All NRI production rates and volumes are based on working interest less 13% royalty volumes. (2) Free cash flow is calculated as (i) revenues less production expenses, general and administrative expense, annual abandonment funding and current income tax expense divided by (ii) the number of NRI barrels of oil sold. About VAALCO VAALCO, founded in 1985, is a Houston, USA based, independent energy company with production, development and exploration assets in the West African region.