Predicting Oxygen Depletion in Reservoir Environments

Abstract

ABSTRACT Iron pyrite (FeS2; ferric sulfide; "Fool’s Gold") is one of the most common of all naturally occurring minerals in sedimentary rock. Any reservoir process involving oxygen (air) might be affected by the aqueous oxidation of pyrite in the reservoir. During this reaction, one mole of pyrite can consume 3.75 moles of oxygen, or the equivalent of all the oxygen present in 6,469 SCF (183 m3) of air. Reservoir pH may be reduced significantly by the major reaction product, sulfuric acid. The precipitation of ferrous hydroxide may cause permeability reduction. This paper discusses the case history of a pilot air storage test, the results of laboratory studies which identified pyrite as the prime suspect for the oxygen loss during the test, the reaction stoichiometry for the reservoir environment studied, laboratory equipment and procedures used to test synthetic sand-pyrite mixtures and core plugs for oxygen consumption, and the empirical relationship which was derived to predict oxygen consumption.