When Opec meets in Vienna on 25 May the group will be facing tougher challenges than it had hoped for in the aftermath of the previous gathering on 30 November. The six-month output curb it agreed on then hasn’t done the trick: inventories, though falling, remain stubbornly high. Despite strong compliance, pressure is mounting for Opec to take firmer action.

But that’s easier said than done. Internal tensions within the grouping – and the 11 non-Opec countries cutting in...