In the latest incident, unknown gunmen seized the two contractors, a Belgian and a Moroccan, in an ambush.
"We gathered that the two men were waylaid on a road in the city, forced out of their car and taken away. We don't know their identities," a spokesman for a major oil firm said.
Late on Tuesday four other foreign oil workers - two Norwegians and two Ukrainians - were seized offshore.
A week earlier a German and three Filipinos were also abducted.
Since the beginning of the year, separatist militants have stepped up their campaign for greater benefits from the oil proceeds for local communities and in protest against environmental devastation.
More than 35 expatriate oil workers have been abducted since then but released after spending days or weeks in captivity.
Nigeria, a nation of 130 million people, is the world's sixth biggest crude exporter with a daily output of 2.6 million barrels, a quarter of which is currently lost to unrest.
A militant group, the Movement for the Emancipation of the Niger Delta (MEND), notorious for series of kidnappings in the past, warned recently that it would carry out deadly attacks on the oil industry.