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
Iron precipitation has been recognized as a significant problem in acidizing operations for over 30 years. Deposits of iron compounds in tubulars combined with iron compounds in the formation determine the amount of iron dissolved by the acid and therefore, the severity of the iron problem. Data from the analysis of recovered acid on previous jobs can be used to predict the concentration of iron that may need to be sequestered on future jobs. Iron sequestering additives may present compatibility problems. Specific types of compatibility problems may be encountered when acidizing limestone formations with HCl, while other compatibility problems may be encountered when acidizing sandstone formation with HCl-HF blends. Iron sequestering additives may lose effectiveness because of degradation. Degradation properties of less stable additives are discussed. The iron sequestering additives currently used are described and their effectiveness is compared using laboratory test data. Iron sequestering in the presence of sulfides present unique problems which are explained. A system for sequestering iron in the presence of sulfides is described. Job descriptions and results from acidizing operations are presented. Introduction The amount of iron that needs to be sequestered in an acidizing operation is difficult to accurately predict because of the influence of variables such as those listed below.Condition of the tubularsAmount of tubular surface areaType of iron compoundsTemperatureType and concentration of acidContact timeType of productionAmount of iron in the formationOther acid-reactive components of the formation. Millscale in New Pipe Several studies of the amount of iron dissolved in acidizing operations have been made. One study of millscale (FeO Fe2O3) in new 2-7/8 in., 6.5 lb/ft tubing indicated that 690 gal of 15% HCl were required to react with the millscale in 10,000 ft of pipe. The acid would contain 85,938 mg/L total iron if this iron remained in solution. The total iron would contain 57,292 mg/L ferric iron, Fe(III), and 28,646 mg/L ferrous iron, Fe(II), Table 1. Iron Compounds in Formation Iron can be found in the matrix of the formation as a component of hematite, magnetite, pyrite, siderite, chlorite clay, and mixed-layer clay. These compounds can contain both Fe(II) and Fe(III). Table 2 shows the oxidation state of the iron in the formation components. An analysis of formation waters show 57 mg/L to 2075 mg/L of ferrous iron may be in solution. The pH of the water samples ranged from 3.1 to 6.0, Table 3. Ferrous iron remained in solution in formation water at the pH of the natural environment. Ferrous iron can precipitate as the hydroxide at a pH of about 7.5. Iron in Recovered Spent Acid Solutions A study was conducted on 17 injection wells. P. 131^
Investigative results from laboratory tests and field jobs show that iron presents a significant and complex problem in stimulation operations. Non-acidic or weakly acidic fracturing fluids are not detrimental from the aspect of dissolving and reprecipitating iron compounds. However, fracturing fluids usually contain a significant cancentration of dissolved and entrained oxygen which makes the fluid incompatible with formation water that contains ferrous ions in solution. The problem presented by an acidizing fluid differs from that presented by non-acidic or weakly acidic fracturing fluid. Acid dissolves iron compounds from equipment and flow lines as it is mixed and pumped to the formation. Acid may dissolve additional iron as it reacts with the formation. If acid does not contain an effective iron control system, dissolved iron precipitates. This precipitate may then accumulate as it is carried toward the wellbore during flowback. This accumulation of solids may plug natural and created permeability and have a detrimental effect on the recovery of the treating fluid and production. Chemicals and techniques that help ensure the compatibility of fracturing fluids and formation water are described. Iron control systems used in acidizing have properties that influence their effectiveness. The properties and effectiveness of the various systems are described so that a judicial selection of the most effective system can be made. Introduction Iron control problems are sometimes encountered when stimulating operations are conducted with weakly acidic or nonacidic fracturing fluids. The fracturing fluid ordinarily does not contain enough acid to dissolve iron and iron compounds from the well equipment or formation. Fracturing fluids, because of blending operations and viscosity of gelled fracturing fluids, will contain dissolved and entrained oxygen, which makes the fracturing fluid incompatible with formation waters that contain ferrous iron, Fe(2). The fracturing fluid mixes with the formation water. Oxygen in the fracturing fluid immediately oxidizes the ferrous iron, Fe (2), to ferric iron, Fe(3). The Fe(2) will remain in solution at pH levels up to about 7.5. The oxidation product, Fe(3), starts to precipitate at a pH of 2.5 and has completely precipitated when the pH reaches 3.5. The pH of the natural environment is usually greater than 3.5 and precipitation of Fe(3) occurs as the Fe(3) is formed by oxidation. Acidizing operations are usually conducted with highly acidic solutions. Acid corrosion inhibitors inhibit the reaction of acid on the metal contacted; however, they do not inhibit the reaction of the acid with iron compounds (rust, mill scale, siderite, and other iron compounds) to an appreciable extent. Acid dissolves these iron compounds as it is mixed and pumped through tubular goods to the formation. Iron content of the acid may possibly exceed 100 000 mg/I by the time it reaches the formation. This will depend on the condition of the pipe, the amount of pipe surface area contacted, concentration of acid used, and the temperature. Conditions may dictate a pre-clean job prior to the formation stimulation operation. This acid, containing iron in solution, flows into the formation, and dissolves additional iron before it is spent.
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).
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