Iron Sulfide Deposition in Sour Gas Wells: A Corrosion Induced Scale Problem

Abstract

Iron sulfide is one of the exotic scales formed in the oil and gas fields, particularly for those deep sour gas wells producing from high temperature and high pressure (HTHP) reservoirs. Compared to the conventional carbonate and sulphate scale, the mitigation of iron sulfide deposition is notoriously difficult. To develop a suitable mitigation strategy, it is essential to understand the formation mechanisms of iron sulfide in the given production system. In this work, we combined laboratorial tests, thermodynamic modelling, and field monitoring for the understanding the source of iron and the mechanisms of iron sulfide deposition in the sour gas well during acidizing treatment and production stage. Study results indicate that iron sulfide deposition in sour gas wells is a corrosion induced scaling problem. During acidizing treatment, high concentration of iron is released from tubular due to acid attack, despite corrosion inhibitor is used in the stimulation fluid package. Large amount of iron sulfide can precipitate when spent acid mixes and reacts with H2S in the reservoir and potentially causes severe formation damage. During the production stage, the iron released from tubular due to corrosion in produced water under high temperature and high pressure is the major contribution of iron sulfide deposited at the surface of tubing. These iron sulfide deposits, although appeared as porous layer, can protect the downhole completion from the highly corrosive fluids, which leads to the unexpected long service lives of mild carbon steel tubular in many wells. However, accelerated corrosion can occur when the protective iron sulfide film or deposit is disturbed or partial. This paper presents a fundamental study to understand the root cause of iron sulfide deposition in sour gas wells. Study results demonstrate that effective corrosion inhibition is key to mitigate the iron sulfide deposition problem in the sour gas wells.