CO2H
Polyaminocarboxylic acids
HO2C HO2C
N N CO2H
Ethylenediaminetetraacetic acid (EDTA)
HO
Hydroxyaminopolycarboxylic acids (HACAs)
N CO2H CO2H
Hydroxyethyliminodiacetic acid (HEIDA)
HO2C N HO2C
N
CO2H N CO2H CO2H
Diethylenetraminepentaacetic acid (DTPA)
CO2H HO HO2C
Hydroxyethylethylenediaminetriacetic acid (HEDTA)
> Chelants. Typical chelants used in the oil field include both polyaminocarboxylic acids and hydroxyaminopolycarboxylic acids (HACAs). These compounds consist of one to three nitrogen atoms surrounded by either carboxylic [CO2H] groups (EDTA and DTPA) or carboxylic and hydroxyl [HO] groups (HEIDA and HEDTA). Molecular weights range from 177 for HEDTA to 393 for DTPA.
N N CO2H
The chelants typically used in oilfield services are complex organic acids (left).9
These
compounds not only bind metals, but also are active dissolution agents in acidizing reactions. Well stimulation with chelants yields several advantages, including retarded reaction rates, low corrosion rates and improved health, safety and environmental benefits. While chelants such as ethylenediaminetetraacetic acid (EDTA) have been widely used for control of iron precipi - tation, hydroxyaminopolycarboxylic acid (HACA) chelants have the additional advantage of high acid solubility, and their primary role is matrix acidizing.
with the carbonate rock, acid-in-oil emulsions have other advantages. Their relatively high viscosity improves distribution in heterogeneous reservoirs, and since the acid does not have direct contact with well tubulars, corrosion is reduced. Although emulsified acid systems have been commonly used for matrix acidizing of carbonates below 93°C, laboratory data indicate
that they can be extended to higher tempera - tures if properly formulated.
0.16 0.18
0.02 0.04 0.06 0.08 0.10 0.12 0.14
0 HEDTA HCl Mud acid
> Corrosion testing. Four-hour corrosion tests at 350˚F were performed on two metallurgies with three acid-stimulation components—a 20% by volume sodium HEDTA chelant, a 15% by volume HCl and a 9-to-1 mud acid (9% by weight HCl to 1% by weight HF). Corrosion rates for the chelant are very low at 0.01 lbm/ft2 [0.049 kg/m2] for both chrome and nickel steels. In contrast, corrosion rates using conventional HCl and HF treatments are 5 to 10 times higher for these metals.
80 Nickel steel 13 Chrome steel
The Schlumberger acid-oil emulsion formulation—called the SXE-HT system—was developed for high-temperature acidizing in carbonate reservoirs. It consists of an acid phase, containing a corrosion inhibitor, and a diesel-oil phase with an emulsifier. These two mixtures are combined at high shear rates to form an oil- external acid emulsion. Laboratory data on the physical properties of this formulation show low corrosion and pitting for a variety of metals, high viscosity retention even up to 177°C [350°F] and good emulsion stability. For example, a typical SXE-HT emulsion is stable for at least two hours at 149°C [300°F], and this stability time can be prolonged by increasing the emulsifier concen - tration. Tests on limestone cores with the SXE-HT fluid at 135°C [275°F] confirm its ability to create wormholes at typical injection rates. Use of a properly formulated acid-oil emulsion is one solution for well stimulation at high temperature. Another approach is to consider a completely different type of reservoir acidizing fluid. Data confirm that a different class of chemicals—chelants—allow well stimulation at conditions that preclude the use of mineral acids. The term chelation is derived from the Greek word meaning claw, and chelants are often used to bind, sequester or capture other molecules— typically metals. Although these agents have been used frequently in the past to control metals or in some cases to dissolve scale, their new focus is well stimulation at elevated temperatures.
The slower reaction rates exhibited by the HACA chelants at high temperatures have important implications. In carbonates, slower rates allow efficient wormhole creation, while in sandstones there is less possibility of damage to sensitive formations. Low corrosion is another important characteristic of HACA chelants. For example, at high temperature, hydroxyethyl - ethylenediaminetriacetic acid (HEDTA) exhibits corrosion rates up to an order of magnitude lower than those of conventional mineral acids (below left).10
Significant health and environ mental benefits include lower toxicity, reduced need for return fluid neutralization and lower concentrations of corrosion products in these fluids. Of all these advantages of HACA chelants, however, the most important may be slower reaction rates at elevated temperatures. Coreflood testing in carbonates at elevated temperatures demon strates the advantage of using a chelant rather than HCl to create an efficient wormhole network (next page).11 Another gauge of chelant effectiveness in carbonates versus that of HCl is the amount of acid required to penetrate a formation—as measured by pore volumes to core breakthrough (PVBT). In one simulation that was scaled up from laboratory data, PVBT values for HCl and HEDTA were predicted for acidizing a carbonate formation at a depth of 2,185 m [7,170 ft], a bottomhole temperature of 177°C, and with damage that extended 0.3 m [1 ft] from the wellbore.12
At a pump rate of 0.95 m3/min
[6 bbl/min], the simulation predicted that the PVBT for HCl was nearly 100 times that for HEDTA—indicating low acidizing efficiency for HCl at high temperature.
As in carbonates, use of HACA chelants in sandstones offers a way to avoid the rapid reaction rates that lead to precipitation. Laboratory tests on West African sandstone with an HACA chelant confirm that proposition.
56
Oilfield Review
Corrosion rate, lbm/ft2
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