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Experimental study of the decomposition of acetic acid under conditions relevant to deep reservoirs
Applied Geochemistry 84 (2017) 306e313
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Experimental study of the decomposition of acetic acid under conditions relevant to deep reservoirs Yuanju Li a, b, Shixin Zhou a, *, Jing Li a, Yu Ma a, b, Kefei Chen a, b, Yuandong Wu c, Yuhong Zhang a, b a
Key Laboratory of Petroleum Resources, Gansu Province/Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou 730000, China University of Chinese Academy of Sciences, Beijing 100049, China c Powerchina Water Environment Governance, Shenzhen, 518102, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 May 2017 Received in revised form 20 July 2017 Accepted 25 July 2017 Available online 27 July 2017
The thermal stability of organic acids is critical in studying the formation mechanism of secondary pores in deep reservoirs (>4000 m). However, the thermal stability of organic acids in geochemical systems is strongly dependent on temperature, minerals and pressure (fluid pressure and lithostatic pressure). To investigate the activity of organic acids and the influence of these factors (temperature, pressure and Kfeldspar), four series of experiments were conducted over a wide range of reaction conditions. Pure acetic acid solutions were selected for this study. The temperatures ranged from 130 C to 380 C, and each experiment was performed for 72 h in steps of 50 C. The results indicate that the decomposition reaction rate was slow but accelerated with increasing temperature. At temperatures higher than 230 C, the decomposition of acetic acid proceeded to a much more significant extent than at low temperatures. The results of experiments also show that the decomposition rates were faster at lower lithostatic pressure, and the lithostatic pressure played a larger role than water pressure in affecting the decomposition of acetic acid. Additionally, the presence of K-feldspar influenced the reaction rates of acetic acid, especially the oxidation rates. This was attributed to the increasing concentration of acetate produced by dissolution. © 2017 Elsevier Ltd. All rights reserved.
Editorial handling by Prof. M. Kersten Keywords: Acetic acid Acetate Decomposition K-feldspar Thermal stability
1. Introduction With the increase of exploration depth, more deep reservoirs (>4000 m) have been found, such as the Dongying depression in the Bohai Bay Basin, NE China (Yuan et al., 2007). Secondary pores are an important storage space for these reservoirs. Previous studies have shown that organic acids play a key role in mineral dissolution and the formation of secondary porosity (Surdam et al., 1984, 1989; Barth et al., 1990). Therefore, understanding the thermal stability of organic acids is critical in studying the formation mechanism of secondary pores in these deep reservoirs. Numerous laboratory experiments have been performed to investigate the thermal stability of organic acids in aqueous solutions (Kharaka et al., 1983; Palmer and Drummond, 1986; Drummond and Palmer, 1986; Crossey, 1991; Bell and Palmer,
* Corresponding author. E-mail address: [email protected] (S. Zhou). http://dx.doi.org/10.1016/j.apgeochem.2017.07.013 0883-2927/© 2017 Elsevier Ltd. All rights reserved.
1994; Bell et al., 1994; Fein et al., 1994; Shock, 1994; McCollom and Seewald, 2003a, b; Ong et al., 2013). These previous studies have shown the possible reaction pathways for the decomposition of organic acids in geologic systems and the factors that influence the thermal stability of organic acids. These observations, however, had only been reported for experiments under relative low fluid pressure, and most of the experiments did not consider the effect of lithostatic pressure. In fact, pressure may play an important role in regulating the reaction process, because the decomposition reaction of organic acids generates lots of gaseous products, which will significantly change the molar volume of the reaction, and because the fluid and lithostatic pressures are distinctly different in natural systems, and their effects need to be assessed individually. What's more, the temperatures of deep reservoirs in different basins vary tremendously. For example, the temperature of the reservoirs at depths of 4000 m is lower than 100 C in the Kela Gas Field, Kuqua Depression, Tarim Basin, China, whereas there are still liquid hydrocarbon accumulations at a depth of 7550 m and 295 C in the
Y. Li et al. / Applied Geochemistry 84 (2017) 306e313
Pre-Caspian Basin, Russia (Sun et al., 2013). However, most previous experiments have been conducted in a very narrow temperature range (Palmer and Drummond, 1986; Bell et al., 1994; McCollom and Seewald, 2003a, b; Ong et al., 2013). Without more detailed knowledge about the changes in stability of organic acids with increasing temperature, it is difficult to use these experimental results to accurately predict the distribution and reactivity of organic acids in deep reservoirs. On the other hand, numerous experiments have been conducted to study the interactions of organic acids and minerals (Palmer and Drummond, 1986; Barth and Riis, 1992; Bell et al., 1994; White and Brantley, 1995; McCollom and Seewald, 2003a, b; Ganor et al., 2009; Shao et al., 2011; Declercq et al., 2013). A wide range of minerals (including stainless steel, quartz, pyrite, calcium montmorillonite, calcite, hematite and magnetite) were chosen to study the catalytic decomposition of acetic acid (Pittman and Lewan, 1994). However, feldspar is far more common than most of the minerals selected in previous studies of sedimentary rock and volcanic rock, but most studies have only focused on feldspar dissolution by water-soluble organic acids (Bevan and Savage, 1989; Welch and Ullman, 1996; Blake and Walter, 1999; Ma et al., 2012; Crundwell, 2015; Yang et al., 2015). If feldspar significantly promotes the decomposition of organic acid, the dissolution reaction might be affected, leading to some changes in the formation of secondary pores. Here, we select K-feldspar to simulate a clastic reservoir, since K-feldspar is more stable than other feldspars, such as albite and anorthite, especially in an acid aqueous solution (Zhang et al., 2009). Above all, due to the abundance of acetic acid in oil field water (Barth and Riis, 1992), we have conducted a series of laboratory experiments examining the reactivity of acetic acid under 45e60 MPa lithostatic pressure and 45e60 MPa water pressure at elevated temperatures (130e380 C). The degradation rate, activation energies and pre-exponential factors were obtained by analyzing the reaction dynamics. In this study, pressure and Kfeldspar were selected to simulate actual geologic environments, such as deep clastic reservoirs in sedimentary basins. Changes in the total concentration of acetate over time were measured to assess the degradation rate under different conditions to address three issues in particular: (1) the influence of water and lithostatic pressure on the thermal stability of acetic acid; (2) the changes in the thermal stability of acetic acid and acetate over a wide temperature range; and (3) the role of K-feldspar in affecting decomposition pathways and rates. 2. Experimental materials and methods 2.1. Materials Dilute pure acetic acid (analytical grade) was chosen as the reaction solution. To avoid changes in the original ionic composition, all solutions were made in degassed deionized water. In Experiment 1 (Ext. 1) to Experiment 4 (Ext. 4), the solute was pure acetic acid (155.4 mmol/kg), and because acetic acid is a weak acid, the main species in these samples was acetic acid (>90%). K-feldspar was included in Ext. 4. A block of pegmatite was obtained from an outcrop in the Xingxingxia Beishan area. After removal of the surface weathered layer, all rocks were fragmentized to screen out the K-feldspar grains. Randomly selected parts of the K-feldspar grains were used for experiments to avoid the effect of mineral heterogeneity. 2.2. Methods The experiments were conducted in a semi-closed high-
307
temperature, high-pressure simulation system (WYMNe3 HTHP, Fig. 1), as used by Sun et al. (2015) and Wu et al. (2016). The instrument consists of a software control system (computer) and a hardware performance system (apparatus). The software control system was used to set experimental conditions and collect data. The hardware performance system included the following systems: reaction cell, heating system, pressure system and collecting systems. The reaction cell was made of titanium alloy and contained within an autoclave (stainless-steel), and the advantages of the reaction cell are the resistance to acid and alkali corrosion with a maximum tolerable temperature of 550 C and pressure of 100 MPa. The accuracy of heating system is ±0.1 C. The pressure system contains a hydraulic control system for lithostatic pressure and confining pressure and a turbocharger for hydrodynamic pressure. The lithostatic pressure accuracy is ±0.1 MPa, water pressure accuracy is ±0.05 MPa. With the aid of pressure system, this instrument can simulate the lithostatic pressure and hydrostatic pressure achieved in reservoir rock. It is widely accepted that high temperatures can be used to model reactions that occur at lower temperatures and over a long period of time in geologic systems, and the experiments in this study investigated six different temperatures: 130 C, 180 C, 230 C, 280 C, 330 C and 380 C. Every experiment at a different temperature was conducted for 72 h. The reaction conditions for each series of experiments are listed in Table 1. The experimental procedure was as follows: (1) loading of K-feldspar (Ext.4) or skeleton (titanium alloy, Ext1-3); (2) vacuuming; (3) equal lithostatic pressure was applied to the top and bottom of the skeleton or mineral; (4) injecting solutions through high pressure liquid pump into the reaction cell until the water press reaches the target pressure. When the water pressure displayed by the pressure gauge exceeds 10% of the initial water pressure, the magnetic valve B is opened to discharge the gas production into the gas-liquid separator until the water press decreases to the initial water pressure; (5) After the separation of the gas and liquid, the gas was extracted using a standard purge-and-trap device and the liquid was injected into the reaction cell again by the liquid pump; (6) The experimental conditions (including temperature, time, lithostatic pressure, and water pressure) were recorded by a computer every minute. Once the experiment was over, the reaction cell was put into a cold trap to separate the gas and liquid. Hydrochloric acid was added into the cell to convert carbonate and bicarbonate to carbon dioxide. Then, all gas samples were extracted using a standard purge-and-trap device. Gases were analyzed on an Agilent 6890 gas chromatograph (Agilent USA) fitted with a flame ionization detector operating at 200 C for gaseous hydrocarbons. One milliliter gas samples were injected at 100 C, with separation performed on a HP-PLOT-Q fused silica 30 m 0.32 mm 20 mm column, with helium as the carrier gas. The oven temperature was programmed to increase from 35 C (5 min hold) to 150 C (3 min hold) at 5 C/ min, then to 270 C (2 min hold) at 10 C/min. Liquid samples collected from the reaction cell were divided into four aliquots. One aliquot was used to determine the abundance of dissolved CO2 (aq) and CH4 (aq) by GC (Agilent 6890). The remaining three aliquots were used to determine the concentrations of Na, K, and acetate by ion chromatography. A Dionex ICS 3000 Ion Chromatography system (Dionex USA) equipped with
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Y. Li et al. / Applied Geochemistry 84 (2017) 306e313
Fig. 1. Schematic diagram of the WYMNe3 HTHP instrument (Wu et al., 2016).
Table 1 Reaction conditions for each series of experiments.
Ext. Ext. Ext. Ext.
1 2 3 4
Temperature ( C)
Time(h)
Water Pressure (MPa)
Lithostatic pressure (MPa)
Materials
130e380 130e380 130e380 130e380
72 72 72 72
60 45 60 60
60 60 45 60
Acetic Acetic Acetic Acetic
IonPac As19 analytical column (250 mm 4.0 mm), IonPac AG19 guard column (50 mm 4.0 mm), IonPac Cs12 analytical column (250 mm 4.0 mm), and IonPac Cs12 guard column (250 mm 4.0 mm) was employed in the experiments. The chromatographic conditions for acetate/acetic acid were an IonPac As19 analytical column, with a column temperature of 30 C and a detector temperature of 35 C. The eluents were 18.2 MU$cm3 water (A) and 30 mmol/L NaOH (B). A gradient elution was employed as follows: 50% B for 0e3 min; 50% B/100% B from 3 to 6 min; 100% B from 6 to 15 min; 100% B/50% B from 15 to 17 min; 50% B from 17 to 25 min. The flow rate was 1.0 mL/min, and the sample size was 20 mL. The chromatographic conditions for Na and K were as follows: an IonPac Cs12 analytical column, with eluents of 18.2 MU$cm-3 water (A) and 20 mmol/L methyl sulfonic acid (B), a current of suppressor at 59 mA, and the other conditions were the same as those for acetic acid. Although a variety of acetate species may be present in our study (i.e., acetic acid, acetate), acetate species will all be converted to acetate during detection because of the NaOH eluent. Here, we P use “ acetate” to represent the total concentration of acetate and acetic acid. We report the average concentrations for the three fluid aliquots. The estimated analytical uncertainty for all compounds was ±1%. 3. Results 3.1. Decomposition of acetic acid without K-feldspar Ext. 1e3 were conducted in the reaction cell without the presence of minerals. The results are listed in Table 2. In this study, the initial concentration of acetic acid was nearly 155.4 mmol/kg. Fig. 2
acid acid acid acid þ K-feldspar
P presents the change in residuary acetate concentration with temperature in Ext. 1e3, the gas production and a decrease in the concentration of acetate indicate that acetic acid decomposed. In Ext. 1e3, almost no acetic acid reduction was observed at 130 C and 180 C, and the concentrations of gas products was extremely low. Due to the relatively short duration in terms of geologic time and the analytical uncertainties, we speculated that the acetic acid may not decompose at temperature under 180 C. At 230 C, a very slight decreasing trend was observed for the acetic acid concentration, but the gas production indicated that decomposition occurred. With increasing temperature, the changes in acetic acid concentrations were significant at 280 C 380 C. There are two main decomposition pathways for acetic acid (Kharaka et al., 1983; Palmer and Drummond, 1986; Drummond and Palmer, 1986; Bell et al., 1994; McCollom and Seewald, 2003b). For the most part, acetic acid is presumed to decompose primarily by decarboxylation:
CH4 COOH/CO2 þ CH4 :
(1)
In Ext. 1e3, the production of CO2 and CH4 in a nearly 1:1 ratio indicates that acetic acid decomposed primarily by decarboxylation (Eqn. (1)). However, Bell et al. (1994) demonstrated that acetic acid decomposition might occur through oxidation under some circumstances due to the extremely low concentration of O2 present in most subsurface geologic environments. The reaction is expressed by:
CH3 COOH þ 2H2 O/2CO2 þ 4H2 : C,
(2)
At temperatures above 230 the concentration of CO2 was slightly higher than CH4, suggesting that some oxidation of acetic acid occurred as well (Eqn. (2)).
Y. Li et al. / Applied Geochemistry 84 (2017) 306e313
309
Table 2 P Changes in the acetate, CO2, and CH4 concentrations with temperature in Ext. 1e3. Temperature ( C) Ext. 1 25 130 180 230 280 330 380 Ext. 2 25 130 180 230 280 330 380 Ext. 3 25 130 180 230 280 330 380
Concentration (mmol/kg) P acetate CO2
CH4
155.40 155.40 155.39 155.29 153.71 133.88 72.68
e \ \ 0.09 0.86 17.81 73.85
e \ \ 0.10 0.88 16.14 71.29
155.40 155.39 155.39 155.28 153.74 136.03 73.79
e 0.01 \ 0.03 0.81 16.8 70.3
e \ \ 0.02 0.67 15.6 69.2
e \ 0.01 0.10 1.20 20.33 103.8
e \ \ 0.07 0.98 19.17 97.23
155.40 155.40 155.40 155.26 153.00 133.19 54.84 P “\” ¼ no data, “-” ¼ no detection, “ acetate” ¼ total acetic acid.
concentration of acetate and
3.2. Decomposition reaction in the presence of K-feldspar P Changes in residual acetate concentration at elevated temperatures in Ext. 4 are shown in Fig. 3. Although the data points also show decay behavior for all samples and the trend was similar to the mineral-free experiments (Ext. 1), the residual concentration was lower than Ext. 1 at the same temperature. In Ext. 4, the decomposition also proceeded extremely slowly (or did not occur) for acetic acid at temperatures under 180 C. Then, the reaction proceeded much more rapidly at temperatures above 230 C. However, the mineral dissolution will consume large numbers of hydrogen ions, but no acetate (Eqn. (3)); therefore, the lower residual concentrations of acetate and acetic acid indicated that heating acetic acid in the presence of K-feldspar led to a faster decomposition rate.
2KAlSi3 O8 þ 2CH3 COOH þ 9H2 O/Al2 Si5 ðOHÞ þ 2K þ þ 4H4 SiO4 þ 2CH3 COO (3) To avoid interference from gas derived from background sources associated with minerals, an additional deionized water experiment was conducted in the presence of K-feldspar. This experiment was also intended to provide a baseline for evaluation of the extent of K-feldspar dissolution. The concentrations of products are listed in Table 3. In this experiment, the concentration of gas as well as the ion concentrations were much lower than in Ext. 4. Therefore, any interference associated with the minerals and deionized water was neglected. Higher concentrations of CO2 and CH4 suggest that more acetic acid was decomposed. The amount of CO2 was nearly equal to CH4, indicating that acetic acid decomposition in the presence of Kfeldspar proceeded primarily through decarboxylation (Eqn. (1)). However, relative to CH4, a small excess of CO2 was produced, suggesting some oxidation of acetic acid occurred (Eqn. (2)). The gap between the concentrations of CO2 and CH4 was larger than
P Fig. 2. Measured concentrations of acetate, CO2, CH4 in Ext. 1e3, (a) Ext. 1, water pressure 60 MPa, lithostatic pressure 60 MPa, (b) Ext. 3, water pressure 45 MPa, lithostatic pressure 60 MPa, (c) Ext. 3, water pressure 60 MPa, lithostatic pressure 45 MPa.
that in Ext. 1, illustrating that acetic acid may be more favorably decomposed by oxidation in Ext. 2 (Eqn. (2)). Fig. 4 shows the change in ion concentration (Naþ, Kþ) in Ext. 2.
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Drummond, 1986; Bell et al., 1994; McCollom and Seewald, 2003a, b). In this study, acetic acid and acetate decompose by decarboxylation and oxidation. The first-order reaction of acetic acid or acetate in our experiments is more appropriately portrayed in the form:
hX i. d acetate dt ¼ keff ½acetate;
(5)
where keff is the effective rate constant for the overall decomposition (Eqn. (6)):
keff ¼ kdecarb þ koxid ;
(6)
where kdecarb is the rate constant for decarboxylation (Eqn. (1)) and koxid is the rate constant for oxidation (Eqn. (2)). Eqn. (5) can be integrated to yield:
hX
i hX i acetate ¼ acetate ekeff t : t
(7)
0
The corresponding expressions for CO2 and CH4 are: P Fig. 3. Measured concentrations of acetate, CO2, CH4 in Ext. 4, water pressure 60 MPa, lithostatic pressure 60 MPa, K-feldspar.
The changes in Kþ and Naþ indicate that K-feldspar was dissolved by acetic acid, and the solubility of feldspar increased with temperature. The dissolution of minerals consumed large amounts of hydrogen ions but little acetate. The concentration of Naþ increased markedly at low temperature (<230 C); then, the dissolution slowed. Different from Naþ, a slight increasing trend was observed for the Kþ concentration. However, the increase in concentration of both ions with increasing temperature indicates that dissolution occurred in the Ext. 2, and it seems that the dissolution was not affected by the decomposition of acetic acid in the duration of experiments. 4. Discussion 4.1. The decomposition rate constants and the role of temperature Previous studies have shown the decomposition of acetic acid and acetate could be modeled with a first-order rate law:
dm=dt ¼ km;
hX
i hX i hX i CO2 ¼ acetate eðkdecarb þ2koxid Þt þ CO2 ;
(8)
i hX i hX i acetate ekdecarb t þ CH4 ¼ CH4 ;
(9)
0
hX
0
0
0
P P P where ½ acetate0 ½ CH4 0 ½ CO2 0 are the initial concentrations of acetate and acetic acid, CH4 and CO2, respectively, and P ½ acetatet is the residual concentration of acetate and acetic acid. Because the residual concentration was measured, we can calculate keff by Eqn. (7) (t ¼ 72 h). Values for koxid and kdecarb in our experiments were determined using Eqns. (8) and (9). We also calculate the maximum oxidation rate constants using Eqn. (6) (koxid ¼ keff kdecarb ), because the concentration of CO2 is easily affected by its water solubility and, for many experiments, degradation primarily occurred by decarboxylation. The koxid values were adjusted in the model for each experiment until a best fit was obtained. Fig. 5 shows changes in rate constants in all four experiments. The reaction rate constants were calculated based on the assumption that the acetic acid and acetate were decomposed at very slow rates at temperatures under 230 C. The general behavior of the curves of the four series experiments was similar across the series.
(4)
where k is the reaction rate constant, t is the elapsed time, and m is the concentration of acetate (Kharaka et al., 1983; Palmer and
Table 3 Fluid compositions during the reaction of Ext. 4. Temperature ( C)
Concentration (mmol/kg) P kþ acetate Naþ
Ext. 4 25 155.40 130 155.39 180 155.38 230 155.28 280 153.21 330 131.83 380 61.41 Experiment with no acids 20 0 330 0
<0.01 0.86 1.11 1.57 1.42 1.70 1.77
<0.01 0.32 0.27 0.41 0.48 0.43 0.45
pH CO2
CH4
e 0.03 0.01 0.15 1.28 24.9 104.8
e \ \ 0.11 1.06 21.6 91.0
2.7 4.3 4.5 4.4 4.7 5.3 5.5
<0.01 <0.01 7 0.1 0.06 0.02 \ 7 P “\” ¼ no data,“-” ¼ no detection, “ acetate” ¼ total concentration of acetate and acetic acid.
Fig. 4. Changes in Na and K concentrations in Ext. 4.
Y. Li et al. / Applied Geochemistry 84 (2017) 306e313
The keffetemperature relationship displayed an increasing trend. In the four series of experiments, the decomposition of acetic acid at 230 C remained within the range of 2.54 109 to 3.28 109, and the rate constants for decomposition at 280 C was 4.21 108, 4.14 108, 5.98 108 and 5.47 108, respectively. In comparing these rate constants, we found that at 230 C, acetic acid decomposed at a rate more than 90% slower than that in experiments at 280 C. The decomposition rates were very low but increased rapidly at temperatures under 280 C. In contrast, the data for these experiments yielded rate constants for acetic acid decomposition at 330 C in the range of 5.13 107 to 6.34 107, whereas the rate constant for decomposition at 380 C ranged from 2.87 106 to 4.02 106. The rates were much higher than in low temperature experiments, but increased slightly more slowly than the lower temperature stage (<280 C). All results of these experiments illustrated that 230 C was the critical temperature; at temperatures above 230 C, acetic acid will decompose significantly and increased rapidly with increasing temperature. High temperature dramatically accelerated the decomposition of acetic acid and acetate. Fig. 6 illustrates the first-order rate constants (logarithm) for P acetate decomposition as a function of temperature expressed as 1000/T. The general behavior for the curves follows a linear tendency (R2 > 0.99). The linear equations for Ext. 1e4 were the Arrhenius equation:
ln k ¼ ln A Ea =RT:
(10)
where A is a preexponential factor, Ea is the activation energy, R is the universal gas constant, and T is the temperature (K). Because almost no acetic acid reduction was observed under 180 C, the calculated effective rate constants at 130 C and 180 C were eliminated from the regression. We have calculated Arrhenius P parameters for decomposition of acetate by linear fitting (Table 4). The calculated activation energies for all four series of experiments were closely indicated that the influence factors in our study all did not significantly affect the decomposition pathways of acetic acid.
4.2. The role of water pressure and lithostatic pressure Table 5 illustrates the changes in keff, koxid and kdecarb in the three series of mineral-free experiments. The results of Ext. 1 and 2 were analyzed comparatively to examine the different changes that
Fig. 6. Effective rate constants for ature expressed as 1000/T.
311
P acetate decomposition as a function of temper-
occur under different water pressures but the same lithostatic pressure. In Ext. 1 the water pressure is 60 MPa, 15 MPa higher than in Ext. 2 (45 MPa), but there was a very slight increase in rates constants of Ext. 1, but only at 330 C was the gap between Ext. 1 and Ext. 2 significant. A previous study has shown that the decomposition of organic acid would be hindered under high fluid pressure (Wang and Zheng, 2012). The results in our study that did not follow a regular behavior should be attributed to the high lithostatic pressure, which covered up the influence of water pressure. The results of Ext. 1 and 3 were analyzed comparatively to examine the different changes that occur under different lithostatic pressures but the same water pressure. Although the lithostatic pressure is 20 MPa lower than that of Ext. 1, the effective rates constants were much higher than Ext. 1, especially at 280 C. The higher lithostatic pressure led to a slower decomposition rate. Apparently, the lithostatic pressure may hinder the decomposition reaction. The decomposition rate in Ext. 3 is the fastest, followed by Ext. 1, and the rate of Ext. 2 (under the lowest water pressure) was the lowest at the specified temperature. Comparing the effective rate constants of Ext. 1, Ext. 2 and Ext. 3, the thermal stability of acetic acid changed very slightly under different water pressure but significantly under different lithostatic pressure. The results indicated that the lithostatic pressure plays a larger role than water pressure in affecting the decomposition of acetic acid. In addition, the acetic acids in all the three series of experiments were mainly decomposed through decarboxylation. The extremely low and close values of koxid of the three series of mineral-free experiments suggested that the lithostatic pressure and water pressure did not significantly influence decomposition pathways. 4.3. The influence of K-feldspar In this study, the higher reaction rates in Ext. 4 at specified temperatures are shown in Fig. 4 and Table 6. As shown in Table 6,
Table 4 P Arrhenius parameters for decomposition of acetate.
Fig. 5. Rate constant (keff) versus temperature.
Experiment
A (S1)
Ext. Ext. Ext. Ext.
8.9 5.3 9.5 1.1
1 2 3 4
105 105 105 106
Ea (kJ/mol) 130.4 127.9 129.4 130.4
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Table 5 First-order rate constants for decomposition of acetic acid and acetate in Ext. 1, 2 and 3 at temperatures above 230 C. Experiment 280 Ext. Ext. Ext. 330 Ext. Ext. Ext. 380 Ext. Ext. Ext.
C 1 2 3 C 1 2 3 C 1 2 3
keff (S1)
kdecarb (S1)
koxid (S1)
4.2 108 4.1 108 6.0 108
4.2 108 4.1 108 6.0 108
<1.0 109 <1.0 109 <1.0 109
5.7 107 5.1 107 5.9 107
5.7 107 5.0 107 5.8 107
<1.0 108 1.0 108 1.0 108
2.9 106 2.9 106 4.0 106
2.8 106 2.8 106 3.9 106
1.0 107 <1.0 107 1.0 107
the decomposition reaction as well as decarboxylation and oxidation proceeded more slowly in Ext. 1 than in Ext. 4, especially at high temperature. In Ext. 4, the general behavior for the curves also increased with temperature. The rate constant (keff) for acetic acid decomposition in the presence of K-feldspar at 230 C was 2.9 109, which was nearly 16% higher than that in Ext. 1 (2.5 109). When temperature was increased to 280 C, the values were 4.2 108 in Ext. 1 and 5.5 108 in Ext. 2, and the comparison of these rates constants suggests that Ext. 1 decomposed at a rate that was 30% slower than that in Ext. 4. At 330 C, Ext. 1 decomposed at a rate that was 10% slower than that in Ext. 4 and 20% slower than that in Ext. 4 at 380 C. No obvious trend was observed for the relationship between temperature and the difference in the effective rate constants between the two series of experiments. Like the effective rates constants, the gap in the data of kdecarb also increased and reached a maximum at 280 C, then narrowed at 330e380 C. Both the difference in the effective and decarboxylation rate constants were not very large. In contrast, although the oxidation reaction rate was very slow at 130 C-230 C, the oxidation also proceeded much more slowly in Ext. 1 than the decomposition of acetic acid in the presence of K-feldspar and the gap between oxidation rate constants rapidly increased with increasing temperature: The oxidation rate constants in Ext. 1 and Ext. 4 were both nearly 1.0 109 at 280 C, whereas the value in Ext. 2 was nearly six times that in Ext. 1 at 380 C. The observed changes in the three kinds of rate constants indicated that the acetic acid in Ext. 4 were decomposed primarily by decarboxylation; however, at temperatures above 330 C, more oxidation of acetic acid occurred, and its role in the decomposition increased with increasing temperature. This may be attributed to two reasons: (1) K-feldspar is catalytic for decomposition, especially for oxidation at temperature above a critical level; (2) there
are lots of acetate produced by K-feldspar dissolution (Eqn. (3)), because the oxidation proceeded more rapidly for acetate than for acetic acid. We prefer the second explanation, since none of its elements were redox sensitive and the previous study also demonstrated that the acetate was more easily decomposed than acetic acid (McCollom and Seewald, 2003a, b). The close calculated activation energies for Ext. 1 and Ext. 4 also indicated that K-feldspar was not a catalyst. What's more, the increasing concentration of acetate should also be a reason why K-feldspar leads to higher decomposition and decarboxylation rates. However, although the role of oxidation in the decomposition increased at 330e380 C, the its contribution was still far less than that of decarboxylation. The relatively small difference in the effective and decarboxylation rate constants suggesting the influence of K-feldspar on the decomposition of acetic acid was not very significant, especially at high temperatures. What's more, as mentioned earlier, it seemed that the dissolution was not affected by the decomposition of acetic acid in the experimental process. In fact, the not very large influence of K-feldspar may not hinder the dissolution over a period of time. In addition, the first-order rate constants for acetic acid decomposition calculated from kinetic parameters given by McCollom and Seewald, (2003a, b) and Bell et al. (1994) are also listed in Table 5. The experimental results of Bell at al. (1994) demonstrated that the activation energies were 270 kJ/mol in titanium and gold vessels. Previous studies have shown that the decomposition of acetic acid and acetate is catalyzed by the vessel surface (Palmer and Drummond, 1986). This could be a reason for the higher Keff and Kdecarb values and lower activation energies in our study, because the reaction cell was made partly of stainless steel. 4.4. Geological application The results of this study have important implications for hydrocarbon exploration, especially for deep oil and natural gas. The thermal stability of dissolved organic acids in deep reservoirs is strongly dependent on lithostatic pressure, temperature and minerals. Lithostatic pressures of 45e60 MPa correspond to conditions at depths of 2000e4000 m in a sedimentary basin. Our experimental results suggested that the decomposition reaction of organic acids will be retarded by lithostatic pressure at deeps of 2000e4000 m. However, water pressures of 45e60 MPa are the freshwater hydrostatic pressures occurring at depths of 4000e6000 m in a sedimentary basin. Our experimental results indicated that the degradation of organic acids will not be affected by water pressure, because the lithostatic pressure is great at such a depth, which can exert a larger influence on the decomposition of organic acids than fluid pressure. The temperatures of deep reservoirs in different basins vary
Table 6 First-order rate constants for the decomposition of acetic acid. Experiment
k (S1)
Ext. 4 K-feldspar, 230 C Ext. 4 K-feldspar, 280 C Ext. 4 K-feldspar, 330 C Ext. 4 K-feldspar, 380 C McCollom and Seewald, 2003a, b 325 C Acetic acid þ pyrite, magenetite, pyrrhotite Na-acetate þ pyrite, magenetite, pyrrhotite Acetic acid þ hematite, magnetite Bell et al. (1994) TiO2 Gold Quartz
2.9 5.5 6.3 3.6
109 108 107 106
5.5 108 9.9 108 3.6 107 4.7 109 1.3 108 3.3 109
kdecarb (S1) 2.8 5.5 5.8 3.0
109 108 107 106
4.7 108 5.0 108 <9.4 109
koxid (S1) 1.0 1.0 5.0 6.0
1010 109 108 107
<1.0 108 4.9 108 3.5 107
Y. Li et al. / Applied Geochemistry 84 (2017) 306e313
tremendously; however, in this study, we observed that there exists a critical temperature, above which the decomposition of acetic acid will proceed to a much more significant extent. This indicated that organic acids may survive for a substantial amount of time if the formation temperature is lower than a certain value, even in the deep reservoirs with depths greater than 4000 m. The presence of K-feldspar does accelerate the decomposition of organic acid; this may suggest that the decomposition reaction will proceed faster in the reservoirs where K-feldspar is abundant. However, K-feldspar was not a catalyst, and its slight influence may be covered by other actors that have a larger influence on the reactivity of dissolved organic compounds, such as iron oxide and iron sulfide minerals. In this study, the temperature, pressure and K-feldspar were taken into consideration to simulate natural geologic environments in deep reservoirs. The results of the experiments suggested that the temperature, lithostatic pressure and K-feldspar all had an influence on the decomposition of acetic acid. However, we did not consider fluid pH values and the acid concentration as variables in this research. In future studies, we plan to focus on the effects of fluid pH values and the concentration of organic acids. 5. Conclusion The results obtained from the evaluation of thermal stability of acetic acid and acetate under elevated temperatures showed that all three factors (temperature, pressure and K-feldspar) influenced the decomposition of acetic acid. The effects of temperature on the decomposition varied greatly. We determined 230 C to be the critical temperature. At temperatures lower than 230 C, the reaction rate was extremely slow so that there was almost no decomposition that occurred in this study. At temperatures above 230 C, the decomposition rates increased with the increasing temperature. Our results also indicated that the lithostatic pressure played a larger role than fluid pressure in affecting the decomposition of acetic acid, and the lithostatic pressure hindered the decomposition reaction, but did not significantly affect the decomposition pathways. Additionally, the presence of K-feldspar can have a slight effect on decomposition and decarboxylation rates, but significantly accelerated the oxidation of acetic acid. This was attributed to the increasing concentration of acetate produced by dissolution. Acknowledgements This research was jointly supported by the Major State Basic Research Development Program of China (Grant No. 41072105), the National Science and Technology Major Projects of Ministry of Science and Technology of China (Grant No. 2016ZX05003002-004, 2016B-0502) and the Key Laboratory Project of Gansu Province (Grant No. 1309RTSA041). The authors would also appreciate two anonymous reviewers who refereed this paper and the Executive Editor Dr. Michael Kersten for all their valuable comments that helped to greatly improve the quality of this paper. References Barth, T., Riis, M., 1992. Interactions between organic acids anions in formation waters and reservoir mineral phases. Org. Geochem 19, 455e482. Barth, T., Borgund, A.E., Riis, M., 1990. Organic acids in reservoir watersdrelationship with inorganic ion composition and interactions with oil and rock. Org. Geochem 16, 489e496. Bell, J.L.S., Palmer, D.A., 1994. Experimental studies of organic acid decomposition. In: Pittman, E.D., Lewan, M.D. (Eds.), Organic Acids in Geological Processes.
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Springer-Verlag, Berlin, Germany, pp. 227e269. Bell, J.S., Palmer, D.A., Barnes, H.L., Drummond, S.E., 1994. Thermal decomposition of acetate: III. Catalysis by mineral surfaces. Geochim. Cosmochim. Acta 58, 4155e4177. Bevan, J., Savage, D., 1989. The effect of organic acids on the dissolution of K-feldspar under conditions relevant to burial diagenesis. Mineral. Mag. 53, 415e425. Blake, R., Walter, L., 1999. Kinetics of feldspar and quartz dissolution at 70e80 C and near-neutral pH: effects of organic acids and NaCl. Geochim. Cosmochim. Acta 63, 2043e2059. Crossey, L.J., 1991. Thermal degradation of aqueous oxalate species. Geochim. Cosmochim. Acta 55, 1515e1527. Crundwell, F.K., 2015. The mechanism of dissolution of the feldspars: Part I. Dissolution at conditions far from equilibrium. Hydrometallurgy 151, 151e162. Declercq, J., Bosc, O., Oelkers, E.H., 2013. Do organic ligands affect forsterite dissolution rates? Appl. Geochem 39, 69e77. Drummond, S.E., Palmer, D.A., 1986. Thermal decarboxylation of acetate. Part II. Boundary conditions for the role of acetate in the primary migration of natural gas and the transportation of metals in hydrothermal systems. Geochim. Cosmochim. Acta 50, 825e833. Fein, J.B., Yane, L., Handa, T., 1994. The effect of aqueous complexation on the decarboxylation rate of oxalate. Geochim. Cosmochim. Acta 58, 3975e3981. Ganor, J., Reznik, I.J., Rosenberg, Y.O., 2009. Organics in watererock interactions. Rev. Mineral. Geochem 70, 259e369. Kharaka, Y.K., Carothers, W.W., Rosenbauer, R.J., 1983. Thermal decarboxylation of acetic acid: implications for origin of natural gas. Geochim. Cosmochim. Acta 47, 397e422. Ma, Q., Liu, Y., Liu, C., He, H., 2012. Heterogeneous reaction of acetic acid on MgO, alpha-Al2O3, and CaCO3 and the effect on the hygroscopic behaviour of these particles. Phys. Chem. Chem. Phys. 14, 8403e8409. McCollom, T.M., Seewald, J.S., 2003a. Experimental study of the hydrothermal reactivity of organic acids and acid anions: I. Formic acid and formate. Geochim. Cosmochim. Acta 67, 3625e3644. McCollom, T.M., Seewald, J.S., 2003b. Experimental constraints on the hydrothermal reactivity of organic acids and acid anions: II. Acetic acid, acetate, and valeric acid. Geochim. Cosmochim. Acta 67, 3645e3664. Ong, A., Pironon, J., Robert, P., Dubessy, J., Caumon, M.C., Randi, A., et al., 2013. In situ, decarboxylation of acetic and formic acids in aqueous inclusions as a possible way to produce excess CH4. Geofluids 13 (3), 298e304. Palmer, D.A., Drummond, S.E., 1986. Thermal decarboxylation of acetate. Part I. The kinetics and mechanism of reaction in aqueous solution. Geochim. Cosmochim. Acta 50, 813e823. Pittman, E.D., Lewan, M.D., 1994. Organic Acids in Geological Processes. SpringerVerlag, Berlin, Germany. Shao, H., Ray, J.R., Jun, Y.-S., 2011. Effects of organic ligands on supercritical CO2induced phlogopite dissolution and secondary mineral formation. Chem. Geol. 290, 121e132. Shock, E.L., 1994. Application of thermodynamic calculations to geochemical processes involving organic acids. In: Pittman, E.D., Lewan, M.D. (Eds.), Organic Acids in Geological Processes. Springer-Verlag, Berlin, Germany, pp. 270e318. Sun, L., Zou, C., Zhu, R., Zhang, Y., Zhang, S., Zhang, B., Zhu, G., Gao, Z., 2013. Formation, distribution and potential of deep hydrocarbon resources in China. Petroleum Explor. And Dev. 40, 641e649 (in Chinese). Sun, L., Tuo, J., Zhang, M., Wu, C., Wang, Z., Zheng, Y., 2015. Formation and development of the pore structure in Chang 7 member oileshale from Ordos Basin during organic matter evolution induced by hydrous pyrolysis. Fuel 158, 549e557. Surdam, R.C., Boese, S.W., Crossey, L.J., 1984. The chemistry of secondary porosity. AAPG Mem. 37, 127e149. Surdam, R.C., Crossey, L.J., Hagen, E.S., Heasler, H.P., 1989. Organic-inorganic interactions and sandstone diagenesis. AAPG Bull. 73, 1e23. Wang, H., Zheng, H., 2012. Research on Raman spectra of oxalic acid during decarboxylation under high temperature and high pressure. Spectrosc. Spectr. Analysis 32, 669e672 (in Chinese). Welch, S., Ullman, W., 1996. Feldspar dissolution in acidic and organic solutions: compositional and pH dependence of dissolution rate. Geochim. Cosmochim. Acta 60, 2939e2948. White, A.F., Brantley, S.L., 1995. Chemical weathering rates of silicate minerals: an overview. Chem. Weather. Rates Silic. Min. 31, 1e22. Wu, Y., Ji, L., He, C., Zhang, Z., Zhang, M., Sun, L., Su, L., Xia, Y., 2016. The effects of pressure and hydrocarbon expulsion on hydrocarbon generation during hydrous pyrolysis of type-I kerogen in source rock. J. Nat. Gas Sci. Eng. 34, 1215e1224. Yang, L., Xu, T., Wei, M., Feng, G., Wang, F., Wang, K., 2015. Dissolution of arkose in dilute acetic acid solution under conditions relevant to burial diagenesis. Appl. 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Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem
Experimental study of the decomposition of acetic acid under conditions relevant to deep reservoirs Yuanju Li a, b, Shixin Zhou a, *, Jing Li a, Yu Ma a, b, Kefei Chen a, b, Yuandong Wu c, Yuhong Zhang a, b a
Key Laboratory of Petroleum Resources, Gansu Province/Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou 730000, China University of Chinese Academy of Sciences, Beijing 100049, China c Powerchina Water Environment Governance, Shenzhen, 518102, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 May 2017 Received in revised form 20 July 2017 Accepted 25 July 2017 Available online 27 July 2017
The thermal stability of organic acids is critical in studying the formation mechanism of secondary pores in deep reservoirs (>4000 m). However, the thermal stability of organic acids in geochemical systems is strongly dependent on temperature, minerals and pressure (fluid pressure and lithostatic pressure). To investigate the activity of organic acids and the influence of these factors (temperature, pressure and Kfeldspar), four series of experiments were conducted over a wide range of reaction conditions. Pure acetic acid solutions were selected for this study. The temperatures ranged from 130 C to 380 C, and each experiment was performed for 72 h in steps of 50 C. The results indicate that the decomposition reaction rate was slow but accelerated with increasing temperature. At temperatures higher than 230 C, the decomposition of acetic acid proceeded to a much more significant extent than at low temperatures. The results of experiments also show that the decomposition rates were faster at lower lithostatic pressure, and the lithostatic pressure played a larger role than water pressure in affecting the decomposition of acetic acid. Additionally, the presence of K-feldspar influenced the reaction rates of acetic acid, especially the oxidation rates. This was attributed to the increasing concentration of acetate produced by dissolution. © 2017 Elsevier Ltd. All rights reserved.
Editorial handling by Prof. M. Kersten Keywords: Acetic acid Acetate Decomposition K-feldspar Thermal stability
1. Introduction With the increase of exploration depth, more deep reservoirs (>4000 m) have been found, such as the Dongying depression in the Bohai Bay Basin, NE China (Yuan et al., 2007). Secondary pores are an important storage space for these reservoirs. Previous studies have shown that organic acids play a key role in mineral dissolution and the formation of secondary porosity (Surdam et al., 1984, 1989; Barth et al., 1990). Therefore, understanding the thermal stability of organic acids is critical in studying the formation mechanism of secondary pores in these deep reservoirs. Numerous laboratory experiments have been performed to investigate the thermal stability of organic acids in aqueous solutions (Kharaka et al., 1983; Palmer and Drummond, 1986; Drummond and Palmer, 1986; Crossey, 1991; Bell and Palmer,
* Corresponding author. E-mail address: [email protected] (S. Zhou). http://dx.doi.org/10.1016/j.apgeochem.2017.07.013 0883-2927/© 2017 Elsevier Ltd. All rights reserved.
1994; Bell et al., 1994; Fein et al., 1994; Shock, 1994; McCollom and Seewald, 2003a, b; Ong et al., 2013). These previous studies have shown the possible reaction pathways for the decomposition of organic acids in geologic systems and the factors that influence the thermal stability of organic acids. These observations, however, had only been reported for experiments under relative low fluid pressure, and most of the experiments did not consider the effect of lithostatic pressure. In fact, pressure may play an important role in regulating the reaction process, because the decomposition reaction of organic acids generates lots of gaseous products, which will significantly change the molar volume of the reaction, and because the fluid and lithostatic pressures are distinctly different in natural systems, and their effects need to be assessed individually. What's more, the temperatures of deep reservoirs in different basins vary tremendously. For example, the temperature of the reservoirs at depths of 4000 m is lower than 100 C in the Kela Gas Field, Kuqua Depression, Tarim Basin, China, whereas there are still liquid hydrocarbon accumulations at a depth of 7550 m and 295 C in the
Y. Li et al. / Applied Geochemistry 84 (2017) 306e313
Pre-Caspian Basin, Russia (Sun et al., 2013). However, most previous experiments have been conducted in a very narrow temperature range (Palmer and Drummond, 1986; Bell et al., 1994; McCollom and Seewald, 2003a, b; Ong et al., 2013). Without more detailed knowledge about the changes in stability of organic acids with increasing temperature, it is difficult to use these experimental results to accurately predict the distribution and reactivity of organic acids in deep reservoirs. On the other hand, numerous experiments have been conducted to study the interactions of organic acids and minerals (Palmer and Drummond, 1986; Barth and Riis, 1992; Bell et al., 1994; White and Brantley, 1995; McCollom and Seewald, 2003a, b; Ganor et al., 2009; Shao et al., 2011; Declercq et al., 2013). A wide range of minerals (including stainless steel, quartz, pyrite, calcium montmorillonite, calcite, hematite and magnetite) were chosen to study the catalytic decomposition of acetic acid (Pittman and Lewan, 1994). However, feldspar is far more common than most of the minerals selected in previous studies of sedimentary rock and volcanic rock, but most studies have only focused on feldspar dissolution by water-soluble organic acids (Bevan and Savage, 1989; Welch and Ullman, 1996; Blake and Walter, 1999; Ma et al., 2012; Crundwell, 2015; Yang et al., 2015). If feldspar significantly promotes the decomposition of organic acid, the dissolution reaction might be affected, leading to some changes in the formation of secondary pores. Here, we select K-feldspar to simulate a clastic reservoir, since K-feldspar is more stable than other feldspars, such as albite and anorthite, especially in an acid aqueous solution (Zhang et al., 2009). Above all, due to the abundance of acetic acid in oil field water (Barth and Riis, 1992), we have conducted a series of laboratory experiments examining the reactivity of acetic acid under 45e60 MPa lithostatic pressure and 45e60 MPa water pressure at elevated temperatures (130e380 C). The degradation rate, activation energies and pre-exponential factors were obtained by analyzing the reaction dynamics. In this study, pressure and Kfeldspar were selected to simulate actual geologic environments, such as deep clastic reservoirs in sedimentary basins. Changes in the total concentration of acetate over time were measured to assess the degradation rate under different conditions to address three issues in particular: (1) the influence of water and lithostatic pressure on the thermal stability of acetic acid; (2) the changes in the thermal stability of acetic acid and acetate over a wide temperature range; and (3) the role of K-feldspar in affecting decomposition pathways and rates. 2. Experimental materials and methods 2.1. Materials Dilute pure acetic acid (analytical grade) was chosen as the reaction solution. To avoid changes in the original ionic composition, all solutions were made in degassed deionized water. In Experiment 1 (Ext. 1) to Experiment 4 (Ext. 4), the solute was pure acetic acid (155.4 mmol/kg), and because acetic acid is a weak acid, the main species in these samples was acetic acid (>90%). K-feldspar was included in Ext. 4. A block of pegmatite was obtained from an outcrop in the Xingxingxia Beishan area. After removal of the surface weathered layer, all rocks were fragmentized to screen out the K-feldspar grains. Randomly selected parts of the K-feldspar grains were used for experiments to avoid the effect of mineral heterogeneity. 2.2. Methods The experiments were conducted in a semi-closed high-
307
temperature, high-pressure simulation system (WYMNe3 HTHP, Fig. 1), as used by Sun et al. (2015) and Wu et al. (2016). The instrument consists of a software control system (computer) and a hardware performance system (apparatus). The software control system was used to set experimental conditions and collect data. The hardware performance system included the following systems: reaction cell, heating system, pressure system and collecting systems. The reaction cell was made of titanium alloy and contained within an autoclave (stainless-steel), and the advantages of the reaction cell are the resistance to acid and alkali corrosion with a maximum tolerable temperature of 550 C and pressure of 100 MPa. The accuracy of heating system is ±0.1 C. The pressure system contains a hydraulic control system for lithostatic pressure and confining pressure and a turbocharger for hydrodynamic pressure. The lithostatic pressure accuracy is ±0.1 MPa, water pressure accuracy is ±0.05 MPa. With the aid of pressure system, this instrument can simulate the lithostatic pressure and hydrostatic pressure achieved in reservoir rock. It is widely accepted that high temperatures can be used to model reactions that occur at lower temperatures and over a long period of time in geologic systems, and the experiments in this study investigated six different temperatures: 130 C, 180 C, 230 C, 280 C, 330 C and 380 C. Every experiment at a different temperature was conducted for 72 h. The reaction conditions for each series of experiments are listed in Table 1. The experimental procedure was as follows: (1) loading of K-feldspar (Ext.4) or skeleton (titanium alloy, Ext1-3); (2) vacuuming; (3) equal lithostatic pressure was applied to the top and bottom of the skeleton or mineral; (4) injecting solutions through high pressure liquid pump into the reaction cell until the water press reaches the target pressure. When the water pressure displayed by the pressure gauge exceeds 10% of the initial water pressure, the magnetic valve B is opened to discharge the gas production into the gas-liquid separator until the water press decreases to the initial water pressure; (5) After the separation of the gas and liquid, the gas was extracted using a standard purge-and-trap device and the liquid was injected into the reaction cell again by the liquid pump; (6) The experimental conditions (including temperature, time, lithostatic pressure, and water pressure) were recorded by a computer every minute. Once the experiment was over, the reaction cell was put into a cold trap to separate the gas and liquid. Hydrochloric acid was added into the cell to convert carbonate and bicarbonate to carbon dioxide. Then, all gas samples were extracted using a standard purge-and-trap device. Gases were analyzed on an Agilent 6890 gas chromatograph (Agilent USA) fitted with a flame ionization detector operating at 200 C for gaseous hydrocarbons. One milliliter gas samples were injected at 100 C, with separation performed on a HP-PLOT-Q fused silica 30 m 0.32 mm 20 mm column, with helium as the carrier gas. The oven temperature was programmed to increase from 35 C (5 min hold) to 150 C (3 min hold) at 5 C/ min, then to 270 C (2 min hold) at 10 C/min. Liquid samples collected from the reaction cell were divided into four aliquots. One aliquot was used to determine the abundance of dissolved CO2 (aq) and CH4 (aq) by GC (Agilent 6890). The remaining three aliquots were used to determine the concentrations of Na, K, and acetate by ion chromatography. A Dionex ICS 3000 Ion Chromatography system (Dionex USA) equipped with
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Fig. 1. Schematic diagram of the WYMNe3 HTHP instrument (Wu et al., 2016).
Table 1 Reaction conditions for each series of experiments.
Ext. Ext. Ext. Ext.
1 2 3 4
Temperature ( C)
Time(h)
Water Pressure (MPa)
Lithostatic pressure (MPa)
Materials
130e380 130e380 130e380 130e380
72 72 72 72
60 45 60 60
60 60 45 60
Acetic Acetic Acetic Acetic
IonPac As19 analytical column (250 mm 4.0 mm), IonPac AG19 guard column (50 mm 4.0 mm), IonPac Cs12 analytical column (250 mm 4.0 mm), and IonPac Cs12 guard column (250 mm 4.0 mm) was employed in the experiments. The chromatographic conditions for acetate/acetic acid were an IonPac As19 analytical column, with a column temperature of 30 C and a detector temperature of 35 C. The eluents were 18.2 MU$cm3 water (A) and 30 mmol/L NaOH (B). A gradient elution was employed as follows: 50% B for 0e3 min; 50% B/100% B from 3 to 6 min; 100% B from 6 to 15 min; 100% B/50% B from 15 to 17 min; 50% B from 17 to 25 min. The flow rate was 1.0 mL/min, and the sample size was 20 mL. The chromatographic conditions for Na and K were as follows: an IonPac Cs12 analytical column, with eluents of 18.2 MU$cm-3 water (A) and 20 mmol/L methyl sulfonic acid (B), a current of suppressor at 59 mA, and the other conditions were the same as those for acetic acid. Although a variety of acetate species may be present in our study (i.e., acetic acid, acetate), acetate species will all be converted to acetate during detection because of the NaOH eluent. Here, we P use “ acetate” to represent the total concentration of acetate and acetic acid. We report the average concentrations for the three fluid aliquots. The estimated analytical uncertainty for all compounds was ±1%. 3. Results 3.1. Decomposition of acetic acid without K-feldspar Ext. 1e3 were conducted in the reaction cell without the presence of minerals. The results are listed in Table 2. In this study, the initial concentration of acetic acid was nearly 155.4 mmol/kg. Fig. 2
acid acid acid acid þ K-feldspar
P presents the change in residuary acetate concentration with temperature in Ext. 1e3, the gas production and a decrease in the concentration of acetate indicate that acetic acid decomposed. In Ext. 1e3, almost no acetic acid reduction was observed at 130 C and 180 C, and the concentrations of gas products was extremely low. Due to the relatively short duration in terms of geologic time and the analytical uncertainties, we speculated that the acetic acid may not decompose at temperature under 180 C. At 230 C, a very slight decreasing trend was observed for the acetic acid concentration, but the gas production indicated that decomposition occurred. With increasing temperature, the changes in acetic acid concentrations were significant at 280 C 380 C. There are two main decomposition pathways for acetic acid (Kharaka et al., 1983; Palmer and Drummond, 1986; Drummond and Palmer, 1986; Bell et al., 1994; McCollom and Seewald, 2003b). For the most part, acetic acid is presumed to decompose primarily by decarboxylation:
CH4 COOH/CO2 þ CH4 :
(1)
In Ext. 1e3, the production of CO2 and CH4 in a nearly 1:1 ratio indicates that acetic acid decomposed primarily by decarboxylation (Eqn. (1)). However, Bell et al. (1994) demonstrated that acetic acid decomposition might occur through oxidation under some circumstances due to the extremely low concentration of O2 present in most subsurface geologic environments. The reaction is expressed by:
CH3 COOH þ 2H2 O/2CO2 þ 4H2 : C,
(2)
At temperatures above 230 the concentration of CO2 was slightly higher than CH4, suggesting that some oxidation of acetic acid occurred as well (Eqn. (2)).
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309
Table 2 P Changes in the acetate, CO2, and CH4 concentrations with temperature in Ext. 1e3. Temperature ( C) Ext. 1 25 130 180 230 280 330 380 Ext. 2 25 130 180 230 280 330 380 Ext. 3 25 130 180 230 280 330 380
Concentration (mmol/kg) P acetate CO2
CH4
155.40 155.40 155.39 155.29 153.71 133.88 72.68
e \ \ 0.09 0.86 17.81 73.85
e \ \ 0.10 0.88 16.14 71.29
155.40 155.39 155.39 155.28 153.74 136.03 73.79
e 0.01 \ 0.03 0.81 16.8 70.3
e \ \ 0.02 0.67 15.6 69.2
e \ 0.01 0.10 1.20 20.33 103.8
e \ \ 0.07 0.98 19.17 97.23
155.40 155.40 155.40 155.26 153.00 133.19 54.84 P “\” ¼ no data, “-” ¼ no detection, “ acetate” ¼ total acetic acid.
concentration of acetate and
3.2. Decomposition reaction in the presence of K-feldspar P Changes in residual acetate concentration at elevated temperatures in Ext. 4 are shown in Fig. 3. Although the data points also show decay behavior for all samples and the trend was similar to the mineral-free experiments (Ext. 1), the residual concentration was lower than Ext. 1 at the same temperature. In Ext. 4, the decomposition also proceeded extremely slowly (or did not occur) for acetic acid at temperatures under 180 C. Then, the reaction proceeded much more rapidly at temperatures above 230 C. However, the mineral dissolution will consume large numbers of hydrogen ions, but no acetate (Eqn. (3)); therefore, the lower residual concentrations of acetate and acetic acid indicated that heating acetic acid in the presence of K-feldspar led to a faster decomposition rate.
2KAlSi3 O8 þ 2CH3 COOH þ 9H2 O/Al2 Si5 ðOHÞ þ 2K þ þ 4H4 SiO4 þ 2CH3 COO (3) To avoid interference from gas derived from background sources associated with minerals, an additional deionized water experiment was conducted in the presence of K-feldspar. This experiment was also intended to provide a baseline for evaluation of the extent of K-feldspar dissolution. The concentrations of products are listed in Table 3. In this experiment, the concentration of gas as well as the ion concentrations were much lower than in Ext. 4. Therefore, any interference associated with the minerals and deionized water was neglected. Higher concentrations of CO2 and CH4 suggest that more acetic acid was decomposed. The amount of CO2 was nearly equal to CH4, indicating that acetic acid decomposition in the presence of Kfeldspar proceeded primarily through decarboxylation (Eqn. (1)). However, relative to CH4, a small excess of CO2 was produced, suggesting some oxidation of acetic acid occurred (Eqn. (2)). The gap between the concentrations of CO2 and CH4 was larger than
P Fig. 2. Measured concentrations of acetate, CO2, CH4 in Ext. 1e3, (a) Ext. 1, water pressure 60 MPa, lithostatic pressure 60 MPa, (b) Ext. 3, water pressure 45 MPa, lithostatic pressure 60 MPa, (c) Ext. 3, water pressure 60 MPa, lithostatic pressure 45 MPa.
that in Ext. 1, illustrating that acetic acid may be more favorably decomposed by oxidation in Ext. 2 (Eqn. (2)). Fig. 4 shows the change in ion concentration (Naþ, Kþ) in Ext. 2.
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Drummond, 1986; Bell et al., 1994; McCollom and Seewald, 2003a, b). In this study, acetic acid and acetate decompose by decarboxylation and oxidation. The first-order reaction of acetic acid or acetate in our experiments is more appropriately portrayed in the form:
hX i. d acetate dt ¼ keff ½acetate;
(5)
where keff is the effective rate constant for the overall decomposition (Eqn. (6)):
keff ¼ kdecarb þ koxid ;
(6)
where kdecarb is the rate constant for decarboxylation (Eqn. (1)) and koxid is the rate constant for oxidation (Eqn. (2)). Eqn. (5) can be integrated to yield:
hX
i hX i acetate ¼ acetate ekeff t : t
(7)
0
The corresponding expressions for CO2 and CH4 are: P Fig. 3. Measured concentrations of acetate, CO2, CH4 in Ext. 4, water pressure 60 MPa, lithostatic pressure 60 MPa, K-feldspar.
The changes in Kþ and Naþ indicate that K-feldspar was dissolved by acetic acid, and the solubility of feldspar increased with temperature. The dissolution of minerals consumed large amounts of hydrogen ions but little acetate. The concentration of Naþ increased markedly at low temperature (<230 C); then, the dissolution slowed. Different from Naþ, a slight increasing trend was observed for the Kþ concentration. However, the increase in concentration of both ions with increasing temperature indicates that dissolution occurred in the Ext. 2, and it seems that the dissolution was not affected by the decomposition of acetic acid in the duration of experiments. 4. Discussion 4.1. The decomposition rate constants and the role of temperature Previous studies have shown the decomposition of acetic acid and acetate could be modeled with a first-order rate law:
dm=dt ¼ km;
hX
i hX i hX i CO2 ¼ acetate eðkdecarb þ2koxid Þt þ CO2 ;
(8)
i hX i hX i acetate ekdecarb t þ CH4 ¼ CH4 ;
(9)
0
hX
0
0
0
P P P where ½ acetate0 ½ CH4 0 ½ CO2 0 are the initial concentrations of acetate and acetic acid, CH4 and CO2, respectively, and P ½ acetatet is the residual concentration of acetate and acetic acid. Because the residual concentration was measured, we can calculate keff by Eqn. (7) (t ¼ 72 h). Values for koxid and kdecarb in our experiments were determined using Eqns. (8) and (9). We also calculate the maximum oxidation rate constants using Eqn. (6) (koxid ¼ keff kdecarb ), because the concentration of CO2 is easily affected by its water solubility and, for many experiments, degradation primarily occurred by decarboxylation. The koxid values were adjusted in the model for each experiment until a best fit was obtained. Fig. 5 shows changes in rate constants in all four experiments. The reaction rate constants were calculated based on the assumption that the acetic acid and acetate were decomposed at very slow rates at temperatures under 230 C. The general behavior of the curves of the four series experiments was similar across the series.
(4)
where k is the reaction rate constant, t is the elapsed time, and m is the concentration of acetate (Kharaka et al., 1983; Palmer and
Table 3 Fluid compositions during the reaction of Ext. 4. Temperature ( C)
Concentration (mmol/kg) P kþ acetate Naþ
Ext. 4 25 155.40 130 155.39 180 155.38 230 155.28 280 153.21 330 131.83 380 61.41 Experiment with no acids 20 0 330 0
<0.01 0.86 1.11 1.57 1.42 1.70 1.77
<0.01 0.32 0.27 0.41 0.48 0.43 0.45
pH CO2
CH4
e 0.03 0.01 0.15 1.28 24.9 104.8
e \ \ 0.11 1.06 21.6 91.0
2.7 4.3 4.5 4.4 4.7 5.3 5.5
<0.01 <0.01 7 0.1 0.06 0.02 \ 7 P “\” ¼ no data,“-” ¼ no detection, “ acetate” ¼ total concentration of acetate and acetic acid.
Fig. 4. Changes in Na and K concentrations in Ext. 4.
Y. Li et al. / Applied Geochemistry 84 (2017) 306e313
The keffetemperature relationship displayed an increasing trend. In the four series of experiments, the decomposition of acetic acid at 230 C remained within the range of 2.54 109 to 3.28 109, and the rate constants for decomposition at 280 C was 4.21 108, 4.14 108, 5.98 108 and 5.47 108, respectively. In comparing these rate constants, we found that at 230 C, acetic acid decomposed at a rate more than 90% slower than that in experiments at 280 C. The decomposition rates were very low but increased rapidly at temperatures under 280 C. In contrast, the data for these experiments yielded rate constants for acetic acid decomposition at 330 C in the range of 5.13 107 to 6.34 107, whereas the rate constant for decomposition at 380 C ranged from 2.87 106 to 4.02 106. The rates were much higher than in low temperature experiments, but increased slightly more slowly than the lower temperature stage (<280 C). All results of these experiments illustrated that 230 C was the critical temperature; at temperatures above 230 C, acetic acid will decompose significantly and increased rapidly with increasing temperature. High temperature dramatically accelerated the decomposition of acetic acid and acetate. Fig. 6 illustrates the first-order rate constants (logarithm) for P acetate decomposition as a function of temperature expressed as 1000/T. The general behavior for the curves follows a linear tendency (R2 > 0.99). The linear equations for Ext. 1e4 were the Arrhenius equation:
ln k ¼ ln A Ea =RT:
(10)
where A is a preexponential factor, Ea is the activation energy, R is the universal gas constant, and T is the temperature (K). Because almost no acetic acid reduction was observed under 180 C, the calculated effective rate constants at 130 C and 180 C were eliminated from the regression. We have calculated Arrhenius P parameters for decomposition of acetate by linear fitting (Table 4). The calculated activation energies for all four series of experiments were closely indicated that the influence factors in our study all did not significantly affect the decomposition pathways of acetic acid.
4.2. The role of water pressure and lithostatic pressure Table 5 illustrates the changes in keff, koxid and kdecarb in the three series of mineral-free experiments. The results of Ext. 1 and 2 were analyzed comparatively to examine the different changes that
Fig. 6. Effective rate constants for ature expressed as 1000/T.
311
P acetate decomposition as a function of temper-
occur under different water pressures but the same lithostatic pressure. In Ext. 1 the water pressure is 60 MPa, 15 MPa higher than in Ext. 2 (45 MPa), but there was a very slight increase in rates constants of Ext. 1, but only at 330 C was the gap between Ext. 1 and Ext. 2 significant. A previous study has shown that the decomposition of organic acid would be hindered under high fluid pressure (Wang and Zheng, 2012). The results in our study that did not follow a regular behavior should be attributed to the high lithostatic pressure, which covered up the influence of water pressure. The results of Ext. 1 and 3 were analyzed comparatively to examine the different changes that occur under different lithostatic pressures but the same water pressure. Although the lithostatic pressure is 20 MPa lower than that of Ext. 1, the effective rates constants were much higher than Ext. 1, especially at 280 C. The higher lithostatic pressure led to a slower decomposition rate. Apparently, the lithostatic pressure may hinder the decomposition reaction. The decomposition rate in Ext. 3 is the fastest, followed by Ext. 1, and the rate of Ext. 2 (under the lowest water pressure) was the lowest at the specified temperature. Comparing the effective rate constants of Ext. 1, Ext. 2 and Ext. 3, the thermal stability of acetic acid changed very slightly under different water pressure but significantly under different lithostatic pressure. The results indicated that the lithostatic pressure plays a larger role than water pressure in affecting the decomposition of acetic acid. In addition, the acetic acids in all the three series of experiments were mainly decomposed through decarboxylation. The extremely low and close values of koxid of the three series of mineral-free experiments suggested that the lithostatic pressure and water pressure did not significantly influence decomposition pathways. 4.3. The influence of K-feldspar In this study, the higher reaction rates in Ext. 4 at specified temperatures are shown in Fig. 4 and Table 6. As shown in Table 6,
Table 4 P Arrhenius parameters for decomposition of acetate.
Fig. 5. Rate constant (keff) versus temperature.
Experiment
A (S1)
Ext. Ext. Ext. Ext.
8.9 5.3 9.5 1.1
1 2 3 4
105 105 105 106
Ea (kJ/mol) 130.4 127.9 129.4 130.4
312
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Table 5 First-order rate constants for decomposition of acetic acid and acetate in Ext. 1, 2 and 3 at temperatures above 230 C. Experiment 280 Ext. Ext. Ext. 330 Ext. Ext. Ext. 380 Ext. Ext. Ext.
C 1 2 3 C 1 2 3 C 1 2 3
keff (S1)
kdecarb (S1)
koxid (S1)
4.2 108 4.1 108 6.0 108
4.2 108 4.1 108 6.0 108
<1.0 109 <1.0 109 <1.0 109
5.7 107 5.1 107 5.9 107
5.7 107 5.0 107 5.8 107
<1.0 108 1.0 108 1.0 108
2.9 106 2.9 106 4.0 106
2.8 106 2.8 106 3.9 106
1.0 107 <1.0 107 1.0 107
the decomposition reaction as well as decarboxylation and oxidation proceeded more slowly in Ext. 1 than in Ext. 4, especially at high temperature. In Ext. 4, the general behavior for the curves also increased with temperature. The rate constant (keff) for acetic acid decomposition in the presence of K-feldspar at 230 C was 2.9 109, which was nearly 16% higher than that in Ext. 1 (2.5 109). When temperature was increased to 280 C, the values were 4.2 108 in Ext. 1 and 5.5 108 in Ext. 2, and the comparison of these rates constants suggests that Ext. 1 decomposed at a rate that was 30% slower than that in Ext. 4. At 330 C, Ext. 1 decomposed at a rate that was 10% slower than that in Ext. 4 and 20% slower than that in Ext. 4 at 380 C. No obvious trend was observed for the relationship between temperature and the difference in the effective rate constants between the two series of experiments. Like the effective rates constants, the gap in the data of kdecarb also increased and reached a maximum at 280 C, then narrowed at 330e380 C. Both the difference in the effective and decarboxylation rate constants were not very large. In contrast, although the oxidation reaction rate was very slow at 130 C-230 C, the oxidation also proceeded much more slowly in Ext. 1 than the decomposition of acetic acid in the presence of K-feldspar and the gap between oxidation rate constants rapidly increased with increasing temperature: The oxidation rate constants in Ext. 1 and Ext. 4 were both nearly 1.0 109 at 280 C, whereas the value in Ext. 2 was nearly six times that in Ext. 1 at 380 C. The observed changes in the three kinds of rate constants indicated that the acetic acid in Ext. 4 were decomposed primarily by decarboxylation; however, at temperatures above 330 C, more oxidation of acetic acid occurred, and its role in the decomposition increased with increasing temperature. This may be attributed to two reasons: (1) K-feldspar is catalytic for decomposition, especially for oxidation at temperature above a critical level; (2) there
are lots of acetate produced by K-feldspar dissolution (Eqn. (3)), because the oxidation proceeded more rapidly for acetate than for acetic acid. We prefer the second explanation, since none of its elements were redox sensitive and the previous study also demonstrated that the acetate was more easily decomposed than acetic acid (McCollom and Seewald, 2003a, b). The close calculated activation energies for Ext. 1 and Ext. 4 also indicated that K-feldspar was not a catalyst. What's more, the increasing concentration of acetate should also be a reason why K-feldspar leads to higher decomposition and decarboxylation rates. However, although the role of oxidation in the decomposition increased at 330e380 C, the its contribution was still far less than that of decarboxylation. The relatively small difference in the effective and decarboxylation rate constants suggesting the influence of K-feldspar on the decomposition of acetic acid was not very significant, especially at high temperatures. What's more, as mentioned earlier, it seemed that the dissolution was not affected by the decomposition of acetic acid in the experimental process. In fact, the not very large influence of K-feldspar may not hinder the dissolution over a period of time. In addition, the first-order rate constants for acetic acid decomposition calculated from kinetic parameters given by McCollom and Seewald, (2003a, b) and Bell et al. (1994) are also listed in Table 5. The experimental results of Bell at al. (1994) demonstrated that the activation energies were 270 kJ/mol in titanium and gold vessels. Previous studies have shown that the decomposition of acetic acid and acetate is catalyzed by the vessel surface (Palmer and Drummond, 1986). This could be a reason for the higher Keff and Kdecarb values and lower activation energies in our study, because the reaction cell was made partly of stainless steel. 4.4. Geological application The results of this study have important implications for hydrocarbon exploration, especially for deep oil and natural gas. The thermal stability of dissolved organic acids in deep reservoirs is strongly dependent on lithostatic pressure, temperature and minerals. Lithostatic pressures of 45e60 MPa correspond to conditions at depths of 2000e4000 m in a sedimentary basin. Our experimental results suggested that the decomposition reaction of organic acids will be retarded by lithostatic pressure at deeps of 2000e4000 m. However, water pressures of 45e60 MPa are the freshwater hydrostatic pressures occurring at depths of 4000e6000 m in a sedimentary basin. Our experimental results indicated that the degradation of organic acids will not be affected by water pressure, because the lithostatic pressure is great at such a depth, which can exert a larger influence on the decomposition of organic acids than fluid pressure. The temperatures of deep reservoirs in different basins vary
Table 6 First-order rate constants for the decomposition of acetic acid. Experiment
k (S1)
Ext. 4 K-feldspar, 230 C Ext. 4 K-feldspar, 280 C Ext. 4 K-feldspar, 330 C Ext. 4 K-feldspar, 380 C McCollom and Seewald, 2003a, b 325 C Acetic acid þ pyrite, magenetite, pyrrhotite Na-acetate þ pyrite, magenetite, pyrrhotite Acetic acid þ hematite, magnetite Bell et al. (1994) TiO2 Gold Quartz
2.9 5.5 6.3 3.6
109 108 107 106
5.5 108 9.9 108 3.6 107 4.7 109 1.3 108 3.3 109
kdecarb (S1) 2.8 5.5 5.8 3.0
109 108 107 106
4.7 108 5.0 108 <9.4 109
koxid (S1) 1.0 1.0 5.0 6.0
1010 109 108 107
<1.0 108 4.9 108 3.5 107
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tremendously; however, in this study, we observed that there exists a critical temperature, above which the decomposition of acetic acid will proceed to a much more significant extent. This indicated that organic acids may survive for a substantial amount of time if the formation temperature is lower than a certain value, even in the deep reservoirs with depths greater than 4000 m. The presence of K-feldspar does accelerate the decomposition of organic acid; this may suggest that the decomposition reaction will proceed faster in the reservoirs where K-feldspar is abundant. However, K-feldspar was not a catalyst, and its slight influence may be covered by other actors that have a larger influence on the reactivity of dissolved organic compounds, such as iron oxide and iron sulfide minerals. In this study, the temperature, pressure and K-feldspar were taken into consideration to simulate natural geologic environments in deep reservoirs. The results of the experiments suggested that the temperature, lithostatic pressure and K-feldspar all had an influence on the decomposition of acetic acid. However, we did not consider fluid pH values and the acid concentration as variables in this research. In future studies, we plan to focus on the effects of fluid pH values and the concentration of organic acids. 5. Conclusion The results obtained from the evaluation of thermal stability of acetic acid and acetate under elevated temperatures showed that all three factors (temperature, pressure and K-feldspar) influenced the decomposition of acetic acid. The effects of temperature on the decomposition varied greatly. We determined 230 C to be the critical temperature. At temperatures lower than 230 C, the reaction rate was extremely slow so that there was almost no decomposition that occurred in this study. At temperatures above 230 C, the decomposition rates increased with the increasing temperature. Our results also indicated that the lithostatic pressure played a larger role than fluid pressure in affecting the decomposition of acetic acid, and the lithostatic pressure hindered the decomposition reaction, but did not significantly affect the decomposition pathways. Additionally, the presence of K-feldspar can have a slight effect on decomposition and decarboxylation rates, but significantly accelerated the oxidation of acetic acid. This was attributed to the increasing concentration of acetate produced by dissolution. Acknowledgements This research was jointly supported by the Major State Basic Research Development Program of China (Grant No. 41072105), the National Science and Technology Major Projects of Ministry of Science and Technology of China (Grant No. 2016ZX05003002-004, 2016B-0502) and the Key Laboratory Project of Gansu Province (Grant No. 1309RTSA041). The authors would also appreciate two anonymous reviewers who refereed this paper and the Executive Editor Dr. Michael Kersten for all their valuable comments that helped to greatly improve the quality of this paper. References Barth, T., Riis, M., 1992. Interactions between organic acids anions in formation waters and reservoir mineral phases. Org. Geochem 19, 455e482. Barth, T., Borgund, A.E., Riis, M., 1990. Organic acids in reservoir watersdrelationship with inorganic ion composition and interactions with oil and rock. Org. Geochem 16, 489e496. Bell, J.L.S., Palmer, D.A., 1994. Experimental studies of organic acid decomposition. In: Pittman, E.D., Lewan, M.D. (Eds.), Organic Acids in Geological Processes.
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