SPE 165142 Properties and Applications of an Alternative Aminopolycarboxylic Acid for Acidizing of Sandstones and Carbonates Enrique A. Reyes, Alyssa L. Smith, and Aaron Beuterbaugh, Halliburton Copyright 2013, Society of Petroleum Engineers This paper was prepared for presentation at the SPE European Formation Damage Conference and Exhibition held in Noordwijk, The Netherlands, 5–7 June 2013. 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 have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract Significant advances in acidizing chemistry have led to the introduction of sequestering agents, such as hydroxypolycarboxylic acids, followed by chelating agents, to mitigate precipitation reactions. The initiative to obtain fluids with an improved environmental footprint has led to the redesign of treatment fluids to possess distinct advantages, such as stability at higher temperature, broader pH activity, and stronger complex formation. In the area of hydrofluoric (HF) acidizing chemistry, the conceptualization of the unique HF acid reactions on clays and silica surfaces—namely, kinetic controls over the so-called primary, secondary, and tertiary reactions—has facilitated fluid designs that can handle such varied reactions. The work presented here describes the development of a new acidizing fluid containing an environmentally relevant chelating agent and an aminpolyocarboxylic acid.

The chelating agent is fully biodegradable, according to the Organization for Economic Co-operation and Development (OECD) protocols, is stable in fluid media from pH 1 to 7 and at high temperatures, and stabilizes the dissolved ions during an acidizing treatment. In HF acidizing, the chelant performance has been tested at 0.6 mol/L and HF acid concentrations from 0.5 to 2%, pH of 2.5 to 4, including a stabilizing agent to mitigate the precipitation of fluorosilicates or fluoroaluminates, and is effective in temperature ranges from 200 to 300°F. Laboratory tests show it to be effective in maintaining in solution dissolved aluminum (3000 to 10 000 mg/L), calcium (5000 mg/L), and iron (6000 mg/L). The use of nuclear magnetic resonance (NMR) spectroscopic analysis revealed additional dissolved-fluoride-containing species that has not been previously reported. Moreover, the chelating agent can also be used when stimulating carbonate rocks in concentrations from 0.2 to 0.6 M with a pH of 1 to 4 and is effective from 125 to 350°F. The representative pore volume breakthrough (PVbt) curves provide an indication of the distinct reactivity of this chelant.

Introduction The use of chelating or sequestering agents in acidizing chemistry has resulted in a distinctive class of fluids that offer myriad properties and significant advantages compared to more conventional treatment fluids (Frenier et al. 2000; Scheuerman 1988; Di Lullo and Rae 1996; Husen et al. 2002; Fredd et al. 1995). The most salient chemical properties that make chelating agents so versatile in this and in many other industries have been amply documented (Katti et al. 1999; McCoy 2007; Hart 1984). In the oilfield industry, these properties include the complexation, or stabilization, of dissolved cations from a mineral matrix as a result of the dissolving action of an acidic fluid (Husen et al. 2002). For instance, preventing the precipitation of ferric ion at pH > 1.5, calcium at pH > 6, aluminum and fluoroaluminates at pH > 2 above their respective solubility limit is feasible. And this can be accomplished without recourse to highly acidic (hydrochloric [HCl], acetic, or formic acid) fluids that might require additional levels of ion control to help prevent precipitation reactions.

The use of chelating agents in matrix acidizing treatments, particularly in sandstone acidizing, encompasses treatment fluids with HF acid (Scheuerman 1988; Di Lullo and Rae 1996; Tuedor et al. 2006; Ali et al. 2008) or free from HF acid (Frenier et al. 2004; Martin 2004) in the main acid stage. This categorization stems from the limitations of HCl/HF acid fluid systems at temperatures in excess of 200°F because of the inherent instability of clay minerals, which are considered the main cause of damage formation, in HCl acid media above such temperature (Shuchart and Gdanski 1996; Gdanski and Shuchart 1998). Fluid formulation in HF sandstone acidizing is one of the most challenging designs because of the large number of chemical reactions, near and far from equilibrium, occurring in a non-homogeneous solid matrix having varied chemical composition. The identification of up to three kinetically controlled reactions stemming from the initial reaction of HF acid on aluminosilicate (clay), feldspars, and quartz surfaces provides the mechanistic basis for fluid design or redesign (Gdanski 1997, 1996; Gdanski and Shuchart 1995). The inclusion of chelating agents, either polyhydroxy carboxylic acids,

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polyaminocarboxylic acids, or even polyphosphonates, led to the introduction of colloquial terms, such as “non-acid” to describe these types of fluids (Frenier et al. 2004; Rozo et al. 2007). The use of chelating agents in general has delivered significant advances to the acidizing field, but there are still lingering issues associated with the stability of the chelant itself at high temperatures, the stability of the complex (chelant-metal ion), and the nature of the potential byproducts from prolonged exposure of the chelant to high temperatures, to name some of the most salient (Brezinski 1999; Saneifar et al. 2011). The use of fluorine NMR spectroscopy was introduced in oilfield research precisely to address limitations of other analytical techniques used in HF acidizing (Shuchart and Buster 1995). Its use with chelant-based HF acid fluids can help establish the type of chemical species present in the spent fluid and, from there, potentially derive a stability criterion. Relying on this specific analytical technique, fluorine-19 NMR spectroscopy, for the purpose of assigning the analyte signal to a specific chemical species requires significant background work, such as identification of the appropriate chemical standards for the purpose of quantifying the signal, eliminating signal artifacts, and corroborating the signal identity. The chemical kinetics of fluoro-aluminate speciation shows that the fluoride ligand will strongly bind to aluminum, a fact already established in HF sandstone acidizing. More importantly, the speciation of Al3+/F- is documented with (Bodor et al. 2000) and without (Martinez et al. 1996) organic ligands. One salient and complicating factor from such chemical speciation is that the solvated fluoride ligand can undergo rapid exchange with the actual fluoride bound to the aluminum, in essence, jumping from one place to another. The consequence in the NMR analysis is that there is a signal (or signals) that appears very broad, compared to the single AlF2

+ or AlF2+, and whose chemical shift is in the most upfield region of the spectrum scale. Such species are denoted HF/F- because it is difficult to chemically distinguish one from the other in the experiment. This will be the focus of additional investigation.

The purpose of this paper is to present the results of testing conducted with a biodegradable chelating agent, an aminopolycarboxylic acid (APCA), that is soluble in low-pH aqueous medium and compatible with HF acid. The testing conducted spanned the range 200 to 300°F with HF acid equivalent concentrations of 0.5 to 2% in the pH range 2 to 4. The preliminary findings are reported stemming from testing with a biammonium quaternary compound that makes the HF acid fluid, specifically the spent HF acid fluid, more resilient against fluorometalate (silicon or aluminum) precipitation. The APCA-based fluid can be used, without HF acid, for carbonate acidizing. The non-HF acid fluid was tested on soft carbonate formation substrates, chalk-type, in the 120 to 220°F range. The APCA has biodegradation and toxicity characteristics that few other chelants possess.

Experimental

Sample Preparation. The clay/sand pack mixtures were prepared by blending, in an industrial mixer, moistened clay and sand in the appropriate ratios for a period no less than 30 min. The mixture was gradually added into the confining sleeve by layering the solid mixture and then compacting it with a hard plastic tool. This procedure typically allows the systematic delivery of 1000 g of material into the sleeve, yielding a well-compacted clay/sand column having a 100-mL pore volume (PV). The kaolinite and the sandstone cores employed were obtained from commercial sources, while the illite was obtained from the Clay Minerals Society clay repository. The packed column length was 10-in. for all tests involving the use of clay/sand packs. All flow tests were conducted under 2,000-psi confining pressure.

The cores employed were Bandera and Berea outcrops of low and medium permeability, respectively. The Bandera sandstone consisted of thinly laminated well-sorted micaceous sediments containing a large amount of carbonate; whereas, the Berea sandstone was a carbonate-free low-clay outcrop. The Bandera core contained, as indicated, substantial quantities of dolomite, CaMg(CO3)2, as well as kaolinite and illite in concentrations of 4 and 5%, respectively; sodium feldspar was also present in this core. The mineralogical analysis for each core is presented in Table 1; note that the general chemical formulas are the most representative of the respective mineral group. Table 2 summarizes the test parameters, fluid properties, and results from the ionic analysis of the effluents samples collected from the flow tests where the Bandera cores were used. The Bandera cores employed had an initial porosity of 14% for Test Core A and 9 to 10% for Test Cores B and C. The Berea core had an initial porosity of approximately 21%. In all core flow tests, each core was allowed to stabilize to the respective brine injection while at test temperature. The clay/sand packs prepared had an approximate porosity of 20%. The initial permeability, as well as the final, to the respective brine was determined from the core dimensions, fluid viscosity at respective temperature, and fluid flow rate, as dictated by Darcy’s law for laminar flow in porous media.

Sample Analysis. The samples collected during each flow test were never acidified for the purpose of sample preservation before elemental analysis, and the effluent collected matched the volume of treatment fluid injected. The pH of the fractional volumes was measured with an HF acid-resistant pH probe for every sample, and the probe was calibrated against certified buffers (pH of 1.68, 4.01, and 7.0). The fraction of sample effluent collected in each flow test was 15 mL for core samples having a PV of 12 mL and 20 mL for core samples having a PV of 50 mL.

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Equipment and Instrumentation. The core holder used was a custom-made device housing a rubber sleeve with a 2-in. or 1.5-in. outside diameter (OD), both of which were capable of sustaining 2,000 psi of confining pressure. A pressure regulator located outside the heated zone supplied 800 to 1,000 psi of backpressure. Temperature control was performed with a Eurotherm regulator by means of PID routines using an internal (oven) and external (J-type, contacting the outside of the stainless-steel body of the core holder) thermocouple. Data acquisition was performed using a custom-built LabView® interface. Analytical quantification and characterization was accomplished by inductively-coupled plasma (ICP) optical emission spectroscopy (OES) in a Varian ICP 735-ES instrument, and by NMR analyses performed in a Bruker 500-MHz Avance spectrometer equipped with a dual-band autotunable probe. All samples were analyzed at the specific pH of elution from the respective column experiment; that is, no sample acidification was performed before ICP or NMR analyses. All NMR measurements were performed at room temperature, with no external signal lock, and the spectrometer’s frequency was externally referenced to a 0.1-M NaF ( = -119 ppm) solution before sample analysis. The samples were typically, but not always, diluted in a 1/3 sample/D2O ratio (the large dilution ratio was necessary to minimize deuterium locking and sample shimming difficulties associated with the high ionic strength of the samples, which resulted in baseline distortion). Pulse parameters were set to 90° excitation pulse, 2,048 points acquired, 2-sec interpulse delay, and 1,200 to 4,096 transients acquired, depending on the magnitude of the signal.

Flow Test Conducted with an Illite-Containing Sandstone Core (Bandera Outcrop).

Test A (Continuous Injection at 225°F). The treatment fluid employed was injected in a continuous fashion during the

entire duration of the treatment phase. The treatment fluid’s pH was 2.5, containing the APCA in a 0.6 M concentration and 1% HF acid with 1% biquaternary ammonium reagent and 0.5% surfactant. The brine used in the pre- and post-flush stages was 3% KCl. The core pore and volume dimensions were 50 mL and 1.5 12 in., respectively.

Test B (Three-Sequence at 200°F). The fluid employed for this test consisted of two different treatments; the first

treatment contained only the APCA in a 0.25- and 0.6 M concentration with no HF acid, while the second treatment contained HF acid. Both treatments were performed on the same core, and the core was shut in overnight after the first treatment while maintained at test temperature. The brine used in the pre- and post-flush stages was 7% KCl in both treatments. Each treatment sequence consisted of 250 mL. The core PV was 50 mL. The HF acid treatment stage contained APCA in a 0.25-M concentration and equivalent 1.4% HF acid (2% ammonium bifluoride [ABF]) at pH 2.5 and also 5% of quaternary ammonium reagent to disrupt or alter the interaction of fluoro-aluminate and fluorosilicates with Na+ and/or K+ ions.

Test C (Two-Sequence at 275°F). The HF acid treatment fluid contained APCA in a 0.6 M concentration and equivalent

0.5% HF acid (0.8% ABF) with 5% of the quaternary ammonium reagent at a pH of 4. The brine used in the pre- and post-flush stages was 5% NH4Cl. The PV was 12 mL, and a backpressure of 500 psi was applied in this case. In this test, all of the acidic fluid introduced contained HF acid. The only difference from the first to the second treatment sequence was the flow rate and volume injected: the first sequence introduced 120 mL, while the second used 60 mL. All test flow rates are noted in Table 3.

Flow Test Conducted with Berea Sandstone Core.

Test D (Continuous at 250°F). The fluid employed in this test consisted of 2.2% HF acid with a 0.6 M APCA and 5% of

quaternary ammonium reagent; the fluid pH was 2.8. A backpressure of 800 psi was applied, and confining pressure was 2,000 psi. The brine employed was 5% NH4Cl. The PV was 110 mL, and the core dimension was 2 5 in. The treatment fluid, which was 5 PV, was injected in continuous fashion at a rate of 5 mL/min. The post-flush was delivered until the pH of the effluent returned to the circumneutral range.

Flow Test Conducted with Clay/Sand Pack. The packed column dimensions were 2 10 in. in all tests involving the use of clay/sand packs. All three of the packed columns tests (E, F, and G) were prepared with 15% clay (illite or kaolinite) in mixed quartz (40/70- and 100-mesh sand in a 15:70 ratio).

Test E (Continuous Illite/Quartz at 250°F). Test temperature was maintained at 250°F, and 1,000-psi backpressure was

applied. The treatment fluid injected was 2.2% HF acid and 0.6 M APCA, including the bis-quaternary ammonium reagent at 5%, and the fluid pH was 2.8. The brine employed was only 6% NaCl. The injected volume was 160 mL, an equivalent of 2 PV at 1 mL/min; pre- and post-flush volumes were 220 and 300 mL at a flow rate of 5 mL/min.

Test F (Continuous Illite/Quartz at 300°F). The fluid employed was exactly the same as that used in Test E, the only

difference being the temperature and a treatment flow rate of 3 mL/min.

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Test G (Continuous Kaolinite/Quartz at 300°F). The kaolinite and sand mixture was maintained at 300°F, and 1,000-psi backpressure was applied. The treatment fluid was the same composition as the one used in Tests E and F injecting at 3 mL/min.

Flow Test Conducted with Carbonate Core (Chalk).

Test H (Chalk at 200°F). Chalk-type carbonate cores with nominal permeabilities of 15 md and sizes of 1 × 4 in. were

used. All of the cores were sourced from quarried rock that corresponds to commercially available chalks. The cores were oven dried in a vacuum at 220°F and vacuum saturated with a 3% KCl solution to determine the PV for each core. The cores were then placed in custom-built Hassler core holders (1- × 6-in.) for testing. All tests had 2,000 to 3,000 psi of confining stress applied to the core and a backpressure of 1,000 psi. Tests were conducted at temperatures ranging from 125 to 220°F at flow rates of 1, 5, and 10 mL/min. The treatment fluids were prepared by diluting a stock solution of the APCA, obtained from a chemical manufacturer in its concentrated form, to concentrations of 0.6 M. The pH of the diluted stock solution was adjusted to the desired pH of 1 to 3 with 35% HCl acid, which was monitored with a Mettler-Toledo pH combination electrode with multipoint calibration (1.68, 4.01, 7.00, and 10.01 pH values). Preceding each core flow test, a flush with 3% KCl was performed to obtain a permeability of the untreated core. Once a stable permeability was reached, the cell was shut in under pressure, and the treatment solution lines were flushed with the treating solution to help ensure no dilution by preflush brine would occur. Treating solution was then flowed until the pump pressure achieved the internal pressure of the cell, at which point the fluid was passed through the core and flowed until breakthrough was achieved. Using the measured PV versus amount of treating volume flowed through the core, PVbt was determined for each test to calculate how much treatment was necessary to generate a wormhole. Different flow rates were conducted to determine the PVbt minima for each core set.

Corrosion testing was conducted following established procedures. Additional details and results will be reported in a subsequent communication. Biodegradation tests were performed by an independent third-party entity.

Results It is important to note that all of the solid matrices employed during the course of testing the HF acid/APCA fluid contained clay minerals. It is well-documented that illite is a non-expandable 1:2 aluminosilicate and can contain iron, magnesium, and potassium in its structure. This characteristic is manifested in the ion concentration of Al, Fe, Mg, and even K found in the effluent from the Bandera core and the illite/sand pack. The Bandera core contains naturally deposited or sedimented illite within the pore matrix, while the illite/sand pack has a distribution of artificially but homogeneously placed illite along the entire length of the column. Kaolinite, also non-expandable, only contains Al and Si in its structural formula, aside from oxygen and hydrogen. Lastly, no experiment contained deliberately introduced or placed CaCO3 in the sandstone core or clay/sand pack; any calcium carbonate was naturally present in the porous matrix.

Flow Test Conducted with an Illite-Containing Sandstone Core (Bandera Outcrop).

Test A (Continuous Injection at 225°F). A summary of flow testing conditions and results with Bandera core is provided

in Table 2. The core flow differential pressure and calculated permeability are presented in Fig. 1. The concentration of aluminum was 3500 mg/L at the mid-point of the test and increased as the treatment fluid continued to contact the mineral surface (Fig. 2). Magnesium’s concentration levels were near constant at 4500 mg/L, and iron was 9000 to 11 000 mg/L. No precipitate was observed in samples collected immediately nor after standing (ICP analysis was conducted after several hours of sample collection). The silicon concentration never exceeded 500 mg/L. The brine used in the pre- and post-flush stages was 3% KCl, and this is the reason for the increase in the K+ after the Na+ decreased—an ionic exchange. The treatment fluid pH was 2.5, while the effluent pH was 5 to 6. These results are presented in Fig. 3.

Test B (Three-Sequence at 200°F). The effect of the two different treatments of the APCA, 0.25- and 0.6 M

concentrations, with no HF acid are shown in Fig. 4. In Fig. 5, the same results are presented, excluding the monovalent ions Na and K for clarity and to facilitate observation of the aluminum and silicon, which were in lower concentration. The HF acid treatment stage effluent composition is presented in Fig. 6. The concentration of the Al ions was 2000 to 2400 mg/L, silicon was below 300 mg/L, iron was 4000 to 6000 mg/L, and magnesium was 2300 mg/L. The dissolved calcium was 5000 to 6200 mg/L, and the Na+ was near constant 16,000 mg/L (not shown).

Test C (Two-Sequence at 275°F). The HF acid equivalent concentration in this treatment fluid was 0.5%, having 0.6 M

APCA and 5% of the quaternary ammonium reagent at a pH of 4. The results of this test are shown in Fig. 7. The concentration of Al3+ was 1000 mg/L in the first sequence and 3000 mg/L in the second sequence, silicon was below 300 mg/L, iron was 5000 mg/L in the first sequence and 2000 to 4000 mg/L in the second sequence, and the magnesium was 1300 to 1400 mg/L in both sequences. The dissolved calcium was 4000 to 5000 mg/L and 3000 to 3600 mg/L in the first and second sequences, respectively. The Na+ was near constant at 40,000 mg/L in the first sequence, but in the second sequence only reached 32 000 mg/L.

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Flow Test Conducted with Berea Sandstone Core. Test D (Continuous at 250°F). In Test D, an equivalent of 5 PVs of the treatment fluid were injected in continuous

fashion at a rate of 5 mL/min. This high flow rate was chosen to show the effect of higher flow rates in a short core, which was still longer than most typical core samples used in HF acid testing. The effluent composition is shown in Fig. 8, and it is apparent that the concentration profile of the major ions oscillated significantly. The fluid also contained the highest concentration of HF acid (2.2%).

Flow Test Conducted with Clay/Sand Pack.

Tests E and F (Continuous Illite/Quartz at 250 and 300°F). The results for the 250 and the 300°F tests are presented in

Figs. 9 and 10, respectively. In Test E, 2 PVs of treatment fluid were delivered, which rapidly produced dissolved aluminum concentrations in the range of 10,000 ppm (see Fig. 9 for specific trend). It is also evident from Fig. 9 that the concentration of sodium corresponded to that of the initial NaCl brine (5.8% measured) and then remained constant during the HF acid stage, between 35,000 and 40,000 ppm Na. In this paper, there is no attempt to construct a mathematically derived kinetic or thermodynamic model, but the results reported are consistent with present understanding of complexation chemistry. The concentration of Fe2+ peaked at 6,500 ppm and began to decrease steadily as the treatment fluid traveled through the core. The concentration of aluminum in Test F actually reached 12,000 to 17 000 mg/L—the high temperature likely played a determining factor—and the silicon was sustained at near 500 mg/L.

Test G (Continuous Kaolinite/Quartz at 300°F). Flow Test G also produced very large quantities, 10,000 mg/L, of

dissolved aluminum and 500 mg/L of silicon. The dissolved sodium and potassium are also shown in Fig. 11. The respective PVs and rates are reported in Table 4.

In addition, testing parameters corresponding to Tests C through G are provided in Tables 2, 3, 5, 6, 7, and 8. Fluorine-19 NMR Analysis. The analyses of all tests will be fully reported separately. In this paper, only the results for

Test A are presented. All signals detected are reported in the following sections and Table 9; for comparison, Table 10 lists the chemical shift of the equilibrium species as a function of pH for the HF/F- pair. No signal appeared at the frequency at which fluorosilicates resonate, in the range of -128 to -129 ppm (with respect to NaF at -119 ppm). All signals detected, from -154 to -168 ppm, correspond to the fluoroaluminates or ligand-bound fluoroaluminates. A signal at -146 ppm could be ascribed to the bifluoride anion. Assignment of the signals at -163 and -167 ppm is based on literature data indicating the exchange of fluoride/HF acid with fluoride-Al3+-ligand (these assignments are still under investigation).

Biodegradation. Biodegradation testing results and toxicity data were obtained independently and are reported herein for

the purpose of disclosure. Other biodegradable chelating agents are reported by Frenier et al. (2003) and more recently by Mahmoud et al. (2010).

• Partition coefficient: log Pow <-4 (25°C). • Adsorption/desorption log Koc: < 1.5 (25°C). • Toxicity: low. • Ready biodegradability: OECD 301.

Corrosion Testing The results from one representative test conducted at 266°F on a chrome alloy (2205 Cr) with the HF acid/APCA fluid showed acceptable corrosion loss (< 0.05 lbm/ft2) using a corrosion inhibitor (0.7%) suitable for this type of application. For the purpose of comparison, the non-HF acid fluid was tested at a higher temperature of 300°F for up to 24 hr; the corrosion loss in this case was 0.0002 for a 0.1% inhibitor loading. Additional testing parameters are presented in Table 11.

Discussion Only results corresponding to Test A differential pressure measurement are presented here. The differential pressure decreased from 185 psi before HF acid treatment to 68 psi after treatment; such measurements were performed with respect to brine. This corresponds to a change in permeability by a factor of 2.5. A notable difference from this test compared to all others reported here is the length of treatment fluid injection. The total volume of fluid was 500 mL (10 PVs) through a 12-in. long core. All other tests were performed on shorter cores, but none was shorter than 5-in. The results from Tests A and B indicate that aluminum was present in the “spent” fluid, or the effluent, in concentrations that were in accordance with HF acidizing standing principles. In Test B, two separate sequences of a non-HF acid-containing treatment fluid, at two distinct concentrations, were injected to corroborate the assertion that, to dissolve aluminosilicate minerals, an HF acid-containing fluid is necessary unless temperatures in excess of 300°F are encountered. Distinctively, the results presented here demonstrate that the fluid employed can mitigate the potential precipitation, and/or formation, of CaF2. The dissolved calcium in the HF acid phase, as found in the effluent from Tests A and B and shown in Figs. 2 and 6, respectively, was

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14000 and 5000 to 6000 mg/L. No solid precipitates were detected in the effluent post-column. Notably, this type of sandstone core, a Bandera core having mixed mineralogy, has been reported to respond negatively to an HF acid/chelating agent (fluid pH of 4) at 300°F (Mahmoud et al. 2011). One difference, aside from temperature, is that the pH of the treatment fluid used in Tests A and B was 2.5. The results of Test A further indicate that the pH of the spent fluid never decreased below a value of 5, always oscillating between pH 5 and 6; these results are summarized in Table 2 and are also presented in Fig. 3. Fig. 3 also shows the concentration of the monovalent ions Na and K, along with the Al and Si.

Test B included two preflushes of APCA only, at a pH of 2, with fluid in a 0.25- and 0.6 M concentration. This test confirmed that a chelating agent fluid, as previously discussed by others, can be used to remove calcium-induced damage in a sandstone matrix whereby only, or preferentially, the calcium mineral is dissolved, leaving the aluminosilicates relatively intact. The concentration of the divalent ions (Fe, Mg, Ca) shown in Fig. 4 clearly demonstrates this. Furthermore, in reference to Test C (Fig. 4), it is of significance that the concentration of all ions in the second phase never stabilized or attained any level of equilibrium or quasi-equilibrium. This observation is derived from the continuous and steep increase of all the ions’ concentration, which also equally rapidly decreased as the treatment fluid was replaced by the brine. The specific consequences of this observation are under investigation. While the testing parameters are different—specifically, temperature and treatment fluid properties—the results and observations point to significant conclusions. Namely, the presence of Ca ions in spent HF acid fluid can be, within the boundaries of the testing implemented, controlled and CaF2, and perhaps other fluorometallates, can be mitigated.

Test C (Two-Sequence at 275°F). The amount of dissolved calcium in Test B’s HF acid phase (between 5000 to 6000 mg/L) was between the levels detected from the two non-HF acid stages—4000 and 8000 mg/L because of the 0.25- and 0.6 M (pH of 2) APCA fluid. The HF acid fluid only contained a 0.25-M concentration of chelating agent. Further analysis of this data is warranted to clearly discern the potential implications stemming from these tests, but, qualitatively, the results indicate that Ca2+ does not precipitate as a result of the HF acid presence. In Test A, where no preflush with APCA was employed and the concentration of the APCA was 0.6 M, the amount of dissolved calcium was 14,000 ppm, and this test was performed at 225°F, contrasting with Test B at 200°F.

Test D (Continuous at 250°F). The dissolved silicon reached the highest concentration in all tests conducted, albeit peaking and then steadily decreasing. Notably, the silicon levels reached 1000 to 1500 mg/L only after post-flush was introduced and began to elute from the core; so, while the aluminum attained near 3000 mg/L levels during the HF acid phase, the corresponding silicon levels were below 500 mg/L. This effect could be caused by residence time transients related to the flow rate and/or the length of the core. The fact that the aluminum levels reached 3000 mg/L, as in the Bandera core tests, indicates that the aluminosilicate minerals were being dissolved. The concentration of the iron peaked rapidly at 5000 mg/L and then decreased. The calcium followed this trend, peaking at a lower level (4000 mg/L). The concentration of potassium was less than 1000 mg/L. The sodium profile showed the most significant demarcation from all other ions, as well as from other tests reported here. Sodium peaked at 40 000 mg/L in an increasing manner, leveled at near 24 000 mg/L while still injecting treatment fluid, and then began to decrease as ammonium chloride exchanged the sodium.

Test F (Continuous Illite/Quartz at 300°F). The test conducted with illite only entailed 2 PVs of treatment and rapidly produced dissolved aluminum concentrations in the range of 10,000 ppm (see Fig. 10 for specific trend). It is also evident from Fig. 10 that the concentration of sodium corresponded to that of the initial NaCl brine (5.8% measured) and then remained constant during the HF acid stage, between 35,000 and 40,000 ppm Na. In this work, no attempt is made to construct a mathematically derived kinetic or thermodynamic model, but the results reported are consistent with the present understanding of complexation chemistry. The concentration of Fe2+ peaked at 6,500 ppm and began to decrease steadily as the treatment fluid was still traveling through the core.

Test G (Continuous Kaolinite/Quartz at 300°F). The effluent analysis serves to show that the concentration of dissolved aluminum and silicon, the only cations that comprise the kaolinite lattice, were present in similar concentrations to organic HF acid blends and even a mud acid (10,000 mg/L of Al3+). Silicon was found between 700 and 500 mg/L, most likely because of the higher temperature of this test, as also found in Test F (illite at 300°F). The potassium in this test came from the brine used as a pre and post-flush, and its concentration, 10,000 mg/L, decreased during the treatment phase as a result of the ion exchange with sodium detected at near 32,000 mg/L.

Fluorine-19 NMR Signal Assignment. The signals detected in the Fluorine-19 NMR spectrum analysis for different fractions collected during the respective flow tests are provided in Table 9. There were three sets of signals detected from all acid flow experiments, which reflect the final equilibrium products from the various reactions of HF acid and chelant occurring at temperatures above 200°F on and with aluminosilicate substrates. These products have already been identified and correspond to fluoroaluminates and fluorosilicates (Shuchart and Buster 1995). The fluorosilicates specifically correspond to penta- and/or hexafluorosilicate species (SiF5

- and SiF62-) resonating at -128 to -129 ppm (low field side of the

frequency scale); the fluoroaluminates resonate in the -150 to -155 ppm range, and these are AlF3, AlF2+ and AlF2+ (the order

of precedence of these species in the NMR frequency scale is exactly this). The fluorosilicate species, SiF5- and SiF6

2-, were

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not detected in any of the experiments conducted above 200°F for the most part; in some instances, there was a signal for such species, but only when the pH of the solution was below 2. This also correlates to observations derived from all NMR analyses, to be reported separately, that corroborate the loss of fluorosilicate species above a pH of 2. Gdanski (1997) concludes that the precipitation of fluorosilicates is not significant at temperatures in excess of 250°F in HCl acid aqueous media; while, at temperatures between 150 and 225°F, fluorosilicate precipitation could be highly problematic. However, this strictly pertained to HCl acid-based fluids. In all testing conducted with the APCA/HF acid fluids, the pH of the starting (treatment) solution was 2 or greater (0.01 mol/L nominal H+ concentration). This essentially corresponds to a “spent HF acid” fluid originating from an HCl/HF acid treatment fluid. The significance of these results is that the actual “spent” fluid contains the AlFx

(3-x)+ species, as well as complexed fluoroaluminate species. Such complexation, or stabilization, of the fluoroaluminate species prevents the precipitation of AlF3 at F/Al > 2.5 or at pH > 2.5 to 3. All signals detected, from -154 to -168 ppm, corresponded to the fluoroaluminates or ligand-bound fluoroaluminates. A signal at -146 ppm could be ascribed to the bifluoride anion, though it is difficult to come to terms with the remaining unreacted “HF acid” in the effluent (spent acid) from the core. The signal detected could also have been exchanging with an Al-ligand (the APCA), but this requires additional investigation. The lack of silicon fluoride signals in the fluorine-19 NMR precisely correlates with the low silicon concentrations in the ICP effluent analysis. This is a consequence of the secondary reaction fully proceeding to completion and converting all silicon fluorides to orthosilicic acid, which then proceeded to condense silicate oligomers until silica was formed (Gorrepati et al. 2010).

Previous work by Shuchart and Buster (1995) ascribes a signal at -85.5 ppm (with respect to trifluroacetic acid) to HF acid (this signal would be at -162 to -161 ppm when referencing NaF 0.1 M). In such work, the aqueous medium was 1.4-M HCl acid, so the presence of free HF acid under such conditions is entirely possible, as reported for a ~48% HF solution (Connick and Poulson 1958). The work reported in this paper corresponds to NMR analyses conducted on effluent samples collected post-acid treatment, and the pH of such samples ranged from a pH of 0.1 to 5. From Table 10, it is evident that the chemical shift of the HF/F- species, as expected, is dependent on pH. The HF species is only present at a pH less than 3; while, at a pH greater than 4, the sole form is fluoride (F-). Near the pH corresponding to the acid dissociation constant for HF acid, with a pKa of 3.1, and at the activity of the fluoride ion, the chemical shift was -139 ppm, reflecting that, in the NMR time scale, the detected species corresponded to the bifluoride ion, HF2

- (Mitra and Rimstidt 2009). Rietjens (1998) reports on the defluorination reaction of AlFx by citrate ion as a function of pH. A single peak was

detected for AlFx complexes from a pH of 3 to 5, though significant line broadening could be observed as the pH of the solution increased. Undoubtedly, at a pH of 5 and 6, the fluorine-19 signal showed signs of extreme exchange, ligand exchange, which corresponds to decomplexation of fluoride from aluminum. The single resonance signal detected (at -78 ppm with respect to trifluoroacetic acid) in the fluoroaluminate region was described as an AlFx species; whether this signal was directly related to the citrate ligand was not explicitly identified. The pH range of all solutions analyzed in this work only spans pH of 0.1 to 5 and, in this range, the fully deprotonated organic ligand cannot realistically form. To generate the deprotonated species of the APCA ligand, the aqueous solution must be at a pH greater than 8, and this circumstance was absolutely outside the working pH of any solution tested.

APCA Low-pH Carbonate Core Flow (Chalk-Type). As with most core flow studies, mineralogy and rock lithology, naturally, played a key role; however, the following test revealed that pH and the corresponding PVbts were dependent as a function of temperature. Trends included core flood treatments performed at temperatures of 125°F and showed optimal dissolution with a lower pH; while treatments at 220°F were even more pronounced, showing again that a low pH was desirable (Fig. 12). Results from this testing matrix highlight the distinct reactivity of this chelant where, regardless of pH and temperature, all tests gave, in orders of magnitude, enhanced permeability, and, in no instance, was core matrix damage observed. Contrarily, to push the boundaries of the flow matrix, excessively high flow rates (greater than 15 mL/min on 1- 4-in. cores; not shown) were conducted in which wormhole generation was still achieved and where minimal facial dissolution was the only negative result observed.

Initial studies included core flood experiments (pending publication), which began with a pH ranging from 1 to 4. From this screening processes, it was determined that tests using a pH of 4 did not provide low PVbt compared to tests at a lower pH; therefore, any further core flow studies involving chalks implemented the optimal pH of 1 to 3 to ascertain optimal PVbts.

Following treatment, characterization of these core flow studies was conducted on the spent fluid by ICP-OES/pH and of the core by computerized tomography (CT) scanning. Analytical characterization of these initial screening tests brought to light the chelant’s ability to produce efficient wormhole generation while maintaining in solution dissolved calcium, a property that is magnified because of the ease with which the acidic fluid spends as it is transported through the matrix. Results from the core flow test of this fully biodegradable APCA are presented in Fig. 13.

Conclusions The initiative to obtain fluids with an improved environmental footprint that possess distinct advantages, such as stability at higher temperature, broader pH activity, and stronger complex formation, has been facilitated by the use of chelating agents. In the area of HF acidizing chemistry, the conceptualization of the unique HF acid reactions on clays and silica surfaces—namely, kinetic controls over the so called primary, secondary, and tertiary reactions—has facilitated fluid (Husen et al.

8 SPE 165142

2002) designs that can handle such varied reactions. The work presented here describes the development of a new acidizing fluid containing an environmentally relevant chelating agent that satisfies the criteria of HF acidizing fluids.

The fluid has the following properties: • APCA-based. • Readily biodegradable. • Effective from 120 to 300°F based on testing reported herein. • Can be efficiently used at a pH of 1 to 4, with a pH of 2 being the optimum for most applications. • Effective concentration can be 0.6 to 0.2 M, depending on pH. • Compatibility with HF acid has been corroborated. • Minimizes corrosion concerns.

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Frenier, W.W., Rainey, M., Wilson, D., Crump, D., and Jones, L. 2003. A Biodegradable Chelating Agent is Developed for Stimulation of Oil and Gas Formations. Paper SPE 80597 presented at the SPE/EPA/DOE Exploration and Production Environmental Conference, San Antonio, Texas, USA, 10–13 March. http://dx.doi.org/10.2118/80597-MS.

Frenier, W., Brady, M., Al-Harthy, S., Arangath, R., Chan, K.S., Flamant, N., and Samuel, M. 2004. Hot Oil and Gas Wells Can Be Stimulated Without Acids. Paper SPE 86522 presented at the SPE International Symposium and Exhibition on Formation Damage Control, Lafayette, Louisiana, USA, 18–20 February. http://dx.doi.org/10.2118/86522-MS.

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Gdanski, R.D. 1997. Kinetics of the Secondary Reaction of HF on Alumino-Silicates. Paper SPE 37214 presented at the SPE International Symposium on Oilfield Chemistry, Houston, Texas, USA, 18–21 February. http://dx.doi.org/10.2118/37214-MS.

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Rietjens, M. 1998. Decomplexation of aluminium-fluoride complexes by citrate-based buffers as a function of pH, aluminium and fluoride concentrations. Anal. Chim. Acta. 368 (3): 265–273. http://dx.doi.org/10.1016/S0003-2670(98)00176-7.

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TABLE 1—XRD ANALYSIS OF SANDSTONE SUBSTRATES USED IN FLOW TESTS

Mineral Composition Bandera (%) Berea (%)

Quartz SiO2 62 89

Na-feldspar NaAlSi3O8 14 1

K-feldspar KAlSi3O8 0 4

Calcite CaCO3 1 0

Dolomite CaMg(CO3)2 14 2

Kaolinite Al2Si2O5(OH)4 4 2

Illite/mica (K,H3O)(Al, Mg, Fe)2(Si, Al)4O10[(OH)2,(H2O)] 5 0

Chlorite (Mg, Fe)5Al(AlSi)3O10(OH)8 0 2

TABLE 2—SUMMARY OF FLOW TESTING CONDUCTED WITH BANDERA CORE

Flow test A B C

Temp (°F) 225 200 275

Fluid injection Continuous Sequential Sequential

— First Second Third First Second

Volume (mL) 500 250 250 250 120 60

PV 10 5 5 5 10 5

Flow rate (mL/min) 2 2 2 2 2 1

Treatment Fluid

Chelant 0.6 M 0.25 M 0.6 M 0.25 M 0.6 M 0.6 M

pH, [HF acid %] 2.5 [1] 2.0 [0] 2.0 [0] 2.5 [1.4] 4.0 [0.5] 4.0 [0.5]

Effluent pH 5.16 to 6.25 6.25 to 7.55 5.68 to 7.29 5.40 to 5.92 6.21 to 7.46 6.72 to 7.34

Effluent ionic Composition

Al (mg/L) 3500 to 4500 250 500 2400 1000 3000

Ca (mg/L) 13 900 to 15 500

3800 8000 6000 5000 3600

Mg (mg/L) 4600 1100 2500 2300 1500 1300

Fe (mg/L) 9300 to 10 900 2600 5400 5600 6000 3800

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TABLE 3—FLUID TREATMENT SEQUENCE FOR FLOW TEST A AT 225°F

Fluid Direction Flow Rate (mL/min) PV

3% KCl Inj 2 4

HF acid-stage Inj 2 10

3% KCl Inj 3 7

TABLE 4—FLUID TREATMENT SEQUENCE FOR FLOW TEST G AT 300°F

Fluid Direction Flow Rate (mL/min) PV

KCl Inj 5 2

HF stage Inj 3 2

KCl Inj 5 5

TABLE 5—FLUID TREATMENT SEQUENCE FOR FLOW TEST B AT 200°F

Fluid Flow Rate (mL/min) PV

7% KCl 3 5

APCA 0.25 M pH 2 2 5

7% KCl 3 10

APCA 0.6 M pH 2 2 5

7% KCla 3 10

HF acid-stage 2 5

7% KCl 3 5 aFollowed by shut-in (16 hr).

TABLE 6—FLUID TREATMENT SEQUENCE FOR FLOW TEST C AT 275°F

Fluid Flow Rate (mL/min) PV

3% KCl 5 50

5% NH4Cl 5 8

HF acid-Stage 1 2 10

5% NH4Cl 5 5

3% KCl 1 8

5% NH4Cl 5 to 20 21

HF acid-Stage 1 1 5

5% NH4Cla 1 to 20 50

TABLE 7—FLUID TREATMENT SEQUENCE FOR FLOW TEST E AT 250°F

Fluid Direction Flow Rate (mL/min) PV

6% NaCl Inj 5 3

HF acid-stage Inj 1 2

6% NaCl Inj 5 5

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TABLE 8—FLUID TREATMENT SEQUENCE FOR

FLOW TEST F AT 300°F

Fluid Direction Flow Rate (mL/min) PV

Organic ammonium

Inj 5 3

HF acid-stage Inj 3 2

Organic ammonium

Inj 5 4

TABLE 9—FLUORO-CONTAINING SPECIES DETECTED IN EFFLUENT FROM TEST A

Chemical Shift (ppm) Assignment

-146.4 HF2-

-154.7 AlFL

-155.7 AlF2L

-163.6 Complexed Al-F

-167.4 Complexed Al-F

TABLE 10—FLUORINE-19 CHEMICAL SHIFT OF HF/F- SPECIES AS FUNCTION OF pH

pH Chemical Shift

1 -159.5

2 -159

3 -139

4 -120

5 -119

7 -119

TABLE 11—CORROSION TESTING COMPARISON

Temperature (°F) Acid pH Time (hr) Alloy Corrosion Inhibitor Additives

Corrosion Loss (lbm/ft2)

266

0.6 M APCA

3 6 2205 (22Cr) 0.7% Derivatized

tall oil

5% Bis-quaternary ammonium

0.049

1.3% HF acid

300 0.6 M APCA 2.5 24 2205 (22Cr) 0.1% Derivatized

tall oil — 0.0002

300 0.6 M APCA 2.5 24 2205 (22Cr) 0.3% Derivatized

tall oil — 0.0004

12 SPE 165142

Fig. 1—Differential pressure and permeability for Flow Test A.

Fig. 2—Effluent composition from Flow Test A, 225°F, using 1% HF acid and 0.6 M APCA fluid.

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Fig. 3—Effluent ionic analysis from Bandera core Flow Test A showing pH of effluent.

Fig. 4—Effluent ionic analysis from Core Flow Test B at 200°F with pH of 2 APCA (1) first sequence at 0.25-M and (2) second sequence at 0.6 M concentration.

14 SPE 165142

Fig. 5—Same as Fig. 4, Test B, without Na and K ions.

Fig. 6—Effluent ionic analysis from Flow Test B, third sequence (pH 2.5, APCA 0.25 M, and HF acid 2.8%).

SPE 165142 15

Fig. 7—Effluent for Flow Test C at 275°F.

Fig. 8—Effluent analysis of Flow Test D, Berea core, at 250°F.

16 SPE 165142

Fig. 9—Effluent analysis Test E on illite/sand pack at 250°F.

Fig. 10—Effluent analysis Test F on illite/sand pack at 300°F.

SPE 165142 17

Fig, 11—Effluent analysis Test G on kaolinite/sand pack at 300°F.

Fig. 12—PVbt curves for chalks at 220°F.

18 SPE 165142

Fig. 13—PVbt curves for chalks at 125°F.