The hostages, who are employees of Swedish company Skanska, Peru's Construcciones Modulares and Peruvian gas company Coga, were building a new plant for gas from the huge Camisea field.
It is hoping this activity will win the attention of the biggest international oil firms, such as Royal Dutch Shell, which discovered Camisea but later sold its Peruvian holdings.
Mr Humala's strategy of focusing the campaign on these issues was helped when the gas pipeline from Camisea suffered a widely publicised rupture last month, the fifth since pumping began in August 2004.