Iron sulfide scale causes many operational problems in the oil industry. For example, accumulation of iron sulfide particles around the wellbore area can result in loss of injectivity or productivity of the wells. Hydrochloric acid can be used to remove some iron sulfide deposits. However, this process produces hydrogen sulfide; a toxic and corrosive gas. In addition, hydrogen sulfide reacts with ferric ions and precipitates elemental sulfur. Elemental sulfur is insoluble in acids, practically impossible to dissolve, corrosive, and can damage the formation. Hydrogen sulfide reacts with ferrous ion and re-precipitates iron sulfide at pH values greater than 2.

An experimental study was conducted to assess acid reaction with iron sulfide. Iron sulfide sticks were ground, sieved and particles of less than 75 μm in diameter were used. Hydrochloric acid was used at various concentrations up to 20 wt%. The acid was added to iron sulfide particles at weight ratios of 5:1 to 20:1. The amount of hydrogen sulfide released was determined using 10 wt% cadmium sulfate solutions. The effects of oilfield chemicals (iron control agents, hydrogen sulfide scavengers, nonionic and anionic surfactants, corrosion inhibitors, clay stabilizers and biocides) on acid reaction with iron sulfide were studied over a wide range of parameters. The reaction temperature was kept constant at 25 °C.

Experimental results indicated that several acid additives adversely affected the reaction between iron sulfide and hydrochloric acid. Corrosion inhibitors (filming amines) formed a thin film on iron sulfide particles. This film acted as a diffusion barrier, which minimized acid contact with iron sulfide. Clay stabilizers (cationic polymers) did affect acid reaction with iron sulfide at high concentrations. Mutual solvent and methanol did not affect acid reaction with iron sulfide. Hydrogen sulfide scavengers (aldehyde-based chemicals) inhibited acid reaction with iron sulfide when they were added to the acid at concentrations greater than 2 wt%.

The results of this work highlight new physico-chemical interactions that can affect acid reactions with iron sulfide. This paper gives procedures to maximize the results of acid treatments in the field.

Subject
Acids, Hydrogen sulfide scavengers, Acid concentration, Metal surfaces, Iron sulfides, Additives, Iron, Surfactants, Solvents, Corrosion reactions, Corrosion inhibitors, Methanol, Chemical reactions
Keywords:
iron sulfide, scale, hydrogen sulfide scavengers
© 2000 Association for Materials Protection and Performance (AMPP). All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of AMPP. Positions and opinions advanced in this work are those of the author(s) and not necessarily those of AMPP. Responsibility for the content of the work lies solely with the author(s).
2000
Association for Materials Protection and Performance (AMPP)
You do not currently have access to this content.