International benchmark Brent crude fell 44 cents to $51.51 per barrel at 2:35 p.m. ET, after hitting a session high of $52.29 a barrel.
U.S. West Texas Intermediate (WTI) was trading at $49.93 per barrel, down 42 cents from the last settlement, after hitting a sessions high of $50.58.
WTI hit a session low of $49.47, as trade volumes during the minute of 9:35 EDT (1335 GMT) soared to 12,932 barrels, the highest since Oct. 13.
Analysts said traders had accumulated a high number of long positions, and were looking to sell ahead of their contracts' expiry date as the U.S. Commodity Futures Trading Commission limits the number they can hold.
"There was a huge net speculative long position," said Kyle Cooper, analyst at ION Energy in Houston. "You do have to get out of positions."
Data from energy monitoring service Genscape showed a draw in crude stockpiles at the storage hub in Cushing, Oklahoma.
But traders said WTI was still under pressure from a report on Friday by oil services provider Baker Hughes, which showed U.S. drillers added four rigs in the week to Oct. 14.