Shell plc (SHEL  ), Chevron Corp. (CVX  ) and BP plc (BP  ) have pulled workers off Gulf of Mexico platforms and shut in production as Hurricane Isaias nears the northern Gulf Coast.

Isaias strengthened into a Category 3 hurricane Friday with 120 mph winds, the National Weather Service said. Landfall is expected Friday evening between Mobile Bay in Alabama and Choctawhatchee Bay (by Destin) in the Florida Panhandle.

Shut-Ins More Than Double

As of Thursday, producers had shut in about 1.3 million barrels per day, or 62.9% of current Gulf oil output, according to Marine Minerals Administration data. On Wednesday, Reuters reported about 25%, or 511,619 barrels a day.

The Gulf region produces almost 15% of the crude pumped in the U.S. each year, according to CNBC.

Who's Offline

Shell evacuated all personnel and shut in its Mars, Olympus, Ursa, Vito and Appomattox assets, Reuters reported. On Wednesday, Chevron said it had started shutting in four of its Gulf facilities. The other five were still running normally at that point.

BP shut its Na Kika and Thunder Horse platforms and evacuated all staff, per the Wall Street Journal. Reuters also pegged Eni SpA (E  ) and Murphy Oil Corp. (MUR  ) as among the most exposed producers.

Refineries Are the Bigger Worry

With oil prices already high, the production cuts are big enough to affect markets, Jim Burkhard of S&P Global Energy told CNBC. Earlier Gulf storms did more damage, though. His bigger concern is whether Isaias comes ashore as a powerful storm near refineries.

Refinery outages can push crude and fuel prices in opposite directions. During past outages at Gulf refineries, fuel prices rose while crude fell, Kevin Book, managing director at ClearView Energy Partners, said on CNBC's "Squawk Box" on Thursday. Plants that go offline stop buying crude, and they stop sending fuel to consumers.

Shipping is a second problem. Andy Lipow, president of Lipow Oil Associates, expects tanker delays on both sides of the refineries.

"Tankers will be delayed delivering crude oil to the refineries while other tankers are delayed loading gasoline, jet fuel and diesel out of the refineries," Lipow said. "Florida will experience delays in receiving gasoline, jet fuel and diesel."

Crude Cools Off

On Thursday, Brent crude jumped 4% to settle at $104.28 a barrel and West Texas Intermediate gained 3.6% to $91.49, CNBC reported. Iran's stepped-up attacks on tankers in the Strait of Hormuz added to the rally.

Prices eased after President Donald Trump said late Thursday that the U.S. would not attack Iran before next month's elections. In midday trading, Brent was down 0.26% to $104.16 and WTI dipped 0.1% to $91.40, according to Trading Economics.

Shares of the United States Oil Fund ETF (USO  ), tracking WTI oil futures, was up 0.43% at $148.21 at the time of publication Friday.