Yes, you can do them in your head, if it is safer for you that way. Be sure to focus like a laser and speak it mentally with intent, as this makes it stronger. You probably know this but I thought I 'd just put a reminder.

Hail Satan.
Hey can I say affirmation, mantras, vibration, or reverse ritual, etc. in my head instead of speaking out loud?
Thank you
Hail Satan!