Iron Control in West Texas Sour-Gas Wells Provides Sustained Production Increases
Authors: Walker Michael L. , Dill Walter R. , Besler Monte R. , McFatridge Darrell G. 
Published: December 2007

Summary
Permian Basin operators have recorded sustained production increases bypreventing production increases by preventing precipitation of iron sulfide andother precipitation of iron sulfide and other sulfur-containing species. Thisimprovement has resulted largely from cleaning out tubing before acidizing andfrom preventing the precipitation of ferrous sulfide and the formation ofelemental sulfur by simultaneous use of iron chelants and sulfide-controlagents. Previously used methods gave only temporary production increases thatterminated production increases that terminated when iron dissolved by thestimulation acid reprecipitated in the pay zone and damaged the formation afterthe stimulation acid was spent. This paper describes a method to optimize ironsulfide control, methods to minimize reprecipitation. and case histories fromthe Permian Basin that show improved methods to control iron in sour-wellenvironments.

Introduction
Stimulating wells with acid was first reported in 1896. Acid was proposed tobe injected into the formation to dissolve the rock and to improve the flow ofoil to the wellbore area. This method offered advantages over the then currentstimulation method of "shooting" the well.
Although the use of an aggressive fluid, HCl, offers many advantages thathave resulted in its widespread use, it does have several disadvantages. Thispaper discusses the disadvantage of the high solubility of iron containingcompounds in HCl: the iron dissolved from the tubulars by the stimulation acidcan be redeposited as a precipitate in the pay zone, damaging the formationwhen the HCl is consumed.
Formation damage in sweet wells occurs when ferric ion is precipitated fromsolution. The iron compounds dissolved during an acid treatment in sweet wellscan place both ferric ion (Fe) and ferrous ion (Fe) in solution. The formationis damaged because the pH of spent acid is about 4.0 and the solubility offerric ion is very high in fluids with pH values below 2.5 and very low influids with pH values above about 3.5. (The values between 2.5 and 3.5 willhold some iron in solution, but to a lesser extent.) Ferrous ion does not causeproblems in sweet wells because its precipitation does not occur until a pH ofabout 7.0 is reached.
In recent years, several methods have been used successfully to control ironreprecipitation in sweet wells. Some common methods include the use ofbuffering agents to hold the pH of the fluid below 2.5, chelating agents toreact with the ferric ion to provide soluble complexes, reducing agents toprovide soluble complexes, reducing agents to modify the oxidation state offerric ion, and combinations of these methods. These systems have found utilitywhen acidizing fluids are applied to sweet wells where ferric ion has beendissolved, but the reprecipitation problem is not fully remedied if sulfidesare present. This results because the buffering systems, some chelating agents, and reducing agents fail to prevent reprecipitation of iron with H S to formiron sulfide and, in some cases, elemental sulfur.

Damage From Scale
Iron sulfide reprecipitation in the formation (from spent-acid solution) isthe most probable reason that acid jobs fail to achieve probable reason thatacid jobs fail to achieve sustained production in sour producing wells orincreased injectivity in injection wells that carry sulfides. The primarysource of the reprecipitated iron sulfide is iron containing sulfide scalesdissolved from the tubulars by the acidizing fluid.
Wells that produce or inject sulfidecontaining fluids contain iron sulfidescales or iron sulfide corrosion products. The type of iron sulfide depositeddepends on a number of considerations, including temperature, brine salinity, and the presence of other gases, such as CO2. Mackinawite (Fe S), troilite(FeS), or pyrrhotite (Fe S) is almost always found on tubular surfaces. Pyrite(FeS) and marcasite (FeS) are also Pyrite (FeS) and marcasite (FeS) are alsofrequently found. Another complication is that one or more types of ironsulfide will precipitate and undergo further reaction with precipitate andundergo further reaction with either H S or the iron surface to create layersof different compositions of iron sulfide. Each compound has its own specificsolubility. The general trend is that compounds with approximately one-to-onestoichiometry will be readily soluble and have rapid reaction rates with HCl, while compounds with higher sulfur stoichiometries will have lower solubilityand much slower reaction rates (Table 1). It is a mistake to assume that wellscontaining moderate to small amounts of H S will form less scale or corrosionproduct on the tubulars than wells with higher H S concentrations. What isknown is that most of these sulfide scales are soluble to some degree in acidicstimulation fluids. These scales or corrosion products can be redeposited inthe formation, products can be redeposited in the formation, causingdamage.
The magnitude of the problem is often not recognized. Iron sulfide scalescan react with HCl to an extent that effectively reduces the acid concentrationto less than 1% HCl content. These fluids, which are high in ferrous iron and HS content, will further spend when contacted with the formation containingcalcium carbonate or other acid-consuming species.
JPT
P. 603