A frac requires a lot of water and can use a combination of fresh and produced water. You need to have a written contract for sale of water, based on a per barrel price. Look at University Lands, Rate and Damage Schedule to see a current rate. Contract needs to specify well requirements including meter on every water well and monthly reports to you of volumes produced from each well. Water use will vary, depending on number and frequency of frac jobs. Is this an oil company limiting to its own use or someone who wants to operate a commercial operation and sell the water to third parties? Your contract needs to contain indemnification language to protect you in case of inuries, damages, claims etc. Do you want to limit number of wells? Can be very remunerative. Ask a lot of questions and for a contract to be reviewed by your attorney.