Yes, you could pray to St. Raphael, or to any other Saint and even to God, in any way in which you choose. You can use any words and you can be as formal or informal as you wish.
But in the Catholic tradition, we have a very long history of pre-written prayers. Those prayers not only teach us theology and how to pray, but they can also be a special way in which to lift our minds to God, and to dispose ourselves to the graces we seek.
I would therefore recommend at least trying to be a bit more formal, and not to limit yourself simply to spontaneous prayers. Yes, we can speak to the Saints and to God as if they are our friends, but they’re also a bit more than just invisible pals. Especially in the case of God, we do need to approach Him with at least some sense of His majesty.