LAREDO — Not all liquefied natural gas exports leave the United States on giant ships from the Gulf of Mexico or Atlantic Coast. Some of them get delivered by 18-wheelers to meet growing demand for energy in Mexico. While the burgeoning U.S. LNG industry has focused on multibillion dollar coastal export terminals, one company from Houston and another from Mexico City have carved out a niche in the market by using tanker trucks to deliver the supercooled fuel to industrial and agricultural customers south of the border. The Houston company, Stabilis Energy, opened a $55 million LNG plant capable of producing 120,000 gallons of LNG a day in the South Texas town of George West in March 2015. The plant initially focused on supplying fuel to portable LNG-powered generators at remote drilling and fracking sites, but it has since added at least 10 Texas frac sand mines, out of reach of pipelines and power grids.