CO2 injection in hydrate-bearing sediments induces methane (CH4) production while benefitting from CO2 storage, as demonstrated in both core and field scale studies. CH4 hydrates have been formed repeatedly in partially water saturated Bentheim sandstones. Magnetic Resonance Imaging (MRI) and CH4 consumption from pump logs have been used to verify final CH4 hydrate saturation. Gas Chromatography (GC) in combination with a Mass Flow Meter was used to quantify CH4 recovery during CO2 injection. The overall aim has been to study the impact of CO2 in fractured and non-fractured samples to determine the performance of CO2-induced CH4 hydrate production. Previous efforts focused on diffusion-driven exchange from a fracture volume. This approach was limited by gas dilution, where free and produced CH4 reduced the CO2 concentration and subsequent driving force for both diffusion and exchange. This limitation was targeted by performing experiments where CO2 was injected continuously into the spacer volume to maintain a high driving force. To evaluate the effect of diffusion length multi-fractured core samples were used, which demonstrated that length was not the dominating effect on core scale. An additional set of experiments is presented on non-fractured samples, where diffusion-limited transportation was assisted by continuous CO2 injection and CH4 displacement. Loss of permeability was addressed through binary gas (N2/CO2) injection, which regained injectivity and sustained CO2-CH4 exchange. OPEN ACCESSEnergies 2015, 8 4074

Natural gas hydrate is a crystallized ice-like substance, consisting of water and natural gas, with methane as the most common gas. Water molecules form cages through hydrogen bonding and encapsulate gas molecules. Natural gas hydrates are found in the earth under high pressure and low temperature where water and gas co-exist, typically in permafrost and submarine environments. Hydrates have been considered a nuisance in the petroleum industry, creating barriers in pipe lines, and effort has mainly been put into preventing hydrate formation. However, natural gas hydrates are in recent decades acknowledged as a potential energy resource for the future; even conservative estimates suggest 10 15 m 3 CH 4 STP present within hydrate.Several methane production scenarios are proposed: thermal-, chemical-and pressure reduction induced dissociation is available, although depressurization is considered the least costly option. The University of Bergen has since 2002 worked on a fourth alternative: exchange of CH 4 molecules with CO 2 . Lab scale experiments have repeatedly shown CO 2 -CH 4 exchange within sediments. These experiments led to a field trial test in Alaska, operated by ConocoPhillips, by utilizing CO 2 injection as a production method. Similar procedures as in the field test were performed in the lab, creating repetitive data for analysis on lab scale. This paper reviews results from both the laboratory and field pilot and discusses challenges and mitigating measures related to production.

Focus is shifted towards renewable energy and sources of natural gas as the demand for cleaner energy continues to increase with global awareness on anthropogenic climate change. Methane (CH 4) provides advantages such as high enthalpy upon combustion and low carbon imprint compared to other fossil fuels. Natural gas is therefore predicted to play an important role as the world moves from coal dependency towards a cleaner and more sustainable energy future. Natural gas hydrate is a solid state of gas and water, where water molecules interconnect through hydrogen bonding to form a cavity which is stabilized by a gas molecule through van der Waals interaction forces. This reaction occurs where water and CH 4 coexist at low temperature and high pressure. In nature, such conditions are typically found in permafrost and submarine environments. Vast energy resources are associated with gas hydrates, where different models suggest that hydrates contain 10 15 to 10 17 m 3 CH 4 at standard temperature and pressure (STP). In comparison, the annual gas consumption in the US is about 7•10 11 m 3. Gas hydrates may therefore become a significant contributor in the future energy mix. Current technological challenges are related to in situ characterization for accurate saturation estimates, further advances in production technologies and continuous improvements of available numerical models through comparison with actual fieldand core-scale data. A synergy between gas production and safe CO 2 storage is achieved through CO 2 sequestration in hydrate bearing sediments, where CO 2 replaces the existing CH 4 molecule within the hydrate crystal. The process occurs because CO 2 offers favorable thermodynamic conditions. Salt was observed to impact the hydrate formation rate and the amount of excess water in Paper 1. Depressurization and diffusion-driven CO 2 exchange were compared, where Magnetic Resonance Imaging (MRI) was used to monitor production in situ. CO 2-CH 4 exchange was more abundant for high residual brine, and therefore sensitive to initial salt concentration. Depressurization was assumed to be limited by permeability and heat transfer. Current opinion on geomechanical issues related to hydrate bearing sediments was addressed in Paper 2. Hydrate decomposition through depressurization resulted in production of associated water with potential loss of structural integrity, as gas