Iron Control Additives for Limestone and Sandstone Acidizing of Sweet and Sour Wells

Abstract

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^

A Systematic Study of Iron Control Chemicals Used During Well Stimulation

“…In the literature, it is generally stated that precipitation of iron hydroxide during acidizing will begin to occur at pH values of 2.2 and will be complete by pH 3.3 3,4 . The effect of temperature and pressure are not taken into account.…”

“…Iron-containing minerals in the formation can also react with the acid 3,4 . The formation brine itself may have significant quantities of iron in solution 8,9 .…”

“…Chelating chemicals such as citric acid or nitrilotriacetic acid (NTA) have been used to form iron (III) complexes which are soluble in spent acid 1,2 . Finally, reducing agents have been used to convert iron (III) to iron (II), which does not form a hydroxide precipitate at pH values of less than 6 3 .…”

“…Several experimental studies of iron control additives have been made 1,2,3,9 . In general, a concentrated acid solution containing iron and iron control additives is heated with marble chips (calcium carbonate) under pressure in a see-through cell.…”

Optimization of Hydrogen Sulfide Scavengers Used During Well Stimulation

“…An effective way to achieve this goal is to use a hydrogen sulfide scavenger and an iron control agent. 6 The effectiveness of iron control chemicals has been studied by several research groups [12][13][14][15] , and is not addressed in the present study.…”