Suggest Remedy For Kidney Stone

I have a 2mm kidney stone in the lower left kidney (not sure of the exact location) which was seen on an ultrasound. What are the chances of passing the stone without pain or even knowing it? I am drinking over 3 liters of water daily. I previously had a stone 5 years ago and ended up in the ER due to severe pain. I passed it in the ER. Also, to prevent future stones, what is your recommendations as to how much water to drink daily? thank you so much in advance. gmg