Natural gas pipelines across the United States are aging and are near capacity, creating a weak link in the nation's electric grid, Energy Secretary Ernest Moniz says.

High-volume transmission pipelines, which carry gas from wells to refineries and storage facilities, remain “underutilized" and still have room to spare, Moniz says. However, smaller distribution lines—which carry gas to customers—are the concern,reports US News & World Report.

The U.S. shale energy boom of the past decade—driven by advances in fracking and horizontal drilling—has sent oil and gas prices plummeting, driving up demand for natural-gas fired electricity and heat. The Environmental Protection Agency's proposed Clean Power Plan, which would curtail emissions from existing power plants, has also encouraged plant operators to switch from coal to cleaner-burning natural gas.U.S. natural gas piepeline network, 2009. Source: EIA

Moniz's comments echo the findings of the first part of the Quadrennial Energy Review, published in mid-April by the Energy Department. The review is a four-year analysis of the nation's electricity grid. While the price-tag for upgrading the distribution network is significant, Moniz says the need is urgent.

“If you look at aging infrastructure, the estimate just to replace all of the very old—like 50 years and older—natural gas distribution pipes, for both environmental methane leaks and safety reasons, is estimated at a quarter trillion dollars," the secretary was quoted as saying. He made the remarks at a breakfast hosted by the Christian Science Monitor.

“Every natural gas utility is focused on upgrading and modernizing its infrastructure," Dave McCurdy, CEO of the American Gas Association, which represents local natural companies, was quoted as saying in a statement.