If you said your intentions outloud and satan or one of his demons knew your intentions it makes no difference because the ‘One who is in you is greater’ (from somewhere in the Gospel according to John).
Whatever power satan may have he is not greater than God. Satan may be so rude and bold as to intrude in the sanctity of your prayers to God, but satan is very intelligent and can perceive a great deal from the exterior of any person though as you rightly say he cannot search your mind, though he can cause intrusive thoughts or delve into placing desires into souls by way of temptation.
So it makes no difference whether you keep your intention in your mind and offer it up to God or you say it outloud.
Personally I don’t give two hoots whether satan knows my intentions or not because it gives me great consolation to know that my prayers drive him wild with anger and please Jesus.
You might like to consider being bold and unafraid. Don’t bother fearing satan because you are in Christ and Christ is greater than all things as you know because He is Lord of all things.
I think though that unless you are saying a vocal prayer like the Most Holy Rosary or partaking in Holy Mass that you might like to consider persuing mental prayer more often as this is a deepening of the spirituality and your prayer life with Jesus.
Go on be bold and fearless and don’t let that evil father of lies interfere with your prayers to Jesus in any way, shape or form!

How satan would love to think that whenever you pray you are also thinking of his presence so vain and exhalted he thinks of himself.
Remember it is the Holy Spirit who prays in you, you say the prayer and God does the work. You know we can do nothing by our own efforts, you know that we are nothing and can do nothing but with God’s grace all work is done. So don’t worry about your own efforts, just trust in God that He will bring about what He desires to bring about by you offering Him your intentions. All God needs is your prayers prayed in the Holy Spirit and He does the rest.
Laugh at satan, I do, he isn’t worth anything else but derision, he may deem himself an enemy beyond match to humans but he is no match for God. So when you experience attack of any sort, stick your tongue out at him, ignore him (don’t converse with him) and laugh at him and hide yourself in the One who is all powerful, Jesus.
In my prayers