the study finds that compared to the depressurization-only method, the combined hot CO2 injection and depressurization method could increase CH4 production by approximately 400%. In addition, this method could sequester about 70% of injected CO2 into solid hydrates, while exhibiting smaller maximum slope of differential displacement. These outcomes highlight the viability and benefits of CH4 hydrate production through CO2 injection, increasing the prospects of this approach.