The number of drill rigs operating in Pennsylvania's Marcellus Shale may be declining, but production keeps going up.

Natural-gas output increased robustly during the first half of this year, according to state Department of Environmental Protection data. And industry observers say the output will continue to rise because so many wells are waiting to be connected to pipelines.

Pennsylvania's "unconventional" shale-gas wells - those that unlock gas entrapped in tight rocks like shale, as opposed to conventional wells that tap into concentrations of free-flowing oil or gas - produced 895 billion cubic feet of gas in the first six months of this year, up nearly 42 percent from the previous reporting period, according to an analysis by the Powell Shale Digest, a trade publication based in Fort Worth, Texas.

That's about 13 times the amount of gas consumed for an entire year in Philadelphia.

The number of producing wells increased by 28 percent, to 2,879. Production is likely to continue increasing because one-third of the wells drilled in the state are not yet selling their output to the market, said Thomas B. Murphy, co-director, energy development, of Penn State University Extension's Marcellus Center for Outreach and Research.

Many wells that have been drilled are waiting to be hydraulically fractured. The controversial process involves high-pressure injection of chemically treated water and sand into the shale formation to stimulate wells.

Other wells that have been fracked are shut-in, or capped, while awaiting completion of pipelines to transport the natural gas to market.

Though the growth rate is slowing from the early years of the shale boom, Pennsylvania Marcellus gas production is expected to increase 78 percent by the end of 2015, according to an August report by Bentek Energy, a market-analytics firm.