Riise has played two matches for Pompey since he moved to Fratton Park last month on a temporary deal which will expire in early November.
He told the club's official website: "Hopefully I can play my part while I'm here
"I want to do as well as possible during my time at Pompey and help the club climb the table.
"I want to play as many games as I can while I'm here. The only way I'm going to do that is by offering something on the pitch.
"If I can do that then hopefully I can stay at Pompey for longer, because I'm enjoying my time here."