Iron Control Provides Sustained Production Increase In Wells Containing Sour Gas
Authors: Walker Michael L. , Dill Walter R. , Besler Monte R. 
Published: March 2010

Abstract
Acid stimulation of wells containing hydrogen sulphide has in some cases resulted in short-lived production increases. The rapid decline in production has been thought to be related to reprecipitation of sulphur-containing species which reduce the flow of hydrocarbons when the well is placed on production. By using agents which interact with both iron ions in solution and hydrogen sulphide, precipitation of iron sulphide and elemental sulphur can be controlled and damage to the formation permeability can be reduced. The result of proper control of these sulphur-containing species is to provide the operator with sustained production increases after acidizing.
This paper will discuss the mechanisms of damage, current iron control methods, solutions to the deficiencies 0f the currently used methods, and case histories showing use of improved methods of iron control in hydrogen sulphide environments.

Introduction
The use of acid to improve the production of hydrocarbons from subterranean formations as an effective method of stimulation was introduced in 1896(1). In these processes, acid was injected into the formation to dissolve the rock matrix to improve the now of hydrocarbons from the formation to the wellbore area for removal.
Hydrochloric acid (HCl), an aggressive fluid, has many advantages which have resulted in its common use as a stimulation fluid. However, as such, an aggressive fluid does have some disadvantages. The major disadvantage pertinent to this paper is the high solubility in HCl solutions of iron containing minerals, many of which will reprecipitate when the acid content of the fluid decreases(2).
Several methods have been used with success in controlling the repredpitation of iron in sweet wells. Some more commonly used materials have included buffering agents, chelating agents, reducing agents, and combinations of the above systems, These systems are designed to control the reprecipitation of ferric ions as hydrated ferric oxides. 'This ion has great solubility in fluids with pH values lower than about 2.5, but rapidly becomes insoluble in fluids with pH values above 2.5(3). This problem occurs because the normal pH of a spent acid fluid is about 4.0.
Buffering agents can maintain a high acid content of the spent acid fluid at pH values below 2.5 for some time, and when used with chelating agents, have allowed spent acid fluids to retain high concentrations of ferric ion in solution(4,5). Reducing agents chemically convert ferric ion to ferrous ion(6,7,8). Ferrous ion does not precipitate at the pH of spent acid fluid. These systems have been shown to be effective in sweet wells; however, the problem becomes severe when sulphides are present. 'This is due to the failure of buffering systems, reducing agents, and some chelating agents to prevent the reprecipitation and reaction of iron solution species with hydrogen sulphide.

Scale Sources
The major source of possible damage to sour gas wells after acidizing is the reprecipitation of iron sulphide in the formation from the spent acid fluid. The source of this compound is from the redissolution, by the stimulation acid, of existing iron-containing sulphides on the tubular(9).