Here is the full list of all 15 Portal achievements. It takes around 10-12 hours to unlock all of the achievements on Windows.
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.
Beat four Portal advanced maps.
Beat all six Portal advanced maps.
Complete Portal.
Fall 30,000 feet.
Knock down a turret with another turret.
Jump 300 feet.
Make the correct party escort submission position decision.
Do whatever it takes to survive.
Acquire the fully powered Aperture Science Handheld Portal Device.