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