Here is the full list of all 15 Portal achievements. It takes around 8-10 hours to unlock all of the achievements on Windows.
Beat four Portal advanced maps.
Beat all six Portal advanced maps.
Earn bronze medals on all Portal challenges.
Earn silver medals on all Portal challenges.
Earn gold medals on all Portal challenges.
Detach security cameras from the walls.
..?
Beat two Portal advanced maps.
Complete Portal.
Jump 300 feet.
Make the correct party escort submission position decision.
Fall 30,000 feet.
Do whatever it takes to survive.
Knock down a turret with another turret.
Acquire the fully powered Aperture Science Handheld Portal Device.