Petersen double lifts Bremen
Bremen created a host of chances before Petersen finally broke the deadlock in the 85th, when Eljero Elia eluded two defenders to set him up.
Three minutes later, Elia did brilliantly again to ghost past two defenders and find Oezkan Yildirim on the left. Yildirim found Petersen, who let fly with a volley to the top right corner off the post.
Hannover goalkeeper Ron-Robert Zieler had made several key saves to keep the sides level.
The win takes Bremen up to 11th, one point behind Hannover, ahead of the rest of the weekend's games.