104 March 2011 SPE Journal

An Environmentally Friendly Stimulation Fluid for High-Temperature Applications

J.N. LePage, SPE, C.A. De Wolf, SPE, and J.H. Bemelaar, AkzoNobel; and H.A. Nasr-El-Din, SPE, Texas A&M University

Copyright 2011 Society of Petroleum Engineers

This paper (SPE 121709) was accepted for presentation at the SPE International Symposium on Oilfield Chemistry, The Woodlands, Texas, USA, 2022 April 2009, and revised for publication. Original manuscript received for review 23 February 2009. Revised manuscript received for review 1 March 2010. Paper peer approved 8 March 2010.

SummaryMatrix acidizing is used in carbonate formations to create flow channels from the formation to the wellbore; in sandstone forma-tions, however, the goal is to dissolve materials that impair well performance. However, the use of acids in deep wells has some major drawbacks, including high reaction rate and corrosion to well tubulars.

We have discovered a new stimulation chemical that can be used as a replacement for or in combination with acid treatments in deep wells. A polyacid whose structure allows for acidification is described. The polyacidic chelate L-glutamic acid, N, N-diacetic acid (GLDA) is manufactured from L-glutamic acid (MSG). The chelate-based fluid very effectively dissolves CaCO3, and it is less corrosive to the equipment and easy to handle.

This paper discusses the reaction of the new chelate, GLDA, with calcite and compares its performance with other available che-lates, including ethylenediaminetetraacetic acid (EDTA), hydroxy-ethylethylenediaminetriacetic acid (HEDTA), nitrilotriacetic acid (NTA), and ethanoldiglycine (EDG). GLDA dissolves calcite over a wide pH interval, although it is less effective than HEDTA at pH > 5. A unique property of GLDA is its high solubility; solutions exceeding 40 wt% can be achieved at a pH of approximately 2, whereas HEDTA solubility is limited to approximately 10 wt%. A mole of GLDA with a natural pH of approximately 1.5 is capable of dissolving up to two moles of CaCO3. Throughout the pH range, GLDA appears to be as thermally stable as HEDTA.

As an additive to hydrochloric acid (HCl), GLDA is as effective as HEDTA in preventing precipitation of moderate levels of Fe3+ in spent acids. At high Fe3+ concentrations, GLDA is slightly less effective than HEDTA on a molar basis; but, to deal with high Fe3+ levels, GLDA may be better because significantly higher concen-trations of it are possible in various acids. In 28 wt% HCl, HEDTA has limited solubility while GLDAs solubility exceeds 40 wt%.

From an environmental standpoint, GLDA is readily biodegrad-able and is made from a renewable raw material, monosodium glutamate. GLDA has low toxicity and aquatic toxicity character-istics. As a replacement for HCl, GLDA is significantly safer and less corrosive.

IntroductionThere are two principal means employed for solubilizing calcite scale downhole: acidizing, which acts on carbonate, and chelation, which acts on calcium ions (Ca2+). The most common means employed is the use of mineral acid solutions, such as 15 wt% HCl and, to a lesser extent, solutions of acetic and formic acids. Under acidic conditions of pH < 4, the solubilization of calcite principally proceeds through pro-tonation of carbonate/bicarbonate ions to CO2 by means of Eq. 1:

2H CaCO H Ca HCO Ca CO H O32 2

2 2+ + + ++ + + +3

1.

. . . . . . . . . . . . . . . . . . . . . . . . (1)

The Ca2+ salt of chloride (or the counter-ion of the acid used) should remain soluble under the treatment conditions. A primary advantage of HCl is its significant dissolving capacity for calcite,

whereby 1 gal of 15 wt% HCl can dissolve nearly 1.8 lbm of CaCO3 (Frenier and Ziauddin 2008).

However, there are drawbacks to the use of strong and concen-trated acids such as HCl, such as too rapid reaction in the vicinity of the wellbore in addition to significant potential for corrosion of iron components and subsequent mobilization of (temporarily) solubilized iron salts. Both reaction rate and corrosion of well tubulars are aggravated at higher downhole temperatures. Various additives can be used to reduce corrosion, but the solubilization of even small amounts of iron can lead to post-precipitation of Fe(OH)3 (Crowe 1985; Taylor et al. 1999) or iron sulfide if H2S is present (Nasr-El-Din et al. 2002). For this reason, iron-control additives such as chelates are often used in conjunction with acids. In addition, an H2S scavenger should be used in sour wells. Addition of a suitable chelate in the live acid can delay/prevent precipitation of Fe(OH)3 as the acid is spent and the pH rises above pH 12 (Taylor et al. 1999).

Use of acids besides HCl, such as acetic and formic acids, provides for a reduction in both reactivity and corrosion rate. Unfortunately, the advantages of these acids are at the expense of calcite-dissolving capacity. Because of the limited solubility of calcium acetate and formate salts, the maximum concentrations of acetic acid and formic acid that can be used are below 13 and 9 wt%, respectively. The dissolving capacity of these organic acids is significantly less than that of 15 wt% HCl.

Because of the problems associated with strong acids, alterna-tive dissolvers such as chelating agents have been explored. Just as chelates are used with acids to prevent precipitation of iron by complexation, they have been used also to dissolve carbonate and sulfate scales in various fields, including fields in the North Sea and the Gulf of Mexico (Jordan et al. 2000; 2006). Fredd and Fogler (1998) were the first to examine the use of EDTA in stimulation of carbonate formations. Frenier and coworkers have been advocates on the use of chelates, particularly HEDTA, for dissolution of calcite (Frenier et al. 2000; 2001; 2003; 2004).

The mechanism involved for calcite dissolution with EDTA (see Table 1 for the structures of EDTA and other chelates used in the present study) is given by

EDTA CaCO EDTA Ca CO4 32

+ [ ] + 32 , . . . . . . . . . . (2)where a complexation constant can be written as (Martell et al. 2003)

K =( ) ( )( ) =

+

EDTA CaEDTA Ca

or l2

4 2105 10 oogK = 10 7. . . . . . . . . (3)

Chelates offer the benefit of lower reaction rates with calcite and less corrosion potential because they are principally used at higher pH values and can be chloride free. But, chelate solutions provide significantly lower calcite-dissolving capacity than acids because of the need to use 1 mole of chelate/mole of calcite. One gallon of a 15 wt% solution of EDTA/Na4 has calcite-dissolving potential that is one-fifth that of HCl.

Perceived problems in using chelates such as EDTA, HEDTA, and NTA either as calcite dissolvers or as iron-control agents are their environmental and safety characteristics. While considered safe, EDTA and HEDTA are not readily biodegradable, although various mechanisms exist in the natural environment to allow for their eventual degradation (Bucheli-Witshel and Egli 2001). On the

March 2011 SPE Journal 105

other hand, while NTA is readily biodegradable, it is classified as possibly carcinogenic to humans and, therefore, creates handling and environmental-discharge concerns. Other chelates, such as EDG [also known as hydroxyethyliminodiacetic acid (HEIDA) (Frenier et al. 2003)], have been identified as readily biodegrad-able. However, EDG is more limited in its ability to control iron and to dissolve calcite.

Blauch et al. (2003) first reported on the multifunctionality of citric acid, whereby the acidity of citric acid protonates carbonate and the citrate formed is capable of chelating Ca2+ and prevents dissolved iron from precipitating. Citric acid is also readily biode-gradable and offers low corrosion potential. A downside of citric acid is the limited solubility of calcium citrate under certain con-ditions (Al-Khaldi et al. 2005, 2007). Also, the results of the first field trials were below expectation, which was explained in terms of low solubility of calcium citrate (Burgos et al. 2004).

Lactic acid (2-hydroxypropanoic acid) is similar to citric acid, but the solubility of its calcium salt is high (Nasr-El-Din et al. 2007). An ester of lactic acid was used in an acid fracturing treat-ment in a carbonate reservoir. Analysis of well-flowback samples did not show any evidence of calcium lactate. A full description of this treatment was given by Nasr-El-Din et al. (2009).

While there are shortcomings in the use of chelateslimited cal-cite-dissolving capacity, slow biodegradabilitychelates play a vital and growing role in stimulation fluids. The ideal chelate for stimula-tion, therefore, should offer environmentally friendly characteristics in addition to multifunctionality (like citric acid) but with enhanced solubility and chelation properties. One such material that has been identified as having many of these positive attributes is GLDA.

GLDA is in the same class of chelating agents as EDTA, called amino polycarboxylic acids (APCs), possessing a single central nitrogen atom, as do NTA and EDG, whereas EDTA and HEDTA both have two. The structure of GLDA is unique in that a major portion of the molecule is derived from monosodium glutamate (MSG), a flavor enhancer used in the food industry. MSG is prin-cipally made from the fermentation of readily available corn sugars and is considered a renewable raw material.

On the basis of an extensive literature survey, current chelating agents either have limited solubility in strong HCl acids or are not environmentally friendly. We developed a new chelating agent, which has the potential to be used in concentrated acids and as a standalone stimulation fluid. The objectives of this study are to (1) measure the solubility of GLDA in water and various acids, (2) determine the ability of the new chelate to dissolve calcium and chelate iron, (3) assess GLDAs biodegradation, and (4) examine its stability and corrosion rates.

Experimental ProceduresChelating agents EDTA, HEDTA, GLDA, NTA, and EDG (also known as HEIDA) were obtained from AkzoNobel. Acetic and formic acid, HCl, FeCl36H2O, and CaCO3 were obtained from Fluka Chemical Company. Sodium hydroxide and CaCl22H2O were obtained from J.T. Baker.

The solubility of GLDA in water was determined by acidifying a GLDA/Na4 solution with HCl to various pH values. All GLDA solutions contained greater than 40 wt% GLDA and remained clear for at least 3 months at room temperature.

The solubility of the GLDA and the remaining chelates in water or acids was determined by stepwise addition of the chelate to the water or acid until an oversaturated solution was reached. Subsequently, the solution was stirred for 48 hours and the con-centration of chelating agent was determined by a complexomet-ric/potentiometric endpoint titration using a ferric chloride solution at pH = 3.

For the determination of the calcite-dissolving capacity, 30-mL bottles with caps were filled with 25.0 mL of chelate solution (2.5 mmol chelate) and a magnetic stirrer. Subsequently, CaCO3 (5 mmol) was added to the bottles. The bottles were stirred for 40 minutes and filtered off. The calcium concentration in the filtrates was analyzed by flame atomic absorption spectroscopy.

The iron-control experiments used 25 g of a 15 wt% HCl solu-tion that contained 2,500 and 25,000 ppm Fe3+ solutions prepared from FeCl36H2O. Chelate acid was then added in an equimolar ratio to the ferric ions. No chelate was added in the blank experi-ment. Subsequently, powdered CaCO3 was added portionwise (in 0.5- or 1-g increments) to the acidic solutions until a precipitate was visible in one of the solutions. Equal amounts of CaCO3 were added to the other solutions. The solutions were filtered after 24 hours, and pictures were taken of both precipitate and filtrate.

Biodegradation was determined using the closed-bottle test described in the Organization for Economic Cooperation and Development (OECD) 1992 Guidelines for Testing of Chemicals, Degradation and Accumulation, Ready Biodegradability, Guideline 301 (van Ginkel et al. 2005).

The corrosion potential of GLDA solutions was tested on com-pletely immersed coupons of cut low-carbon steel (SAE 1020) at 158F (70C) for 1 week under static nonaerated conditions. The corrosion rate in mm/a was calculated on the basis of weight loss of the coupons.

The thermal stability of the chelates was determined by heating 10 mL of a 0.1-M chelate solution in a 100-mL Teflon pressure vessel for 4 hours at 149C (300F) or 177C (350F), using a temperature-controlled microwave oven, resulting in a pressure

TABLE 1CHELATE MOLECULAR WEIGHTS, PROTONATION CONSTANTS, AND METAL STABILITY CONSTANTS

Chelate Molecular Weight p Ka Values goL K

Abbreviation Structure Acid Sodium Salt pK1 pK2 pK3 pK4 Ca2+ Fe3+

EDTA

292 380 10.2 6.2 2.7 2 10.7 25.1

HEDTA

278 344 9.9 5.4 2.6 8.1 19.7

GLDA

263 351 9.36 5.0 3.5 2.6 5.9 11.7*

NTA

191 257 9.7 2.5 1.8 6.4 15.9

EDG

177 221 8.7 2.2 4.7 11.6

* Value is not listed in the National Institute of Sciences and Technology Standard Reference Database 46 version 7.0 for Critically Selected Stability Constants of Metal Complexes. Experimental results indicate that the Fe3+ stability constant of GLDA is comparable to NTA.

106 March 2011 SPE Journal

of 4.5 and 9 bar, respectively. The thermal stability was measured in one series per temperature to avoid differences in the heating profile. The stability was calculated from the chelate concentra-tion before and after heating, as determined by complexometric/potentiometric endpoint titration using a ferric chloride solution at pH 3. The remaining solutions were analyzed by nuclear magnetic resonance (NMR).

Results and DiscussionThe chemical properties of APCs can be characterized by the acid-dissociation constants and the stability constants for specific metals ions. The acid-dissociation constants are listed in Table 1 as logKa or pKa. It is the pKa values that define the acidic nature of these materials; the lower the pKa is, the more acidic or read-ily ionizable the acid is. As will be discussed later, the pKa value when coupled with the solubility of the chelate influences which salt types and physical forms of these chelates are available on a commercial scale.

Table 1 also shows logK complexation constants of a number of chelate products with Ca2+ and Fe3+. As described earlier, the complexation constants define how a fully ionized APC (such as EDTA4 or HEDTA3) associates with the free metal ion. The con-cept of the conditional stability constant K allows one to provide a

means to combine the thermodynamic complexation constants with side reactionssuch as protonation of the chelatethat occur as a function of pH. Fig. 1 shows a plot of log conditional stability constants of these chelates as a function of pH. Expectedly, the plot predicts the maximum strength of chelation of Ca2+ with APCs will occur at high pH values, and full ionization of the APC will occur when pH exceeds pK1 listed in Table 1. Note that complexation of APCs with Fe3+ is more complex and maximum logK binding occurs at pH of almost 3. LogK binding constants of GLDA fall below the strong chelates EDTA and HEDTA, reside above EDG, and are almost equal to NTA in the ability to complex Ca2+ and Fe3+ at any pH value.

Solubility of ChelatesFig. 2 shows the solubility of chelates over a wide pH range. It is from such data that strategies on formulation stability of fluids can be deduced. Additionally, the solubility profile affects what commercial forms of these products are available. A common property among the APCs shown is their solubility at high pH, which is expectedly high because of the complete ionization of the carboxylate groups. This is fortunate because the commercial manufacture of APCs almost always starts with the production of the sodium salts of chelates at high pH. Another shared solubility

0

2

4

6

8

10

12

0 2 4 6 8 10 12 14pH

Log

Con

d. S

tabi

lity

Con

stan

ts C

a2+

, log

K'

EDTA

HEDTA

GLDA

NTA

EDG

Fig. 1The calculated conditional stability constants (K) of various chelates with Ca2+ as a function of pH.

0

10

20

30

40

50

60

0 2 4 6 8 10 12 14pH

Solu

bilit

y, w

t% a

s ac

id

EDTAHEDTAGLDANTAEDG

Fig. 2The solubility of various sodium chelates at 20C as a function of pH expressed as the weight percentage of the cor-responding chelate acid. At pH < 4, the long-term solubility of GLDA is ensured up to 10 wt% as acid.

March 2011 SPE Journal 107

propertywith one exceptionis the rather low solubility at low pH because of the protonation of these carboxylate groups, which suppresses ionization of the chelate.

The poor solubility of EDTA at low pH clearly has its draw-backs in using EDTA in low-pH stimulation fluids. However, this property does allow for efficient manufacture of two important EDTA solid products that are used in the oilfield industry: diso-dium EDTA (solubility is approximately 10 wt% at pH 4.5) and EDTA acid (solubility is approximately 0.05 wt% at pH 2). EDTA can be readily converted to highly soluble ammonium and potas-sium salt solutions, both of which are used for descaling/comple-tion, but they will behave like sodium salt when used in HCl solutions. NTA acid is another material that is efficiently isolated from simple acidification of the NTA trisodium salt solution.

While the solubility of HEDTA is also suppressed at low pH, it does have a significantly higher solubility (approximately 1015 wt% at pH 23), a fact that has made Frenier and Ziauddin (2008) advocates for the use of HEDTA in low-pH formulations. Note that commercial HEDTA is limited to the approximately 40-wt% trisodium salt solution at approximately pH 13.5, which needs to be pH adjusted with HCl.

In Fig. 2, the notable exception to the generally low solubility of APCs at pH 23 is GLDA, where the solubility exceeds 40 wt%. Such high solubility makes it difficult to isolate the solid acid on

a commercial scale. However, the potential use of a concentrated acidic fluid containing pure chelate for stimulation and other pur-poses is significant.

Fig. 3 shows a comparison of the solubility of these chelates in HCl, formic acid, and acetic acid, where they might be useful as iron-control agents. Both HEDTA and EDG offer considerable solubility in 15 wt% HCl, but their solubility is significantly sup-pressed in 28 wt% HCl. NTA is moderately soluble also, while EDTA has limited utility in these acidizing fluids. GLDA is a notable exception, showing significant solubility in all the acids examined. It is important to mention that a similar high solubility of GLDA is also found in sulfuric, phosphoric, and nitric acids. GLDAs high solubility under extremes in pH, high ionic strength, and low free-water content offers great flexibility in its use in vari-ous oilfield applications (stimulation, scale removal, and cleaning of drilling-fluid filter cake).Dissolution of Calcite With Chelates Fig. 4 shows the ability of chelates to dissolve CaCO3 over a range of pH values. At pH 5 and greater, the dissolving capacity of che-late-containing fluids is principally driven by the chelates ability to complex Ca2+. EDTA and HEDTA are both strong chelates for Ca2+ and show a near 1:1 molar efficiency in their ability to dissolve cal-cite. Less efficient for dissolving calcite are those with lower stability

0

10

20

30

40

50

60

EDTA HEDTA GLDA NTA EDG

Solu

bilit

y, w

t% a

s ac

id

15% acetic acid

28% acetic acid

15% formic acid

28% formic acid

15% hydrochloric acid

28% hydrochloric acid

Fig. 3The solubility of APCs in various acids at 20C. The listed acid concentrations reflect the concentrations before dilution with the chelate.

0

1

2

3

0 2 4 6 8 10 12

Initial pH of Solution

Dis

solu

tion

of C

aCO

3,m

oles

CaC

O3

/mol

e ch

elat

e

EDTA

HEDTA

HEDTA,Frenier

GLDA

EDG

Fig. 4The dissolution of CaCO3 with various chelates as a function of initial pH at 20C.

108 March 2011 SPE Journal

constants, such as GLDA, NTA, and EDG, even at pH 1112 where their ability to complex Ca2+ is at a maximum (see Fig. 1).

At pH < 5, APCs initially behave as normal acids, and the calcite is dissolved by a protonation reaction; but, when the acid is spent, the chelate inside provides the means to complex Ca2+ or Fe3+, preventing the deposition of the latter. As a result, chelates such as GLDA that have high solubility at low pH can offer both high calcite-dissolving power and chelation ability.

As shown in Fig. 4, a mole of GLDA/H3Na with a starting pH of approximately 3.0 can dissolve approximately 1.5 moles of calcite, whereas 1 mole of GLDA/H4 having a pH of approximately 1.5 can dissolve up to 2 moles of calcite. Calculated on a per-gallon basis, a 20 wt% solution of these GLDA fluids can dissolve approximately 1.1 and 1.5 lbm calcite/gal, respectively. By contrast, the calcite-dissolving capacity of a 20 wt% EDTA/Na4 (pH 12) or of 20 wt% HEDTA/Na3 (adjusted to pH 2.5 with HCl) is 0.52 and 0.74 lbm calcite/gal dissolved, respectively.

A more potent dissolution fluid for calcite can be obtained by dissolving chelates in more-concentrated acids. Here, the principal role of the chelate is to control and prevent Fe(OH)3 precipitation as the pH of the spent acid increases. Figs. 5 and 6 show residual CaCO3 filtered from the spent HCl solutions that contained 2,500 and 25,000 ppm of Fe3+, respectively. Where indicated, GLDA and HEDTA were initially present in the HCl at the same molar concentration as the Fe3+. Fig. 7 shows the filtrates from the 2,500-ppm-Fe3+ experiment. The color or lack of color of the filter and filtrate is directly related to the Fe+3 ion content. Expectedly, with no chelate present (blank), Fe(OH)3 readily precipitates (dark color of filter) with little to no soluble Fe present in the filtrates (color-less solution). However, as noted by the nearly white color of the filter from the 2,500-ppm-Fe3+ test solutions, both the HEDTA and GLDA are equally effective at keeping Fe(OH)3 from precipitating. At a concentration of 25,000 ppm Fe3+, a small amount of Fe(OH)3 is observed in the spent HCl solution containing GLDA, whereas essentially no Fe(OH)3 precipitate is formed with HEDTA. Clearly, this latter condition is extremevery high Fe coupled with the use of an exact stoichiometric amount of the GLDA. Under such high Fe+3 conditions, an excess of GLDA would likely be needed to reduce formation and precipitation of Fe(OH)3.

Biodegradation of ChelatesAn important aspect of the use of chemicals in the oil field is related to their environmental, health, and safety effects. The replacement of highly reactive, corrosive, and volatile fluids containing HCl with a nonvolatile chelate is highly advantageous. A further favorable environmental characteristic of GLDA is that it is readily biodegradable. Fig. 8 shows the degradation of GLDA in fresh water vs. time in an OECD 301D test using unacclimated bac-teria. After an induction period, the GLDA is shown to biodegrade readily by achieving > 60% degradation within a 28-day period, similar to what is found for NTA and EDG. Fig. 8 also shows the general lack of degradation of HEDTA (similar to EDTA) under these testing conditions.

GLDA HEDTA Blank

Fig. 5Excess CaCO3 in spent HCl acid in the presence of 2,500 ppm Fe3+. GLDA and HEDTA give a nearly white precipitate indicative of CaCO3 with low precipitated Fe(OH)3. The brownish precipitate in the blank solution without chelate is caused by Fe(OH)3 precipitation.

GLDA HEDTA Blank

Fig. 6Excess CaCO3 in spent HCl acid in the presence of 25,000 ppm Fe3+. GLDA gives a slight orange precipitate caused by a small quantity of precipitated Fe(OH)3, whereas HEDTA gives a nearly white precipitate indicative of CaCO3 with low precipitated Fe(OH)3. The brown precipitate in the blank solution without chelate is caused by large quantities of Fe(OH)3 precipitation.

GLDA HEDTA no chelate

Fig. 7Filtrates of spent HCl acid in the presence of 2,500 ppm Fe3+. The color in the chelate-containing acids is caused by com-plexed Fe3+ ions, whereas the lack of color in the blank filtrate with no chelate indicates the absence of dissolved Fe3+ ions.

March 2011 SPE Journal 109

Stability and Corrosion Rate To be effective, chelate fluids used for stimulation need to be stable at high temperatures for several hours. Fig. 9 shows the percentage of GLDA and HEDTA remaining after heating for 4 hours at 300 and 350F (149 and 177C, respectively). With the possible excep-tion of a single data point at pH 1.5, the thermal stability of GLDA appears to be on the same order as HEDTA. NMR analyses showed that the major breakdown products of the chelates are formic acid and cyclic GLDA or cyclic HEDTA, respectively. Under these conditions, no precipitate was formed during heating.

The corrosion potential of GLDA solutions was tested [National Association of Corrosion Engineers (NACE)] on completely immersed coupons of cut low-carbon steel at 158F (70C) for 1 week under static conditions. The corrosion rate as mm/a was calculated on the basis of weight loss of the coupons. A 40 wt% solution of GLDA/H3Na at pH 3 resulted in a corrosion rate of 11.2 mm/a (0.3438 lbm/ft2 over a 1-week period) on the low-car-bon-steel coupons. At lower temperatures and higher pH values, significantly less corrosion to low-carbon steel was found. There is a need to use a suitable corrosion inhibitor if GLDA is used at high temperatures or used at low temperatures for a longer period of time (more than a day). The same recommendation applies to other chelates that are used at low pH values and high tempera-tures. More details on the solubility and use of GLDA for matrix acidizing are given by Mahmoud et al. (2010).

ConclusionsOn the basis of an extensive literature survey, current chelating agents either have limited solubility in strong HCl acids or are not environmentally friendly. We developed a new chelating agent, GLDA, which has the potential to be used in concentrated acids and as a standalone stimulation fluid. This new chelate offers sig-nificant advantages over conventional chelating agents: It is a fairly good chelate for both Ca2+ and Fe3+ and offers high

solubility over a wide pH range; and, most significantly, aqueous solutions of up to 40 wt% are possible at pH 2.

It is highly soluble in acids, even in 28 wt% HCl where HEDTA solubility is limited.

The monosodium salt (pH 3) or free acid (pH 1.5) GLDA by itself offers significant dissolving capacity for calcite.

It is as effective, or nearly as effective, as HEDTA in preventing Fe(OH)3 precipitation in spent HCl acids.

GLDA is readily biodegradable vs. conventional chelates cur-rently used in stimulation fluids.

Solutions of GLDA are less corrosive than HCl and will be safer to handle. And, because high concentrations of GLDA are possible in 28 wt% HCl, the control of high levels of Fe should be possible.

From a manufacturing standpoint, we are actively developing the process needed to manufacture GLDA-solution products hav-ing a low pH. Such products will be chloride free (having lower corrosion potential) and will have a low sodium content and, thus, will be more compatible with hydrofluoric-acid-based stimulation fluids that are used in sandstone formations.

AcknowledgmentsThe authors wish to acknowledge AkzoNobel for granting permis-sion to publish this paper and the contributions of coworkers Ingrid Huybens, Kees van Ginkel, Martin Heus, and Arjen Reichwein. The anonymous reviewers of this paper are acknowledged for many useful comments.

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100

80

60

40

20

00 10 20 30

Time, days

Bio

degr

adat

ion,

%

Fig. 8The percentage of biodegradation of GLDA () and HEDTA () vs. time in OECD 301 D closed-bottle tests inoculated with unacclimatized activated sludge. The dashed line represents the pass level of the ready biodegradability test.

0

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80

100

0.6 1.3 1.5 2.8 8 12.5 0.6 1 2.1 12.3pH

Che

late

afte

r Hea

ting,

%

149C (300F)

177C (350F)

HEDTA

149C (300F)

177C (350F)

GLDA

Fig. 9The percentage of GLDA and HEDTA remaining after heating for 4 hours at 149C (300F) and at 177C (350F).

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Jordan, M.M., Marlow, D., Johnson, T.L., and Johnston, C. 2006. The Evalu-ation of Enhanced (Carbonate/Sulfate) Scale-Dissolver Treatments for Near-Wellbore Stimulation in Subsea Production Wells, Gulf of Mexico. Paper SPE 100356 presented at the SPE International Oilfield Scale Symposium, Aberdeen, 31 May1 June. doi: 10.2118/100356-MS.

Mahmoud, M., Nasr-El-Din, H.A., De Wolf, C.A., and LePage, J.N. 2010. Evaluation of a New Environmentally Friendly Chelating Agent for High-Temperature Applications. Paper SPE 127923 presented at the SPE International Symposium and Exhibition on Formation Damage Control, Lafayette, Louisiana, USA, 1012 February. doi: 10.2118/127923-MS.

Martell, A.E., Smith, R.M., and Motekaitis, R.J. 2003. NIST Standard Reference Database 46: Critically Selected Stability Constants of Metal Complexes, Version7.0. Gaithersburg, Maryland: Standard Reference Data Program, National Institute of Standards and Technology.

Nasr-El-Din, H.A., A.A. Zahrani, A.A., Garzon, F.O., Giraldo, C.A.F., Al-Hakami, I.M., and Al-Marri, H.M. 2009. Acid Fracturing of Gas Wells by Use of an Acid Precursor in the Form of Solid Beads: Les-sons Learned From First Field Application. SPE Prod & Oper 24 (2): 320335. SPE-110895-PA. doi: 10.2118/110895-PA.

Nasr-El-Din, H.A., Al-Humaidan, A.Y., Fadhel, B.A., Frenier, W.W., and Hill, D.G. 2002. Investigation of Sulfide Scavengers in Well-Acidiz-ing Fluids. SPE Prod & Fac 17 (4): 229235. SPE-80289-PA. doi: 10.2118/80289-PA.

Nasr-El-Din, H.A., Al-Zahrani, A., Still, J., Lesko, T., and Kelkar, S. 2007. Laboratory Evaluation of an Innovative System for Fracture Stimulation of High-Temperature Carbonate Reservoirs. Paper SPE 106054 pre-sented at the International Symposium on Oilfield Chemistry, Houston, 28 February2 March. doi: 10.2118/106054-MS.

Taylor, K.C., Nasr-El-Din, H.A., and M. Al-Alawi, 1999. Systematic Study of Iron Control Chemicals Used During Well Stimulation. SPE J. 4 (1): 1924. SPE-54602-PA. doi: 10.2118/54602-PA.

Taylor, K.C., Nasr-El-Din, H.A., and Saleem, J.A. 2001. Laboratory Evalu-ation of Iron-Control Chemicals for High-Temperature Sour-Gas Wells.

Paper SPE 65010 presented at the SPE International Symposium on Oilfield Chemistry, Houston, 1316 February. doi: 10.2118/65010-MS.

van Ginkel, C.G., Geerts, R., and Nguyen, P.D., 2005. Biodegradation of L-glutamate diacetate by mixed cultures and an isolate. In Biogeochemistry of Chelating Agents, ed. B. Nowack and J. Van Briesen, Chap. 10, Vol. 910, 183194. New York: ACS Symposium Series, Oxford University Press.

James N. LePage is technical service and development man-ager for AkzoNobel Functional Chemicals chelate operations. email: jim.lepage@akzonobel.com. He has more than 30 years of experience in research and development and marketing of chelating agents. In the past 5 years, LePages focus has been directed to the areas of biodegradable chelates and the use of chelates in the oil field. He holds a BS degree in chem-istry from the University of Massachusetts and a PhD degree in analytical chemistry from Northeastern University. LePage began his career at WR Graces Hampshire Chemical divi-sion in Nashua, New Hampshire, during his college years and became manager of Analytical R&D and Technical Service laboratories while there. In 1997, he joined AkzoNobel when it acquired Hampshires chelate business. LePage received AkzoNobels 2008 Environmental Sustainability Award for his work on chemical processing of oil field chemicals. Corine A. de Wolf is a project leader at the research and development laboratory of the AkzoNobel sub-Business Unit Chelates. email: corine.dewolf@akzonobel.com. Her main fields of interest include the functionality and properties of chelates in oilfield applications, cleaning, micronutrients, and food fortification. Previously, de Wolf worked at TNO, a Dutch research institute, as a research specialist on parts and precision cleaning. She holds an MS degree in chemistry from Utrecht University and a PhD degree in heterogeneous catalysis and surface chemistry from Leiden University, both in the Netherlands. She has con-tributed to two patents and 14 technical papers on various subjects. Josine H. Bemelaar is an employee at the research and development laboratory of the AkzoNobel sub-Business Unit Chelates. email: josine.bemelaar@akzonobel.com. She holds a BS degree from the Saxion Hogeschool Enschede in organic chemistry. Bemelaars research interests are on the properties and functionality of chelates. On this subject, she was coauthor of two patents and a technical paper. Hisham A. Nasr-El-Din is a professor and holder of the John Edgar Holt Chair at Texas A&M University in petroleum engineering. Previously, he worked for 15 years as principal professional and team leader of the Stimulation Research and Technology Team, Saudi Aramco. Before joining Saudi Aramco, he worked for four years as a staff research engineer with the Petroleum Recovery Institute in Calgary. He also worked as a research associate with the University of Saskatchewan, the University of Ottawa, and the University of Alberta. His research interests include well stimulation, formation damage, cementing, drilling fluids, two-phase flow, enhanced oil recovery, rheology, con-formance control, interfacial properties, adsorption, and non-damaging fluid technologies. Nasr-El-Din has several patents and has published more than 400 technical papers. He is an adjunct professor with the University of Alberta and has super-vised several MS and PhD students. He has received numer-ous awards within Saudi Aramco for significant contributions in stimulation and treatment-fluid technologies and stimula-tion design and for his work in training and mentoring. Nasr-El-Din holds BS and MS degrees from Cairo University and a PhD degree from the University of Saskatchewan, all in chemi-cal engineering. He serves on the SPE steering committees on corrosion and oilfield chemistry, is a review chairperson for the SPE Journal and is a technical editor for the SPE Production & Operations and SPE Drilling & Completion journals. He was invited to give keynote presentations in various SPE and NACE conferences. He received the SPE Regional Technical Discipline Award for Production and Operations in 2006, was named a Distinguished SPE Member in 2007, and received SPE awards for Outstanding Associate Editor (SPE Journal) and Outstanding Technical Editor (SPE Production & Operations) in 2008. In addi-tion, he received SPE Production and Operations Award and Outstanding Associate Editor Award (SPE Journal) in 2009.