Donald Trump wants a wall along Mexico’s northern border. Energy companies are perforating it with pipelines. They are laying steel pipes, some beneath the Rio Grande riverbed, to export billions of cubic feet more US shale gas to markets in Nuevo León, Guanajuato and other Mexican states. The shipments could quietly uncork a glutted US market, rivalling volumes of much-hailed new liquefied natural gas (LNG) exports from the coasts. Bigger gas sales to Mexico could not come soon enough for energy producers. The warmest winter on record left 2.5tn cubic feet of gas in US storage, the most ever for the end of the heating season. The US Energy Information Administration believes gas prices will average $2 per million British thermal units this year, the cheapest since 1998.