Kell, most states have a statute of limitation covering unpaid past royalties.  In LA. that is three years.  Your question is not sufficiently explicit to get a good answer.  You need to include the state where the minerals/well is located as mineral law is different in each.  Also the serial number, operator and well name, or section-township-range is in LA.  The site is divided up into groups and you'll likely get better answers if you post in the correct group.  That group may be a specific parish or county or it could be in a more general group such as Texas Mineral law.  You can review all the groups by clicking on the "All Groups" link at the top of the Main Page.