Sour Environments: Protecting Production and Equipment-Case Studies
Authors: Nasr-El-Din H.A. , Metcalf A.S. , Luft H.B. 
Published: March 2010

Abstract
Acid stimulation of sour wells, including gas, oil, and water injection, presents special challenges. Add high temperatures and specialty equipment, and sometimes permanent production damage can result. Avoidance of damaging precipitates from unwanted reactions, which can permanently plug a producing interval while providing protection of all production equipment, is sometimes a tenuous balance of design.
Precipitation of elemental sulphur, reaction products from incompatible additives used in an acid stimulation fluid, and reprecipitation of iron sulfide scale dissolved during the pumping of acid through corroded tubulars are possible sources of potential permeability restrictions. Corrosion protection to the proper level of weight loss [&lt; 0.0976 to 0.2441 kg/m2 (0.02 to 0.05 lb/ ft2) over the anticipated exposure time period] and no localized surface defects (pitting and stress cracking) both in live and spent acid solutions are essential to protecting the production equipment and treating tubulars.
This paper reviews two case studies. The first from Saudi Arabia discusses damage to the outside surface of a coiled tubing string due to acid and H2S corrosion. A Canadian case history describing a mechanical failure of coiled tubing as the result of hydrogen embrittlement is the second case study presented. Guidelines are offered for coiled tubing usage to obtain successful stimulation while controlling risks of tubing failures.

Introduction
Types of workover treatments vary from conventional overbalanced acid fracturing and bullhead matrix acid jobs to underbalanced washing or wellbore cleanout operations. When treatments are being performed underbalanced or when iron sulfide scales are being dissolved, the coiled tubing and the metal tubulars will be exposed to H2S. Another occasion of concern is during the recovery of spent or partially spent acids, which have been depleted of inhibitors(1).
Equipment failures do occur during workover operations; Schlumberger(2) reported from 1995 to 1998 that 33% of their coiled tubing failures were due to corrosion. The corrosion mentioned being broken out into storage, acid, and H2S. BJ Services found a similar 31% failure rate due to corrosion from 1996 to 2001(3). In 1998, 3,200 coiled tubing strings were produced with a 10% per annum growth rate(4). As of 2002, worldwide 1,043 coiled tubing units were in service, with approximately half of them in Canada and the United States(5).
This paper covers testing essential to providing a safe environment where both equipment and production of the well are protected during a workover. Case histories will be provided to substantiate needs for guidelines of tubing usage in the treating of sour wells. Finally, a set of guidelines are presented to provide a checklist of decision points that should be considered prior to rigging up and pumping fluids where H2S may contaminate the wellbore fluids.

Background

Acid Corrosion Issues in Sour Environments
Corrosion protection is typically requested over a variety of exposure times, temperatures, and fluid conditions.