The Potential Role of Pyrite Oxidation In Corrosion And Reservoir Souring
Authors: Hutcheon I. 
Published: March 2010

Pyrite (FeS2) is a common and ubiquitous constituent of reservoir rocks and forms geologically under extremely reducing conditions. Pyrite is easily oxidized by exposure to moderately oxidizing conditions, including de-oxygenated injection or process water. When pyrite is oxidized in water, the resulting products include sulphuric acid (H2SO4) and sulphate ion (SO42-). In the near well bore, the resulting waters are far from equilibrium with Portland type well cements. A small amount of pyrite oxidation can result in total removal of well cement and the replacement of the cement by structurally less stable products, such as calcium sulphate. In addition, the sulphuric acid may result in corrosion of well tubing or above ground facilities. Casing leaks, particularly in injection wells, may lead to failures of casing due to cement collapse, tubing corrosion, or both. In the reservoir, during water flooding or steam injection, pyrite oxidation may release sulphate ion (SO42 -) which can promote the activity of sulphate reducing bacteria, resulting in "souring" and the production of the acids gases H2S and CO2, where no production of acid gas was noted before injection. The results of pyrite oxidation are examined using reaction path modeling, in which the effects on reservoir fluids and rocks of these complex chemical reactions, including corrosion and scaling, can be determined.

Introduction

Pyrite (FeS2) is an ubiquitous constituent of carbonate and clastic reservoir rocks. Pyrite is particularly abundant in shales, and may be an unrecognized cause of well bore or casing failures in shaly intervals. Pyrite is only stable at very low oxidation states, partly depending on the pH and temperature, and consequently is very susceptible to oxidation during injection of fluids of relatively high oxidation state, such as steam or water from water floods. The objective of this paper is to show the possible consequences of pyrite oxidation for well bore cement stability and the effect on the composition of formation fluids. Chemical models are used to determine whether the changes in fluid chemistry can have significant effects on the stability of well cement (portlandite) and common reservoir minerals, such as calcite.

One of the consequences of oxidation of a sulphide mineral is the release of sulphate to the aqueous fluid. In the typical hydrocarbon reservoir environment this provides a sulphate source for anaerobes that reduce sulphate to sulphide, resulting in the creation of sour gas in water flooded reservoirs that previously were not subject to sour production. Limited data from steam and water flood operations suggest that pyrite oxidation may occur, leading to the possibility that such pilots may become subject to sour production as production proceeds.

Pyrite Oxidation and Cement Dissolution

The oxidation of pyrite does not require a source of "free" oxygen, but can utilize the oxygen in water. Observation of Reaction (1) below shows that, in addition to the release of sulphate, oxidation of pyrite will release hydrogen ions, producing acidic conditions. Effectively, the product of pyrite oxidation and dissolution is sulphuric acid.

Equation 1 (available in full paper)