Gunmen seized the 14 workers at the start of the month on a road between the towns of Omoku and Elele, around 50 km (30 miles) from Port Harcourt.
Kidnapping for ransom is a common problem in some parts of Nigeria and the southern Delta energy hub has seen an increase in crime since the start of attacks by militants calling for more oil wealth to go to the impoverished region.
Gunmen seized the 14 workers at the start of the month on a road between the towns of Omoku and Elele, around 50 km (30 miles) from the city of Port Harcourt, Rivers state.
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The workers are employees of Nigerian oil company Nestoil.
"We secured their release in batches. Five were released yesterday while seven were released in the early hours of today," said Nnamdi Omoni, spokesman for police in Rivers state. The remaining two had been released earlier.
"As a result of our pressure on the kidnappers our men had a shootout with them but none of the kidnappers was killed or arrested," he said, without giving further details. "No ransom was paid."