Jamieson, Fausset and Brown on Genesis 21:25-31

25-31. And Abraham reproved Abimelech because of a well--Wells were of great importance to a pastoral chief and on the successful operation of sinking a new one, the owner was solemnly informed in person. If, however, they were allowed to get out of repair, the restorer acquired a right to them. In unoccupied lands the possession of wells gave a right of property in the land, and dread of this had caused the offense for which Abraham reproved Abimelech. Some describe four, others five, wells in Beer-sheba.