Shell says a fire has forced it to close a key oil pipeline feeding Nigeria's strategic Bonny Export Terminal, which militants attacked last week.
The ongoing challenges are losing oil multinationals billions of dollars in what used to be Africa's biggest petroleum producer.
SBM Intelligence risk analysts estimate that renewed militant attacks, low oil prices and weak refinery margins have cost Dutch-British Shell and U.S.-based Chevron and ExxonMobil $7.1 billion in the first half of the year, representing about 70 percent of earnings.
Shell spokesman Precious Okolobo says the Trans Niger Pipeline was shut down Monday to investigate a fire.
Shell has refused to comment on reports that militants bombed its Bonny crude pipeline Friday, crippling exports days after they resumed following months of repairs from a May bomb attack.