Be very careful here. As others have pointed out, lending based on projected future production is a highly risky proposition. Not only will production steadily decline, but oil and gas prices are highly volatile (as we have recently seen). To make matters even worse, your client is a royalty owner and, as such, has no control over the operation of the well(s) in question.
There are only a dozen or so banks in the US that do oil and gas based lending and they typically have in-house engineers to help analyze the wells, and even then will only loan on a fraction (~60%) of the value.