Precipitation of Chromium Acetate Solutions

Abstract

Summary Chromium(III)-polymer systems are often used in permeability modification treatments. The transport of chromium(III) in-depth in porous rocks is sometimes limited due to precipitation. Precipitation is caused by the increase in the solution pH because of fluid-rock interactions, particularly in carbonate rocks. This paper presents experimental data on the rate of precipitation of chromium from chromium(III) acetate solutions. Experimental data were obtained for the precipitation of chromium from chromium acetate solution at constant pH. Data were obtained as a function of pH, temperature, salinity, and OAc/Cr mole ratio. The pH range studied was between 7 and 10, and the temperature range was between 25 and 45°C. Precipitation of chromium was preceded by an induction period where there was no change in concentration. The induction period decreased from 1,200 minutes at pH 7 to less than 200 minutes at pH 10 at 2°C. A higher temperature increased the precipitation rate. Precipitation is delayed by the addition of acetate ion. Both the salinity and salt type have significant effects on the rate of precipitation. A kinetic model was developed to describe both the induction period and the rate of precipitation of chromium from a chromium acetate solution in 1% KCl at 25°C for a pH range of 7 to 10 with OAc/Cr mole ratios between 3 and 9. Background Several investigators1–4 have studied the application of chromium-crosslinked gel systems to carbonate (or carbonate-containing) reservoirs. Results showed that the polymer propagates through carbonate (or carbonate-containing) rock but poor propagation was observed for chromium(III). It was concluded that the retention of chromium was caused by precipitation brought on by the elevated pH environment because of the dissolution of carbonates into the injected fluids. This interpretation is supported by solubility calculations of chromic hydroxide. Fig. 1 shows the solubility of chromic hydroxide using solubility constants from Rai et al.5 McCool et al.1 studied the interaction between dolomite cores and xanthan-chromium(III) gel systems. Significant amounts of chromium(III) precipitated when chromium was injected as a chromium acetate solution. McCool et al. concluded that the chromium was retained due to precipitation at elevated in-situ pH levels. Seright2 studied the propagation of chromium acetate and chromium chloride in Indiana limestone cores. More chromium propagated when the counter-ion was acetate. Seright suggested that chromium was removed from solution by precipitation. Stavland et al.3 found precipitation was the primary mechanism for chromium retention in Berea and Brent sandstone cores. Precipitation was caused by the dissolution of carbonate minerals that increased the pH of the injected solution. The pH of unbuffered solutions that are injected into dolomite matrix rock has been shown to increase due to carbonate dissolution.6 Carbonate dissolution and the corresponding effect on pH behavior in carbonate rock were studied by injecting potassium chloride brine at selected pH values through a 6-in.-long Baker dolomite rock. The pH and magnesium concentration in the effluent (for contact times of a few hours) as a function of the pH of the injected potassium chloride brine are shown by the symbols in Fig. 2. The lines in Fig. 2 are the calculated pH and magnesium concentrations for a simulation of geochemical reactions using the computer program PHREEQE.7 The effluent pH exhibited a three-stage behavior when the pH of the injected solution increased from 1 to 13. In the typical pH range of injected gelant, the effluent pH reached around 10 for a nonbuffered solution. This study of fluid-rock interactions showed the dramatic increase in pH that can occur by carbonate dissolution when brine solutions that do not have strong buffering capacities are injected through carbonate rocks. Polyacrylamide-chromium acetate is a commonly used gel system. A study of aqueous chromium acetate8 showed that dissolving solid chromic triacetate in water produced a complex mixture in which bridge-structured trimers were the dominant species, including both cyclic and linear trimers. Increasing the pH of the trimeric chromium(III) solution causes hydroxyl groups to replace the bridging acetate group. Hydroxyl substitution converts the cyclic structure to a linear structure. Additional hydroxyls cause precipitation of the complex. Bryant et al.4 indicated that the precipitation of chromium involved the slow, irreversible formation of insoluble chromium(III) colloids such as Cr(OH)3(H2O)3. Burrafato et al.9 studied the stability of chromium(III) solution with various carboxylate ligands. It was observed that chromium acetate precipitates at moderate pH in unbuffered solutions. Chromium precipitation is a kinetic process1 and the precipitation rate and chromium solubility are dependent upon many parameters. In this work, the effects of pH, temperature, salinity, acetate ligand concentration, and salt type on chromium precipitation were studied using constant pH titrations. The investigation covered experimental conditions for a pH range between 7 and 10. The experimental study was concentrated between pH 7 and 10 because the pH of a nonbuffered chromium acetate solution usually increases to this range after being injected through dolomite core. The temperature range studied was between 25 and 45°C, the salinity range was between 0 and 5% KCl, and the ratio of acetate to chromium was between 3 and 9. The effect of salt type on precipitation was also studied for monovalent and divalent cations. Experiment Materials. The chromium triacetate was obtained from Alfa Products. The empirical formula of the material is Cr(OAc)3·H2O. Chromic chloride, CrCl3·6H2O, was obtained from Fisher Scientific. The titratant was 0.1 M NaOH (Fisher Scientific) unless described otherwise. Reagent grade potassium chloride was used to adjust the ionic strength in the solution. Distilled, de-ionized, and de-aerated water was used to prepare all solutions. Except for those specified otherwise, all chromic acetate solutions were prepared by dissolving solid Cr(OAc)3·H2O in a 1% potassium chloride solution. The initial pH of the Cr(OAc)3 solution (200 ppm chromium) was about 4.5 and was not adjusted during the aging period prior to titration. The aging period for the chromium solution was at least 7 days. Acetic acid (1.0 M) prepared from reagent-grade glacial acetic acid was used to adjust the ratio of acetate to chromium.

Leak-off During Placement of Cr(III)-Partially Hydrolyzed Polyacrylamide Gelling Solution in Fractured Porous Media

“…Also, after a shut-in period of 7 days, brine injection into the core resulted in expulsion of a quarter of a pore volume of polymeric solution. At this time, the permeability was reduced by three orders of magnitude with first (Zou et al 1999), and hindered crosslinking and further growth of aggregates.…”

Development of Polymer Gel Systems to Improve Volumetric Sweep and Reduce Producing Water/Oil Ratios

“…and Martell A.E., 1964]. Zou et al [2000b] modeled chromium precipitation in bulk experiments using two equations, one that described an induction time period with no precipitation followed by precipitation according to Eq. 6.10.…”

“…The rate constant, k 0 and the exponents, α, β, γ in the equation are fitting parameters. The following values were proposed by Zou et al [2000b]: k 0 = 2.6x10 -3 (min M α+γ+β-1 ) -1 , α = 1.0, β = 0.37, γ = -1.2. These parameters were adjusted to have a better match to the experimental data.…”

“…Precipitation has been identified as an important retention mechanism impeding the transport of chromium(III) in laboratory flow experiments [Seright, 1992;Stavland et al, 1993Stavland et al, , 1995Bartosek et al, 1994;McCool et al, 1995]. Zou et al [2000b] measured the rate of chromium(III) precipitation in chromium(III) acetate solutions as a function of pH, temperature, salinity, chromium salt type, salt type and acetate-tochromium(III) ratio. Chromium precipitation was characterized by an induction period followed by precipitation.…”

Development of Polymer Gel Systems to Improve Volumetric Sweep and Reduce Producing Water/Oil Ratios

“…Zou et al [2000] modeled chromium precipitation in bulk experiments using two equations, one that described an induction time period with no precipitation followed by precipitation according to Equation 22. Induction time periods were not observed in flow experiments so chromium precipitation was modeled with Equation 22 only.…”