Bromate Oxidation of Ammonium Salts: In Situ Acid Formation for

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Bromate Oxidation of Ammonium Salts: In Situ Acid Formation for Reservoir Stimulation Katherine L. Hull,* Amy J. Cairns, and Marium Haq Aramco Services Company: Aramco Research Center − Houston, 16300 Park Row, Houston, Texas 77084, United States

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S Supporting Information *

ABSTRACT: A redox chemistry approach has been employed to synthesize an assortment of acids in the subterranean environment for the purpose of enhancing productivity from hydrocarbon-bearing rock formations. Experimental studies revealed that bromate selectively oxidizes a series of ammonium salts NH4X where X = F−, Cl−, Br−, SO42−, and CF3CO2− to produce 5−17 wt % HX. Importantly, the in situ method allows strategic placement of the acid in the zone of interest where the fluid is heated, and the reaction is triggered. Ammonium counteranions are shown to influence the kinetics of the bromate-ammonium reaction, and the conditions are tailored to promote oxidation of ammonium at reservoir temperatures. The reaction is observed to be acid-catalyzed, where the formation of bromous acid (HBrO2) is involved in the rate-limiting step. As a result, an induction period that scales with the pKa of the acid being formed is followed by rapid formation of the reaction products. products. In sandstone rock acidizing,16,17 by contrast, the procedure is inherently complex as it typically consists of a three-stage process involving a series of chemical and physical interactions between the fluid and the reservoir rock. First is a preflush of HCl, then a main flush consisting of an HCl-HF mud acid stage, followed by an after/postflush of HCl or ammonium chloride (NH4Cl). In both cases, the corrosivity, potential to form insoluble products, and rapid reaction kinetics, coupled with concerns over safety and environmental protection, continue to pose a challenge for controlled acid placement to specific zones of interest in the subterranean environment. To address these limitations, the oil and gas industry has identified various strategies to delay acid release. These include but are not limited to acid-in-diesel emulsions (i.e., emulsified acids)18−20 or encapsulation methods such as polymer core− shell21 and nanoparticle-based systems.22 Additionally, uses of polymers and gelling agents, organic acids, acid-generating systems, and delay additives have been extensively reported.23−27 Regardless of the chemical approach, the objective is the same, that is, to provide a means of minimizing the reaction between acid and the metal tubulars or slowing the reaction between acid and rock to promote deeper penetration of live acid into the desired pay zone and restore/increase permeability. Herein, we unveil a new stimulation strategy that leverages the attractive attributes of the ammonium oxidation process as a means of enhancing production via the development of a so-called two-in-one in situ acid system, that is, acid generation on-demand in the presence of gaseous

1. INTRODUCTION Oxidation of methane and ammonium, two of the simplest species in nature, are highly relevant processes across many industries, including oil and gas. Methane oxidation to form methanol,1,2 light olefins,3 benzene,4 or other products is an active area of development and utilization. Meanwhile, ammonium (NH4+) oxidation is an important process that affects not only terrestrial ecosystem productivity5−8 but also a number of other applications including ammonia scrubbing via electrochemical hypochlorination9 and energetic materials.10,11 A fundamental understanding of these processes will continue to drive technology development across industries. Recently when exploring oxidizing systems for reservoir rock stimulation applications in the subterranean environment, it was observed that a reaction occurs between bromate (BrO3−), a strong oxidizer, and NH4+ whereby H+ is liberated. The acid generated from this process was then tailored for potential applications through which in situ acid generation was beneficial for placement of reagents at relevant subterranean sites. Reservoir stimulation using acid systems, whether mineral acids, organic acids, or combinations thereof, has been extensively studied for an assortment of downhole applications as a means of restoring permeability. These applications range for example from carbonate acidizing12−14 to formation damage removal in sandstone rock formations to mitigation of acid-soluble forms of scale.15 Accordingly, the type of acid(s), concentration and required volume are known to vary with each application, and not surprisingly proper selection is critical for the success of the treatment. In carbonate rock acidizing, for example, hydrochloric acid is the most prevalent mineral acid used due to its availability, low cost, low viscosity, high dissolution power, and soluble reaction © XXXX American Chemical Society

Received: October 11, 2018

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DOI: 10.1021/acs.inorgchem.8b02891 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry products using cost-effective chemical reagents that are readily used in the industry. The approach was tailored to produce a series of acids, having a wide range of pKa values, by reacting NaBrO3 with alternative ammonium-based salts, producing in situ generated acids that are relevant for subterranean applications. To gain a deeper understanding of reactivity, a series of anions were explored including sulfate, acetate, and other polyatomic ions as well as various halides. As the ammonium salts were oxidized with bromate, their products were analyzed by electrospray ionization mass spectrometry (ESI-MS), and acid formation was quantified via acid−base titration methods. Reagent concentrations were optimized to maximize acid yield, and reaction conditions were tailored to encourage acid formation in the presence of various counteranions. The role of counteranion on the kinetics of the bromate-ammonium reaction system is shown in relation to the pKa of its conjugate acid.

2. EXPERIMENTAL SECTION 2.1. Chemical Reagents and Materials. All chemicals and solvents used in the preparation of the compounds described herein were of reagent grade and used without further purification (Table S1). (WARNING: This series of reactions should be handled with caution. Acidifying solutions containing ammonium salts and sodium bromate at room temperature in an open vessel in some cases resulted in autoignition of the gas evolved from the solution.) 2.2. Series A: Reaction of NH4X (X = Cl, Br, and H2PO4) and NaBrO3. In a typical experiment, a solution consisting of NH4X and NaBrO3 in 25 mL of DI (deionized)-H2O was prepared in a 120 mL Ace Glass pressure tube. The tube was then capped and heated in a programmable silicone oil bath at temperatures up to 150 °C for a time frame up to 3 h to give a transparent solution. The generation of acidic solutions led to the evolution of bromine (Br2) gas as a side product as clearly evident from the distinct orange color. In the case of each counterion, a series of independent experiments were performed, whereby the ratio of NH4+ to BrO3− was systematically varied (using 5 mmol BrO3− as the baseline) in order to determine the optimal stoichiometry required to potentially reach the theoretical acid concentration. The ratios tested ranged from 1:1, 1.33:1, 1.67:1, 2:1, 5:1, and 10:1 (Figures 1 and S1). All solutions were cooled to room temperature, after which an acid−base titration was conducted to determine the acid concentration (mmol). 2.3. Series B: Reaction of NH4X (X = F, HF2, PF6, BF4) and NaBrO3. In a typical experiment, a solution consisting of NH4X and NaBrO3, DI-H2O (25 mL) was prepared in a 125 mL Parr (#4748) acid digestion vessel equipped with a polytetrafluoroethylene (PTFE) liner. The capped liner containing the solution was then placed into the reaction vessel. One corrosion disc (#310AC) was then placed on top of the PTFE liner lid followed by the addition of one rupture disc (#311AC). To complete the assembly, the pressure plate (#306AC) was added in addition to two spring washers and a compression ring. The cap was then carefully secured with compression screws. The secured vessel was then heated at temperatures up to 150 °C for a time frame up to 3 h to give a transparent solution. The ratios, i.e., NH4+ to BrO3−, evaluated under these conditions were 1:1 and 2:1 (refer to Figure S3). All solutions were cooled to room temperature, after which an acid−base titration was conducted using an indicator to determine the acid concentration (mmol). 2.4. Series C: Reaction of (NH4)2X (X = SO4, HPO4) and NaBrO3. In a typical experiment, a solution consisting of NH4X and NaBrO3 in DI-H2O (25 mL) was prepared in a 120 mL Ace Glass pressure tube. The same procedure was followed as described for Series A, except the ratios tested were limited to 1:1 and 2:1. In the case of the (NH4)2X (X = HPO4) system, a negligible amount of acid was generated in the case of both ratios after soaking for 3 h at 150 °C. Conversely, where X = SO4, the amount of acid generated was

Figure 1. (a) Amount of acid generated based on using variable amounts of NH4Cl in the presence of 5 mmol of sodium bromate as acid generating precursors. Under analogous conditions, no reaction was observed at 60 °C. (b) Concentration of generated acid as a function of ammonium concentration upon reacting 2:1 NH4+/ BrO3−. measured and found to be 1.6 and 9.2 mmol for the 1:1 and 2:1 ratios under analogous conditions. 2.5. Series D: Reaction of (NH4)2S2O8 and NaBrO3. In this series of experiments, a solution consisting of two inorganic oxidizers, i.e., (NH4)2S2O8 and NaBrO3, in DI-H2O (25 mL) was prepared in a 120 mL Ace Glass pressure tube. The same heating procedure was followed as described for Series A and C. However, the uniqueness of this system, with respect to the unexpectedly high concentration of acid generated under analogous testing conditions, led to a more indepth evaluation into the impact of the reagent ratio on acid yield. Accordingly, the tested ratios included 1:1, 3:1, 5:1, 10:1, 20:1, and 30:1. 2.6. Series E. Reaction of NH4Cl and NaX / KX (X = ClO3 and IO3). In order to validate that NaBrO3 is the requisite oxidizer needed to produce acid under these conditions, two independent experiments were performed whereby NaBrO3 was substituted for alternative inorganic oxidizers having similar properties. Accordingly, a solution consisting of NH4Cl and NaX or KX, DI-H2O (25 mL) was prepared in a 2:1 ratio in a 120 mL Ace Glass pressure tube and heating for 3 h at 150 °C. As expected, the solutions remained colorless with no evidence of acid generation as the pH values for the heating solutions were measured to be 6.88 and 6.84 for the chlorate and iodate analogues, respectively.

3. RESULTS AND DISCUSSION NaBrO3 (1.5 eV) and ammonium persulfate ((NH4)2S2O8, 1.8 eV), commonly used oxidizers in upstream applications, were recently exploited as a mixed oxidizer system for degrading B

DOI: 10.1021/acs.inorgchem.8b02891 Inorg. Chem. XXXX, XXX, XXX−XXX

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frame. Initially, upon visual inspection the transparent solution remained colorless, and then suddenly the solution and surrounding space in the vessel became orange (signature of Br2 formation). It should be noted that the time scale of this induction period varied according to temperature and concentration conditions. Lowering the pH to the pKa range of HBrO2 also reduced or nearly eliminated the induction time, as will be discussed later. Reaction solutions were typically only observed in one of two states, regardless of the amount of time allowed for the reaction: (1) colorless with neutral pH (essentially no reaction) or (2) orange with stoichiometric acid formation. The ratio of NH4+ to BrO3− was systematically varied, and the optimal reaction stoichiometry was established to be 2:1 NH4/BrO3−. Figure 1a shows the results of combining 5 mmol of NaBrO3 with varying concentrations of NH4Cl. The solutions were analyzed via titration with NaOH (1 M) to determine the amount of H+ generated by the reaction. Five mmoles of NaBrO3 and at least 10 mmol of NH4Cl produced 8.36 (±0.21) mmol of H+. In other words, for every NH4+ molecule oxidized, 0.83 protons are released. By contrast, microbial NH4+ oxidation produces 2 protons per ammonium molecule.32 Concentrations of NH4Cl and NaBrO3 were varied by almost 10-fold, and the generated acid scaled linearly as shown in Figure 1b. Reactions were also performed under saturated conditions (2:1 NH4Cl/NaBrO3) and yielded stoichiometric acid. Notably, substitution of sodium chlorate (NaClO3) or sodium iodate (NaIO3) for NaBrO3 did not produce any acid under analogous experimental conditions. ClO3− and IO3− appear in this case to be inhibitors rather than coreagents, confirming the unique mechanistic pathway for producing acid using NaBrO3 as an oxidizer. Longer reaction times up to 24 h and higher concentrations also did not result in NH4+ oxidation by these isoelectronic oxidizing compounds. The potassium salts of ClO3− and IO3− were also not reactive under the same conditions, although KBrO3 yielded the same results as NaBrO3. This verifies that the monocation is a spectator in these oxidation reactions. The nonreaction of IO3− under source-rock conditions is significant here, in contrast to prior related systems. For example, although not reactive toward NH4+, iodate-based systems have been used to oxidize CH4 and other light alkanes in the presence of Cl−.33 Solution characterization was performed to identify other reaction products present. Electrospray ionization mass spectrometry (ESI-MS) was performed on product solutions after heating 2:1 NH4Cl/NaBrO3 for 3 h at 100 °C. Interestingly, no nitrogen-based species such as nitrite (m/z 46) or nitrate (m/z 62) could be identified in any spectra, in positive or negative ion mode. This indicates that all the nitrogen has evolved as a gas from the reaction mixture. Similarly, Fourier transform infrared (FT-IR) spectra of the solutions show the complete loss of the N−H peak and no appearance of N−O peaks. Figure 2 shows the ESI-MS spectra obtained both directly after the reaction and also after being titrated with NaOH. The reaction solution is dominated by bromine-based species, which is consistent with the orange color characteristic of Br2. As is typical of halogens in the ESIMS environment, a variety of polyhalide ion clusters are readily formed.34 Table S2 provides a list of potential m/z peaks for the systems studied. Peaks that were observed are highlighted in Figure 2.

organic matter to improve productivity from unconventional reservoirs.28 The hydrocarbon-generating source material, kerogen, in source rock formations has been shown to exhibit ductile behavior that is problematic for hydraulic fracturing. In a new approach to stimulating these reservoirs, the kerogen is made more brittle and/or dissolved by strong oxidizers to improve fracturing and long-term fracture stability. Interestingly, a synergy was observed between the reaction of NaBrO3 and (NH4)2S2O8, suggesting an interaction between the two systems. In the presence of a stronger oxidizer, BrO3− is known to oxidize to perbromate (BrO4−), where the oxidation state of Br is +7, although it has proven difficult to isolate.29,30 Scheme 1 highlights the complex chemistry of oxybromine systems. Scheme 1. Oxidation States of Br, Ranging from Perbromate (+7) to Bromide (−1), Starting with Bromate Reagent

When heating aqueous solutions of NaBrO3 and (NH4)2S2O8, bromine (Br2) is formed as clearly evident from the evolved orange color of the resultant gas. If BrO3− had disproportionated to form Br2 and BrO4−, then electrospray ionization mass spectrometry (ESI-MS) could be used to identify its presence.31 However, ESI-MS solution characterization of reacted BrO3−-persulfate solutions demonstrated the absence of BrO4− (m/z 143, 145) as shown in Figures S5−S6. Further investigation showed that the ammonium cations are readily oxidized by bromate, thereby leading to significant acid generation. The large amount of acid generated using small quantities of bromate prompted a deeper study to understand whether this NH4+ oxidation chemistry could be extended to other counteranions such as Cl−, Br−, SO42−, etc. in order to generate a series of acids for potential use in applications involving upstream stimulation of rock formations. Presented here is a study demonstrating the utility of this approach for a number of acid systems and the inhibiting effects or delayed acid release mechanisms of certain ammonium counteranions. Given the widespread use of HCl in upstream stimulation applications, a series of reactions were first performed between NaBrO3 and NH4Cl in order to determine first whether BrO3− was reactive toward NH4+ in the presence of Cl− and second to establish the suitable reagent stoichiometry required to yield the optimal acid concentration. The two reagents were combined in 25 mL of water in a glass pressure tube and placed in a preheated oil bath. To gain insights into the reaction mechanism under reservoir temperature conditions, while taking into consideration the cooling effects during the pumping process, temperatures of 60, 100, and 150 °C were explored. In the absence of induction time accelerators, reactions occurred at 100 and 150 °C within a 3 h time C

DOI: 10.1021/acs.inorgchem.8b02891 Inorg. Chem. XXXX, XXX, XXX−XXX

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consequently there exists a need for in situ-generated mechanisms as they are beneficial for safety purposes. Interestingly, attempts to oxidize NH4+ in the presence of counteranions other than Cl− led to a wide range of reactivities. As expected, NH4Br behaved in a similar fashion to NH4Cl where NH4+ was easily oxidized at 100 and 150 °C, but no reaction was observed at 60 °C in the 3 h time frame. Induction times for NH4Cl and NH4Br reactions with NaBrO3 were nearly identical, suggesting that Br− does not play a catalytic role in the reaction. Trace to stoichiometric quantities of Br− added to NaBrO3 and NH4Cl reactions also had no effect on the induction time. Therefore, the three-step Landolt type kinetic model can be ruled out for this system.39 Further, titration measurements confirmed the same amount of acid was generated with Br− as with Cl− counterion (Figure S1) over similar time frames (Figure S2). In contrast, F− counteranion resulted in a different apparent reactivity. NH4+ oxidation in the presence of F− required higher temperatures and extended heating times in order to produce the acid. Typical reaction conditions where acid was generated involved heating for 24 h at 150 °C or 3 h at 200 °C. Yet, if the reaction occurred, the same amount of acid was formed. Finally, BrO3− reactions with ammonium iodide (NH4I) were also attempted, although it was quickly determined that I− itself is readily oxidized by BrO3−, so the reactivity of BrO3− toward NH4+ was obscured. The oxidation potential of I− (0.54 V) is much lower than Br− and Cl− (1.09 and 1.36 V, respectively). The brown reaction solution color clearly demonstrates the presence of I2, consistent with other reports that iodine species are readily oxidized by BrO3− in aqueous solution.40 By contrast, oxidation of Br− by BrO3−, although possible, appears to be slower than NH4+ oxidation. Scheme 3 summarizes the various reactions between 2:1 NH4+/BrO3− in the presence of four different halides after heating for 3 h at 100−150 °C.

Figure 2. ESI-MS negative ion mode spectrum of solutions obtained after reacting 2:1 NH4Cl/NaBrO3 for 3 h at 100 °C (blue) and after titrating this reaction product (orange).

The absence of nitrogen species in solution suggests the formation of gaseous products such as N2 or NOx compounds. A number of thermal decomposition studies have been conducted to determine the composition of gaseous products formed when heating solid samples of ammonium salts of strong oxidizers such as ClO4−, BrO4−, BrO3−, NO3−, etc. For example, NH4ClO4 is rather stable until it is heated to ∼130 °C, at which point it begins to slowly decompose to a variety of products including N2O and N2.35 Meanwhile, the ultimate nitrogen-based products of solid NH4BrO4 and NH4BrO3 decomposition are dominated by N2.36,37 Analogous high pH hypochlorite and hypobromite solutions also oxidized NH4+ predominantly to N2 along with minor amounts of N2O that increase slightly with temperature.38 For the present system, FT-IR analysis was performed on the gaseous products formed during a reaction of NaBrO3 and NH4Cl at 80 °C and revealed an approximately 90:1 ratio of N2 to N2O, with no evidence of NO or NO2 having formed. Further it is presumed that some H2 is formed, as some ammonium-bromate oxidation experiments performed in an open vessel were observed to autoignite. Considering the range of potential N2/N2O molar ratios along with formation of H2, eqs 1−4 in Scheme 2 are presented as viable solutions. Given that N2 dominates the nitrogen-based products, it appears that eq 1 is the most representative. Beyond HCl, other mineral and organic acids such as HF and acetic acid are highly relevant for applications in the subterranean environment, e.g., improving permeability of carbonate and sandstone rock formations by removing natural or induced forms of damage. In addition to placement issues, acids such as HF are undesirable to handle on the surface, and

Scheme 3. Reaction Schemes for NH4+ and BrO3− (Molar Ratio 2:1) in the Presence of Halides after Heating for 3 h at 100−150 °C

Similarly, apparent differences in reactivity were observed among the polyatomic ions such as sulfate, phosphate, and acetate. For example, the same concentration conditions and heating times applied to solutions of bromate and ammonium salts with different counteranions produced different results. In the presence of PO43− or HPO42−, no reaction was observed at 60, 100, or 150 °C. The presence of other polyatomic anions such as SO42−, however, appeared to inhibit reactivity at 60 and 100 °C within the 3 h time frame but not at 150 °C. Counteranions BF4− and PF6−, when present at 100 or 150 °C, also did not inhibit the redox reaction between bromate and ammonium ions, though they were observed to hydrolyze under elevated temperature conditions, producing significant additional acid.41 NH4+ oxidation by BrO3− was found to be acid-catalyzed. This observation is consistent with the long induction times,

Scheme 2. Reaction Schemes for the Reaction between NH4+ and BrO3− and the Products Formeda

a

Note the variable gas formation. D

DOI: 10.1021/acs.inorgchem.8b02891 Inorg. Chem. XXXX, XXX, XXX−XXX

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excess H+ (19.4 vs 18.3 mmol) is observed in the case of NH4HF2. In the former case, although the solution color changed after approximately 1 h of heating, the intensity of the Br2 color was notably less than all other reactions described previously. By Scheme 2, the molar ratio of Br2/H+ shifted from 1/10 to 0.5/11. Notably, NH4+ oxidation occurs under acidic conditions, rather than under basic conditions where the more available nitrogen center of ammonia is present in solution. A recent study on the oxidation of NH3 over Ru/TiO2 catalyst demonstrated that conversion of NH3 to N2 is >99% when the pH is >12, while only 20% conversion was observed at pH 5.5.43 However, the pH drops while forming HNO2 as an intermediate and the HNO2, in turn, reacts with NH4+ to form N2. In the bromate-ammonium redox system, a significant drop in the reaction rate occurs as the pKa of the acid being formed increases. In particular, in the time frames tested, no reactions were observed above the pKa of HF (3.14). This suggests that the oxidation kinetics is not driven by HBrO3 (pKa −2) but rather by HBrO2 (pKa 3.4). Interestingly, kinetics similar to the bromate-ammonium system have previously been observed between ferrocyanide and BrO3− in acidic medium, where the bromide-bromate couple dominates the kinetic behavior.44 It has been shown that the direct reaction between BrO3− and a reducing agent (Red) is slow and does not contribute to the reaction rate.45,46 However, BrO3− and Br− in the presence of H+ can form HOBr and HBrO2 or vice versa.47−49 Therefore, a more complex reaction scheme for bromate’s oxidation of NH4+ (Red) may be realized as BrO2· is formed and reacts with NH4+ (Scheme 4) as it has been described in generalized

followed by rapid formation of stoichiometric acid quantities. It was further established by combining small quantities of HCl or other acid with NaBrO3 and NH4Cl. The reaction was significantly accelerated, such that even at room temperature, reactions are complete within minutes. The induction times for reactions between NH4Cl and NaBrO3 at variable temperatures were captured. Solutions were prepared in glass tubes, and the sealed tube was inserted in the preheated recirculating silicone oil bath. The induction time was noted by visual inspection, that is, upon the solution changing from colorless to orange. For the temperature range of 75−150 °C, the induction times were observed to be 46 min (75 °C), 13.6 min (100 °C), 10.5 min (125 °C), and 4.5 min (150 °C). Similarly, the induction times for reactions between BrO3− and NH4+ in the presence of various counteranions were captured by the same method, with identical concentrations at 150 °C. The reactivity trend shown in Table 1 clearly follows the pKa series Table 1. pKa Values of Counteranions and Reactivities of NH4+ in Their Presence When Exposed to Aqueous Solutions of BrO3− at Elevated Temperaturea

Scheme 4. Reaction Schemes between the Active Solution Oxidant BrO2· and Reducing Agent (Red) and the Regeneration of HBrO2.50

a

Induction times were captured for each reaction where 2:1 NH4+/ NaBrO3 are heated to 150 °C.

trend, where bromate was the least reactive in the presence of high pKa anions that bind H+ as it is generated in the early stages of ammonium oxidation. Once the oxidation reaction proceeds to where protons are readily available, the process is catalyzed and quickly proceeds to completion. In a recent example of BrO3− oxidation of thiosulfate, the observed autocatalysis resulted from bisulfite rather than from protons.42 NH4+ oxidation by BrO3− in the presence of counteranions of strong acids including Br−, Cl−, and trifluoroacetate (CF3CO2−) all proceeded to completion within 30 min. Upon appearance of the bright orange color, reaction tubes were immediately removed from the oil bath, cooled, and titrated to find most reactions were >75% complete. A few minutes of extra heating, on the other the hand, was sufficient to ensure the reaction was complete. Reactions performed with (NH4)2SO4 were faster than expected given the higher pKa of 1.92. However, reactions were performed under conditions of 2:1 NH4+/BrO3−, so the concentration of SO42− was half that of all other reactions. Increasing the concentration of the reaction by a factor of 2.5 reduced the induction time from 3.5 h to 20 min. Reactions performed with protic counteranions of weak acids yielded different reaction product distributions. Less acid was formed when combining NaBrO3 with ammonium phosphate monobasic (NH4H2PO4) and ammonium bisulfate (NH4HSO4). Anticipated (actual) mmoles of H+ titrated with NaOH (1 M) were 28.3 (19.4) and 18.3 (9.7). By contrast,

form50 and as in a recent example by Olangunju and Simoyi.51 Consistent with this proposed scheme is the fact that the pKa of isoelectronic HClO2 is 1.92. This may explain why no reaction is observed between NH4+ and ClO3−. Since the pKa is much lower than HBrO2, the reaction kinetics are much slower. To demonstrate the utility of this approach for acid stimulation of carbonate reservoirs, a series of static dissolution tests were performed under ambient conditions. Indeed, upon combining the in situ-generated acid with carbonate core sample (mineralogy in Figure S7), the rock is fully dissolved with no evidence of precipitation, and the orange solution rapidly goes colorless as both the acid and the Br2 are spent (see Figure S8). The rise in pH (i.e., >4) coupled with the solution changing to colorless is indicative that both species, i.e., H+ and Br2, are fully spent. Importantly, this feature helps to manage risk in terms of corrosion prevention and safety of the personnel at the wellsite. Although the cost and availability of HCl make it very difficult to replace in acidizing carbonate reservoir rock, the in situ-generated approach provides a means of placing reagents deeper into the formation before the acid forms and reacts with the rock. Accordingly, the success of this approach is critically dependent on the design of the field E

DOI: 10.1021/acs.inorgchem.8b02891 Inorg. Chem. XXXX, XXX, XXX−XXX

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bromine continues to be an active area of investigation, across the various oxidation states including Br−,58 Br2,59 BrO−,60−63 BrO2−,63−65 and BrO3−.51,66,67 Their presence in natural environments and utility in chemical transformations continue to drive the various avenues of interest.

treatment, i.e., with respect to the manner in which the reagents are delivered into the reservoir. To effectively stimulate the pay zone, a two-stage injection may be considered whereby an aqueous solution of the ammonium salt is first injected into the reservoir followed by injection of the sodium bromate, the latter of which is delivered in an encapsulated or partially encapsulated form to promote ondemand acid generation in the zones of interest. In situ methods for generating HF are highly desirable in order to avoid surface handling of this toxic material. For example, the quantities of HF generated by this method are more than sufficient for sandstone reservoir stimulation.52 Given that the acid yield scales linearly with the concentration of ammonium, the maximum concentration of acid to be formed from NaBrO3 and NH4Cl, NH4F, etc. corresponds to the solubility of these reagents. NaBrO3 has the limiting solubility (∼35 g/100 mL) in most ammonium salt systems. With 2:1 ratios of NH4+ to BrO3−, the potential acid concentrations are 7 wt % HCl, 5 wt % HF, 17 wt % HBr, etc. These concentrations extend well into the range needed for sandstone stimulation where mud acid systems commonly contain 3 wt % HF and are comingled with up to 12 wt % HCl.53 This can now be achieved in situ by combining both NH4F and NH4Cl with NaBrO3 in the stimulation fluid and injecting into the formation. The in situ acid system may also be considered for acid fracturing in unconventional hydrocarbon-bearing formations to stimulate production.54 Low concentrations of acid (e.g., 3 wt %) may require a means of delaying acid generation in order for the well to obtain the full benefit of the treatment. Otherwise, the acid is likely to be spent in the well or when first reaching the formation rather than extending into the unpropped fractures where etching can occur. Unconventional formations with carbonate content between 10 and 30% have been shown to be well-suited for the acid concentration ranges achieved by this system.55 Beyond the acid formation from this in situ-based system, a mole of gas is formed for every mole of H+ generated. Further, since the reaction is acid-catalyzed, the reaction proceeds very quickly to completion once a critical concentration of acid is formed. As a result, the gas is released rapidly to the formation. In other words, as the fluid is pumped into the natural fractures, the temperature of the fluid rises from the heat of the formation to trigger oxidation of the ammonium. Acid is released into the fractures, while gas is simultaneously formed with the potential to also stimulate the rock. Although other bromate-based systems have been shown to produce gas,56 the unique pathway provided by the NH4+ to BrO3− coupling is the simultaneous formation of both acid and gas. Understanding the role of pH and specific anions in controlling the reaction kinetics of ammonium oxidation has applications which extend well beyond those involving in situ acid generation for reservoir stimulation. The oxidation of ammonium or ammonia in nature is important in nitrification processes, and oxidants across the series of oxyhalogens and nitrogen-based compounds can provide insights into the reaction mechanisms at work. Although NH4+ oxidation has been an active area of exploration for many decades, little is known about the role of specific ions in these processes. Specific ion effects have been observed to play a role not only in macromolecular biological systems such as proteins, but also in polymers and colloids, and may find an important place among environmental processes.57 Further, the chemistry of

4. CONCLUSIONS The reactivity of BrO3− toward NH4+ in the presence of a wide range of counteranions has been demonstrated. This in situ acid-generating method represents an excellent platform for synthesizing acids such as HCl, HBr, and trifluoroacetic acid. As these acids can be spent quickly in the wellbore due to their high reactivity, the proposed in situ method provides a customizable technique for delaying their formation until placement in the region of interest. Importantly, controlled delivery of the acid is triggered either by temperature or through release of a pH-altering agent. The acid formation is accompanied by gas formation that may further stimulate the fractured formation. Future work should be able to validate the utility of this approach under reservoir conditions.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02891. Materials and methods for reacting NaBrO3 and NH4X where X = F−, Cl−, Br−, I−, SO42−, HSO4−, HPO42−, H2PO42−, CF3CO2−, and CH3CO2−. Additional characterizations of the products including ESI-MS, titration data from acidic product solutions as a function of temperature/time/counteranion (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Katherine L. Hull: 0000-0002-6211-0921 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Chris Pennington (Rice University) and Brent Cooper (Aramco Services Company) for their assistance with electrospray ionization mass spectrometry experiments and ICP analysis, respectively.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b02891 Inorg. Chem. XXXX, XXX, XXX−XXX