SPE 37281 Society of Petroleum Engineers

Chemical Study of Organic-HF Blends Leads to Improved Fluids

Chris E. Shuchart, SPE, Halliburton Energy Services

Copyright 1997, Society of Petroleum Engineers [nco

This paper was prepared for presentation at the 1997 SPE [nternationa[ Symposium on Oilfield Chemistry held in Houston, Texas, February 18·21.

This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessan[y reflect any position of the Society of Petroleum Engineers, its officers, or membe~s. Papers prese~ted at SPE meetings are subject to publication review by Editoria[ Committees of the Society of Petroleum Engineers. Permission to copy is restricted to an abstract of not more than 300 words. [liustrations may not be copied. The abstract should contain conspicuous acknow[edg· ment of where and by whom the paper is presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A, fax 01-972-952-9435.

Abstract Organic-HF blends have successfully stimulated sandstone for­mations with corrosion concerns, HCI-sensitive mineralogy, and/orcrude oil incompatibilities, three conditions in which the use of typical HCI-based fluids can result in severe damage. Despite the success of such blends, very little work has been published on the reactivity of organic-HF fluids with alumino­silicates. Recent work on the chemistry ofHF acidizing focused on HCI-HF systems. [ This paper discusses the chemistry of organic-HF systems as determined by laboratory reactivity tests, 19F nuclear magnetic resonance (NMR) studies, and or­ganic-HF flowback analyses.

Laboratory reactivity testing revealed severe precipitation problems associated with the currently used acetic-HF and formic-HF systems. Flowback analyses after organic-HF field treatments fully supported the laboratory results. A 19F NMR spectroscopic study of the fluoride distribution throughout the primary, secondary, and tertiary reactions helped explain the precipitation and when it might occur. From this work, new organic-HF systems were developed that prevent precipitation while maintaining all the advantages associated with the acetic­HF and formic-HF fluids. The retarded nature of these systems is discussed in this paper.

Introduction Common problems associated with HCI-based fluids include high reactivity,2 high corrosivity,3 sludging tendencies when fluids contact crude oils,4 and the HCI sensitivity of clay, minerals and zeolites,S These problems are aggravated at higher temperatures. Organic acids, such as acetic acid, formic acid,

References at the end of the paper. 675

and combinations have overcome many of these problems. The recent realization that organic acids act like fresh water has led to the addition of 5% NH4CI to organic acids used in formations containing clays and zeolites that are capable of ion exchange and/or susceptible to swelling or fines migration."

Acetic-HF and formic- HF fluids were developed because of the problems associated with HCI-based HF fluids, However, very few studies have investigated the chemistry of these systems, Recently, we discovered that HCI-HF fluids obey a chemical equilibrium during the primary and secondary reac­tions,7 As a result, the fluoride distribution, and in particular the F/AI ratio, depends upon the acid (H+) concentration. For example, a solution containing 13.5% HCI-1.5% HF reacted with alumino-silicates will have a F/AI ratio of approximately 1,3 during the primary and secondary reactions. However, at the low acid (H+) concentrations of organic acids (pH 1 through 4), the FI Al ratio of organic-HF fluids during these reactions would be 2,5 to 3.0. The dominant aluminum fluoride species in spent organic-HF solutions would be AlF/ and AIF3, The acid depen­dence on the F/Si ratio is only slight. In HCI-based fluids, the F/Si ratio is considered to be constant at 5.0; the dominant species is HSiFs' In organic-HF systems, the F/Si ratio is about 5.5, and the dominant species are HoSiFo and HSiFs,l Based on the fluoride distribution, the prirri'ary reaction (Eq. 1) and secondary reaction (Eq. 2) for organic-HF fluids can be written in the general terms below.

HF + M - Al- Si ~ H 2SiF6 + HSiFs

+ AIF;+ + AlF3 +M+ ................................................. (l)

H2SiFr, + HSiFs + M - Al- Si + H+ ~ Si02 • Hp

+ A IF; + + AlF3 + M+ ................................................. (2)

As reported in a previous paper,s flowback analyses from acetic-HF and formic-HF field treatments showed that alumi­num fluoride precipitation had occurred, Laboratory spending tests and flow tests showed that AIF3 precipitated from the acetic-HF and formic-HF fluids quickly upon spending because of the high F/AI ratio and the poor solubility of AlF3, Two new

2 CHEMICAL STUDY OF ORGANIC-HF BLENDS LEADS TO IMPROVED FLUIDS SPE 37281

acid systems were developed to prevent precIpItation. This paper presents additional details concerning the chemistry of the acetic-HF and formic-HF systems as well as the new acid systems.

Results and Discussion Acid-Returns Analyses. Analyses offlowbacks after acidizing treatments highlighted the precipitation problems associated with acetic-HF and formic-HF fluids. Flowbacks from several 7.5% acetic acid- I.5% HF acid treatments in Thailand showed low silicon and aluminum concentrations, high F/AI ratios, and pH values of approximately 4. BHSTs were approximately 200°F. Silicon levels of 50 to 200 mg/L revealed that the secondary reaction went to completion, as would be expected in HCI-HF fluids. 19F NMR spectroscopy of the samples showed that no silicon fluorides remained in solution. The low alumi­num concentrations (160 to 560 mg/L), and subsequently low fluoride levels indicated aluminum fluoride precipitation in the formation during the treatments. After accounting for dilution with non-HFfluid stages and formation brine, we calculated that at least 75 to 95% of the fluoride had precipitated. The F/AI ratios in the flow backs were 2.5 to 2.8.

Flowback analyses from 9% formic-3% HF treatments in Colombia showed similar results. With 3% HF, much higher aluminum concentrations would be expected without precipita­tion. However, only 120 to 1440 mg/L of aluminum were observed in solution. The F/AI ratios and pH values were only slightly lower than those observed in the acetic-HF returns. Again, over 75% of the fluoride had precipitated within the matrix of the formation.

New Acid Systems-Laboratory Spending Tests. To prevent the aluminum fluoride precipitation, we investigated the use of alternative acid systems. The purpose of the new system would be to prevent AIF3 precipitation while maintaining all the advantages of acetic-HF and formic-HF fluid systems. Ion­exchanging ability and compatibility with multivalent ions, such as Fe, Ca, and Mg, would also be required. The understand­ing of these requirements and the HF reaction processes led to the development of new proprietary systems.

The key components of the new systems, which will be referred to as "new acid systems" (NAS), are new organic acids that can coordinate aluminum fluoride complexes and buffer the spent HF fluids. The coordination of aluminum fluoride com­plexes helps alter the aluminum-fluoride equilibria, which mini­mizes the amount of AIF3 in solution and forces the HF reactions to lower FI Al ratios.

Batch reactions with kaolinite at 200°F were conducted as a means of studying the precipitation of aluminum fluoride complexes from acetic-HF and formic-HF fluids and develop­ing new organic acid systems. Before tests were conducted, 25 g of kaolinite was added to 100 mL of acid to ensure adequate fresh clay for the fluids to react. Various fluids were tested, including HCI-HF, acetic-HF, formic-HF, and the two new acid

676

systems, NAS-I and NAS-II. The fluids and clay were mixed at room temperature and heated to the required test temperature. Samples were removed at various time intervals, were filtered hot, and then analyzed by inductively coupled plasma (ICP) for Si and AI. 19F NMR Spectroscopy was conducted on all samples as a means of determining the fluoride distribution and calculat­ing the percentage of precipitated fluoride.

Testing was conducted with fluids prepared with either ammonium bifluoride (ABF) or ammonium fluoride (AF). Table 1 shows representative results of fluids prepared with ABF. Almost complete precipitation of aluminum fluorides occurred in the reaction of kaolinite with acetic-HF fluids (Test 1). The secondary reaction had nearly gone to completion (as noted by the low silicon content) and the aluminum concentra­tion was expected to approach or exceed 5000 mg/L. However, the aluminum concentration was only 677 mg/L after 1.5 hours, giving 79% precipitation of the fluoride. Continued precipita­tion and lower aluminum concentrations were noted at longer reaction times. Solutions prepared with AF were somewhat more susceptible to precipitation because the consumption of acid to protonate the fluoride salt gave a slightly higher pH system and subsequently resulted in a higher F/AI ratio in solution. Formic-HF fluids (Test 2) performed substantially better, but 2 I % of the fluoride precipitated. The formic-HF returns analyses described previously, where 75% or more ofthe fluorides precipitated, suggest that the laboratory results are very conservative.

Both NAS-I and NAS-II fluids completely prevented alu­minum fluoride precipitation. At I.5 to 2 hours of reaction, the aluminum concentrations exceeded 5000 mg/L. When the reac­tions were allowed to proceed over 24 hours, aluminum concen­trations increased to over 10 000 mg/L, since these improved fluids dissolved more alumino-silicates without any aluminum fluoride precipitation.

New Acid Systems-Preventing Precipitation. The new acid systems prevented aluminum fluoride precipitation throughout the primary and secondary reactions. Several samples from the laboratory spending tests were analyzed by 19F NMR spectros­copy to help reveal the mechanisms behind the new acid systems. 19F NMR spectroscopy has proven to be an excellent tool to allow us to observe the distinct silicon fluoride and aluminum fluoride species in solution during HF reactions. I

Fig. la shows the room-temperature NMR spectrum of a NAS-II solution reacted with kaolinite at 200°F for 1 hour. At this point in the experiment, the primary reaction was complete and the secondary reaction was 90% complete. The figure shows only the aluminum fluoride region of the spectrum. The peaks are very broad, which suggest multiple aluminum fluoride species in equilibrium and fast exchange. Similar spectra were observed with the formic-HF and acetic-HF fluids that were reacted with clay. The anticipated aluminum fluoride species were AIF/ and AIF3.

SPE 37281 C.E. SHUCHART 3

As a means of slowing down the exchange processes, low­temperature NMR experiments were then conducted. LiCI was added to the samples, which prevented the aqueous solutions from freezing and the equilibria from changing. Fig. Ib shows the low-temperature spectrum of an acetic-HFfluid reacted with kaolinite at 200°F. The fluid was prepared with 10% acetic acid and ABF with HCI to give 1.5% HF. (The addition of the HCI provided a lower FI AI ratio and limited the amount of AIF3 precipitation. About 20% of the fluoride still precipitated at this point in the test). Four much sharper signals were observed. The assignment of the signals was then made based on numerous NMR experiments with various standards and controlled mix­tures. The two sharp signals result from AIF/. In an octahedral environment, the fluorides on aluminum could be opposite each other (trans) or next to each other (cis) giving two possible isomers for AIF/. The small, sharp signal between these two peaks was identified as AlP+, and the broad signal to the left was identified as AIF 3" The broadness of the AIF 3 signal suggested multiple isomers. From this data, a FI AI ratio of 2.4 was determined. This ratio is slightly lower than expected for this fluid, which has a pH of2.7, but precipitation of AIF3 did occur, which caused a lower F/AI ratio in solution.

Fig. Ie shows results from a similar experiment with the same NAS-II fluid used in Fig. la. At low temperature, as many as 20 signals were observed. Each signal represents a different fluoride environment, which indicates as many as 15 aluminum fluoride products. NAS-ll's chelating ability allows this many species to form when isomerization and degrees of substitution are taken into account. The chelation of the aluminum fluoride species alters the aluminum-fluoride equilibria, which mini­mizes the amount of AIF, in solution, thus preventing its precipitation.

Live HF Studies. During this study, the 19F NMR spectra of "live" organic-HF fluids were obtained. The results were quite surprising: the position of the signalfor the "live HF" changed depending on the fluid. For example, in NAS-II and an acetic­HF solution prepared with ABF, the signals appeared at -71 and -68 ppm, respectively (Fig. 2). However, when a mixture of the two acids was prepared with AF as the fluoride salt, the signal was observed at -59 ppm. Control studies showed that actual HF appears at -86.8 ppm, while free fluoride (from AF) appears at -42.5 ppm.

The disparity between these results was quite surprising, but it can be explained if we consider the pKa values of HF acid and the organic acids. Most organic acids, including acetic acid, formic acid, NAS-I, and NAS-II are not strong enough to fully proton ate fluoride. The actual degree of protonation of a organic acid and fluoride mixture can be determined based on its pKa values. Results for several acid systems that use AF as the fluoride salt are shown inFig. 3. Similar curves can be generated for systems that use ABF.

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These studies show that the 19F NMR chemical shift of a organic-HF solution can be used for determining the amounts of free fluoride and HF. This information correlates extremely well with the calculations when pKa values are used. The chemical shift is simply an averaging of the chemical shift of the two species. For example, 10% acetic-l % HF prepared with ABF showed an NMR signal at -68 ppm. This result corresponds to 58% HF and 42% F, which agrees well with pKa calculations.

These results reveal a distinct advantage for organic-HF fluids. Since the HF reaction on sand is the square function of the HF concentration,9 the reaction rate of organic-HF systems on sand is greatly reduced compared to HCI-HF systems that have the same fluoride concentration. As the HF reacts, more HF is quickly generated from the free fluoride and organic acid. The ultimate amount of HF reacting is the same as in HCI-HF systems. Therefore, organic-HF fluids act as retarded HF sys­tems that allow deeper clay-damage removal. In fact, when used in a core flow test, NAS-II improved permeability throughout the entire length of a 12-in. core.s This effect was not apparent in acetic-HF and formic-HF systems, likely because of the almost immediate precipitation of AIF3, which negated any of the fluid's stimulation effects.

Conclusions Significant precipitation of aluminum fluorides occurs with acetic-HF and formic-HF fluids during a stimulation treat­ment. Therefore, these fluids should be avoided. NAS-I and NAS-II strongly chelate aluminum fluoride complexes, thereby forming numerous species and prevent­ing AIF3 precipitation. The low acidity of most organic-HF fluids, including NAS­I and NAS-II, results in only partial protonation of the fluoride salts. As a result, these fluids act as retarded HF systems.

References I. Shuchart, C.E. and Buster, D.C.: "Determination of the Chemis­

try ofHF Acidizing with the Use of 19F NMR Spectroscopy," SPE 28975 presented at the 1995 SPE International Symposium on Oilfield Chemistry, San Antonio, Feb. 14-17.

2. Van Domelen, M.S. and Jennings, A.R.: "Alternate Acid Blends for HTHP Applications," SPE 30419 presented at the 1995 Offshore Europe Conference, Aberdeen, Sept. 5-8.

3. Dill, R.W. and Keeney, B.R.: "Optimizing HCl-Formic Acid Mixtures for High-Temperature Stimulation," SPE 7567 pre­sented at the 1978 Annual SPE Technical Conference and Exhi­bition, Houston, Oct. 1-3.

4. Norton, S.1. and Smith, C.D.: "Sand Control Installation and Mechanical Design," OTC 7886 presented at the 1995 Annual Offshore Technology Conference, Houston, May 1-4.

5. Van Domelen, M.S., Ford, W.G.F., and Chiu, T.1.: "An Acid Expert System for Matrix Acidizing Treatment Design," SPE 24779 presented at the 1992 Annual SPE Technical Conference and Exhibition, Washington, DC, Oct. 4-7.

4 CHEMICAL STUDY OF ORGANIC-HF BLENDS LEADS TO IMPROVED FLUIDS SPE 37281

6. Gdanski, R.D.: "Fractional PoreVolume Acidizing Flow Experi­ments," SPE 30100 presented at the 1995 SPE European Forma­tion Damage Control Symposium, The Hague, May 15-16.

7. Gdanski, R.D. and Shuchart, C.E.: "Newly Discovered Equilib­ria Control HF Stoichiometry," lPT(Feb. 1996) 145-149.

8. Shuchart, C.E., Gdanski, R.D.: "Improved Success in Acid Stimulations with a New Organic-HF System," SPE 36907 pre­sented at the 1996 SPE European Petroleum Conference, Milan, Oct. 22-24.

9. Gdanski, R.D.: "Kinetics of the Primary Reaction of HF on Alumino-Silicates," papers SPE 37459 to be presented at the 1997 Production Operations Symposium, Oklahoma City, Mar. 9-11.

Table 1-Reactions of Organic-HF Acids with Kaolinite at 200°F

Test Fluid [AI] [8i] Fluoride

(mglL) (mg/L) ppt (%)

1 10% acetic-1.0% HF 677 277 79

2 10% formic-1.0% HF 4278 240 21

3 NAS-I! 5416 258 0

4 NAS-I" 7540 439 0

"Data at 2 hours; remainder of data at 1.5 hours.

(c) NAS~I, -3O'C

(b) acetlc-HF, -30'C

FI ==;====;:~==--'----"'--""""::::=I;===;====;I (a) NAS-II, RT

-70 -75 -80 -85 -90 o

Fig. 1-19F NMR spectra of "spent" organic-HF fluids

678

I

I

J I I

-50 -60 ·70 o

I -80

'-- NAS·I with HCI

NAS·((

10%A cetic·1% HF ed with ABF prepar

NAs·n + I prepare

Acetic·HF d with AF

-90

Fig. 2-'9F NMR spectra of "Jive" organic-HF fluids

100

u. J: 75 ... 0 c 0 ;:

50 IV C 0 -~

0.. ;;e 25

0

0.0 0.5 1.0 1.5 2.0 2.5 Organic Acid Conc (M)

Fig. 3-Degree of fluoride protonation in organic-HF fluids prepared with ammonium fluoride (AF)