The Energy Information Administration (EIA) has posted its Annual Energy Outlook for 2013 - see here.  Discussing natural gas production growth, it states, for example:  "The 44-percent increase in total natural gas production from 2011 through 2040 in the AEO2013 Reference case results from the increased development of shale gas, tight gas, and coalbed methane resources (Figure 91). Shale gas production, which grows by 113 percent from 2011 to 2040, is the greatest contributor to natural gas production growth. Its share of total production increases from 34 percent in 2011 to 50 percent in 2040."

Graphically:

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