Futures prices for West Texas Intermediate oil settled at their lowest level in about three months Thursday, dogged by worries over a fresh crude-oil glut.

August West Texas Intermediate crude fell 50 cents, or 1%, to settle at $50.91 a barrel on Nymex, the lowest settlement for a most-active contract since April 9. On Wednesday, prices slid 3.1%. But Brent crude’s August contract, supported reports of an oil-field outage and contract expirations, ended higher.

August Brent crude on London’s ICE Futures exchange, however, rose 46 cents, or 0.8%, to end at $57.51 a barrel on the contract’s expiration day. But September Brent the new front-month contract, settled at $56.92, down 20 cents after earlier gains. Brent prices had climbed after news reports said a power outage shut the United Kingdom’s largest oil field. But a spokeswoman for Nexen, which operates the Buzzard oil field, said the company “does not publish production or maintenance operations information on an individual asset basis.” The field, located in the North Sea, is the U.K.’s highest-producing field, according to Nexen.

WTI oil failed to rally Wednesday despite a hefty decline in weekly U.S. crude supplies and data provider Genscape Thursday reportedly said inventories at the futures delivery point of Cushing, Okla., have climbed almost one million barrels since Friday. Part of the reason for WTI’s failure to rally Wednesday was Federal Reserve Chairwoman Janet Yellen, “who seemed very upbeat and very confident that the Fed will raise rates this year,” said Phil Flynn, senior market analyst at Price Futures Group. Yellen offered her twice-yearly testimony before Congress on Wednesday.

Higher interest rates tend to support the U.S. dollar and a stronger greenback can weigh on dollar-denominated oil. “Oil can’t fight the Fed and it can’t fight the perception that the market is massively oversupplied—a situation that they say can only get worse when Iranian oil comes back on line and the fact that the summer-driving season is past its peak,” Flynn said in a note.

Market participants were still assessing the details of the Iranian nuclear accord and its impact on oil markets. The general consensus is it will take several months for any significant volumes of additional Iranian oil to hit the market. Oil prices appeared to find some support after eurozone finance ministers agreed to Greece’s request for a three-year bailout. That helped ease worries about risks to energy demand from Europe.