The Marcellus and Eagle Ford shales have exactly what the U.S. gas market doesn’t need right now: more gas on the way, according to natural gas analysts at Barclays Capital. They titled their latest note — an analysis of the effects of upcoming pipeline debottlenecking in the two plays — “Unleashing a Caged Monster.”

The firm said Tuesday pipeline projects in both supply regions would debottleneck some supply during the last four months of this year and into 2013. “Our analysis indicates that this is likely to unleash a sizable amount of incremental supply as both regions have gas waiting to flow,” wrote Barclays analysts Shiyang Wang and Michael Zenker.

It’s difficult to pin down how much more gas would be flowing from the Marcellus, they said, for several reasons. They don’t know how much gas is ready to flow; midstream capacity might not be in place in all locations to serve the capacity; and some new pipeline capacity coming on line would merely send gas downstream to the next bottleneck. “Nevertheless, this debottlenecking represents the biggest source of upside risk to our supply outlook,” they said.

Gas supply has been flat since peaking earlier this year, despite dropping rig counts, the analysts said. “Leaving aside debottlenecked gas arriving to market, our analysis indicates that production should fall through the end of 2012 and into 2013, even considering associated gas production growth,” Wang and Zenker wrote.

“…[B]ased on our conversations with producers and infrastructure developers, there appears to be more gas waiting on pipeline than we have included in our outlook. In particular, a cluster of pipeline capacity additions exceeds our outlook for gas attributable to debottlenecking alone. If there is enough supply ready to flow, this will add meaningfully to aggregate U.S. gas production.”

The analysts did not enumerate the pipeline projects in their note or go into details about infrastructure. They did, however, exclude projects seen moving gas from one full pipeline to another. They also excluded projects that did not look as if they would come to fruition.

The Marcellus holds the majority of latent gas supply that could be hitting the market in the coming months. Barclays said debottlenecking of the Marcellus and Eagle Ford could add between 0.9 Bcf/d to 2.2 Bcf/d of incremental supply between now and the end of 2012. Those numbers represent the firm’s “low” and “high” cases. The base case is for an additional 1.8 Bcf/d. The majority of anticipated debottlenecking in the base case (1.3 Bcf/d) should occur in November, the firm said.

Next year the biggest capacity additions are expected for the end of the first quarter and then in November. “The Eagle Ford, in our base outlook, could see slightly under 1 Bcf/d of supply coming from pipeline completions by March 2013,” the analysts said, “while November 2013 sets the stage for another 800 MMcf/d of supply from the Marcellus.”

Overall, next year pipeline projects are expected by Barclays to add about 3.4 Bcf/d of incremental supply. “…[T]his supply should come on stream even if the gas-directed rig count drops to zero, although this supply addition will be somewhat offset by supply declines in other locations (e.g., Canada).

“These supply additions, if realized, are likely to take U.S. gas production to new highs by the end of 2012 and should place 2013 on a supply growth path.”

The analysts said they are awaiting “a few more data points” before updating their U.S. supply outlook. The current outlook projects that Lower 48 production would be 64.48 Bcf/d this year and 62.94 Bcf/d in 2013.