HOUSTON — For the second time in two weeks, a powerful hurricane is forcing oil companies to shut down major production and refining facilities, driving up gasoline prices in the United States even as the price of oil continues to slide on global markets.

As Texas leaders ordered mandatory evacuations along the coast, oil companies on Thursday decided to shut down or curtail operations in as many as half of the state’s 26 refineries. Energy experts predicted that as much as 15 percent of the nation’s refining capacity would be knocked out for a week or more if Hurricane Ike produced major flooding and power failures.

Meteorologists are forecasting that the hurricane could make landfall early Saturday morning just south of Galveston, Tex., as a strong Category 3 storm, potentially putting industry headquarters in Houston, vital port facilities and six or more major refineries near the eye of the storm.

“We’re concerned about wind, we’re concerned about storm surge and we’re concerned about our ability to get our employees evacuated,” said Bill Day, a spokesman for Valero Energy, which is shutting down at least two Texas refineries. The company had just succeeded in getting a Louisiana refinery back online after it was knocked out by Hurricane Gustav, which tore through that part of the country on Labor Day weekend.