FROZEN HEAT | Volume 2

et al. (2011) and Rutqvist and Moridis (2012) have reviewed first-order assessment of the first four years of production and the associated geomechanical response for hypothetical marine and permafrost gas hydrate deposits. As shown in Figure 3.6, these simulations suggest substantive changes in water and gas production over time, as well as significant surface displacement. A critical element in the field production testing phase is the detailed evaluation and monitoring of associated environ- mental impacts. This evaluation will require study of base- line conditions within the environment prior to the test, as well as monitoring of any changes in these conditions during and after the test (Fujii et al. 2012; Nagakubo et al. 2011). Pa- rameters that will be monitored include the impact of subsid- ence or other geomechanical instability and possible release of methane or other substances into the ocean or atmos- phere. An important environmental issue is the impact of the release of colder, anoxic, and low-salinity water (originat- ing from the dissociation of marine hydrates) near the ocean floor, with potentially significant consequences for chemos- ynthetic communities there (Moridis and Reagan 2007a, b). 3.4.6 Potential for extending production beyond sand-dominated gas hydrate reservoirs At this time, only gas hydrate deposits in which the hydrate occurs as a pore-fill within clay-poor sediments of high per- meability are seen as well suited to sustained production with

currently available technologies employed for production from conventional oil and gas resources. However, gas hydrates in such reservoirs are likely to represent only a small fraction of the global gas hydrate inventory. The bulk of global gas hydrate occurrences probably consists of dispersed, low-concentration gas hydrate accumulations (perhaps occupying five per cent or less of sediment pore space) in fine-grained marine sedi- ments. These are unlikely to be candidates for commercial production due to low resource density, limited permeability, and low sediment strength (Moridis and Sloan 2007). Recent drilling investigations carried out in offshore India (Collett et al. 2008) and Korea (Park 2008), however, have identified thick sedimentary sections containing a variety of macroscopic gas hydrate forms, including fracture fillings and nodules. Gas hydrate concentrations in these marine settings can be in the range of 20 to 40 per cent of bulk sedi- ment pore space, making them plausible production candi- dates if significant geomechanical challenges can be over- come (Moridis et al. 2013). In addition, highly concentrated gas hydrate occurrences associated with cold vent features have been observed within 100 metres of the seabed in sev- eral offshore locations, including offshore Korea (Bahk et al. 2009), the Cascadia margin (Riedel et al. 2006a, b), and the Gulf of Mexico (Sassen et al. 2001). While such depos- its may hold potential as future production targets, they are not suited to conventional oil and gas production methods. Thus, it is likely that new technologies and approaches will be required to achieve economic production of gas hydrates in fine-grained, unconsolidated marine sediments.

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