All wells in the state, new and existing, should have a registration number beginning with 55- followed by six digits. A.R.S. § 45-593(B). For a new well, the registration number is assigned when the Notice of Intent to Drill or Application for a Permit to Drill is received by the Department. For existing wells, the registration is assigned when the Late Registration form is received. To understand how well registration numbers are assigned, read about the Wells 55 Index Numbering System.