ABSTRACT. An experimental study of aluminum-oxalate complexing at 80 °C indicates that the presence of oxalate in sedimentary basin fluids may play an ...
Experimental study of aluminum-oxalate complexing at 80 °C: Implications for the formation of secondary porosity within sedimentary reservoirs Jeremy B. Fein* Illinois Slate Water Survey, Office of Environmental Chemistry 2204 Griffith Drive, Champaign, Illinois 61820
ABSTRACT An experimental study of aluminum-oxalate complexing at 80 °C indicates that the presence of oxalate in sedimentary basin fluids may play an important role in creating secondary porosity in reservoir rocks. The solubility of gibbsite was measured as a function of oxalate concentration. Geologically realistic oxalate concentrations were used in order to simulate the aluminum mobilities that occur in reservoir fluids. Although the stoichiometry of the important aluminum-oxalate species cannot be determined from these data, the results are used to quantify the minimum extent of aluminum-oxalate complexing that occurs in solution. Assuming that Al(Ox) 3 3- is the only important aluminum-oxalate complex, the data limit the log of the dissociation constant of Al(Ox)33~ to be ^-16.5. Thermodynamic modeling of aluminum systems with fluids that contain NaCl, acetate, and oxalate illustrates that aluminum-oxalate complexing is much more important than aluminum-acetate complexing, even at relatively low oxalate concentrations and at very high acetate concentrations. In addition, these calculations show that aluminum-oxalate complexing can greatly increase aluminum mobility in formation waters and, therefore, may increase aluminosilicate mineral dissolution within sedimentary basin fluid-rock systems. INTRODUCTION Textural evidence from sedimentary basin rocks indicates that, in many cases, porosity enhancement has occurred as a result of aluminosilicate grain dissolution (e.g., Schmidt and McDonald, 1979; Stone and Lumsden, 1984; Surdam et al., 1984). Current knowledge of inorganic fluid-rock interactions cannot account for the high degree of aluminum mobility that is required by the textural observations (e.g., Giles and Marshall, 1986). However, the presence of high concentrations of carboxylic acid anions in oil-field formation waters has led to speculation that organic-inorganic fluid-rock interactions may hold the key for understanding porosity enhancement (Surdam et al., 1984). Specifically, aqueous complexing between rock-forming cations and carboxylic acid anions may enhance mineral solubilities enough to create secondary porosity in sedimentary basin rocks. In order to ascertain the validity of these speculations, the existence of these aqueous complexes must be tested experimentally. If they do exist, thermodynamic properties of the geologically important complexes must be determined in order to quantitatively model organic-bearing fluid-rock systems. Only two experimental studies have quantified the interactions between aqueous aluminum and a geologically important organic-acid anion at temperatures greater than 25 °C (Palmer et al., 1990; Fein, 1991). These studies have focused on acetate, a monofiinctional carboxylic-acid anion. It does not appear that the stabilities of aluminum-acetate complexes are high enough to explain the extent of observed porosity en*Present address: University of Edinburgh, Grant Institute of Geology, West Mains Road, Edinburgh EH9 3JW, Scotland. GEOLOGY, v. 19, p. 1037-1040, October 1991
hancement, even at the maximum acetate concentration that has been measured in oil-field brines. Experimental studies of feldspar dissolution in oxalate-bearing fluids have suggested that aluminum-oxalate complexes may be present in sedimentary basin fluids (Surdam et al., 1984; Stoessell and Pittman, 1990). However, because of the design of these experiments, the results of these studies are qualitative and cannot be applied to quantitative geochemical models. In this paper I present results from thermodynamically wellconstrained experiments. These results are the first to place quantitative constraints on the complexing behavior between aluminum and a difunctional carboxylic-acid anion at a temperature greater than 25 °C. Oxalate was chosen as the organic-acid anion because of its natural abundance in some oil-field formation waters. MacGowan and Surdam (1988) reported the presence of oxalate in oil-field waters from several North American basins in concentrations up to nearly 500 ppm. In addition, experiments have indicated that oxalic acid is the dominant dicarboxylic acid that is produced from thermal alteration of kerogen, and that concentrations of oxalic acid up to 3000 ppm may be generated in this way (Kawamura and Kaplan, 1987). The objective of this study was to determine if aqueous aluminum-oxalate complexes exist at geologically realistic oxalate concentrations at 80 °C, and, if so, to quantify thermodynamically the nature and extent of complexing. EXPERIMENTAL PROCEDURES The experiments in this study consisted of batch-type solubility measurements (the general procedures and materials were described in detail in Fein, 1991). A very simple chemical system was studied in order to isolate, as much as possible, the complexing effects on the solubility. The solubility of gibbsite [Al(OH)3] was measured at 80 °C in sodium-oxalate-oxalicacid solutions of differing total oxalate concentrations. The concentrations of the starting solutions are compiled in Table 1. T A B L E 1. G I B B S I T E S O L U B I L I T Y M E A S U R E M E N T S Sample no.
G1 G2 G3 G4 G5 G6 G7 G8
Total oxalate
Total sodium
dog molality)
dog molality)
-3.00 -2.60 -2.20 -2.00 -1.80 -1.60 -1.40 -1.20
-2.90 -2.50 -2.10 -1.90 -1.70 -1.50 -1.30 -1.10
Total Al, Final * (log molality) -3.67 -3.26 -2.90 -2.72 -2.47 -2.31 -2.08 -1.89
Calculated pH # 4.9 5.1 5.3 5.4 5.5 5.6 5.7 5.8
*Averaged over five samples taken after concentration plateau attained. #From speciation calculations using the thermodynamic parameters given in Table 2. Calculated pH increases with increasing oxalate concentration due to the inclusion of Al(Ox)33' in the calculations.
1037
The initial solutions did not contain aqueous aluminum. Therefore, the systems approached equilibrium from undersaturation. Samples were taken periodically over the course of 41 days. This was more than enough time to demonstrate the existence of a concentration plateau, suggesting that equilibrium was closely approached in these experiments. In addition, because equilibrium was approached from undersaturation, the measured aluminum concentrations provide minimum constraints to the equilibrium values. Therefore, at worst, the results provide conservative estimates of the stability of the aluminum-oxalate complexes that are present in the experimental systems. Fluid samples were filtered and acidified immediately after sampling. The solutions were analyzed for aluminum concentrations by means of a flameless atomic adsorption (A A) technique. Although the matrices of the standards that were used were closely matched to those of the samples, the presence of oxalate affected somewhat the reproducibility of the aluminum analyses. Analytical reproducibilities ranged from less than ±0.05 log molality units for the runs with the lowest oxalate concentrations to ±0.10 log molality units for the runs with higher oxalate concentrations. Sodium concentrations, measured by using a flame AA procedure, remained constant in the experimental fluids throughout the course of the experiments. All of the final samples (taken after 41 days), as well as several intermediate samples, were analyzed by ion chromatography to measure total oxalate concentrations. All of the measured oxalate concentrations were within 1% of the starting concentrations, indicating that decarboxylation of the oxalate did not occur. RESULTS Figure 1 illustrates the solubility data (compiled in Table 1). Each triangle in Figure 1 represents a concentration plateau that was sampled at least five times over the course of 41 days. The apex of each triangle represents the average aluminum concentration for samples taken after the concentration plateau was reached. The figure illustrates smoothly increasing aluminum concentrations with increasing oxalate concentrations. Figure 1 also depicts the solubility of gibbsite at 80 °C in solutions identical to the ones used in the experiments, calculated assuming that no aluminum-oxalate complexing occurs in the systems. The observed aluminum molalities are from 2.3 to 3.1 orders of magnitude higher than those calculated without aluminum-oxalate complexes. For example, at a log molality of total oxalate of -2.5 (-300 ppm), the calculated solubility without aluminum-oxalate complexing is 20 ppm. The high observed solubilities provide compelling evidence of very strong aluminum-oxalate complexing behavior at 80 °C. THERMODYNAMIC CALCULATIONS Mineral solubilities, measured as a function of anion concentration at fixed pressure and temperature, can define the stoichiometrics and thermodynamic properties of aqueous complexes. However, in this study the aluminum-oxalate stoichiometrics and thermodynamic properties are not the only controls on the solubility behavior. The stability of the aluminumoxalate complex is so high that mass-balance restrictions exert a limiting effect on the solubilities. For example, if the dominant aqueous aluminum species is an aluminum-oxalate complex with a 1:3 stoichiometry, then mass-balance restrictions limit aluminum concentrations to be less than or equal to one-third of the total oxalate concentration in the system. Figure 1 depicts the observed solubilities relative to this limiting condition (uppermost dashed line). Because the observed solubilities are very close to this line, mass-balance restrictions prevent the determination of the stoichiometry of the aluminum-oxalate complex that is present in the experimental solutions. If aluminum concentrations were lower, the average stoichiometry of the aluminum-oxalate complex could be determined by comparing the observed aluminum vs. oxalate solubility slope to those calculated by using specific aluminum-oxalate stoichiometrics. However, because of mass-balance restrictions, the calculated slopes at these very high aluminum concentrations are effectively independent of the aluminum-oxalate stoichiometrics that are used in the computations. Although the stoichiometry of the dominant aluminum-oxalate species cannot be determined unequivocally, the data indicate clearly that aluminum-oxalate complexing exerts a very large influence on aluminum mobility in aqueous fluids at 80 °C. In order to place quantitative bounds on that influence, I assume that the dominant aluminum-oxalate complex is A1(Ox)33". (Ox 2 " represents the completely deprotonated oxalate anion.) At 25 °C, Al(Ox)33_ appears to be an important, if not the dominant, aluminum-oxalate complex at pH values and oxalate concentrations similar to those in this study (Stary, 1963; Bottari and Ciavatta, 1968; Sjoberg and Ohman, 1985). Alternatively, it is possible that mixed aluminum-hydroxide-oxalate complexes are dominant in the experimental fluids. These mixed complexes, if present, would increse aluminum mobilities in solutions with pH ranges higher than that associated with A l ( O x ) 3 3 ~ . Further experimental work is in progress to better determine the aluminum speciation at elevated temperatures. However, the speciation assumption that is made allows for conservative quantitative estimates of aluminum mobility, because for a fixed oxalate concentration, Al(Ox)33_ transports less aluminum than does a complex with a higher Al:Ox ratio. Thus, regardless of the stoichiometry assigned to the aluminum-oxalate species, this study demonstrates the importance of aluminum-oxalate complexing. The results enable conservative quantitative estimates to be made regarding aluminum mobility in reservoir fluids. In order to apply the results of this study to models of more realistic fluid-rock systems, the data are used to quantify the dissociation reaction for A1(Ox)33": A1(Ox) 3 3_ ~ Al 3 + + 3 Ox 2 ".
Log M o l a l i t y
Total
Oxalate
Figure 1. Measurements of gibbsite solubility at 80 °C. Dashed curves represent calculated solubility behaviors (see text). 1038
(1)
The procedure for determining the dissociation constant of equilibrium 1 (Kj) is similar to the one I used (Fein, 1991) to solve for aluminum-acetate dissociation constants. Details of the calculation procedure and of the standard states that are employed were given in Fein (1991). Values of the equilibrium constants that are used in the calculations, and their sources, are compiled in Table 2. A set of solubility curves as functions of oxalate concentration is generated by using different values for K| to produce each curve. The solubility curve that best fits the data defines the value of Kj that is GEOLOGY, October 1991
consistent with the observed solubilities. The results of these calculations are depicted in Figure 1, which illustrates the solubility behaviors calculated by using values of-11.0, -13.5, and -16.5 for log Kj. Equilibrium 1 was neglected in order to calculate the solubility of gibbsite without aluminum-oxalate complexes (lowest dashed curve). The curve generated by using a value of -16.5 provides a conservative bound on the extent of aluminum-oxalate complexing required by the data. As Figure 1 shows, the differences between the calculated solubility behaviors decrease as log K j decreases. This is a result of the solubility limitations that are imposed by mass-balance restrictions. Clearly, the curve that is calculated by using a value of-13.5 for log Ki does not St the data. However, the solubilities that are calculated by using values that are less than or equal to -16.5 all provide reasonable fits to the data. The experiments can only provide an upper bound to the value of log Kj. Therefore, a value of -16.5 for log K | represents the most conservative estimate of the extent of aluminum-oxalate complexing at 80 °C. The data require a stability at least this high, but they do not preclude significantly higher stabilities. GEOLOGIC APPLICATIONS Using the results from this study, it is now possible to quantify the role of aluminum-oxalate complexing in reservoir diagenesis. The discussion below is based on the most conservative estimate for the importance of aluminum-oxalate complexing. There are two reasons that the estimate is conservative: the experiments quantified the minimum stability of aluminum-oxalate complexes, and A l ( O x ) 3 3 ~ (which has a low Al:Ox ratio) was assumed to be the dominant complex. Therefore, the results of these geologic applications represent minimum equilibrium estimates for the degree of aluminum mobility in 80 °C sedimentary basin fluids. The examples that follow represent calculated gibbsite solubilities in realistic
oxalate- and acetate-bearing sodium-chloride fluids. The solubility of gibbsite is used to model aluminum mobility in oil-field waters. Figure 2 illustrates the solubility of gibbsite at 80 °C, in 0.1 molal NaCl fluids that contain no oxalate, 100 ppm oxalate, and 500 ppm oxalate. These oxalate concentrations coircspond to the range of concentrations observed in oil-field formation waters by MacGowan and Surdam (1988). Therefore, this example demonstrates the range of potential aluminum concentrations in actual reservoir fluids. Figure 2 shows the results of aluminum-oxalate complexing on the solubility of gibbsite. The experimental systems were not pH buffered. Thus, although the systems were thermodynamically Well delimited, pH effects as well as aluminum-oxalate complexing effects contributed to the observed solubility behavior. Figure 2 separates these two effects. As Figure 2 indicates, the presence of oxalate significantly increases the solubility of gibbsite at 80 °C in fluids with pH values between - 3 . 0 and 6.5. Within this range, the presence of oxalate can enhance markedly the amount of aluminum that sedimentary basin fluids can transport. For example, at a pH value of 4.5, the solubility of gibbsite in oxalate-free solutions is 0.02 ppm aluminum. The presence of 100 ppm oxalate increases the solubility to be greater than 5 ppm aluminum, and the presence of 500 ppm oxalate causes the solubility to increase further, to be greater than 40 ppm aluminum. Note that other cations may compete with aluminum to form complexes with oxalate and thereby reduce aqueous aluminum concentrations. However, the results of this study indicate that the presence of oxalate in formation waters can lead to drastically increased aluminosilicate mineral solubilities and may enhance porosity within sedimentary basin rocks.
o < 75 4-J
TABLE 2. EQUILIBRIUM CONSTANTS USED IN CALCULATIONS Equilibrium 3 f T + Gibbsite
-
4H + + Al(OH)4"
-
Al 3+ + 3 H 2 0 Al 3+ + 4 H 2 0
3H* + Al(OH)3°
-
Al s + + 3 H 2 0
2H* + Al(OH) 2 +
-
Al 3+ + 2 H 2 0
H* + Al(OH) 2+
"
Al 3+ +
H 2 Ox°
»
H
Source
5.2
Fein (1991)
17.5
Wolery (1983)
11.8 7.3
Wolery (1983) Wolery (1983)
3.6
Wolery (1983)
-12.6
Wolery (1983)
0.2
Wolery (1983)
h20
H 2 0 « H* + OH" NaOH° - Na + + OH" +
log K
+ HOx"
-1.4
Daniele et al.(1981)*
HOx" » H + + Ox2" NaOx" - Na + + Ox2"
-4.6 -1.0
Daniele et al. (1981)*
o
Figure 2. Calculated gibbsite solubility, as function of pH, in 0.1 molal NaCl fluids at 80 °C. Solid curve represents fluids that do not contain oxalate; dashed curves represent systems containing oxalate.
- 1
500
1000
3f
-3
Acetate
/
o
A,3*
/
O)
o
-6
-7
No
3
-
— X /
-5
OOO ppm
Log Molality 1038 GEOLOGY, October 1991
75
A I ( O X )
E
