Marking the first time natural gas will have flowed westbound on the Rockies Express Pipeline (REX), the pipeline’s Seneca Lateral was expected to begin initial service Wednesday with 0.25 Bcf/d of capacity, according to the Energy Information Administration (EIA), which cited a customer notice posted on the pipeline’s website.

The 14-mile lateral runs from the MarkWest Seneca gas processing plant in Noble County, OH, to the REX mainline, where a new compressor station will allow gas to be delivered westward to Ohio, Indiana and Illinois. When fully operational, the lateral will have capacity of 0.6 Bcf/d, according to EIA.

REX operator Tallgrass Energy Partners LP did not immediately respond to a request for more information on the lateral’s status.

“According to pipeline operator Tallgrass Energy, the remaining capacity on the Seneca Lateral is fully booked and will come into service in the fourth quarter of this year,” EIA said Wednesday. “In addition, REX has received binding commitments to deliver another 1.20 Bcf/d of Appalachian production west on its existing mainline as part of the Clarington West Project [see Shale Daily, Jan. 30], which REX plans to bring into service in either 2016 or 2017.”

The long-planned 1,679-mile REX pipeline, which went into service just as the shale revolution in North America was starting to take off, was originally constructed as a west-to-east pipeline that could deliver bountiful Rockies-produced natural gas to thirsty northeastern U.S. markets. The first east-bound flows came almost five years from the day the pipeline delivered its first Rockies gas into Ohio (see Daily GPI, June 30, 2009).

Tallgrass is the majority owner of REX, whose other owners are Sempra U.S. Gas & Power (25%) and Phillips 66 (25%).