Description: The recovery of natural gas from CH4-hydrate deposits in sub-marine and sub-permafrost environments through injection of CO2 is considered a suitable strategy towards emission-neutral energy production. This study shows that the injection of hot, supercritical CO2 is particularly promising. The addition of heat triggers the dissociation of CH4-hydrate while the CO2, once thermally equilibrated, reacts with the pore water and is retained in the reservoir as immobile CO2-hydrate. Furthermore, optimal reservoir conditions of pressure and temperature are constrained. Experiments were conducted in a high-pressure flow-through reactor at different sediment temperatures (2 °C, 8 °C, 10 °C) and hydrostatic pressures (8 MPa, 13 MPa). The efficiency of both, CH4 production and CO2 retention is best at 8 °C, 13 MPa. Here, both CO2- and CH4-hydrate as well as mixed hydrates can form. At 2 °C, the production process was less effective due to congestion of transport pathways through the sediment by rapidly forming CO2-hydrate. In contrast, at 10 °C CH4 production suffered from local increases in permeability and fast breakthrough of the injection fluid, thereby confining the accessibility to the CH4 pool to only the most prominent fluid channels. Mass and volume balancing of the collected gas and fluid stream identified gas mobilization as equally important process parameter in addition to the rates of methane hydrate dissociation and hydrate conversion. Thus, the combination of heat supply and CO2 injection in one supercritical phase helps to overcome the mass transfer limitations usually observed in experiments with cold liquid or gaseous CO2.

Description: Hydrocarbon-rich fluids expelled at mud volcanoes (MVs) may contribute significantly to the carbon budget of the oceans, but little is known about the long-term variation in fluid fluxes at MVs. The Darwin MV is one of more than 40 MVs located in the Gulf of Cadiz, but it is unique in that its summit is covered by a thick carbonate crust that has the potential to provide a temporal record of seepage activity. In order to test this idea, we have conducted petrographic, chemical and isotopic analyses of the carbonate crust. In addition a 1-D transport-reaction model was applied to pore fluid data to assess fluid flow and carbonate precipitation at present. The carbonate crusts mainly comprise of aragonite, with a chaotic fabric exhibiting different generations of cementation and brecciation. The crusts consist of bioclasts and lithoclasts (peloids, intraclasts and extraclasts) immersed in a micrite matrix and in a variety of cement types (microsparite, botryoidal, isopachous acicular, radial and splayed fibrous). The carbonates are moderately depleted in 13C (δ13C = − 8.1 to − 27.9‰) as are the pore fluids (δ13C = − 19.1 to − 28.7‰), which suggests that their carbon originated from the oxidation of methane and higher hydrocarbons, like the gases that seep from the MV today. The carbonate δ18O values are as high as 5.1‰, and it is most likely that the crusts formed from 18O-rich fluids derived from dehydration of clay minerals at depth. Pore fluid modelling results indicate that the Darwin MV is currently in a nearly dormant phase (seepage velocities are 〈 0.09 cm yr− 1). Thus, the thick carbonate crust must have formed during past episodes of high fluid flow, alternating with phases of mud extrusion and uplift. Highlights ► Results of pore fluid modelling indicate low seepage activity at localised sites. ► Pore fluids are supersaturated with respect to hydrocarbons of thermogenic origin. ► AOM supports vent fauna and results in the formation of authigenic carbonates. ► The carbonate crust has a brecciated appearance and mainly consists of aragonite. ► The crust formation seems to be regulated by changes in fluid and mudflow activity.

Description: The accumulation of methane hydrate in marine sediments is controlled by a number of physical and biogeochemical parameters including the thickness of the gas hydrate stability zone (GHSZ), the solubility of methane in pore fluids, the accumulation of particulate organic carbon at the seafloor, the kinetics of microbial organic matter degradation and methane generation in marine sediments, sediment compaction and the ascent of deep-seated pore fluids and methane gas into the GHSZ. Our present knowledge on these controlling factors is discussed and new estimates of global sediment and methane fluxes are provided applying a transport-reaction model at global scale. The modeling and the data evaluation yield improved and better constrained estimates of the global pore volume within the modern GHSZ ( ≥ 44 × 1015 m3), the Holocene POC accumulation rate at the seabed (~1.4 × 1014 g yr−1), the global rate of microbial methane production in the deep biosphere (4−25 × 1012 g C yr−1) and the inventory of methane hydrates in marine sediments ( ≥ 455 Gt of methane-bound carbon).

Description: SUGAR (SUbmarine GAs hydrate Reservoirs) is a collaborative R&D project with 20 partners from SMEs, industry and research institutions. It was launched in 2008 and is now successfully continuing in its second phase, running until summer 2014. The portfolio of technologies developed in SUGAR includes state-of-the-art hydro-acoustic, 3-D seismic and electromagnetic devices for the exploration of marine gas hydrate deposits as well as the monitoring of hydrate exploitation operations. The novel joint inversion technique combines the interpretation of seismic and electromagnetic data and was successfully applied to hydrate accumulations off New Zealand and in the Black Sea. New autoclave systems for drilling and recovering marine hydrates under in situ pressure have been designed that are suitable for deployments from both drilling vessels and small research vessels. A further outcome of the project is a unique 3-D basin modeling software for the prediction of the formation of gas hydrate deposits in marine and permafrost settings. Exploitation strategies for marine hydrate deposits are being developed in laboratory experiments as well as in numerical reservoir simulations. Their primary focus is on the production of methane by injection of CO2, combining natural gas recovery with the safe sequestration of carbon dioxide in CO2 hydrates below the seafloor. While the reservoir simulations test field-scale strategies and assess these in terms of gas production rates and economics, the laboratory experiments focus on the optimization of the hydrate conversion reaction by application of supercritical CO2, heat supply via in situ combustion and addition of polymers. In the 2nd SUGAR phase, a new subproject started developing novel drilling technologies specialized for marine hydrate deposits which are significantly shallower below the seafloor than standard oil and gas reservoirs.

Description: The recovery of natural gas from CH4-hydrate deposits in sub-marine and sub-permafrost environments through injection of CO2 is considered a suitable strategy towards CO2-neutral energy production: CO2 activates the release of CH4 from the gas hydrate and is retained in the reservoir as immobile CO2-hydrate. In our experiments we could show that the injection of hot, supercritical CO2 is particularly promising. The addition of heat dissociates the CH4 hydrate and leads to a fast and continuous release of the encaged methane. However, the total production yield depends strongly on the structural properties of the surrounding hydrate/sand matrix. Additional CH4 can be liberated if percolating cooled, liquid CO2 gets in contact with CH4-hydrate triggering a direct exchange of the guest molecule. Furthermore, the transport of CH4 to a production well requires sufficient permeability of the sediment matrix and is hindered by the reformation of gas hydrates during the process. We present experimental data from a high-pressure flow-through reactor at different sediment temperatures (2 ?C, 8 ?C, 10 ?C) and hydrostatic pressures (8 MPa, 13 MPa). The efficiency of both CH4 production and CO2 retention was evaluated and best results were achieved at 8 ?C, 13 MPa. This behavior can be explained by the different percolation properties of the mobile phases at the various sediment temperatures. To substantiate these findings, we performed magnetic resonance imaging experiments that provided spatially resolved information on the fate of liquid CO2 in the sand/CH4-hydrate matrix. The results confirm the good accessibility of the pore space for liquid CO2 at 8 ?C, 13 MPa.

Description: In order to proceed with speculative modelling of the impacts of potential leakage of geologically stored carbon, it is necessary to develop plausible scenarios. Here a range of such scenarios are developed based on a consensus of the possible geological mechanisms of leakage, namely abandoned wells, geological faults and operational blowouts. Whilst the resulting scenarios remain highly speculative, they do enable short term progress in modelling and provide a basis for further debate and refinement.