U.S. oil prices are falling well behind their international rivals, as booming shale production has created pipeline constraints, driving the biggest discount to North Sea Brent in three years.

On May 31, U.S. benchmark West Texas Intermediate’s (WTI) discount to Brent crude moved above $11 a barrel (bbl), a level not seen since 2015, in the latest sign inland U.S. crude markets have become swamped by rampant production.

Brent was near $78/bbl while WTI traded closer to $66, before recovering.