Corrosion Protection Of Sour Gas Wells With Chemical Inhibition
Authors: Yeganeh M. 
Published: December 2007

The importance of corrosion prevention by chemical inhibitors and material selection in M.I.S. gas wells containing 25–30% H2S, 9% CO2 and approximately 1 bbL of water per mmSCFd or 5.6 × 10-6 Vol/Vol of Gas, both during the design and production stages is examined in this paper.

A formalism to describe the system, its chemical inhibition programme as written during the design stage and carried out in the production period, and the system's response to the inhibition treatment has been made; with data showing the corrosivity of the system when inappropriate corrosion inhibitor was used.

Corrosion Monitoring methods employed since the start of production in 1970, both in tubings flow streams and annuli of the wells are described. Application of the newer and more modern methods in the later years, with stress on their effectiveness in evaluating the corrosivity of the system have been discussed; this includes graphical analysis of the results to display the efficiency of the inhibition programme. The conclusion reached is that the programme. The conclusion reached is that the chemical inhibition programme has been effective, and this trend will be continued in future and monitored.

Introduction

M.I.S., is an old oilfield in Southwest of Iran, and discovered in 1908. The Jurassic gas reservoir, some 4267 m deep was discovered in 1967/68. In 1968/ 69 five gas wells, numbers 309, 310, 312, and 314 were drilled in Jurassic to supply sour gas feed to Shahpour Petrochemical Plant some 120 km south of M.I.S., and an average 240–300 mmSCFd or 6.8 × 10 to the 6th to 8.5 × 10 to the 6th m3/d is produced, dehydrated and transported via a 20" line produced, dehydrated and transported via a 20" line to the chemical plant.

The operational importance of these wells made it necessary that close attention be given to the corrosion prevension program, both during the design and operation stages.

INTRODUCTION (Cont'd)

The system description is as follows:-

Field Area- M.I.S. Sour Gas Reservoir Formation- Jurassic Depth- 4267 m Closing Pressures:1. Bottom hole- 441.26 Bar 2. Wellhead- 310.26 Bar Flowing W.H.P.- 114.45 Bar B.H. Temperature- 137.7 degrees C W.H. Temperature- 48 degrees C Environment- 25–30% H2S (Volume percent)- 9–10% CO2 (Volume percent)- 19.6 × 10-6 Vol/Vol Condensate or 3.5 bbL per mmSCFd5.6 × 16-6 Vol/Vol water or 1 bbL per mmSCFd Producing zone Producing zone thickness- 182.88 m

CORROSION PREVENTION MEASURES DISCUSSED DURING THE DESIGN STAGE

It was obvious that an environment containing acid gases, moisture, and dissolved salts, when operated at high temperatures and pressures has a potential of initiating electrochemical reactions, potential of initiating electrochemical reactions, when metal cathode and anodes are present. And as the majority of metals, especially the ferrous alloys, are in a state of high activity level due to their inherent composition along with inherent and induced stresses, many prospective anode-cathode sites were hence, thought to be present on the surfaces of the tubing and wellhead equipment of the M.I.S. system, leading to corrosion and metal loss. It was then decided that a barrier to this natural reaction should be incorporated to reduce the metal loss and, hence, maintain the integrity of the system.