FROZEN HEAT | Volume 2

2.2 WHAT ARE THE MOST PROMISING ACCUMULATIONS FOR PRODUCTION?

The criteria that determine the attractiveness of a gas hy- drate accumulation for production are similar to those ap- plied in other hydrocarbon-rich basins around the world. An extensive discussion on the subject can be found in Moridis et al. (2009). Initial production will likely target large and richly concentrated occurrences that can be produced safely and at rates that make the venture profitable. It is not suffi- cient that the in-place resource simply be there. There must also be a way to extract the gas that is economically viable, safe, and environmentally responsible. As discussed in Chapter 3, the cumulative results of work in the field, in the lab, and through application of numerical models – conducted mainly within the past decade – indicate that the richest gas hydrate occurrences discovered to date, as well as those most likely to support economically viable

production of methane, are found in sand-rich sediments (Fig. 2.2). Production would proceed through specially-tai- lored applications of well-drilling technologies used in con- ventional oil and gas production. While it appears that, glob- ally, most gas hydrates are housed in clays, assessments in the highest-studied areas (the Alaska North Slope, the Gulf of Mexico, and offshore southeastern Japan) show large po- tential resources in sand-rich deposits (Boswell and Collett, 2011). What makes sand reservoirs attractive is the sedi- ment’s high natural permeability, a measure of the ease with which fluid can flow. This permeability is key to enabling gas hydrates to accumulate at high concentrations. Once hy- drates form, that permeability is dramatically reduced, but it is still sufficient to allow pressure (and even advection- based thermal) changes to be delivered into a reservoir from a drilled well.

Figure 2.2: Permeability of gas-hydrate host sediments. Right: The most promising gas hydrate occurrences are distinguished primarily by the nature of the enclosing sediment (after Boswell et al. 2011). The high initial permeability of sand-rich units enables gas hydrates to accumulate to high saturations. After gas hydrate is emplaced measurable permeability is maintained in the sediments sufficient to enable existing well-based extraction technologies. Left: Gas hydrates limited to thin sand intervals in cores obtained from the Ulleung basin, Korea, in 2010 (from Bahk et al. 2011b with permission).

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