IP RESEARCH REPORT THE EFFECT OF NITRATE ANTI-SOURING TREATMENT ON CORROSION OF MILD STEEL

IP RESEARCH REPORT THE EFFECT OF NITRATE ANTI-SOURING TREATMENT ON CORROSION OF MILD STEEL

IP RESEARCH REPORT THE EFFECT OF NITRATE ANTI-SOURING TREATMENT ON CORROSION OF MILD STEEL November 2006 Published by ENERGY INSTITUTE, LONDON The Energy Institute is a professional membership body incorporated by Royal Charter 2003 Registered charity number 1097899

The Energy Institute gratefully acknowledges the financial contributions towards the scientific and technical programme from the following companies: BG Group BHP Billiton Limited BP Exploration Operating Co Ltd BP Oil UK Ltd Chevron ConocoPhillips Ltd ENI ExxonMobil International Ltd Kuwait Petroleum International Ltd Maersk Oil North Sea UK Limited Murco Petroleum Ltd Nexen Shell UK Oil Products Limited Shell U.K. Exploration and Production Ltd Statoil (U.K.) Limited Talisman Energy (UK) Ltd Total E&P UK plc Total UK Limited Copyright 2006 by the Energy Institute, London: The Energy Institute is a professional membership body incorporated by Royal Charter 2003. Registered charity number 1097899, England All rights reserved No part of this book may be reproduced by any means, or transmitted or translated into a machine language without the written permission of the publisher. The information contained in this publication is provided as guidance only and while every reasonable care has been taken to ensure the accuracy of its contents, the Energy Institute cannot accept any responsibility for any action taken, or not taken, on the basis of this information. The Energy Institute shall not be liable to any person for any loss or damage which may arise from the use of any of the information contained in any of its publications. The above disclaimer is not intended to restrict or exclude liability for death or personal injury caused by own negligence. ISBN 978 0 85293 458 6 Published by the Energy Institute Further copies can be obtained from Portland Customer Services, Commerce Way, Whitehall Industrial Estate, Colchester CO2 8HP, UK. Tel: +44 (0) 1206 796 351 email: sales@portland-services.com

CONTENTS Acknowledgements... vii Page Executive summary... viii 1 Introduction and project objectives... 1 1.1 Project objectives... 2 2 Technical background... 3 2.1 Effect of nitrate/nitrite biological treatment... 3 2.2 The denitrifying bacteria... 4 2.3 - Sulphide removal in the presence of NO 3- /NO 2... 4 2.4 The effect of nitrate/nitrite treatment on corrosion... 4 2.5 The detrimental role of nitrite... 6 3 Environmental, health and safety considerations... 7 4 Financial considerations... 9 5 Experimental work... 11 5.1 Preliminary tests... 11 5.2 Water composition... 12 5.3 Replenishment model... 12 5.4 Shear stress modelling... 13 6 Results and discussion... 15 6.1 Worst-case scenario... 15 6.2 Microbial enumeration... 16 6.3 Ammonia production... 17 6.4 Thiosulphate measurements... 17 6.5 Effect on redox potential (Eh)... 17 6.6 Effect on ph... 17 6.7 Nitrate monitoring... 17 v

Contents Cont... Page 6.8 Biological generation of nitrite... 18 6.9 Carbon source and sulphate... 18 6.10 Biological generation of sulphide... 18 6.11 Weight loss corrosion measurements... 19 6.12 Surface film (ESEM/EDX and XRD analyses)... 20 6.13 Microscopic examination... 21 6.14 LPR corrosion rate... 21 6.15 Electrochemical impedance spectroscopy... 22 6.16 Electrochemical noise measurements... 22 7 Summary of findings... 23 7.1 Corrosion rate effects... 23 7.2 Surface film effects... 24 7.3 Biochemical effects... 24 8 Conclusions... 27 9 Guidelines with respect to corrosion control during nitrate treatment... 29 10 Glossary of specialist terms... 31 11 References... 33 Figures... 35-52 Annexes Annex A: ph Adjustment procedures... 53 Annex B: Sulphide testing and calibration curve... 55 vi

ACKNOWLEDGEMENTS The Energy Institute wishes to record its appreciation of the work carried out by Dr Tony Rizk of CAPCIS Ltd (currently works for Saudi Aramco) as principal author of The effect of nitrate anti-souring treatment on corrosion of mild steel. This project was steered through to completion by the Energy Institute s Microbiology Committee, chaired by Mr Graham Hill of ECHA Microbiology Ltd. Members of the Microbiology Committee include: Ms P A Carberry Prof M Coleman Dr B Crook Dr C Devine Dr B Dunsmore Dr B N Herbert Mr G Hill Ms K Hoad Mr D Kadlecek Mr A Kitson-Smith Dr J W J Koenig Ms E McFarlane Mr M Maeso Dr T Rizk Dr J Stott QinetiQ The University of Reading Health & Safety Executive Commercial Microbiology Ltd Oilplus Ltd Consultant ECHA Microbiology Ltd (Chairman) Energy Institute ExxonMobil Aviation International Ltd ExxonMobil Aviation International Ltd Consultant Shell Energy Institute CAPCIS Ltd CAPCIS Ltd Further contributions were provided by: Mr D Thrasher BP The Energy Institute would also like to thank the following companies for their input: CAPCIS Shell Yara vii

1 INTRODUCTION AND PROJECT OBJECTIVES The last few years have seen a major shift toward nitrate treatment for the selective control of sulphate-reducing bacteria (SRB) and associated biogenic generation of hydrogen sulphide (H 2 S). The change in operator practice was boosted by the recent field successes reported in the North Sea. Currently, nitrate is used by many operators and in almost all major regions of oil production including North America, South America, Middle East and additional operators in the North Sea. Carbon steel is still by far the most commonly used material in injection systems and the petroleum industry in general. Highly toxic organic biocides have been the traditional method of controlling corrosion in the injector. However, tougher health and safety regulations and increasing produced water re-injection (PWRI) practices have brought new challenges to, in many cases, ageing installations. The use of nitrate to both prevent and mitigate biogenic reservoir souring in the water zones of oil reservoirs has raised concern about the possible effect on the integrity of tubulars. This was further fuelled by the conflicting information available on the effect of nitrate on corrosion. The Microbiology Committee of the Energy Institute (previously the Institute of Petroleum) commissioned this multi-phase study to investigate the effect of nitrate on corrosion. If unquantified, the concern about corrosion could lead to undesirable practices such as the concurrent injection of biocide (to control injector corrosion) and nitrate (to control reservoir souring). Such a practice would divest the nitrate technology of its two most valuable features of being environmentally friendly and comparatively inexpensive. This independent study was a part of the Microbiology Committee s leading role in promoting good practices, greener alternatives and improved understanding of industrial microbiology. It was conducted in three consecutive stages over a three-year period: Phase I was an assessment of the worst-case scenario in terms of ph and carbon availability on nitrite generation. In Phase II, the effect of the worst-case scenario on corrosion obtained from Phase I, was further evaluated under conditions of continuous fluid replenishment. Phase III was an assessment of the worst-case scenario and continuous replenishment on corrosion under the simulated hydrodynamic conditions of the injector including shear stress (t w ). Both real seawater and real produced water were used in the investigation programme. The produced water was provided by an oil company s production field in the North Sea and selected because it was free of additives that could confuse the work and affect bacterial activity. The study also assessed the effect of various PWRI scenarios including: 1. Control (nitrate-free), SRB population. 2. Continuously nitrate-treated from onset. 1

THE EFFECT OF NITRATE ANTI-SOURING TREATMENT ON CORROSION OF MILD STEEL 3. Delayed treatment, nitrate addition to an SRB biofilm. 4. Interrupted nitrate treatment. This document provides a summary of the experimental work and findings with guidelines to operators on the use of the nitrate to combat SRB and associated reservoir souring. For further details of the experimental work, please refer to the interim reports produced at the end of each phase. Phase III is regarded the most representative of field conditions and as such it is given more weight ininterpreting the data particularly when some discrepancy between the results of different phases was observed. 1.1 PROJECT OBJECTIVES The aim of the evaluation programme was to investigate the effect of nitrate anti-souring treatment on the corrosion of carbon steel injection pipework. The study was supported by chemical and microbial analyses to ensure an active microbial population representative of the commingled produced water and seawater. 2